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395 Cards in this Set

  • Front
  • Back
plasma membrane
allows the cell to maintain cytoplasmic composition (lipid, permeability barrier), contains proteins (enzymes, ion channels, receptors and antigens), composition can vary from cell to cell
constituents of the ECF (serum)
Na+: 142 mM/L
K+: 4.2 mM/L
Ca2+: 2 mM/L
Cl-: 108 mM/L
HCO3-: 28 mM/L
glucose: 85 mg/dL
pH: 7.4
Y- (HCO3-, PO43-, proteins, nucleic acids, metabolites): 40
Mg2+: 1-2
constituents of the ICF (cytoplasm)
Na+: 5-15 mM/L
K+: 140 mM/L
Ca2+: 1*10^-4 mM/L
Cl-: 5-15 mM/L
pH: 7.2
Y- (HCO3-, PO43-, proteins, nucleic acids, metabolites): 130
Mg2+: 0.5
lipid composition of the plasma membrane
1. phospholipid (most numerous component)
2. cholesterol (the amount of cholesterol may vary with the type of membrane)
3. glycolipids (5% of lipid molecules)
urea
a hydrophilic molecule (polar), the + and – regions of water molecules orient in relation to polar solutes (readily dissolves in water), the free energy of the system is reduced by these interactions
hydrophobic molecules
are nonpolar molecules, where water molecules can not form hydrogen bonds with these substances (ex: hydrocarbons), prevent interactions between water and the molecules and so there is an INC in the free energy of the system
phospholipids
fat derivatives in which one fatty acid has been replaced by a phosphate group and one of several nitrogen containing molecules, is an ampiphatic (amphiphilic) molecule, hydrophobic part is the hydrocarbon chain (fatty acids), hydrophilic part is the phosphate, glycerol and amino groups (they have charges)
type of double bonds found in unsaturated fatty acids
cis-double bonds
what effects membrane fluidity
the length and saturation of the fatty acid tails as well as temperature, thin plasma membranes are indicative of unsaturated phospholipids (more fluid, phase transition occurs at lower temperature) while thick plasma mebranes are indicative of saturated plasma membranes (ex: shpingomyelin, reduce fluidity)
phase transition
occurs when phospholipid bilayers are exposed to temperature changes that causes a change from the fluid state to a gel state, occurs with colling
what happens to the energy of a solution of amphipathic molecules after the nonpolar regions clump together
it reduces the free energy of the solution
micelles or bilayers
both minimize contact between water and nonpolar groups
self-sealing and self repairing plasma membranes
free edges are energetically unfavorable, they spontaneously rearrange to elimate the free edge(seal up), the creation of a living cell resulted from the shape and amphiphatic nature of the phospholipid
properties of the phospholipid bilayer
1. barrier to charged or polar molecules (things that are soluble in water don’t cross the wall well)-energetically unfavorable (delta G >> O) for these particles to enter the nonpolar, hydrocarbon core of the bilary
2. a lipid bilayer has self-healing and membrane fluidity properties
lipid rafts
stable structure in the plasma membrane composed from sphingolipids (long and saturated hydrocarbon chains) and cholesterol, are thicker than the bulk membrane (rest of the membrane), have different lipids and proteins vs. bulk membrane
purpose of having different proteins on lipid rafts
they give the rafts a different biochemical activity, some include immune mechanisms or cell signaling mechanisms
types of phospholipids
1. phosphatidyl-ethanolamine-found on the cytoplasmic side (inner leaflet) of the plasma membrane
2. phosphatidyl-serine-found on the cytoplasmic side of the plasma membrane, negatively charged COO-
3. phosphatidyl-choline-found on the matrix/serum/ECF side (outer leaflet) of the plasma membrane
4. sphingomyelin-found on the matrix/serum/ECF side of the plasma membrane, forms lipid rafts
lipids found in different types of membranes
1. liver cell-
a. P_choline: 24
b. P_ethanolamine: 7
c. P_serine: 4
d. Sphingomyelin: 19
e. cholesterol: 17
f. glycolipids: 7
g. other: 22
2. myelin-
a. P_choline: 10
b. P_ethanolamine: 15
c. P_serine: 9
d. Sphingomyelin: 8
e. cholesterol: 2
f. glycolipids: 28
g. other: 8
3. mitochondria
a. P_choline: 39
b. P_ethanolamine: 25
c. P_serine: 2
d. Sphingomyelin: 0
e. cholesterol: 3
f. glycolipids: trace
g. other: 21
why do we need a variety of phospholipids
some membrane proteins can reveal function only in the presence of specific phospholipids, some cytosolic enzymes bind to specific lipid head groups exposed on cytosolic face of a membrane
use of lipids in signaling
membrane lipids are a source of 2nd messengers, phosphatidylserine and phosphatidylinositols are part of a signal cascades that link signal molecules to cellular responses
phosphatidylserine and signaling
negatively charged lipid in the cytoplasm leaflet, required for the activation of protein kinase C (PKC, the phospolipid and Ca+ dependent protein kinase), PS exposed on the cell sureface serves as a signal to induce neighboring cells (macrophages) to phagocytose the dead cell and digest it
PKC
a soluble protein in the cytoplasm, after Ca2+ binds to it, PKC moves to the membrane where it binds to phosphatidylserine (PS), No PS -> No binding -> No function
phosphatidylinositols and signaling (PI)
is a substrate for a number of kinases activated by the binding of signal molecules to trnasmembrane receptors, the phosphorylated lipid recruits specifc proteins to the membrane and things happen (enzyme cascade), hydrolyzied by a number of phospholipaes activated by the binging of signal molecules to transmembrane receptors, splits PI into diacylglycerol and inositol 1,4,5-triphosphate (IP3)
mechanism for PI degradation into IP3 and DAG
1. DAG stays in the membrane and activates PKC
2. IP3 is soluble and activates IP3-receptors in the ER, these channels release Ca2+ into the cytoplasm, Ca2+ activates and inhibits many proteins
cholesterol
cholesterol is a major constituent of plasma membranes, lies parallel to the fatty acids, reduces fluidity (lateral mobility) and improves the barrier, the steroid rings of cholesterol are very rigid, interacts specifically with the outer portion of the phospholipid (polar head)
glycolipids
are sugar-containing lipid molecules present in small quantities, some lipids mainly in the outer leaflet of the plasma membrane are modified by the addition of fairly complex carbs, these glycolipids are thought to partition into the lipid rafts
functions of glycolipids
1. chemical protection (blood group antigens)
2. negative charged glycolipids (gangliosides) attracts Ca2+ and alters electrical fields
3. cell recognition (cells have specific glycolipids for membrane-bound carbohydrate-binding proteins)
4. entry mechanism for some toxins (cholera toxin must bind gangliosides to enter a cell and prolonged INC of cAMP and efflux of Na+ and water
membrane proteins
the phospholipid bilayer is the primary structure with some active functions, the membrane proteins is primarily responsible for functional properties with some structural contributions (100 different proteins, enzyme, transport protein, receptors, channels, antigens), the amounts and types of proteins are highly variable
integral membrane proteins
proteins embedded in the phospholipid bilayer including transmembrane protein, many of the proteins associated with the plasma membrane are tightly bound to it, some are attached to lipids in the bilayer, poorly soluble or insoluble in water, isolated and purified only with the help of detergents that destroy the membrane
transmembrane proteins
type of integral membrane protein, where the polypeptide chain actually traverses the lipid bilayer, some span the bilayer several times to form a hydrophilic channel through which certain ions and molecules can enter or leave the cell, are amphipathic
peripheral membrane proteins
associated with the surface of the phospholipid bilayer, are more loosely associated with the membrane, are usually attached noncovalently to the protruding portions of integral membrane proteins, do not extend into the hydrophobic interior of the lipid bilayer, freq. associate with integral membrane proteins or hydrophilic surfaces of the bilayer (protein-protein interaction), relatively easy to isolate protein by gentile extraction methods
type of bond for transmembrane proteins
covalent link to one leaflet, mainly the cytosolic monolayer, cross the membrane as either an alpha helix or beta barrel
restriction of movement in membrane proteins
some exposed at the interior face are tethered to cytoskeltal elements like actin microfilaments, some exposed at the exterior face are anchored to components of the extracellular matrix like collagen
spectrin
heterodimers associate head to tail, many proteins bind to spectrin, enable RBC to endure the stress passing through capillary (maintaining structural integrity and biconcave shape), absence of spectrin can lead to anemia because of spherical shape (fragile)
hereditary spherocytosis
speroidal RBC, less deformable and vulnerable to splenic sequestration, defect in cytoskeleton (spectrin) of RBC membrane, hemolytic anemia
glycoproteins
plasma membrane proteins do not protrude naked from the exterior of the cell, are decorated by carbohydrates (oligosaccharide chains), as with the lipids, membrane proteins on the extracellular surface are commonly glycosylated, this is a marker for an extracellular domain
oligosaccharides and proteins
can be added to both glycolipids and extracellular domains of membrane proteins
glycocalyx
carbohydrate zone of the cell membrane
functions of the glycocalyx
1. protection against mechanical and chemical damage
2. recognition by friends or foes
3. cell to cell adhesion (lectin)
4. prevent undesirable protein-protein interactions
transport of small molecules across membranes
an essential function of a cell membrane is to control the movement of solutes between two compartments (permeability barriers), the result is to make the contents of one compartment different from the other (maintain concentration of solutes in the cytoplasm)
molecules that can cross the membrane through diffusion
1. hydrophobic molecules (O2, CO2, N2, benzene), diffuse easily
2. small uncharged polar molecules (H20, urea, glycerol), have relatively high permeability
molecules that can cross the membrane through protein-mediated transport
can be either facilitated transport or active transport
1. large uncharged polar molecules (glucose, sucrose), very little transport across, depends on conc.
2. ions (H+, Na+, H2CO3, K+), cannot cross the membrane
simple diffusion
refers to aprocess where a substance passes through a membrane without the aid of an intermediary such as an integral membrane protein, diffusion is due to the intermingling of molecules because of their random thermal motion
water transport across lipid bilayer
water crosses phospholipid bilayers reasonably well, about a million times more permeable than Na+ (polar and charged) by diffusion
rate of diffusion
= gradient X temp X surface area X solubility
distance X sq. rt of the molecular mass
things that influence the rate of diffusion
concentration gradient
temperature
solubility of the molecule
SA of the membrane
size of molecule involved
distance the molecule has to travel
osmosis
flow of water across a semipermeable membrane from one compartment in which the solute concentrate is lower to the other compartment which the concentration is higher, water moves into a volume containing a higher concentration of solute
semi-permeable membrane
permeable to water but impermeable to solutes
amplitude of pressure
depends on the concentration of the solute, the number of osmotically active particles per molecule of solute is i
for 1 NaCl, i=2
for 1 glucose, i=1
for 1 CaCl2, i=3
for 1 protein, i=1
1 M glucose
has 0.5 M NaCl = 0.333 CaCl2
osmolarity of a solution
= osmotic coefficient * i * conc.
what are the major factors of osmotic pressure
degree of ionization and concentration
what does tonicity depend on
the concentrations of impermeant solutes
what is the osmolarity for hypertonic solutions
> 282 mOsm/L
what is the job of the kidney, liver and circulatory system in terms of osmosis
osmotically active components of the extracellular milieu include ions and plasma proteins and it is these organs responsibility to keep this constant, failure can result in changes in cell volume, cell function and cell survival
edema
cell shrinkage, interstitial swelling, found in albumin and liver disease
protein-mediated transport
water soluble molecuesl are essential for cell survival but do not corss membrane at appreciable rates by simple diffusion, ions and charged or polar molecules have low permeability across phospholipid bilayers, so need to used specialized transmembrane proteins, carrier proteins and channel proteins
importance of metallic cations
cell membrane can store potential energy in the form of electrochemical gradients
their high concs. make them important osmolytes in setting cell volume
they function in electrical signaling by nerve, muscle and other cells
important pathologically
two types of protein mediated membrane transport
facilitated (passive) transport-used to equilibrate concentration and requires no energy
active transport-used against electrochemical gradients and requires energy
properties of protein-mediated membrane transport
transport is more rapid than that by simple diffusion
transport rate shows saturation kinetics (early time-transport fast, when certain concentration reaches, transport rate increases no further)
mediating protein has chemical specificity (D-glucose transports well, L-glucose is poorly transported)
saturation kinetics
all transporters (passive and active) have binding sites for their substrates just like enzymes, the affinity, on-rate, and concentration of substrate will determine fraction of sites occupied and transport rate
channels
gated pores, when the gate is open the solute rips through, fast, millions of solute particles per second, cannot couple to an energy source, solute can only be transported down an energy gradient
carriers
bind the solute, undergo a conformational change, release the solute on the other side, relatively slow (0.5 to 100 cycles/sec), monosaccharides enter cells by faciliatated diffusion, D-glucose (insulin relation), galactose, arabinose but not L-glucose, mannitol
common properties of facilitated transport
no energy requirement
no depressed by metabolic inhibitors
can not move uncharged substance against concentration gradients or ion gradients
equalize concentration
active transport
mechanism by which soltes are carried against concentration and voltage gradients, they require energy (which is linked to energy metabolism), can be inhibited by interfering energy metabolism
types of energy sources for active transport
ion gradients-come from electrochemical gradients, DG < 0 is run down of the gradient
ATP-DG < 0, ATP -> ADP +Pi
light-DG < 0, the reaction in the core of the sun
Na+-driven active transport
a secondary active transport for glucose and amino acids, can be an exchanger with H+, Cl—HCO3-, or Ca2+, couples movement of Na+ down its electrochemical gradient with the movement of glucose up its chemical gradient, Na+ is reduced and glucose transport decreases, get Na+ back out of the cell through an ATP-driven Na+ pump
H+-driven active transport
powers coupled transport of many solutes across plasma membrane of bacteria and yeast and intracellular membranes of eukaryotes
types of carrier-mediated transport
uniport, symport or antiport, symport and antiport are considered coupled transporters
Na+-H+ exchanger
pumps H+ out of the cytoplasm (antiport), Na+ in for H+ out, important in controlling cytoplasmic pH
Na+-driven Cl—HCO3- exchanger
brings HCO3- into the cell, influx of Na+ and HCO2- (NaHCO3), efflux of Cl- and H+ (HCL)
methods for maintaining intracellular pH
1. pump out H+ from the cell through an Na+-H+ exchanger
2. bring HCO3- into the cell (Na+-driven Cl—HCO3- exchanger or Na-independent Cl- -HCO3- exchanger
3. ATP-driven H+ pump
Na+-Ca2+ pumps
Na+-Ca2+ exchange to lower free Ca2+ in the cytoplasm (antiport), control of cytoplasmic free Ca2+ is very important because the Ca2+ activates numerous enzymes and activites, 3 Na+ in for 1 Ca2+ out at negative resting potential with low intracellular Na+, if the cell is depolarized and/or intracellular Na+ is high 1 Ca2+ in for 3 Na+ out, important in heart
glucose transport from the intestinal lumen to the extracellular fluid
facilitated through first an Na+-driven glucose symport then by a Na+-K+ pump or carrier protein mediating passive transport of glucose (glucose concs. highest in the intestinal epithelium)
types of ATP-driven Carrier proteins
1. Na+-K+ pump
2. Ca2+ pump
3. H+-K+ pump
concentrations of Na+ and K+ utilized by the Na+-K+ pump
cytoplasm: Na+ (5-15) and K+ (140)
serum: Na+ (145) and K+ (5)
Ex: Na+ (+70 mV) and K+ (-87 mV)
Na+-K+ pump
uses ATP as its power source to move solutes against the electrochemical gradients, autophosphorylation, 3 Na+ out plus 2K+ in per ATP (osmolarity), net transfer of positive charge out of cell and thus contribute resting membrane potential (RMP), there is around 10% of electrical negativity inside of cell, essential in maintaining the normal volume of a cell
role of phosphorylation and dephosphorylation for the Na+-K+ pump
phosphorylation is used for binding Na+, dephosphorylation is used for binding K+
what happens if the Na+-K+ pump stops
Na+ will leak in, in our bodies things don’t leak out fast enough to compensate, this causes the cell to swell and burst, cells die
amount of calories that go into running the Na+-K+ pump
about 1/3 of consumed calories goes into running this
cardiac glycosides
ouabain, acetylstrophagnthidin, strophanthdin (estimation of Na-K-pump involvement on RMP), they compete with K+ at the extracellular binding site and lock the pump, modest inhibition of the Na+-K+ pump INC intracellular Na+, reducing activity of Na+-Ca2+ exchange raising intracellular Ca2+ and INC the ability of the heart to pump blood
Ca2+ ATPse pumps
plasma membrane ones pump Ca2+ out of the cytoplasm, ER/SR ones used in Ca2+ reabsorption into the SR or ER
ways to maintain a very low conc. of free Ca2+ in the cytosol
1. Na+-driven Ca2+ exchanger
2. Ca2+ pump
3. Ca2+ pump in ER membrane
ABC transporters
have two ATP-binding cassettes that bind ATP as part of the transport reaction, ATP-binding and hydrolysis drives conformational changes, these are the largest and most diverse family of transporters
multidrug resistance protein (MDR)
a class of ABC transporters, pump hydrophobic cytotoxic compounds out of bacterial and eukaryotic cells, expression is often bad news
cancer therapy and MDR
expression of MDR proteins produce tumors that resist chemotherapy
bacteria and MDR
expression of MDR proteins produces antibiotic resistant infections
malartia and MDR
resistance to antimalaria drug (Chloroquien), pump out through ABC transporter
what determines resting and action potentials
ion gradients, ion channels and the Nernst Equation
current
movement of charge (coulombs/sec), flux of charge
voltage difference between two points of work
equal to force * distance, voltage is the amount needed to move a charge from one point to another
charged solutes and diffusion
will diffuse in response to both concentration and voltage gradients
forces acting on a given ionic species
1. chemical gradients-the chemical concentration gradient tends to move ions down this gradient
2. electrical gradient-electrostatic force due to the charge separation across the membrane tends to move ions in a direction determined by its particular charge
K+ movement
amount that moves is small, the “extra” K+ outside and the “extra” A- on the inside are plastered against the membrane, a dipper of solution will have equal + and –
Nernst equation
describes voltage and concentration gradients that have the same energy
voltage gradient = RT/zF ln ([Cl]o/[Cl]i) = 60/z log ([Cl]o/[Cl]i)
R (gas constant), F = faraday constant, z = valence of the ion
Nernst equilibrium
where the net gradient of a membrane is nil, for Cl-, occurs when [Cl]i = 10, [Cl]o = 110, charge is -62 mV inside, occurs when the concentration gradient and the voltage gradient have the same energy, therefore there is no energy change as Cl- moves from left to right or from right to left
Ek for K+
[K+]o = 5, [K+]i = 140, Ek = -87 mV (inside)
membrane potential
critical for maintenance of the integrity of the cell membrane including the protein complexes within the lipid bilayer, and for signal transduction in extiable cells such as nerve and muscle, to understand how nerves conduct information or how muscles are induced to contract, one must first understand the origin of the membrane potential
resting potential
negative in all cells, -67 mV
what generates the -67 resting potential
large neatively charged molecules inside the cell too large to pass through, K+ leaks out easier (leaves a positive charge outside) (Na+ and A- don’t cross readily, but can a very little amount that’s why the resting potential is -67 and not -87), Na-K exchange pump leaves a + charge outside
anions (intracellular vs. extracellular)
extracellular anions are mostly Cl-, intracellular anions are mostly amino acids, proteins and phosphates
if K+ is diffusing out of the cell, why isn’t there a detection of a change in concentration
the movement of a very few ions gives a hug voltage, K+ might fall to 139.99999 mM instead of 140, this is undetectable chemically
movement of other ions in real cells
ions are generally no at equilibrium, they passively leak in or out of the cell and are actively pumped to maintain steady state gradients, K+ (leaks out and is pumped in), Na+ (leaks in and is pumped out), Cl- (can either leak in/out and pumped out/in or can even be at equilibrium)
Nernst equilibrium and other ions
each ion gradient contributes its Nernst equilibrium potential to the resting potential, contribution is weighted such that ions with higher relative permeabilities contribute more
high permeability
indicates that particle mass moves easily through a membrane
high conductance
indicates that electrical charge moves easily through a membrane, is the inverse of electrical resistance
Goldman, Hodgkin, Katz (GHK) equation
a quantitative and unambiguous version of all the permeabilites of each ion
Em = RT/F ln (P[K]o + P[Na]o + P[Cl]i / P[K]i + P[Na]i + P[Cl]o)
what parameters can give a resting potential of -67 mV
K+ intra 140, extra 5, relative permeability of 1
Na intra 5, extra 140, relative permeability of 0.01
Cl intra 145, extra 145, relative permeability of 0.032
electrical signals
are receptor potential, synaptic potential and action potential, are all caused by transient changes in the current flow into and out of the membrane, drives the electrical potential across the plasma membrane away of its resting condition
kidney malfunction
can lead to an elevation in extracellular [K+]
which cell manipulate the membrane potential
skeletal, cardiac, and smooth muscle, they activate contraction or secretion, the signal travels quickly along long cells
what is an action potential
a transient depolarization from the normal resting negative membrane potential, the properties vary from cell to cell, duration can be a few ms to a few seconds, amplitude can be from 20 to 120 mV, and it can be evoked by a stimulation or spontaneous, extracellular 0 mV and intracellular is -60 mV
mechanism for action potential
basic mechanism that is pretty constant from cell to cell, changes in membrane permeability mediated by opening and closing of ion channels, but the details vary a lot from cell to cell including the identity of the channels and ions involved an the mechanisms that control the gating of the channels
nerve cell vs. cardiac myocyte action potential
nerve cell, has a very rapid INC then DEC in membrane potential while cardiac myocytes have just as fast of an INC, but a much slower, gradual DEC in potential
giant squid axons
Hodgkin-Huxley model, very much like mammalian axons, fewer kinds of channels than in cardiac muscle, smooth muscle and neurons (GNa, Gk, and leak K+ conductance)
properties of action potentials
1. threshold (all or none)
2. refractory period
3. conduction
depolarization
occurs when + charge is injected, makes the membrane potential less negative
hyperpolarization
occurs when – charge is injected, makes the membrane potential more negative
threshold
injections of small amounts of current result in small shifts in membrane potential, when the injection stops, the membrane potential recovers more or less directly to the resting potential unless threshold is reached
level of hyperpolarization for -8 microAmps
11 mV level of hyperpolarization
level of hyperpolarization for -4 microAmps
6 mV level of hyperpolarization
level of hyperpolarization for -2 microAmps
3 mV level of hyperpolarization
level of depolarization for +2 microAmps
3 mV level of depolarization
level of depolarization for +4 microAmps
6 mV level of depolarization, has slower recovery
level of depolarization for +8 microAmps
93 mV level of depolarization
level of depolarization for +16 microAmps
97 mV level o depolarization, about the same has +8 (supramaximal stimulation)
subtreshold stimulus
stimuli that fail to elicit an action potential
suprathreshold stimulus
stimuli that elicit an action potential
all or none response
the fact that action potentials are (nearly) independent of the stimulus, you either get an action potential or you don’t
mechanisms of the action potential in the giant squid axon
1. K+ leak channels-major conductance during resting state, sets the K+ leak channels (Pk > PNa)
2. voltage-gated Na+ channels-during onset of AP, 500 fold INC in Na+ conductance
3. voltage-gated K+ channels-opening slowly after Na+ channel open
opening of Na+ channels
causes an INC in PNa and shift membrane potential toward ENa (79.0)
opening of K+ channels
causes an INC in Pk and shift membrane potential toward Ek (-86.8)
activation of voltage-gated Na+ channels
they respond to a small depolarization (between -70 and -50 mV, this INC PNa 500-5000 fold, this causes Na+ to enter the cell down its electro-chemical gradient causing depolarization, very little Na+ will cause a large depolarization, this depolarization causes more Na+ channels to open, INC Na+ influx and INC depolarization
inactivation of voltage-gated Na+ channels
after depolarization, Na+ channels inactivate spontaneously causing PNa to fall, membrane potential begins to recover toward the resting membrane potential, inactivation gate will not reopen until the membrane potential returns to the original RMP level (recovery from inactivation)
what is the tetrodotoxin effect?
it is a Na+ channel blocker, tetrodotoxin is present in pupperfish, the blue-ringed octopus, harlequin forgs, some snails, brabs and roughskinned newts, it affects the nervous system in such a way as to prevent the propagation of nerve impulses
voltage-gated K+ channels
open in response to depolarization INCing Pk, open more slowly than the Na+ channels, results in INC K+ efflux and helps restore the resting potential, INC in PNa drives the depolarization, fall in PNa (inactivation) and Pk (activation) drive the repolarization, the leads to full recovery of the RMP within a few ms
What is tetraethylammonium chloride effect?
K+ channel blocker, the membrane would depolarize very slowly from very slow Na+ leakage and faster K+ leakage
different phases of the AP in ventricular myocytes or Purkinje cells
1. phase 0-rapid depolarization to a threshold voltage of about -70 mV, caused by a transient INC in fast Na+ channel conductance, this causes a fall gK+, moves membrane potential closer to ENa and away from Ek
2. phase 1-represents an initial repolarization that is caused by the opening of a special type of K+ channel
3. phase 2-plateau phase, occurs because of the large INC in slow inward gCa+ delaying the repolarization, inward calcium from long-lasting (L-type) calcium channels that open up when the membrane potential depolarizes to about -40 mV
4. phase 3-repolarization, occurs when gK+ INC and gCa2+ DEC
5. phase 4-phase of RMP, remains near the equilibrium potential for K+
actional potential in non-pacemaker cells
primarily determined by changes in fast Na+, slow Ca2+ and K+ conductances
refractory period
occurs because Na+ channels are unable to respond to a stimulus right after they are inactivated repolarization, the cell remains this way until the channels recover from inactivation, this limits the frequency at which a cell can generate action potentials, two types (absolute and relative)
absolute refactory period
occurs right depolarization when repolarization begins, no matter how strongly the cell is stimulated it is unable to fire a second action potential, due to inactivated Na+ channels, prevents action potentials until Vm is negative enough to open channels
relative refractory period
occurs during the last part of the action potential, the cell is able to fire a second action potential, but a stronger than normal stimuli is required, too much repolarization, need a higher stimulus until voltage gated K+ channels close (found in premature contraction in heart)
how long does the refractory period last
about 6 to 7 ms, before 7 ms, a 30 microAmp current will not stimulate an action potential
conduction of the AP
an action potential started at one end of a long cell will be conducted along the length of the cell, the amplitude and other characteristics are pretty constant, e.g. motor neuron to skeletal muscle fibers (sometimes 1 meter)
electronic conduction
normal resting nerve fiber, nerve fiber has been exited in mid-portion (INC Na+ permeability), if past threshold then a progressive depolarization is produced and propagates along a nerve or muscle fiber
what does the rate of conduction depend on?
size of the cell (larger cells conduct faster), size of the Na+ influx (larger Na+ influx means faster conduction, this in turn depends on the number of Na+ channels that are opening), and expression of Na+ channels
what determines the speed of electrotronic conduction
1. electrical resistance of the plasma membrane to current flow
2. the resistance of the longitudinal path down the inside of the fiber-larger diameter of axon offers a greater cross-sectional area to the internal flow of the current, which reduces resistance by providing many parallel for currents to continue down the interior of the axon (power cable)
distances role on depolarization
depolarization falls off with distance along the axon from a region where the AP is occurring because membrane is a leaky insulator
myelination
greatly INC rate of conduction in some axons in vertebrates, the AP jump from node to node (Saltatory conduction through node of Ranvier), unmyelinated (0.25 m/sec) vs. myelinated (100 m/sec)
electrophysiology
extracellular recording, intracellular recording, whole-cell recording, single channel recording
patch clamp
how to study electrophysiology
structure of voltage-gated ion channels
the first to be cloned, tetramer, hydrophaty plots and other data indicate 6 transmembrane alpha helices plus a bit of beta helix in each monomer
voltage-clamp
a feedback amplifier is used to set the membrane potential to a desired value, measure currents or channel behavior, want to control the voltage and record the current
general properties of ion channel proteins
1. most ion channels have narrow and highly selective pores that can open and close
2. ion transport through ion channel is faster than carrier protein
3. the transport of ion is passive or facilitated via electrochemical gradients
4. ion channel proteins have “ion selectivity” (selective filter) and “gating properties” (not continuously open)
classification of ion channels
gated by voltage, ligands (Ca2+, neurotransmitters) or mechanically
examples of voltage-gated ion channels
1. Na+ channel
2. K+ channel
3. Ca2+ channel
function of Na+ channel gating
generate (upstroke) and propagate self-regenerating action potential localization: clustered at axon initial segments, nodes of Ranvier and post-synaptic folds of neuromuscular junction
Na+ channels
found in the nerve and muscle cells and is used in the rapid electrical signaling found in these cells, principle subunit is a polypeptide chain of more than 18 amino aids, structure of alpha subunits (4 repeated motifs each with 6 transmembrane spanning subunits)
blocerks of Na+ channel
tetrodotoxin, Saxitoxin, lidocaine
subtypes of alpha subunits
SCN5A (Nav1.5)
structure of the Na+ channel
one portion of the channel determines its ion selectivity, quite selective for sodium channels, two different gates, one is controlled by a voltage sensor which responds to the level of the membrane potential the other is an inactivation gate that limits the period of time the channel remains open, despite steady stimulation
voltage gating in other cell types
pretty much the same in all excitable cells, squid axon, mammalian axon or neuron, mammalian cardiac myocyte
Na+ channel related diseases
generalized epilepsy, long QT syndrome 3, progressive cardiac conduction defect (PCCD2), congential non-progressive heart block, myasthenia, muscular dystrophy
types of K+ channels
1. voltage-gated
2. inward rectifying
3. Ca2+ sensitive
4. ATP-sensitive
5. mecahno-sensitive
6. Type A
7. receptor-coupled
K+ channel gating
activated by membrane depolarization, in the squid they activate slowly (delayed rectifier K+ channel) and stay open, in mammals, currents that inactivate by a mechanism are similar to the inactivation of Na+ channels (rapid inactivation by ball and chain model), excitable cells use K+ channels with different protperties to tune their AP, the activity of K+ channels sets membrane potential that controls the activity of voltage-gated Ca2+ channels and hence Ca2+ influx
types of Ca2+ sensitive K+ channels
1. high conductance (BK) chanels-gated by internal Ca2+ and membrane potential, conductance = 100 to 220 picoSiemens (pS)
2. intermediate conductance (IK) channels-gated only by internal Ca2+, more sensitive than BK channels, conductance = 20 to 85 pS
3. small conductance (SK) channels-gated only by internal Ca2+, more sensitive than BK channels, conductance = 2 to 20 pS
SK and IK channels
have a relatively low single-channels conductance and are activated by an INC in cytosolic free calcium, contain six transmembrane domains and one pore loop, the functional channels are tetramers
SK blockers
charybdotoxin
Kv channels
are voltage dependent (activated by depolarization) that are expressed in a wide range of cells and tissues, they are composed of four alpha subunits that form the pore of the channel, each subunit has six transmembrane domains, four beta subunits are closely associated with the alpha subunits, so that the functional channel has an octameric structure, nine Kv channel alpha subunit families have been identified: Kv1-Kv9, each family consists of a number of structurally and functionally similar channels
inwardly rectifying K+ channel
2 transmembrane regions (M1 & M2, corresponds to S5 & S6 in Kv channel), 4 subunits surround central pore, P region separates M1 and M2, non-conducting at positive membrane potentials, blocked by external Ba++
Kir channels
characterized by their inward-rectifying current-voltage relationship, they are widely expressed in mammalian cells and their main role is though to be in maintaining membrane potential, subdivided into 7 subfamilies (Kir1-Kir7), regulated by a diverse range of mechanisms
currents property
large inward at potentials negative to K+ equilibrium potential and small outward currents at more positive potentials
blockers of Kir channels
LY97241, Gabbon viper venom, Sr++, Ba++, Cs+
regulation of Kir channels
external K+, internal Mg, intracellular polyamines, ATP or G-proteins
is it the inwardly rectifying K channel that is primarily responsible for repolarization of a cardiac muscle AP?
repolarization of a ventricular muscle cell involves activation of delayed rectifier K channels, as repolarization proceeds these channels close and inwardly rectifying K channels open
G-protein-coupled inwardly-rectifying K+ channels (GIRK)
PTX-sensitive G-alpha-i/o-coupled receptors (release of G-beta-gamma-bind to GIRK-increase channel activity, PTX-insensitive G-alpha-q inhibition of channel activity
KATP channel
KATP INC causes a decrease in channel opening, pancreatic type or cardiac type
KNDP channeld
NDP INC causes an INC in channel opening in the presence of Mg2+, smooth muscle type
characteristics of KATP
1. octameric-four alpha subunits (KIR 6.1 or KIR 6.2) and four beta subunits (SUR1, SUR2A, SUR2B
2. smooth muscle type-KIR6.2/SUR2B
3. sulfonyurea agents-blibenclamide, tolbutamide inhbit channel activity
4. pharmagoligcal KATP activator-pinacidil, cromokalim, lemakalim, dizoxide, minoxidil, nicorandil (induce hyperpolarization)
Voltage gated Ca2+ channels
contain 4 or 5 distinct subunits, alpha-1 subunit, alpha 2 delta, beta and gamma subunits
alpha-1 subunit of voltage gated Ca2+ channels
contains voltage sensitive and Ca2+ selective pore, consists of 4 internal repeated domains (I-IV), each domain contains 6 alpha-helical transmembrane regions (S1-S6)
Domain I of voltage gated Ca2+ channel
responsible for channel activation kinetics, S4: positively charged, forms part of the voltage sensor, domains between S5 & S6 form pore region of channel
L-type (long lasting) Ca2+ channels
1.blockage: dihydropyridine (DHP) also blocked by phenylalkylamines (verapamil), benzothiazepines (diltiazem) and calciseptine
2. activation of L-type Ca2+ channels-strong depolarization (high threshold)
3. inactivation of L-type Ca2+ channels-a little bit by depolarization
N-type Ca2+ channels
• Blockade: w-conotoxin GVIA (Strong; Irreversible) & MVIIA
• DHP insensitive
• Activation: Strong depolarization
• Inactivation: Slow
• Neuronal localization: Presynaptic
P-type Ca2+ channel
• Activation: High threshold, strong depolarization , Inactivation: Slow
• Blockade: Funnel web spider venom; w-agatoxin IVA; w-conotoxin MVIIC
• Insensitive to DHP & w-conotoxin GVIA
• Localization:Neuronal presynaptic,cerebellum: Purkinje cells, neuromuscular junctions
Q-type Ca2+ channels
• Activation: High threshold, strong depolarization
• Blockade: ? more sensitive to w-conotoxin MVIIC than P-type
• Location: Cerebellar granule cells; Hippocampal pyramidal neurons
• Function: Transmitter release
R-type Ca2+ channel
• Activation: High threshold, strong depolarization
• Inactivation: Voltage dependent; Rapid kinetics
• Blockade: SNX-482 peptide from African tarantula, Hysterocrates gigas
• Function: Transmitter release
T-type (transient) Ca2+ channel
1. activation-by depolarization near resting potential, low voltage activation (LVA) threshold
2. inactivation-rapid, window current: small range of voltages, conductance low (8pS)
3. blockade-nickel ions, miberfradil, kurtoxin, peptide from South African scorpion, do not bind dihydropyridines
4. tissue localization-cardiac and vascular smooth muscle, nervous system
5. function-ryhthmic action (pacemaker) potentials in cardiac muscle and neurons, brust firing mode of action potentials, regulate intracellular Ca2+ concentrations
ligand-gated ion channels
1. acetylcholine-gated cation channel-must have ligand and be activated to be open
2. glutamate-gated Ca2+ channel
3. serotonin-gated cation channel
4. GABA-gated Cl- channel
5. glycine-gated Cl- channel
sequential activation of neuromuscular transmission
1. Nerve impulse reaches the nerve terminal. Depolarization opens voltage-gated Ca2+ channels. Increase in Ca2+ triggers release of ACh into the synaptic cleft.
2. ACh binds to Ach receptor in muscle cell membrane, and transiently opening cation channels (localized depolarization)
3. Local depolarization opens voltage gated Na+ channel, and further depolarized
4. Generalized depolarization of the muscle cell membrane activates voltage-gated Ca2+ channels.
5. This causes CICR (Ca2+-induced Ca2+ release from SR).
Muscle cell contract.
Neruomuscular junction
1. presynapse-membrane depolarization, Ca2+ influx, Ach release
2. postynapse-binding Ach receptor, cation influx, end-plate potential, Na channel activation, action potential
3. synaptic cleft-Ach by cholinesterase, choline and acetate reabsorption to synaptic vesicle
4. blockage-curare, alpha-bungarotoxin, myasthenia gravis (treat with anticholinesterase)
cell cytoskeleton
a complex network of protein filaments throughout the cytoplasm of eukaryotic cells
functions of the cytoskeleton
1. control cell shape and movement
2. movement of molecules and organelles within the cell
3. physical support for plasma membrane and mechanical linkages between cells
4. provide communicatiton paths to allow the cell to have specialized services concentrated in different areas
types of cytoskeletal filaments
1. actin filaments-composed of actin monomers
2. microtubules-composed of tubulin
3. intermediate filaments-composed vimentin
these are formed form thousands of identical protein molecules assembled to form a single linear filament (polymerization)
actin filaments
AKA microfilaments (MF), are double stranded helical polymers of actin, 5-9 nm in diameter, can be organized into linear bundles, 2D networks, 3D gels, essential for cell movement
microtubules
composed of tubulin dimmers, 25 nm diameter, form a web-like network within the cytoplasm, one end attached to a microtubule organizeing center (MTOC) or centrosome, form the mitotic spindle during cell division
intermediate filaments
ropelike fibers with 10 nm diameter, composed of intermediate filament proteins, a large and heterogenous family, share similar structural and functional properties, Ifs forms the nuclear lamina beneath the inner nuclear membrane, other types of Ifs extend across the cytoplasm for mechanical strength
characteristics of the cytoskeletal filaments
1. form as helical assemblies of subunits which self-associate
2. differences in subunits structure and attractive forces between them produce differences in stability and mechanical properties
3. held together by weak non-covalent interactions
4. therefore, assembly and disassembly can occur rapidly (covalent bonds aren’t formed or broken, other biological polymers (DNA, RNA, proteins are held together by covalent linkages between subunits))
protofilaments
single strand, single protofilaments are thermally unstable (a break causes just one broken bond), multiple are stable (removing one from the end breaks three total breaks)
disadvantages of single filaments
1. a single string held together by non-covalent interactions is not strong and would require tight binding of each subunit to its neighbor to prevent breaking
2. such tight binding would limit the rate at which the filaments could disassemble, making the cytoskeleton a static structure
3. one potential disadvantage of a protofilament structure: short oligomers can assemble spontaneously but are unstable and disassemble readily
advantages of protofilaments
1. ends are dynamic-subunits can be added or removed rapidly
2. the filament structure is resistant to thermal breakage
nucleation
the process of nucleus assembly, subunits must first assemble into a nucleus (is stabilized by many subunit-subunit contacts before it can rapidly elongate
rate limiting step of nucleation
formation of cytoskeletal filaments
steps of polymerization
1. lag phase-no filaments observed, instability of small aggregates creates a kinetic barrier to nucleation, nuclei are slowly assembling, phase eliminated if nuclei are added to the solution
2. rapid elongation phase-more subunits coming on than off
3. equilibrium phase-has subunits coming on and off at = rates
advantage of nucleation
cells use proteins to catalyze nucleation at specific sites, allows the cell to regulate where new cytoskeletal filaments are assembled, primary way cells control shape and movement
tubulin
is a heterodimer of alpha and beta subunits, each alpha and beta tubulin has a binding site for one molecule of GTP, GTP is permanently bound to alpha tubulin, beta tubulin is exchangeable (GTP hydrolysis to GDP has an important effect on microtubule dynamics, alpha tubulin at the minus end
structure of microtubules
hollow cylinders of 13 parallel protofilaments, composed of alternating alpha and beta tubulin, tubulin subunits all point in the same direction, microtubules have structural polarity, addition or loss of subunits occurs almost exclusively at the ends, alpha tubulin is exposed at the minus end and beta is exposed at the plus end, tubulin polymerization requires GTP bound beta tubulin (T-tubulin)
structure of microfilaments
actin subunits assemble head to tail, has structural polarity, actin binds and hydrolyzes ATP, two parallel protofilaments form a right-handed helix, the ATP-binding cleft points toward the minus end, actin polymerization requires ATP-bound actin (T actin)
structural polarity
leads to kinetic polarity, kinetic rate constants (kon and koff) are much greater at one end, excess subunits can assemble and the plus end elongates much faster, if solution is rapidly diluted, the plus end also depolymerizes faster
coupling of polymerization/depolymerization to energetically unfavorable processes
in the cytosol, polymerization and depolymerization occur spontaneously, can couple this with an energetically unfavorable process, ex: elongating MT’s help push out membranes, shirking MTs pull mitotic chromosomes, elongating MFs protrude the leading edge of motile cells
filament depolymerization
requires nucleotide hydrolysis, destabilizes the filament and causes subunits to fall off, this occurs preferentially at the plus end of each filament, so this gives rise to treadmilling for MFs and dynamic instability for MTs
MT dynamic instability
after GTP binds to beta-tubulin , it is hydrolyzed slowly, during rapid growth, tubulin molecules are added on faster than the GTP can be hydrolyzed, this results in a GTP cap, tubulin molecules carrying GTP bind to one another with higher affinity than those carrying GDP, so the GTP cap encourages a MT to continue growing, conversely, once a microtubule has lost its GTP cap it will start to shrink, until another heterodimer carrying GTP happens to be added
MT nucleotide experiments
tubulin containing non-hydrolyzable GTP analogues forms MTs normally, therefore GTP binding is required for MT polymerization but its hydrolysis is not, these do not depolymerize like normal MTs when the free tubulin concentration is lowered or when treated with colchicine
role of GTP binding to beta tubulin
it is to stabilize the MT and allow polymerization
role of GTP hydrolysis of MT
allows MTs to depolymerize causing destabilization of the protofilament, causes MT to grow in a curved conformation that can not readily pack
actin filaments and ATP hydrolysis
ATP hydrolysis occurs slowly, when microfilaments grow, actin added to one end faster than the ATP they are carrying can be hydrolyzed creating an ATP cap (this encourages elongation), if ATP hydrolysis rate is greater than on rate then the MF loses its ATP cap and starts to lose actin and shrinks
shinrking vs. treadmilling
shrinking/elongation occurs for MTs while treadmilling occurs for MFs, although both filaments undergo both kinds of behavior
what determines whether a filament undergoes dynamic instability or treadmilling
1. the rate of sunit addition at each end (kon)
2. the rate of nucleotide hydrolysis
3. free subunit concentration
treadmilling occurs for C > Cc(T, plus end) but < Cc(D, minus end)
keratin in epithelial cells
there are 16-18 keratin filaments in epithelial cells
intermediate filaments
many types of IFs, different cells express different IFs, major function of Ifs is physical strength, desmosomes connect epithelial cells through Ifs
structure of intermediate filaments
1. individual polypeptides are monomers with a helical region
2. 2 monomers align in parallel to form dimer
3. 2 dimers aling antiparallel to form staggered tetramer
4. 2 tetramers associated end to end
5. 8 tetramers twist together to form a ropelike structure that is easy to bend and hard to break
Ifs lack structural polarity
protein phosphoylation and IF structure
regulate assembly and disassembly of the IF structure
Types of Ifs in vertebrates
1. nuclear-located in nuclear lamin composed of lamins
2. vimentin-like-cells of mesenchymal origin (composed of vimentins), muscle (desmin), glial cells (glial fibrillary acidic protein), certain neurons (peripherin)
3. epithelial-epithelial cells and derivatives (hair, skin, nails), composed of keratins
4. axonal-composed of nuerofilament proteins
keratins
an IF family, forms a structural framework between cells of epidermis and basal lamina via hemidesmosomes
epidermolysis bullosis
caused by a maturation in keratin genes, very slight mechanical stress causes basal cells to rupture leading to blisters, three primary forms: (1) epidermolysis bullosa simplex (EBS), (2) recessive dystrophic epidermolysis bullows (RDEB), (3) recessive junction epidermolysis bullosa (RJEB)
neurofilaments
aligned IF arrays with croossbridges along axons, smooth Ifs in glial cells, accumulation of NF is associated with ALS
how do drugs attack the cytoskeleton
all bind to either free subunits or the filament, disrupts the balanced assembly-disassembly of the filament network leading to cell death
latrunculin
from a sea sponge, binds to actin monomers to prevent MF elongation, causes existing MF to depolymerize
phalloidin
from the amanita muchroom, binds to existing MFs to prevent their depolymerization
colchicine
from the meadow saffron, binds to free tubulin to prevent MT elongation, causes existing MT networks to disassemble
taxol
from the bark of the pacific yew, binds to existing MT and prevents depolymerization, drives free tubulin into MTs, preferentially kills dividing cells (cancer cells)
centrosome
MTOC called the centrosome, is found in cytoplasm near the nucleus, composed of gamma-tubulin ring complex protein and other unknown proteins, nucleates MT growth, MTs are attached to centrosom by minus ends
actin nucleation at the leading edge of migrating cell
make MFs, fibroblasts permeabilized, MF ends labeld with rhodamine labeled actin for 5 mins, then labeled with fluorescein-phalloidin (all filaments-A)
cell cortex
are the MFs under the plasma membrane that determines cell shape and movement
cytoskeletal-binding proteins
there are several hundred known, fundamental to the functions of the cytoskeleton including:
1. regulate the spatial distribution and the dynamic behavior of filaments
2. determine the sites of new filament assembly
3. regulate the partitioning of polymer proteins between filament and subunit forms
4. change the kinetics of filament assembly and disassembly
5. harness energy to generate force
6. link filaments to one another or to other cellular structures (ex: organelles or plasma membrane)
7. bring cytoskeletal structure under the control of extracellular and intracellular signals
proteins that bind to free subunits
they regulate MF elongation, include thymosin and profiling
thymosin
a negative regulator, binds actin monomer, blocks monomer addition to the MF, prevents nucleotide hydrolysis or exchange
profilin
a positive regulator, competes with thymosin for actin monomer binding, forms complex with actin monomer, profiling-actin easily binds plus end of MF, then profiling falls off
stathmin
binds and sequesters tubulin heterodimers, also binds and blocks plus end MTs
microtubule associated proteins (MAPs)
bind to existing filaments to modify filament stability, stabilize against disassembly, ex: MAP2 and Tau, these are neuronal MAPs that regulate MT spacing by projecting arm length
what is Alzheimer’s disease
the most common of the dementias, about 50-70% of all cases, dementia is the loss of memory, intellect, rationality, social skills and normal emotional reactions, early phase symptoms include memory loss, vagueness, taking longer to do routine tasks, losing conversations or repeating oneself, in the late phase of the disease, patients can no longer care for themselves
why is Alzheimer’s disease a problem
around 4 million people in the U.S. currently have the disease, 10% of people over 65 and about 40% of people are over 80 are affected, with an aging population the number of affected people is rising, there is no cure or effective treatment for this serious disease
causes of alzheimer’s disease
deposits in the brain, called amyloid plaques and neurofibrillary tangles
amyloid plaques
were found to be composed mostly of extracellular deposits of beta-amyloid (also called Abeta), a small 40-42 amino acid peptide
neurofibrillary tangles
were found to contain highly phosphorylated tau
alzheimer’s vaccine
antibodies against beta amyloid, early human trials were halted due to the development of potentially lethal brain inflammation in some participants, Newer anti-amyloid antibodies that have been developed do not have the adverse side effects of the original ones and show preliminary evidence of improved cognitive function in participants and reduced toxic amyloid accumulation.
alzheimer’s drugs
-Alzhemed™ is a new drug that prevents the soluble form of beta amyloid from forming the insoluble form and preventing the toxic build-up of insoluble amyloid. This drug is currently in clinical trials
-Cholinesterase inhibitors: are drugs that help preserve the ability of damaged nerve endings to transmit nerve messages at the synapse and increase the life span of nerves as a result. Cholinesterase inhibitors are effective during early stage Alzheimer Disease.
-Memantine: Memantine and other NMDA (N-methyl-D-aspartate) receptor blockers act by hindering the re-uptake of the glutamate into the nerve endings allowing just enough to get in for the endings to use it to transmit impulses across the junction between nerve cells. These drugs prevent the massive uptake of glutamate that would eventually kill a nerve cell. Memantine is effective for moderate to later stage Alzheimer Disease.
stem cells
Embryonic and stem cells have the capacity to differentiated into any cell type including nerve cells. Current Federal funding has been limited research to less than 100 stem cell lines. Stem cell research holds significant potential for the treatment of Alzheimer’s disease.
capping and filament dynamics
uncapped MFs add or lose subunits at both ends, so elongate or shrink rapidly, if plus end is capped only the minus end can add or lose subunits, so changes occur more slowly, can be regulated by extracellular signals (ex: INC in PIP2 uncap MF plus ends and so MFs more likely to elongate), in skeletal muscle cells, filaments are capped at both ends, CapZ at plus end, tropomodulin at minus end (tropomodulin only binds to MFs that are stabilized by tropomyosin)
gamma-tubulin ring complex (gamma TuRC)
nucleates MTs, gamma TuRC binds MTs at minus ends (embedded in an MTOC), gamma TuRC is thought to serve as a template for MT nucleation
regulation of MT growth-shrinkage transition
MAPs binding stabilizes MT end and encourages elongation, catastrophin (& other Kinexin family members) pry protofilaments apart, opposing molecules are highly regulated
plectin
a type of filament cross-linking proteins, bundles vimentin, links the Ifs to MTs, links MF bundles to plasma mamebrane, plectin KO is lethal-mice die within a few days of birth
filaggrin
a type of filament cross-linking proteins, bundles keratin filaments in the outermost cells of the epidermis
actin arrays
arrangement of MFs in crawling cell, stress fibers are contractile and exert tension, cortex underlies plasma membrane, pilopodium allow cell to explore environment
actin crosslinking proteins
2 types which have 2 actin binding sites (bundling proteins and gel-forming proteins), spacing and placement of these proteins determines the structure MFs will form
fimbrin
makes tightly packed parallel bundles found in filopodia and the leading edge of a cell, too close together for myosin to get in
alpha-actinin
makes wider bundles of actin filaments found in stress fibers, spacing between alpha-a ctinin and actin bundles is just right for myosin to fit between
microvilli
finger-like extension on the cell surface, INC cell SA (ex: intestinal epithelia), formed by bundles or parallel actin filaments, MFs are crosslinked by villin and fimbrin, cillin induces microvillus formation when overexpressed
spectrin
a gel-forming MF binding protein, actin binding sites are far apart, forms a 2-D web held together by short actin filaments, links to the PM via interactions with peripheral membrane proteins, hereditary spherocytosis (mutation that causes a change in the structure of spectrin) RBCs have spherical shape
filamin
a gel-forming MF binding protein, forms loose gel-like networks, required for cell motility, melanosomas lacking filamin are less invasive and do not metastasize as easily
why break up existing filaments into many smaller ones
1. generate new ends to nucleate a large number of new filaments
2. promote depolymerization of old filaments so the cytosol becomes more fluid
katanin
sword in Japanese, hydrolyzes ATp to sever MTs at the point of attachment to the centrosome (for rapid depolymerization during mitosis)
gelsolin
family of actin-severing proteins, activated by high Ca2+ (no ATP requirement), after severing gelsolin caps the MF to prevent elongation, local INC in PIP2 removes gelosolin
model for cytoskeletal reorganization
1. platelet receives clotting signal (damaged blood vessel or thrombin)
2. influx of [Ca2+]i activates gelsolin
3. slow rise in PIP2 inactivates gelsolin
4. rapid actin filament growth
5. actin filament crosslinking into a gel (filamin), others are bundled (fimbrin and alpha-actinin)
6. activated platelet extends lamellipoids and filopoida
7. attachment of platelet via integrins to form clot
ERM protine family
includes Ezrin, Radixin, and Moesin, links MFs to PM transmembrane proteins (ex: CD44), activated thorugh phophorylation or PIP2 binding, loss of the ERM family protein merlin results in a genetic form of neurofibromatosis (multiple benign tumors develop in auditory system)
focal contacts
specialized attachment points between MFs and the ECM, serve as anchors for the cell, allow cells to pull on the substratum and generate tension, at focal contacts, stress fibers link to integrins which bind to the ECM, integrins clustered here activate FAK to relay signals from the ECM into the cell
FAK
helps regulate cell attachment to the stustratum, influences morphology, proliferation, differentiation and movement
G proteins and actin rearrangement
most receptors that rearrange MFs signal through activation of small, monomeric GTPases, are Rho, Rac and Cdc42 types, differences arise because of the different downstream targets of these G proteins
Rho
induces the formation of stress fibers and focal contacts
Rac
induces actin polymerization at the cell periphery, forms lamellipodia and membrane ruffles
Cdc42
triggers actin polymerization, bundling to form filopodia and/or microspikes
motor proteins
bind to either an MT or MF, use ATP hydrolysis to move along the filament, identified by the type of filament they bind, the direction in which they move, and the type of cargo they carry, myosin is a motor protein that binds to and moves along microfilaments
kinesin
type of MT motor protein, move toward the plus end of a MT
dyneins
type of MT motor protein, move toward the minus end
cilia
hairlike membrane bound appendages that extend from the surface of many kinds of cells, cilia beat with a whiplike motion, have a bundle of MTs at their core, move fluid over the surface of the cell, propel single cells through a fluid, found on the epithelial cells lining the human respiratory tract (sweep mucus, dead cells and trapped particles of dust we breathe in toward the mouth and out of the bronchia), cilia help to sweep eggs along the oviduct
flagella
also have a bundle of MTs at core, move the cell by creating a wave of constant amplitude from the base to the tip of the flagellum, propels sperm
axoneme
structure used by cilia and flagella for movement, a core of MT, composed entirely of MTs and MT associated proteins, arranged in the famous 9+2 pattern (9 doublet MTs from ring around 2 single MTs), the ring of doublet MTs allows ciliary dynein to bend the axoneme
ciliary dynein
generate the bending motion of cilia and flagella, motor head domain hydrolyzes ATP, moves along a MT toward minus end, tail end carries a cargo, because of the regular placement of crosslinking proteins, MT doublets cannot slide past each other so they bend instead
primary ciliary dyskenesia (Kartagener’s Syndrome)
a mutation in dynein, found in 1/20 000 – 1/40 000 births, causes male sterility and high susceptibility to lung infections, common presentations include neonatal respiratory distress, recurrecnt otitis media that may lead to deafness, situs inversus, bronchiectesis, digital clubbing, nasal polyps and male infertility, negative sweat test, diagnosis is confirmed by EM examination of ciliary ultrastructure in nasal epithelial cells
common development dealing with cell shape and locomotion
1. coordinated migrations of epithelial cell sheets
2. neural crest cells migrate to a variety of distant sites within embryo
3. growth cones of neurons are fundamental to the the architecture of neurons, and thus the entire nervous system
examples of locomotion in cells
1. macrophages and neutrophils crawl to sites of infection and foreign pathogens
2. osteoclasts and osteoblasts form bone tissue
3. fibroblasts aid in wound healing
4. cancer cells form a primary tumor crawl into blood or lymph vessels and get carried to other sites and form metastases
cell crawling
dependent upon the actin cortex beneath the plasma membrane, involves 3 processes
1. protrusion-actin filament polymerization and elongation push the front of the cell forward
2. attachment-actin filaments form focal contacts with the outside surface
3. contraction-the rest of the cell is pulled forward by contraction of contractile bundles
three structures of protrusion
1. filopodia-found on growth cones, contain a core of bundled actin filaments undergoing elongation
2. lamellipodia-a 2-D sheet-like structure containing a mesh of actin filaments but undergoing elongation, found on epithelial cells and fibroblasts
3. pseudopodia-3-D projections filled with an actin filament gel, found on neutrophils, requires coordinated and spatially regulated actin filament polymerization and depolymerization
lamellopidia
ARP (actin related protein) complexes nucleate new MFs from old ones at 70 degree angles (forming a mesh), newly formed MFs contain ATP (not yet hydrolyzed) so cofilin does not bind, older filaments contain ADP, so cofilin binds to them, so the older MFs depolymerize and provide supply of actin for polymerization of new MFs at the PM to push the cell forward
cytotoxic T cells
kill cells carry foreign antigens on their surface and are a critical component of the immune response to infection. When the T cell receptors recognize antigen on the surface of the target cell, Rho-family GTPases are activated, Causes actin polymerization under the zone of contact between the two cells, creating a specialized region of the cortex. This specialized site causes the centrosome to reorient, moving the microtubules to the zone of T-cell-target cell contact. The microtubules position the Golgi apparatus right under the contact zone, focusing the killing machinery onto the target cell.
extracellular matrix (ECM)
once thought of as a relatively inert scaffold that served to stabilize physical structure of tissue, now known to have more active and complex roles including survival, development, migration, proliferation, shape and function, is an intricate network that surrounds the cells in tissue, composed of proteins and polysaccharides, amount and composition help determine tissue’s physical properties, major component of bone and cartilage but a minor component of the brain
types of functional cell junctions
1. occluding junctions-seal cells together in an epithelium to prevent molecules passing through, AKA tight junctions in vertebrates
2. anchoring junctions-anchor cell cytoskeleton to other cells (adherens junctions) or to ECM (focal adhesions)
3. communicating junctions-mediate passage of signals between cells, gap junctions and chemical synapses
focal adhesions
anchoring junctions formed by integrins, link ECM to actin filaments, ex: myotendinous junctions, integrins cluster at sites of contact with ECM to form focal adhesions, FAK is recruited by anchoring proteins, FAK molecules phosphorylate each other, recruit Src family kinases, FAK appears to be important for focal adhesion disassembly
local macromolecules that make up the ECM
1. GAGs (usually covalently linked to proteins to form proteoglycans)
2. fibrous proteins (collagen, elastin, fibronectin, laminin)
glycosaminoglycans (GAGs)
GAGs are unbranched chains of repeating disaccharide units, one of the two repeating units is nearly always a sulfated amino sugars (N-acetylglucosamine or N-acetylgalactosamine), the second unit is usually a uronic acid (glucouronic or iduronic), are highly negatively charged
main groups of GAGs
distinguished by sugar composition and number and location of sulfates, except for hyaluronan, all GAGs are covalently attached to proteins to form HUGE proteoglycans
1. hyaluronan
2. chondroitan sulfate and dermatan sulfate
1. heparin sulfate
2. kertain sulfate
hyaluronan
simplest GAG, up to 25,000 nonsulfated units, abundant in embryos (space filler, used to force a change of shape), role in resisting compressive forces (tissues, joints), produced in large quantities during wound healing, noncovalently attach to ~100 aggrecan monomers
proteoglycans
contain up to 95% carbohydrate by weight (compared to 1-60% for glycoproteins), ex: decorin (MW ~40,000), aggrecan (MW ~ 3*10^6), and ribonuclease (~MW 15,000)
collagen family
major component of ECM (esp. skin and bone), long stiff fibrous proteins, 3 alpha chains wound like ropes to form triple helical structure (proline ring stabilizes helices, glycine every 3rd residue for tight packing)
types of collagen
1. fibrillar collagens (Types I, II, III, V, XI)-assemble into long cable like fibers
2. fibril-associated collagens (Types IX, XII), link fibrils to ECM
3. network-forming collagens (Types IV, VII), assemble into felt-like mesh
4. anchoring fibrils (Type VII) attach basal lamina of epithelia to connective tissue
scurvy
vitamin C deficiency common in 19th century sailors, prevents proline hydroxylation, defective pro-alpha chains cannot form stable triple helices and are degraded, blood vessels become fragile and teeth become loose, suggests collagen turnover is fast in these tissues, slower in bone (persist 10 yrs)
collagen synthesis
1. pro-alpha chains are synthesized as precursors (with additional aa at both termini)
2. prolines and lysines are hydroxylated in the ER
3. glycosylation of selected hydroxylysines
4. self-assembly of three pro-alpha chains
5. procollagen triple-helix formation
6. after secretion, proteolytic enzymes cleave ends
7. cleaved molecules 1000X less soluble than procollagen molecules
8. fibril formation is driven by tendency for collagen molecules to self assemble
9. aggregation of collagen fibrils to form a collagen fiber (collagen fibrils have cross striations every 67 nm viewed by EM, reflects staggered packing of individual collagen molecules)
functions of propeptides
guide intracellular formation of triple helices and prevent formation of large collagen fibrils inside the cell
importance of lysine side chains in collagen
covalent intermolecular cross links form between the lysine side chains, if this is inhibited, tensile strength of fibrils is drastically reduced (tissues become fragile, skin, blood vessels and tendons tear, especially highly crosslinked in achiles tendon
osteogenesis imperfecta
mutations affect Type I collagen, weak bones that fracture easily
chondroplasias
mutations affect Type II collagen, abnormal cartilage, bone and joint deformities
Ehlers-Danlos syndrome
mutations in Type III collagen, fragile skine and blood vessels, hypermobile joints
elastin
highly hydrophobic protein, rich in Gly and Pro, not glycosylated, tropoelastin is the soluble secreted precursor, assembles into network of elastic fibers close to cell surface, 5X more elastic than rubber bands, gives tissues like skin, blood vessels, lungs resilience (comprises 50% of arterial dry weight), usually interwoven with collagen to limit tissue stretching
microfibrils
sheath that covers elasin core, forms scaffold on which elastin fibers assemble
fibrillin
large glycoprotein that is major component of microfibrils, essential for elastin fiber integrity
Marfan Syndrome
relatively common genetic disorder, caused by mutations in Fibrillin-1 gene, affects connective tissues, aorta is prone to rupture, Abe Lincoln
fibronectin
large glycoprotein, dimer of 2 subunits joined by disulfide bonds at c-termini, important for cell adhesion and migration
structure of fibronectin
two polypeptides form the dimer, similar but not identical polypeptides, synthesized from the same gene, each chaing has 5-6 domains, each domain allows specialized binding to a different molecule
Type III fibronectin repeat
main type of module, ~90 aa long, occurs at least 15X per subunit, contains RGD sequence important for cell adhesion and recognized by several integrins on cell surface
plasma fibronectin
soluble, in bloodstream, enhances clotting, wound healing, phagocytosis
fibrillar adhesion
associate with integrins
uterglobin
binds fibronectin to block fibril formation in kidney
actin filaments and fibronectin fibril assembly
regulated by actin filaments, promote assembly of fibrils, cytochalasin treatment dissociates fibrils, influence fibril orientation (mainly mediated by integrins)
laminin
main ECM component during early development, composed of alpha, beta and gamma chains linked by disulfide bonds, several isoforms of each, so larg family, functional domains allow laminin to bind many molecules including perleacan, nidogen, integrins and dystroglycan
congenital muscular dystrophy
autosomal recessive causing neuromuscular disorders, onsent from congential or infancy (<1 year), includes diffuse weakness, contractures and CNS problems, common in severe forms of CMD, may be subclinical, disorders of myeling or neuronal migration
basal laminae
thick mats of specialized ECM, organized into 3 ways, (1) surround individual muscle, fat and Schwann cells, underlie all epithelial sheets, can separate cells from connective tissue or function as filter (kidney)
composition of basal laminae
exact composition varies, but most contain Type IV collagne, perlecan, laminin and nidogen (entactin), cell surface receptors organize basal lamina assembly (integrins and dystroglycans)
basal lamina and NMJ
controls synaptic components on both sides, when both nerve and muscle are damaged and if only the nerve is allowed to regenerate, the basal lamina directs the nerve to the original site of the NMJ and only the muscle is allowed to regenerate, the basal lamina causes AChRs to cluster at the original synaptic site
mechanism for cell orietntation in a tissue
1. oreitns the assembly of secreted ECM molecules in the vicinity, the oriented ECM reaches cells and orients the cytoskeleton of those cells, these cells can now secrete an oriented matrix in their vicinity, in this way the ordering of cytoskeletons is propagated to later cells
survival and anchorage dependence
extend of cell spreading on matrix is critical for survival, anchorage dependence is mediated by signaling integrin signaling, helps cells fill vacated space for regeneration
ECM degradation
turnover of ECM important for many biological process such as involution of uterus after childbirth, leukocytes migrate from blood vessel to site of injury or infection, or meastasis of cancer cells
proteolytic enzymes and ECM degradation
prteolytic enzymes are secreted locally to degrade ECM, MMPs, serine proteases and collagenases
proteolysis and cell migration
it clears a path through the ECM, expose cryptic sites for cell binding to ECM molecules, promote cell detachment and release signaling proteins
mechanism to regulate ECM degradation
1. local activation-secreted precursors can be activated locally when needed, (ex: plasminogen), inactive protease abundant in blood, cleaved by plasminogen activators to plasmin, which helps break up clots, tPA is a plasminogen activator given after stroke or heart attack
2. confinement by cell surface receptors, (ex: uPA), plasminogen activator on tips of axons, metastatic cells, clears path for migration
3. secretion of inhibitors-inhibitors that bind specific proteases to block their activity (ex: TIMPs and serpins)
integrins
heterodimeric TM proteins, principle ECM receptors in animal cells, most connect to actin filaments via anchoring proteins, act as cell-cell adhesion molecules, low affinity, so attachment depends on many receptors, important for outside in and inside out signaling, bound by cysteine bond
integrin activation
extracellular signal activates intracellular signaling cascade, ex: integrins inactive in white blood cells (do not adhere), platelets activated by damaged vessel or soluble molecules, integrin conformations changes to allow adhesion (beta3 rapidly activated), platelets bind clotting protein fibrinogen, form plug to stop bleeding
Glanzmann’s Disease
genetic deficiency in beta3 integrin, characterized by excessive bleeding
Duchenne Muscular dystrophy (DMD)
is the most common X-linked genetic disease affecting 1/3500 male births, suffer from severe, progressive muscle wasting and are confined to a wheelchair in their early teens and die in their early twenties, caused by an absence dystrophin, muscle wasting disease
treatment of DMD with gene therapy
vrial mediated gene transfer, problems and limitations include the size of the dystophin gene (2.6*10^6 bp), immune response to viral proteins and to dystrophin, efficient delivery to large organ, direct DNA injection
treatment of DMD with gene repair
chimeroplasts (DNA/RNA oligonucleotides to direct the repair mechanisms to correct point mutations), antisense spliceosomes (restores registration in reading frame), aminoglycoside induced repair (gentamicin suppresses internal stop codons during translation
treatment of DMD with repair by muscle precursor cells
myoblast/satellite cell transfer, stem cells from bone marrow, embryonic stem cells, INC muscle mass (myostatin blockade)
treatment of DMD with inhibiting muscle fiber destruction
deplete or inhibit cells that cause immune-mediated muscle destruction, anti-inflammatory drugs (Prednisone), alter proteolytic degradation of contractile apparatus
treatment of DMD with complementery gene therapy
enhanced expression of genes with complementary functions, (e.g. alpha7-beta-1, utrophin), good gene is already present, drug induced therapy, circumvents immunological problems, can be used in both viral and cell based therapies, integrin gene is small (2.7*10^4 bp) but many not give complete rescue
muscle mechanics
due to intermolecular interactions, myosin motors behave differently when they function in groups than they behave when they function as single molecules, involves a cyclical interaction of actin and myosin driven by actin-myosin ATPase reaction
muscle myopathies
caused from defect in myosin type II (MYHC2), clinically there is muscle weakness, atrophy near shoulders, back, hand and thigh muscles, opthalmoplegia, pathogenesis (mutations primarily to SH1 helix in myosin, thought to alter actin-myosin ATPase activity
types of myosin
there are many types of myosin, all have similar motor domains, differences in tail correspond to differences in cargo and regulation, all myosins are actin based motors and all but myosin VI are plus-end directed
Griscelli syndrome
caused from defect in myosine type V, mysoine Va point mutation leads to hypopigmentations and neurological defects
Hearing loss
caused from defect in myosin type VI and VII
myosin IIa
involved in cell division
myostin V
involved in melanosom transport and certain neurological functions, is a high (>50%) duty ratio “processive” motor, transports vesicles in cells, like a child across the monkey bars, long neck (6IQ motifs), organelle motor, functional unit has two heads
myosin VI and VII
involved in mainting organization of actin-filled sterocilia
Usher syndrome
defect in certain myosin VII
myosin II
is muscle myosin, is a dimer, each heavy chain has a motor domain and a tail, has an actin binding site, active site, ELC and RLC, is a low duty ratio (<10%) motor, short neck (2IQ motifs), drives muscle contraction, functional unit of ~20 heads, myosin II molecules are assembled into a thick filament, filaments form by association of hydrophobic regions in the tail
how many ATP needed to move 22.5 lb (100 newtons) 1 meter?
work = 100 N * 1 m = 100 joule
deltaG = -14 kcal/mol = -59 kJ/mol
N = 100/59,000 mole = 1.7 * 10^-3 mole = 1*10^21 molecules ATP
myosin’s are ~50% efficient, so actually twice this many ATP
how many myosin steps needed to lift 22 lb?
work = 5 pN * 10 nm = 50*10^-21 J
N = 100 J/(50*10^-21) = 2* 10^21 steps
attached state of myosin and actin
at the start of the cycle shown in this figure, a myosin head lacking a bound nucleotide is locked tightly onto an actin filament in a rigor configuration, in an actively contracting muscle, this state is very short-lived, begin rapidly terminated by the binding of a molecule of ATP
released state of myosin and actin
a molecule of ATP binds to the large cleft on the back of the head and immediately causes a slight change in the conformation of the domeains that make up the actin binding site, this reduces the afficinity of the head for actin and allows it to move along the filament
cocked state of myosin and actin
the cleft closes like a clam shell around the ATP molecule, triggering a large shape change that causes the head to be displaced along the filament by a distance of about 5 nm, hydrolysis of ATP occurs but the ADP and inorganic phosphate (Pi) produced remain tightly bound to the protein
force-generating state of myosin and actin
a weak binding of the myosin head to a new site on the actin filament causes release of the inorganic pohsophate produced by ATP hydrolysis, conconmitantly with the tight binding of the head to actin, this release triggers the power stroke, the force-generating change in shape during which the head regains its original conformation, in the course of the power stroke, the head loses its bound ADP, thereby returning to the start of a new cycle
attached state of myosin and actin at the end of the cycle
the myosin head is again locked tightly to the actin filament in a rigor configuration, note that the head has moved to a new position on the actin filament
distance of myosin’s working step
in myosin II is 5 to nm swing of lever arm, in myosin V, 30 to 40 nm swing of lever arm
sarcomere
is the fundamental contractile unit in muscle, contracts when myosin thick filaments and actin thin filament slide past each other
myofibril
consists of a series of sarcomeres, light (I) and dark (A) banding pattern gives rise to striated muscle
sarcomere shortening
i.e. muscle contraction, shortens the I band but not the A band, crowns of corssbridges project from thick filament at 14.3 nm intervals and successive crowns are rotated, the result is a thick filament with six rose of crossbridges along its length
accessory proteins in muscle
1. CapZ and tropomodulin cap ends of actin to keep filament length constant
2. Z disc contains alpha-actinin and other proteins that stably join sarcomeres
3. titin maintains thick filament position in the sarcomere
4. nebulin sets the length of the thin filaments
muscle regulation
two types of regulation: (1) thin filament (skeletal and cardiac) and (2) thin filament (smooth muscle)
smooth muscle regulation
an inactive myosin is phosphorylated by myosin light chain kinase (which is activated by calcium-calmodulin) to form myosin-P, myosin-P is inactivated by myosin light chain phosphatase
tropomysoin
wraps around actin filament blocking myosin binding sites on actin, calcium binding to troponin C results in torpomysoin movement away from myosin binding sites
what effect does Ca2+ have on actomyosin ATPase and force
as myoplasmic [Ca2+] INC then force or ATPase activity INCs sigmoidally
types of muscle growth
hypertrophy-adding new myofibrils within a cell
hyperplasia-formation of new cells
lengthening-adding more sarcomeres in series
what happens when adding more sarcomeres in parallel (hypertrophy or hyperplasia)
force is doubled, velocity and shortening capacity is unchanged
what happens when adding more sarcomeres in series (lengthening)
force is unchanged, velocity and shortening capacity is doubled
myosin composition in body
myosins make up 10-15% of the protein in the body, myosin mutations are bound to lead to muscle disorders
distal arthrogryposis, Freeman-Sheldon syndrome, Sheldon-Hall syndrome
caused from mutations to embryonic MyHC (MYH3), joint contractures with predominant distal involvement, pathogenesis (mutations in troponin I, troponin T, tropomyosin, perinatal myosin and embryonic myosin, through to disrupt sarcomere development
laing myopathy
mutations to B-cardiac myosin (MyHC7), clinical features include weakness of ankle dorsiflexion and hanging big toe, pathogenesis (mutations are in the LMM region of myosin, thought to disrupt myosin filament formation or disrupt interactions with myosin binding proteins like titin
familial hypertrophic cardiomyophaty (FHC) and dilated cardiomyophathy (DCM)
caused by point mutations to beta-cardiac myosin, actin, troponin and tropomyosin, may cause sudden death as well, FHC mutations enhance myosin force generation, DCM mutations decrease myosin force generation
sources of ATP in muscle
ATP -> creatine phosphate -> glycogen (anaerobic (lactic acid) then aerobic) -> lipolysis
type I muscle fibers
aerobic, red, small, slow, many mito, many capillaries, myoglobin
type IIB fibers
anaerobic, white, large, fast, few mito, few capillaries, no myoglobin
how is calcium released into muscle cytoplasm and then removed
muscle membrane structures, calcium stores, neuromuscular junctions, calcium channels, and action potentials
sarcoplasmic reticulum (SR)
surrounds myofibrils, store calcium
transverse tubules
formed from invaginations of plasma membrane
SR and T-tubules
association is formed between them, the lumen of the SR contains granular material called calsequestrin (a protine that binds large amounts of Ca2+
action potential and calcium
action potential trigger release of calcium from the SR into the cytosol
acethylcholine
contained within the vesicles, is liberated by exocytosis into the synaptic cleft, additionally calcium ions are pumped out of the axon terminal, binds to receptor sites of chemically-regulated ion channels on the motor end plate, this causes the channels to open permitting an influx of sodium ions and a small efflux of potassium ions, this ion exchange causes a local depolarization of the motor end plate
breakdown of acethylcholine
after a brief period, acethylcholine diffuses away from its receptor site and the ion channel closes, acetycholine is then broken down by the enzyme acethylcholinesterase
action potential propagation
depolarization of the motor end plate initiates an action potential which propagates along the sarcolemma in all directions down the T tubules
calsium release from terminal cisternae
the AP causes the release of calcium ions from the terminal cisternae into the cytosol
contraction of the muscle cell
calcium ions trigger a contraction of the muscle cell
curare
compete with acetylcholine, block neuromuscular transmission and thus paralyze skeletal muscles, can be used to immobilize patients during surgery
neostigmine
INC the availability of acetylcholine by preventing its breakdown, can be used to treat disease such as myasthenia gravis, characterized by muscle weakness due to inadequate acetylcholine stimulation, in large amounts this type of drug will cause severe muscle spasms and possibly death
nicotine
binds to the acetylcholine receptor sites and opens the ion channels mimicking acetylcholine, nicotine is not immediatlely hydrolyzed by acethylcholinesterase and therefore occupies to cause depolarization of the motor end plate, acts as a stimulant and may cause visible muscle tremors
motor unit
a motor neuron and all of the muscle cells it stimulates
recruitment
when a strong contraction is needed, the nervous system may cause more than one motor unit to be stimulated, the stimulation of additional motor units for INC strength of ctonraction is called recruitment
small motor units
responsible for producing precise movements, contain a few muscle cells, are found where precise movements are needed, as in muscles of the eye
large motor units
repsponible fore producing gross movements, found in large muscles, have large motor units in which a single neuron is connected to a large number of muscle cells
muscle tone
in a muscles relaxed state, random asynchronous motor unit contractions provide a nearly constant state of low-level tension and resistance to stretch called muscle tone, asynchronous motor unit activity maintains a nearly constant tension in the total muscle
factors that affect muscles ability to generate force and to contract
1. frequency of stimulation-can increase the force of the muscle contraction if frequency is fast enough resulting in tetanus
2. number of motor units stimulated
3. degree of stretch (Frank-Starling relationship)
4. whether muscle is allowed to shorten (force-velocity relationship)
phases of a muscle twitch
latent period, contraction period and relaxation
temporal summation
additional influx of Ca2+ promotes a second contraction, which is added to the first contraction, second peak is higher than the first
multiple stimuli
individual twitch responses start to fuse and sum as the frequency INC, leading to a complete or fused tetanus, is also incomplete tetanus, temporal summation and treppe
number of motor units stimulated
the strength of a muscle contraction is determined not only by the frequency of stimulation but also the number and size of motor units recruited, in vivo, the number of motor units that are recruited is determined by the number of motor neurons that are stimulated by the central nervous system, by varying the number and size of recruited motor units, the nervous system can control the degree of contraction of a particular muscle
length-tension relationship (Frank-Starling mechanism)
the strencht of a muscle contraction can laso be altered by chaning the starting length of a muscle, max strength of contraction when the sarcomere is 2.0 to 2.2 micrometers in length, if longer then tension decreases
load and twitch response
the heavier the load, the shorter the duration of the stretch, the longer the time before start of stretch and the shorter the distance shortened
force-velocity curves
shows the relationship between shortening velocity and load, if have max load, have zero velocity, if have zero load have max shortening velocity