Exam 1 - Action Potential

Question 1

contrast graded (electrotonic) potentials vs. action potentials

Answer 1

graded potentials:

• amplitude proportional to injected current
• fast
• can be depolarization or hyperpolarization
• can get temporal, spatial summation
• travel by passive spread
• decremental
• triggered by external stimuli
• no refractory period
• begin in dendrites or soma

action potentials:

• slow
• triggered only by depolarization
• all or none (must reach threshold)
• regenerates and propagates w/o decrement
• absolute and relative refractory periods
• begins in axon hillock

Question 2

7 characteristics of an action potential

Answer 2

• triggered by depolarization (about 10 to 20 mV)
• all or none (must reach threshold)
• frequency of discharge = fxn of stimulus strength
• regenerative, progagate w/o decrement at relatively slow speed
• overshoot (at peak, cell interior becomes positive)
• undershoot (when repolarizing, transiently becomes more negative than normal)
• absolute refractory period (1 ms) - limits maximum firing rate to 1000 Hz

Question 3

3 pieces of evidence that the inward current in an AP is caused by Na+

Answer 3

1- Nernst: if you measure Em w/ applied I and graph -> find no inward current when Em is clamped at +55 mV. Nernst predicts the Eion for Na+ to be +55 mV. Therefore, this suggests Na+ is responsible for the inward current.

2-Ionic substitution: a 90% decrease in external Na+ eliminates the early inward current -> therefore, this current is due to transmembrane movement of Na+

3-TTX: neurotoxin that only blocks Na+ channels -> when added to a cell, no longer get inward current. Therefore, inward current = Na+

Question 4

2 pieces of evidence that the outward current in an AP is caused by K+

Answer 4

1. Ionic substitution: replacement of intracellular K+ with Cs+ eliminates the outward current -> suggests K+ is responsible for the outward current
2. TEA: selectively blocks K+ currents -> when added to a cell, no longer get outward current. Therefore, outward current = K+

Question 5

describe Hodgkin and Huxley’s contributions to figuring out the mechanism of generation of an AP using voltage clamp experiments

Answer 5

voltage clamp permits determination of how the membrane potential (Em) influences ionic current flow across membrane. When they:

• hyperpolarize Em -> brief redistribution of charge across membrane, then no other current flows
• depolarize Em -> again brief capacitance current, but then rapid increase in inward current followed by a slower, delayed outward current

Question 6

what is the resting membrane potential in most nerve cells?

Answer 6

-60 mV

Question 7

two-voltage dependent membrane conduction (g) model of the AP: 3 important phenomena

Answer 7

• both conductances are voltage dependent -> both gNa and gK increase progressively as neuron is depolarized
• both conductances change over time -> inward flow of Na+ precedes the outward flow of K+
• activation saturates

Question 8

describe how the changes in Na+ and K+ conductance (g) cause an action potential

Answer 8

• selective increase in gNa initiates AP by causing Na+ to enter cell -> depolarizing it
• rate of depolarization falls due to decreased electrochemical driving force on Na+ and gNa inactivating
• depolarization slowly activates gK and K+ leaves cell -> repolarizing it
• hyperpolarization causes gK to turn off -> membrane potential returns to resting level

Question 9

describe how the effects of an AP on Na+ conductance are an example of positive feedback

Answer 9

regenerative: depolarization -> increase in gNa -> increase in inward Na+ current -> leads to more depolarization -> repeat
- occurs until Na+ channels close

Question 10

describe how the effects of an AP on K+ conductance are an example of negative feedback

Answer 10

not regenerative: depolarization -> increase in gK -> increases outward K+ current -> leads to repolarization

-self-limiting

Question 11

what is membrane voltage at any point on the membrane a function of?

Answer 11

the distance from the point of maximum depolarization - aka the potential change decays exponentially w/ increasing distance from the site of stimulation

Question 12

electrotonic potentials decay the farther they go… so how is it that APs make it all the way down a neuron?

Answer 12

AP causes such a large local depolarization that passive spread is still sufficient to depolarize adjacent regions of the axon above threshold -> AP is generated at a point adjacent to original -> repeats until travels entire length of axon

Question 13

why is the rate of conduction of an AP not instantaneous?

Answer 13

b/c it is an electrochemical response

Question 14

what causes the refractory period?

Answer 14

in the wake of a wave of depolarization, Na+ channels are inactivated and K+ channels are activated for a brief time -> makes it near impossible to produce another AP

Question 15

why do APs travel in one direction only?

Answer 15

the refractory period of the membrane following the wave of propagation

Question 16

difference b/w causes of absolute and relative refractory period

Answer 16

absolute: when Na+ channels open and then close
relative: after gNa changes have finished and the membrane voltage is being changed by outward flow of K+

Question 17

factors influencing conduction velocity

Answer 17

• axon size (larger = faster)
• internal resistance (lower = faster)
• membrane resistance (higher = faster: less leaks out)

Question 18

how does saltatory conduction make propagation a more efficient process?

Answer 18

• enhances conduction velocity

- axons have Na+ and K+ channels localized to nodes of Ranvier -> less metabolic drain