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73 Cards in this Set
- Front
- Back
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Flux equation (Fick's law for movement of a solute through solution). |
or, Jx=Px(Xo-Xi) |
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Px |
permeability coefficient, combines lipid/ water partition coefficient, diffusion coefficient, membrane thickness, and a standard 1-micron squared area. specific for a given molecule/ ion and largely affected by the number and types of channels that the ion can pass through. |
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Gated channels |
opened/closed by chemical, voltage, mechanical, or other forces. |
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Non-Gated Channels |
pores, passive channels, leaky channels. |
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Leaky Channels |
Ion-Specific important in determining the resting properties of the cell. Include membrane resistance and resting membrane potential. |
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Jmax |
maximum flux. at this point, movement of ions through channels may not follow fick's law (which says the increase in concentration gradient leads to a linear increase in flux). due to limited number of channels for ion to move through. Never happens physiologically. |
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Ohm's Law |
V=IR |
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Conductance (G) |
G=1/R |
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Current |
I=GV I=V/R |
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Permeability vs. Conductance |
Permeability: ability of an ion to move across membrane. Dependent on number of channels. Conductance: actual electrical measurement of the movement of ions. Directly related to flux. |
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Current units |
nanoampere (nA) picoampere (oA) |
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Conductance units |
picosiemens nanosiemens |
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electrochemical equilibrium |
electrical force equals the diffusion force so no net change in concentration. |
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Nernst Equation |
Tells us voltage at which an ion is in electrochemical equilibrium, given initial conditions. |
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Resting membrane potential |
Potential due to all the ions electrochemical equilibriums. Sum of currents must equal 0. Ik+INa = 0 Ik=-INa |
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Goldman-Hodgkin-Katz equation |
measure resting potential of cells (multiple ions) P is permeability of that ion. |
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Permeability |
dependent on number of ion channels for that specific ion. |
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Equilibrium vs. Resting Membrane Potential |
Equilibrium: does NOT require energy to reach. given ion simply flowing in and out at equal rate. Resting membrane potential: steady state. requires energy, usually in form of ATPase pump. ion not in equilibrium. |
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Conductance Equation for I, current |
I = G (Vm -Eeq) Use this to calculate the current of a specific ion through a membrane. (Vm-Eeq) is the driving force for ion movement. If resting membrane potential is equal to the equilibrium voltage for a given ion, that ion will not have a net current. |
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reversal potential |
If resting membrane potential is equal to the equilibrium voltage for a given ion, that ion will not have a net current. |
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Resting membrane potential at steady state |
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Types of channels include |
-Voltage gated -Ligand gated -Pressure/stretch gated -Non-gated (passive leak) -Water channel (aquaporins) |
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structure of Ion Channels |
derived from primary AA sequence. multiple subunits, each with distinct transmembrane segments (domains) that fulfill distinct roles. Each subunit in VG channel consists of 6 transmembrane helices. |
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Electrical Circuit Model for membrane |
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threshold |
voltage at which a depolarization can trigger an action potential. |
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depolarization |
to make voltage less negative |
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hyperpolarization |
to make voltage more negative |
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decreasing potential. |
refers to absolute value, meaning making less negative. |
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tetrodotoxin (TTX) |
toxin that blocks the sodium gated ion channels and will cause respiratory failure quickly. effects irreversible. |
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voltage clamp experiments |
membrane voltage is artificially held static at certain voltage and current is observed. Because I=GV, and V held constant, measuring conductances, which are related to number of certain channels on membrane (permeability). |
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depolarization phase caused by |
opening of voltage gated Na+ channels, allowing Na+ to rush into cell, making membrane potential less negative to a positive maximum called overshoot. |
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Depolarization starts when |
Na+ channels inactivate and delayed rectifier voltage gated K+ channels open. |
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undershoot |
slight hyper polarization at the end of the AP. due to long lasting activation of the K+ channels after depolarization. |
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excitable cells |
include cells that display APs, heart, muscle, nerve. typically have lots of Na+ channels. voltage at the peak of the AP is closest to equilibrium potential of Na+, ENa. |
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voltage clamp technique |
allows one to hold membrane voltage at a fixed value, even when conductance is changing to measure the current. Because voltage cannot change, any change in current is direct result of change in conductance. |
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inward negative |
positive ions flow into cell making negative currents draw downward. sodium entering the cell. |
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TTX (Tetrodotoxin) STX (Saxitoxin) |
TTX-produced by puffer fish. STX- produced by algae responsible for red tide. both are specific sodium channel blockers. |
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TEA (tetraethyl ammonium) |
selectively blocks outward flow of potassium. |
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current clamp |
holding current constant and measuring voltage. sticking an electrode in a cell and measuring changing membrane potentials during AP is done with current clamp. |
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patch clamp |
variant of voltage clamp. allows us to voltage clamp a tiny piece of membrane "patch" to study the current produced by a single channel activity. patch can be attached to cell or excised. |
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Sodium channels |
-fast acting. -exhibit inactivation. -3 conformations: open, closed-inactivated, closed- essential for AP to travel down axon. stimulus given, channel senses this and opens. opening increases conductance, increase in current. just after peak, the channel closes and enters closed-inactivated (depends on both time and voltage). channel remains inactive until cell repolarizes and enters closed confirmation. |
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closed-inactivated |
Na-channel conformation that's dependent on both time and voltage. During this conformation, another spike cannot happen. |
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closed state |
Na-channel conformation. another AP can be initiated, but requires stronger depolarizing stimulus. |
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K channels vs. Na channels |
-K channels open slower than sodium channels. -do not exhibit inactivation-remain open as long as cell is depolarized. |
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Action Potential sequence of events |
-Membrane depolarization opens VG sodium channels.
-sodium rushes into cell. -cell depolarizes, opening more sodium channels. -membrane potential driven up to Erev > Na channels inactivate (cause decrease in GNa) and deploy opens VG K channels to increase GK. -K rushes out to depolarize cell. -Local circuit currents produced which allow propagation to near by membrane. |
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Action Potential |
self-sustaining, regenerative current driven by influx of sodium and efflux of potassium. |
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absolute refractory period |
another AP cannot occur because Na channel is closed-inactive. |
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relative refractory period |
due to persistent activation of K channels which keep the cell hyper polarized longer than it would normally be. another AP can occur, but stimulus must be stronger. |
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threshold |
voltage at which an AP can start. one cannot define threshold for single channel current. |
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extracell Ca2+ effects on VG Na channels |
-Ca2+ changes the way the VG Na channel senses changes in membrane potential because charge shielding effects on voltage sensor. -higher than normal extracell Ca2+ > decreased excitability -lower than normal extracell Ca2+ > hyper excitability. |
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Big drop in Ca2+ may lead to |
-tetany of peripheral nerves -paralysis of respiratory muscles. |
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space constant |
how far an AP can passively travel due to decrement in voltage before it reaches 37% of initial strength. Rm=membrane resistance Ri=internal resistance given as inverse of the dam of cell. |
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increase space constant.. |
increase space constant, increase the speed and distance an AP will travel. you can increase the constant by: -increasing axon diameter, thereby dec internal resistance. -increasing membrane resistance, so reducing amt of current lost. |
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What affects membrane resistance? |
-properties of phospholipids -number of leak channels -presence of myelin |
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capacitance |
-inherent property of membrane. -membrane acts as capacitor by separating and storing the intra and extracellular charge. -in case of current clamp, response to injected current will be slowed by capacitance. -voltage clamp-no effect of capacitance. |
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time constant |
amount of time required to charge and discharge the membrane capacitance. time constant=Rm x Cm (membrane resistance x membrane capacitance) |
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Action potentials and capacitance |
-During action potential, flow of ions charge membrane and so the action potential becomes weaker. -if we could decrease capacitance, we could speed the signal propagation. |
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Myelination |
-decrease time constant and increase space constant. -oligodendrocytes in CNS and Schwann cells in the PNS wrap axons with myelin, thereby decreasing membrane capacitance and increasing resistance. successive layers of myelin thicken membrane and increase distance b/w places of capacitor. -Thus, increases Rm and decreases Cm. -reduction in CM outweighs increase in RM, having an overall effect of reducing the time constant and increasing conduction velocity. |
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Nodes of Ranvier |
-points along axon in which the myelin is absent. -contain concentrated grouping of the voltage gated Na and K channels that are absent in the myelinated sections. -nodes create saltatory conduction. (AP jumps from node to node by getting boost at each node). |
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eukaryotic membranes contain |
1) phospholipid bilayer matrix (hydrophobic interior) 2) intrinsic membrane proteins (can be extracell or intracell, cannot be removed w/o destroying membrane). 3) extrinsic membrane proteins (only extracell, can be removed w/o disrupting membrane integrity). 4) glycolipids, glycoproteins, and cholesterol. |
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Membrane Fxns |
-serves as reactive surface -compartmentalizes the cell -controls entry and exit of molecules. |
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Px, permeability coefficient |
from Fick's Law, Jx=Px(Xo-Xi) Px made up of 4 components: -Lipid/Water Partition Coefficient (B)-ratio of molecule's solubility in oil to its solubility in water. (0=soluble in water, 1=soluble in lipid, more flux) -Diffusion Coefficient (Dx)-increasing Dx will increase Jx. -Membrane thickness (X)-assumed constant for most membranes. -Area considered. typically 1 sq/micron. |
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Pore (non-gated channel) |
also called leakage channels. number and type of pore channels responsible for the resting membrane potential. |
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Channel (gated-pore) |
channels are conduits gated by a door, and responsible for all other potentials on a membrane such as action potentials and synaptic potentials. open in response to sound, light, mechanical stretch, voltage or chemicals. |
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3 examples of gated channels |
1) Voltage-gated sodium channels. 2) Gap Junction Channel 3) Ligand-gated Channel |
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Voltage-gated sodium channels |
-opens in response to voltage change in membrane. -multimeric protein and forms an ion channel, selecting for sodium. |
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Gap Junction Channel |
-formed when two hemichannels from adjacent cells simultaneously open and ions pass b/w adjacent cells. |
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Ligand-gated channel |
Binding of ligand (acetylcholine, for example) to its receptor causes channel to open and allow sodium and potassium to move through channel. |
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Components of a typical ion channel |
1. Gate 2. Ion Selectivity Filter 3. Typically they are glycoproteins 4. Have anchor proteins to keep them stabilized to lipid bilayer. 5. Voltage sensors or ligand binding site. |
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Ion channel structure |
-multiple-pass transmembrane domain-containing chains of AAs. -some have alternating b/w hydrophobic and hydrophilic AAs, telling you it spans membrane multiple times. -N terminal inside membrane. -P-loop referred to as pore domain-loose binding of ions. |
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Tetramers |
-voltage gated Na+ -voltage gated Ca2+ -voltage gated K+ |
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Pentamers |
-Nicotinic ACh receptor channel (ligand-gated channel in synapse). |
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Hexamers |
-Connexon- 6 unit structure that makes up half an entire gap junction channel. -A connexon in a cell's membrane aligns with adjacent cell's connexon hexameter to propagate electrical potential. |
