Question

part f (6 of 6)
what is the first change to occur in response to a threshold stimulus?
voltage - gated na+ channels change shape, and their inactivation gates close.
voltage - gated na+ channels change shape, and their activation gates open.
voltage - gated k+ channels change shape, and their activation gates open.
voltage - gated ca²+ channels change shape, and their activation gates open.

Explanation:

When a threshold stimulus is reached, the first event is related to voltage - gated sodium (\(Na^{+}\)) channels. Voltage - gated \(Na^{+}\) channels have activation and inactivation gates. At rest, the activation gate is closed and the inactivation gate is open. When the threshold is reached, the voltage - gated \(Na^{+}\) channels change shape, and their activation gates open. This allows \(Na^{+}\) to rush into the cell, depolarizing the membrane. Voltage - gated \(K^{+}\) channels open later during repolarization. Voltage - gated \(Ca^{2 +}\) channels are more relevant in processes like neurotransmitter release at synapses (not the first response to a threshold stimulus in a typical action potential initiation in a neuron). The option with inactivation gates closing is incorrect as the first change is activation gate opening.

Answer:

Voltage - gated \(Na^{+}\) channels change shape, and their activation gates open.