Problem 18 - Entrance Test

The absolute refractory period is a crucial phase in action potential propagation, ensuring unidirectional transmission and setting a limit on the maximum firing rate of a neuron. During this period, no amount of stimulus, no matter how strong, can trigger another action potential. The underlying molecular mechanism is related to the state of the voltage-gated Na+ channels: 1. Activation State: Upon reaching threshold, voltage-gated Na+ channels open rapidly, allowing Na+ influx and depolarization. 2. Inactivation State: These channels have an inactivation gate. Shortly after opening, this inactivation gate swings shut, blocking the channel. This happens during the peak of depolarization and lasts through most of the repolarization phase. Once in the inactivated state, the channel cannot be opened again by further depolarization, even if the voltage-sensing gate is open. 3. Reset State: Only after the membrane has repolarized sufficiently (back to or below resting potential) can the inactivation gate open again, allowing the channel to return to its 'closed but ready to open' state. Let's evaluate the options: A. Voltage-gated K+ channels are involved in repolarization, and they open during this period (though more slowly than Na+ channels). They are not inactivated in a way that prevents a new action potential; their opening facilitates repolarization. So, this is incorrect. B. Correct. During the absolute refractory period, the majority of voltage-gated Na+ channels are in an inactivated state. This means they are closed and cannot be re-opened by a new depolarizing stimulus, regardless of its strength. This prevents the rapid Na+ influx necessary for generating another action potential. C. The Na+/K+ pump works continuously to maintain ion gradients, but its activity is not responsible for the absolute refractory period. It restores the resting membrane potential slowly over time, not rapidly for this specific period. D. Ligand-gated ion channels are typically found at synapses and respond to neurotransmitters, not directly involved in the refractory period of action potential propagation along the axon. Also, their continuous opening would prevent repolarization, which is the opposite of what's needed for recovery.