Biological Systems MCAT Practice Exam

What characterizes the absolute refractory period?

1. Neuron can fire a new action potential

2. Na+ channels are inactivated

3. K+ channels are closed

4. Only K+ channels are open

The correct answer is: Na+ channels are inactivated

The absolute refractory period is characterized by the inactivation of sodium (Na+) channels. During this period, a neuron cannot generate a new action potential, regardless of the strength of the stimulus applied. After an action potential is generated, Na+ channels open to allow sodium ions to flow into the cell, causing depolarization. Shortly after, these channels inactivate, preventing any further influx of sodium ions. This inactivation is crucial because it ensures that once an action potential has occurred, there is a brief period during which the neuron is incapable of firing again, thereby preventing overlapping signals and allowing for proper signal propagation in one-directional flow along the neuron. This distinct phase plays a vital role in maintaining the integrity and timing of neuronal signaling. While other channels may have different states during this period, it is the inactivation of Na+ channels that primarily defines it. Hence, this is the critical factor that identifies the absolute refractory period, allowing the neuron to reset and recover before it can respond to subsequent stimuli.