Neuro L2

Front 1

CURRENT

Back 1

net flow of charges measured in amperes (A)

Front 2

POTENTIAL DIFFERENCE (E)

Back 2

the WORK needed to move a unit of test charge in a frictionless matter from one point to another

measured in volts (V)

to move 1 coulomb of charge ax a 1 volt difference requires 1 Joule of work

(potential = voltage = voltage difference)

Front 3

ELECTRICAL CONDUCTANCE (G)

Back 3

conductance is defined by OHM'S LAW

measures the ease of flow of current between two points where I = GE
I (current) =
G (conductance) E (voltage difference)

measured in units of siemens (S)

Front 4

OHM'S LAW

Back 4

V = IR or I = GV (aka I=gE)

voltage = current x resistance
current = conductance x potential difference

Front 5

Vrest

Back 5

RESTING MEMBRANE POTENTIAL

the voltage drop (Vm) across a membrane due to a separation of charge across the membrane (capacitor) in the absence of any other stimulus or input

Front 6

the membrane functions as what in terms of charge separation?

Back 6

A CAPACITOR

Front 7

components of plasma membrane

Back 7

a. lipid bilayer (phospholipids, cholesterol, glycolipids)
b. ion channels (allow ions to flow down electrochemical gradient)
c. active transporters/pumps: move ions against their concentration gradients

Front 8

role of plasma membrane in Vrest

Back 8

1. separates charged substances
2. Imparts capacitance
3. Provides the stuctural framework for subcellular localization (lipid rafts)
4. Is a substrate for enzymes of specific signaling cascades

Front 9

Typeical Vrest:

Back 9

-40 to -90 mV (=10^-3 V)

Front 10

DEPOLARIZATION

Back 10

reduction in charge separation

(i.e. making it less negative;
e.g. -70 mV to -30 mV)

Front 11

HYPERPOLARIZATION

Back 11

an increase in charge separation

(e.g. -70 mV to -100 mV)

Front 12

FREE IONIC CONCENTRATION for a vertebrate SPINAL CORD NEURON
Na+

[Na+]out/[Na+]in
ratio out:in =
Eion =

Back 12

[117]out/ [30]in

Ratio out:in = 4
Eion= +36mV

Front 13

FREE IONIC CONCENTRATION for a vertebrate SPINAL CORD NEURON
K+
[K+]out/[K+]in
ratio out:in =
Eion =

Back 13

[3]out/ [90]in

Ratio out:in = 0.033
Eion= -90 mV

Front 14

FREE IONIC CONCENTRATION for a vertebrate SPINAL CORD NEURON
Cl-
[Cl-]out/[Cl-]in
ratio out:in =
Eion =

Back 14

[120]out/[4]in

Ratio out: in = 30
Eion = -90mV

Front 15

FREE IONIC CONCENTRATION for a vertebrate SPINAL CORD NEURON:
Anions-

[anions-]out/[anions-]in

Back 15

[0]out/[116]in

Front 16

NERNST EQUATION

Back 16

used to find membrane potential with respect to ion concentrations inside and outside the cell

E (k+) = constant x log [K+] out/ [K+]in

Front 17

PRINCIPLE OF MACROSCOPIC ELECTRICAL NEUTRALITY

Back 17

equal number of positive and negative charges

Front 18

given the principle of macroscopic electrical neutrality, how then does membrane potential arise from a separation of charges?

Back 18

1. membrane is SELECTIVELY PERMEABLE (at rest it will allow some ions to cross more readily than others)
2. There is an UNEVEN DISTRIBUTION OF INDIVIDUAL ION SPECIES on the inside versus the outside of the cell

Front 19

what provides the pathway for ions that mediate rapid signalling to move across the membrane?

Back 19

ION CHANNELS

Front 20

what paintains the concentration gradients?

Back 20

ION PUMPS (TRANSPORTERS)

Front 21

ION CHANNELS

Back 21

integral membrane proteins: hetero-oligomeric complexes

a) high probability of being opened at Vrest (non-gated or leak channels)

b) opened only by specific stimulus (gated)
-voltage-gated
-ligand gated
-mechanosensory (pacinian corpuscles)

Front 22

characteristics of ion channel function

Back 22

SELECTIVE PERMEABILITY (let only specific ions through)
CONDUCTANCE: ~10^8 ions/second (much greater than conductace of pumps/transporter)
MEAN OPEN TIME: 0.1-100 msec
PASSIVE FLUX: no energy required

Front 23

main differences between functions of ion channels and pumps

Back 23

channels- greater conductance, passive flux, open longer

Front 24

Patch clamp technique

Back 24

allows measurement of current flow through a single ion channel or an ensemble of active ion channels (e.g. synaptic currents or whole cell currents)

Front 25

Vrest arises because:

Back 25

1. there are unequal concentrations (and therefore unequal driving forces) for ions inside and outside the cell

2. because certain ion channels (mainly K+ channels) have a higher probability of being open at rest

Front 26

for a neuron at Vrest, membrane has many channels open for what type of ion?

Back 26

K+

very few types of other channels are open

Front 27

for a neuron at Vrest what forces influence K+ movement

Back 27

CONCENTRATION GRADIENT (pushes K+ ions out of cell)
ELECTRICAL GRADIENT (pushes K+ ions into the cell)

Front 28

Ek

Back 28

EQUILIBRIUM POTENTIAL

definied point at which the concentration gradietn pushing K+ out will be exactly equal to the electrical gradient holding them in

Front 29

Equilibrium Potential is predicted by what?

Back 29

NERNST EQUATION

Eion = 58log [ION]out/[ION]in

Front 30

At equilibrium potential for any given ion (Eion) describe current

Back 30

NO NET CURRENT FLOW FOR THAT ION

Front 31

Ek (Equilibrium potential for K+) in a typical neuron

Back 31

Eion= -90mV

Front 32

What percentage of intracellular K+ ions must leave cell to generate Vrest?

Back 32

<<<<1% (10^-12 moles)

Front 33

charge separation is limited to what area?

Back 33

limited to the area at the faces of the bilayer?

Front 34

electrical neutrality is conserved in terms of:

Back 34

concentrations of ions in bulk solution

Front 35

RESTING MEMBRANE POTENTIAL IS PRIMARILY (but not completely) DETERMINED by:

Back 35

Ek

Front 36

Contribution of Chloride ions to Vrest

Back 36

in most nerve cells conductance for chloride (Gcl) is very small, and Cl- ions are passively distribtued according to Vrest est. by Ek

Vrest of muscle cells SIGNIFICANTYL DEPENDS UPON RESTING Cl- conductance

Front 37

Contribution of sodium ions to Vrest

Back 37

permeability of Na+ (at rest) is small compared to permeability to K+ at rest

Pna: Pk = ~0.03

Front 38

MEMBRANE POTENTIAL (Vm) will be determined by:

Back 38

a weighted average of the NERNST POTENTIALS for each permanant ion and their conductances (Gion: think # of open channels) of these ions

Front 39

For most neurons at rest, consider these ions in determining Vm

Back 39

K+ and Na+

Front 40

Why does Vrest end up being so much closer to Ek than to Ena?

Back 40

because conductance of sodium is so much less than conductance of potassium ions

Gna << Gk

Front 41

At Vrest
Itot (current flow) =

Back 41

Itot = 0

by definition this system is at rest, there is NO NET CURRENT FLOW

Front 42

At Vrest
which ions are at their individual equilibrium potential?

Back 42

NONE OF THE INDIVIDUAL IONS IS AT ITS EQUILIBRIUM POTENTIAL

current flows through each class of channels; magnitude of this current is determined by the:
1) CONDUCTANCE (similar to permeability, i.e. # of open channels, and rate of flux through each)
2) DRIVING FORCE equation (how much the Vrest differs from the Eion)

Front 43

At Vrest
current of Na+
current of K+

Back 43

constant inward current of Na+ ions
constant outward current of K_ ions

however, net current = 0

Front 44

what structures are required to maintain ionic gradients?

Back 44

PUMPS OR MEMBRANE TRANSPORT PROTEINS
ATPases (Na+/K+)
ION EXCHANGE PUMPS
(Na+/Ca++ and
Na+/H+ exchangers)

Front 45

Na+/K+ pump

Back 45

1. integral membrane protein
2. active transporter (requires ATP)
3. maintains long-term membrane potential
4. controls cell volume and drives active transport of sugars and aas
5. 20-40% of brain's energy use is to power this pump
6. in electically active nerve cells, it is 70% of cell's energy requirement
7. digitalis glycosides (e.g. ouabain) inhibit the pump from the outside by competing with K+ for its binding site
8. for every molecle of ATP hydrolysed, 3 Na+ are pumped out and 2 K+ are pumped in (pump is ELECTROGENIC- will slightly hyperpolarize Vrest from predicted value)
9. pump is NOT needed to generate individual action potentials (these occur by passive flux of ions running down their electrochemical gradients)
10. sometimes pump activity will alter electircal signalling (eg in small axons, high frequency stimulation will lead to a prolonged hyperpolarization as the pump extrudes Na+)

Front 46

disease associated with abdnormal resting potential

Back 46

MYOTONIA CONGENITA (fainting goats?)

Front 47

What does it mean, that the Na/K+ pump is ELECTROGENIC?

Back 47

that it will slightly hyperpolarize Vrest from what is predicted based solely on ion channel activities

Front 48

what generates individual action potentials or graded potentials (receptor potentials, synaptic potentials)

Back 48

PASSIVE FLUX of IONS running down their electrochemical gradients

Front 49

Energy requirements of Na/K+ ATPase

Back 49

20-40% of brain's energy is used to power this pump

in electrically active cells, it is 70% of cell's energy requirement

Front 50

ouabain inhibits what?

Back 50

the Na/K+ ATPase, by competing with K+ for its binding site on the outside of cell

Front 51

In skeletal muscle Vrest is primarily dependent upon:

Back 51

Gcl (conductance of Chloride ions)

(Gcl >> Gk)

Front 52

what kind of mutation gives rise to myotonia congenita?

Back 52

mutation in CLCN1 gene (encodes subunit of the predominant Cl channel expressed in skeletal muscle)

Front 53

what happens in myotonia congenita?

Back 53

temporary stiffness induced by forceful, abrupt movement

muscle is characterized by an abnormally low resting Gcl: does not allow rapid repolarization following an action potential and relaxation of contraction in the muscle following an initial contraction/movement

Front 54

what characterizes muscle in myotonia congenita?

Back 54

an abnormally low resting Gcl

does not allow rapid repolarization following an action potential and relaxation of contraction in the muscle following an initial contraction/movement.