L9: Ionic Bases Of Action Potential

Question 1

What is the definition of the action potential?

Answer 1

• It Is the electrical changes that occur in the resting membrane potential as a result of stimulation by an effective stimulus.
• In the case of nerve fibers, it is transmitted as a self-propagated disturbance known as the nerve impulse.

Question 2

What are the components of the action potential?

Answer 2

The AP consists of 2 main stages:
• (depolarization and repolarization) that are followed by 2 other stages:
• after-depolarization and after-hyperpolarization.

Question 3

What is the definition of depolarization?

Answer 3

It is a loss of the normal resting polarized state of the membrane.

Question 4

What is the ionic basis of depolarization?

Answer 4

• The stimulus increases the permeability of the cell membrane to Na+ ions, which diffuse inside causing the gradual change in the membrane potential from the resting potential (-70m.v) to the isoelectric line (zero) and exceeding it to +35 mv (overshot) by 2 steps separated by the firing level which equals –55 mv
• The first step is slow depolarization in which the membrane potential shifts from -70 mv to -55 mv (firing level) due to the opening of some Na+ channels

-The second step is Rapid depolarization in which the membrane potential shifts from -55 mv to +35 mv (overshot) due to the opening of all Na+ channels and it results in A.P. having a magnitude of
105 mV (from -70 to + 35mV).

• In the resting state, only the inactivation gates of Na+ channels are open, so the membrane permeability to Na+ is low and When the nerve is stimulated, the Na+ activation gates also open thus the membrane permeability to Na+ and Na+ influx markedly increased

Question 5

Which Na+ Gates are open during resting state and what does this cause?

Answer 5

In the resting state, only the inactivation gates of Na+ channels are open, so the membrane permeability to Na+ is low.

Question 6

Which Na+ gates are open when the nerve is stimulated?

Answer 6

When the nerve is stimulated, the Na+ activation gates also open thus the membrane permeability to Na+ and Na+ influx markedly increased

Question 7

What is the definition of depolarization?

Answer 7

• It is the restoration normal resting polarized state of the membrane.
• It is recorded as a fall of the membrane potential in the negative direction from +35 to – 70, producing the descending limb of action potential

Question 8

What are the steps of repolarization?

Answer 8

1) RP proceeds immediately and rapidly after the overshoot is reached
2) When RP is 70% completed, its rate decreases by about 4 msec. (after-depolarization or (negative after-potential).
3) After RP is completed, the membrane potential overshoots to the negative side (by 1-2 mV) leading to hyperpolarization of the membrane (after-hyperpolarization or positive after-potential).
4) It lasts about 40 msec but its magnitude gradually declines till normal resting membrane potential is restored.

Question 9

What is the ionic basis of repolarization?

Answer 9

1-Stoppage of Na+ influx due to:-

A-Closure of Na+ inactivation gates.
B- Reversal of direction of the electrical gradient for Na+ (which becomes from inside to outside the membrane)

2-K+ efflux (exit):

A-occurs through specific K+ channels that contain a single gate located toward the inside of membrane.
B-The decrease in membrane polarity during depolarization leads to the opening of K+ gates and K+ efflux

3- The negative after-potential stage:

• is due to slowing of the rate of K+ efflux,
• while the positive after-potential is due to sow return of K+ channels to the closed state (which allows prolonged K+ efflux).

4- Following the AP, the normal distribution of Na+ and K+ ions across the cell membrane is restored by the action of the Na+-K+ pump

Question 10

What is the compound action potential?

Answer 10

This is the AP recorded from peripheral nerves when stimulated by maximal stimuli.

Question 11

In Which type of nerve fibers is the conduction the fastest?

Answer 11

• These nerves contain different types of nerve fibers that vary in their speeds of conduction (the thicker the nerve fiber, the more rapid is its rate of conduction of nerve impulses and vice versa.
• Thus, the activity in fast conducting fibers arrives at recording electrodes earlier than activity in slower nerve fibers resulting in a cAP that have multiple peaks