CN - Chapter 4 - The Action Potential

Question 1

rising phase

Answer 1

The first part of an action potential, characterized by a rapid depolarization of the membrane.

Question 2

overshoot

Answer 2

The part of an action potential whemn the membrane potential is more positive than 0 mV.

Question 3

falling phase

Answer 3

The part of an action potential characterized by a rapid fall of membrane potential from positive to negative.

Question 4

undershoot

Answer 4

The part of an action potential when the membrane potential is more negative than at rest; also called after-hyperpolarization.

Question 5

after-hyperpolarization

Answer 5

The hyperpolarization that follows strong depolarization of the membrane; the last part of an action potential, also called undershoot.

Question 6

threshold

Answer 6

A level of depolarization sufficient to trigger an action potential.

Question 7

absolute refractory period

Answer 7

The period of time, measured from the onset of an action potential, during which another action potential cannot be triggered.

Question 8

relative refractory period

Answer 8

The period of time following an action potential during which more deploarizing current than usual is required to achieve threshold.

Question 9

voltage clamp

Answer 9

A device that enables an investigator to hold the membrane potential constant while transmembrane currents are measured.

Question 10

voltage-gated sodium channel

Answer 10

A membrane protein forming a pore that is permeable to Na⁺ ions and gated by depolarization of the membrane.

Question 11

patch clamp

Answer 11

A method that enables an investigator to hold constant the membrane potential of a patch of membrane while current through a small number of membrane channels is measured.

Question 12

channelopathy

Answer 12

A human genetic disease caused by alterations in the structure and function of ion channels.

Question 13

tetrodotoxin (TTX)

Answer 13

A toxin that blocks Na⁺ permeation through voltage-gated sodium channels, thereby blocking action potentials.

Question 14

voltage-gated potassium channel

Answer 14

A membrane protein forming a pore that is permeable to K⁺ ions and gated by depolarization of the membrane.

Question 15

saltatory conduction

Answer 15

The propagation of an action potential down a myelinated axon.

Question 16

spike-initiation zone

Answer 16

A region of the neuronal membrane where action potentials are normally initiated, characterized by a high density of voltage gated sodium channels.

Question 17

Question 1, part A:

Define membrane potential (V_m)

and

sodium equlibrium potential (E_Na)

Answer 17

The membrane potential (V_m) is the voltage across the neuronal membrane at any moment in time. The potential of the resting membrane is –75 mV.

The sodium equilibrium
potential (E_Na) is the steady equilibrium potential achieved when the membrane is permeable
only to sodium ions. The value of E_Na is 62 mV.

Question 18

Question 1, part B:

Which of these (membrane potential, V_m; or, sodium equilibrium potential, E_Na), if any, changes during the course of an action potential?

Answer 18

In its resting state, the membrane is not permeable to sodium. During the application of action potential, sodium channels open and sodium rushes into the cell. The large sodium current takes the membrane potential from its negative resting state toward E_Na.

Sodium channels are deactivated after 1 msec, and the
membrane repolarizes due to potassium efflux, which takes the membrane potential back toward the equilibrium potential of potassium.

Question 19

Question 2:

Which ions carry the early inward and late outward currents during the action
potential?

Answer 19

During the early part of the action potential, the influx of sodium ions across the membrane briefly depolarizes the membrane.

The brief inward sodium current is a
consequence of opening the voltage-gated sodium channels for only 1 msec.

Membrane repolarization is the result of potassium efflux, which is the outward potassium current because of opening voltage-gated potassium channels after a delay of 1 msec.

Question 20

Question 3:

Why is the action potential referred to as “all-or-none”?

Answer 20

Action potential is termed “all-or-none” because no partial action potentials exist.

A physical or electrical event opens sodium permeable channels, but the resulting influx of sodium ions and the resulting depolarization – called a generator potential — must reach a critical level before the axon generates an action potential. The critical level is called a threshold.

After achieving threshold depolarization, the cell fires an action potential.

Question 21

Question 4:

Some voltage-gated K⁺ channels are known as delayed rectifiers because of the timing of their opening during an action potential. What would happen if these channels took much longer than normal to open?

Answer 21

Voltage-gated potassium channels open 1 msec after membrane depolarization.

The resulting potassium conductance rectifies, or resets, the membrane potential. This conductance is called the delayed rectifier because of the 1 msec delay in rectifying the membrane potential.

If these channels took longer than normal to open, the action potential would be wider, which means that it would take longer to restore the resting membrane
potential.

Question 22

Question 5, part A:

Imagine you have labeled tetrodotoxin (TTX) to be able to see it with a microscope.

What
would be the consequence of applying TTX to the neuron?

Answer 22

TTX is a natural toxin that interferes with the function of voltage-gated sodium channels.

TTX blocks the sodium permeable pore by binding tightly to a specific part outside the channel and blocking all the sodium-dependent action potentials.

Applying TTX to a neuron would block all impulses in that nerve, preventing it from firing any action potential,
regardless of input.

Question 23

Question 5, part B:

Imagine you have labeled tetrodotoxin (TTX) to be able to see it with a microscope.

If we wash the TTX on to a neuron, what parts of the cell would you expect labeled?

Answer 23

Labeled TXX could be visualized on the cell’s axon, where voltage-gated sodium channels are concentrated.

Question 24

Question 6:

How does the conduction velocity of action potential vary with axonal diameter?

Why?

Answer 24

The speed of action potential depends on how far depolarization spreads ahead of action
potential. This, in turn, depends on the physical characteristics of axons. The two paths that a
positive charge can take are inside an axon and across the axonal membrane.

When the axon is narrow with many open pores, more of the current flows across the axonal membrane and
is lost. When the axon is wide with a few open pores, the current flows inside the axon.

The farther down the axon the current flows, the farther ahead of the action potential the membrane will be depolarized and the faster the action potential will propagate. As a result, the conduction velocity of axons increases with the diameter of axons.