Neuron Structure and Action Potentials - Anatomy and Physiology

1. What does the all-or-none principle state about action potentials?

   Once threshold is reached, an action potential fires at full amplitude. Stimulus strength is coded by firing frequency, not by spike size.

2. Why can no stimulus trigger a second action potential during the absolute refractory period?

   Voltage-gated Na+ channels are open or inactivated, and inactivated channels cannot reopen until the membrane repolarizes.

3. Which two axon properties increase action potential conduction velocity?

   Larger axon diameter and myelination. Both reduce current loss and resistance, so depolarization spreads farther and faster.

4. A neuron rests at -70 mV and has a threshold of -55 mV. How much depolarization must summed inputs produce to trigger an action potential?

   15 mV. The membrane must move from -70 mV to -55 mV, a change of 15 mV, to reach threshold.

5. What is required to fire an action potential during the relative refractory period?

   A stronger-than-normal stimulus. Some Na+ channels have recovered, but the membrane is hyperpolarized and K+ channels are still open.

6. What is saltatory conduction?

   Conduction in myelinated axons in which the action potential is regenerated only at the nodes of Ranvier, so it appears to jump from node to node.

7. A toxin blocks voltage-gated Na+ channels. Which phase of the action potential is prevented?

   The depolarization (rising) phase. Without Na+ influx the membrane cannot reach the spike, so no action potential is generated or conducted.

8. At which region of a typical multipolar neuron is an action potential usually initiated?

   The axon hillock and initial segment (trigger zone). It has the highest density of voltage-gated Na+ channels, so threshold is reached there first.

9. Which ion movement produces the repolarization phase of an action potential?

   K+ efflux through voltage-gated K+ channels, which open more slowly than Na+ channels and let K+ leave the cell.

10. What causes the after-hyperpolarization (undershoot) at the end of an action potential?

    Voltage-gated K+ channels close slowly, so K+ keeps leaving and the membrane briefly becomes more negative than resting potential.

11. Which ion movement produces the rising (depolarization) phase of an action potential?

    Na+ influx through voltage-gated Na+ channels. Na+ rushes in down its electrochemical gradient once threshold opens the channels.

12. Why does an action potential normally travel in only one direction along an axon?

    The membrane just behind the active region is refractory, so the impulse can only depolarize the resting membrane ahead of it.

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