SDL-3 Action Potential

Question 1

What are the five stages of an action potential?

Answer 1

I. Rising phase
II. Overshoot
III. Peak
IV. Falling phase
V. Hyperpolarization (Undershoot)

Question 2

(T/F) The membrane potential remains negative throughout an action potential.

Answer 2

False. During the Overshoot phase, the membrane potential depolarizes past the threshold needed to elicit an action potential, peaking at approximately +30mV.

Question 3

On average, the hyperpolarization phase lasts __-__ milliseconds.

Answer 3

10-15 ms

Question 4

A sudden influx of ___ ions causes the rising phase of the action potential.

Answer 4

Na+

Question 5

____________ is a toxin found in puffer fish used to block Na+ channels for use in measurement of membrane permeabilities.

Answer 5

Tetrodotoxin

Question 6

____________ is a toxin used to block K+ channels for use in measurement of membrane permeabilities.

Answer 6

Tetraethylammonium (TEA)

Question 7

How is an action potential propagated the length of an axon?

Answer 7

As Na+ ions enter the cell through voltage-gated Na+ channels, they cause eddy currents from outside to inside the membrane. The entering Na+ ions are attracted to the nearby negative region of the membrane. This in turn depolarizes the nearby region of the membrane, opening more Na+ channels. The newly entering Na+ ions continue the process further and further down the axon, allowing for propagation of the action potential.

Question 8

How does a myelinated fiber increase action potential propagation?

Answer 8

In myelinated fibers, the eddy currents generated by Na+ ions skip the internodal myelinated portions, only depolarizing the membrane at the intervening nodes where Na+ channels are congregated. This process is referred to as saltatory conduction. Also, the voltage-gated K+ channels are sequestered underneath the myelinated portions of the fiber. This prevents hyperpolarization, allowing for generation of a new action potential sooner.

Question 9

In myelinated fibers, repolarization of the membrane is due to ____________ that are not actively opened and thus, there is no hyperpolarization.

Answer 9

K+ leakage channels

Question 10

Conduction of an action potential away from the soma is referred to as (antidromic/orthodromic).

Answer 10

Orthodromic

Question 11

Conduction of an action potential toward the soma is referred to as (antidromic/orthodromic).

Answer 11

Antidromic

Question 12

Changes in membrane potential that occur in a small region of the cytoplasmic membrane are referred to as _____________.

Answer 12

Electrotonic potentials

Question 13

The length of membrane at which an electrotonic potential is decreased to 1/e (about 37%) of it’s initial amplitude is the membrane’s ______________.

Answer 13

Length constant, λ (Lambda)

Question 14

The time required for the amplitude of an action potential to decrease to 37% of its initial value is the membrane’s _____________.

Answer 14

Time constant, τ (Tau)

Question 15

On average, the time constant of a membrane is __-__ milliseconds.

Answer 15

1-10 ms

Question 16

Larger diameter nerve fibers have (smaller/larger) length constants and (smaller/larger) time constants due to their lower internal resistance to the flow of ions.

Answer 16

Larger length constants; smaller time constants

Question 17

Larger diameter fibers have (lower/higher) thresholds for generation of an action potential. The opposite is true of smaller diameter fibers.

Answer 17

Larger diameter = lower threshold

Smaller diameter = higher threshold

Question 18

A depolarizing stimulus that accumulates slowly but does not result in an action potential due to the inactivation of Na+ channels before a cascade may be reached is referred to as _______________ to the stimulus.

Answer 18

Accomodation

Question 19

Local anesthetics prevent generation of action potentials by blocking ___________.

Answer 19

Na+ channels