When a neuron receives a stimulus, an action potential is generated and travels along the axon as an electrical signal. Let us understand this mechanism at the ionic level.
Basic Knowledge
Mechanism of action-potential generation:
① Depolarization: When a stimulus above the threshold is applied, voltage-gated Na⁺ channels open simultaneously, and Na⁺ flows into the cell. The potential inside rapidly rises to about +40 mV.
② Repolarization: Na⁺ channels become inactive, and voltage-gated K⁺ channels open. K⁺ flows out of the cell, lowering the potential inside.
③ Hyperpolarization: Too much K⁺ flows out before the K⁺ channels close, making the inside slightly more negative than the resting potential.
④ Recovery: The Na⁺-K⁺ pump becomes active and returns the potential to the resting potential (-70 mV).
All-or-none law: With a stimulus above the threshold, an action potential of the same magnitude is generated regardless of the strength of the stimulus. No action potential is generated below the threshold.
Conduction of excitation: The site where an action potential has occurred temporarily enters a refractory period and cannot be reexcited in the reverse direction. Therefore, excitation travels in one direction. In myelinated nerves, it is accelerated by saltatory conduction between nodes of Ranvier.
Explain the relationship between stimulus strength and action potential in the “all-or-none law.”
Answer: Once the stimulus reaches the threshold, an action potential of the same magnitude (about 110 mV) is generated even if the stimulus is made stronger or weaker. Below the threshold, no action potential is generated. Stimulus strength is encoded by the frequency of action potentials (firing frequency).
- Depolarization = Na⁺ channel opening → Na⁺ inflow → inside reaches +40 mV
- Repolarization = K⁺ channel opening → K⁺ outflow
- All-or-none law = a constant action potential above the threshold
- Refractory period = the neuron cannot be reexcited immediately after an action potential
- Saltatory conduction = excitation jumps between the nodes of Ranvier in a myelinated nerve
- Excitation is conducted in one direction (because of the refractory period)
Points of Caution
① Depolarization is caused by Na⁺ inflow, and repolarization by K⁺ outflow. Do not reverse them. ② The all-or-none law applies to individual neurons. ③ Because of the refractory period, excitation can travel in only one direction.
Practice
- Which ion mainly flows into the cell during the depolarization phase of an action potential?
- What is the “all-or-none law”? Explain it briefly.
- What is the name of the phenomenon in which excitation jumps between nodes of Ranvier in a myelinated nerve?