Which structure initiates an action potential?

Which structure initiates an action potential?

• A. Axon Hillock
• B. Soma
• C. Dendrite
• D. Myelin sheath

Answer: (A)

Initiation of an action potential happens in the region of the neuron that is most excitable, which is the axon hillock. This area has a high density of voltage-gated sodium channels and a relatively low threshold, so when graded inputs from dendrites and the soma summate to reach that threshold, a rapid depolarization is triggered. Once the threshold is reached, Na+ channels open in a positive feedback loop, producing the all-or-none spike that travels along the axon. The soma and dendrites are where inputs are received and integrated, generating graded potentials that may or may not reach threshold. The myelin sheath, on the other hand, speeds signal conduction by insulating the axon and reducing leakage, but it does not initiate the action potential itself; initiation still occurs at the axon hillock, with regeneration of the spike typically occurring at subsequent nodes of Ranvier along the myelinated segments.

Initiation of an action potential happens in the region of the neuron that is most excitable, which is the axon hillock. This area has a high density of voltage-gated sodium channels and a relatively low threshold, so when graded inputs from dendrites and the soma summate to reach that threshold, a rapid depolarization is triggered. Once the threshold is reached, Na+ channels open in a positive feedback loop, producing the all-or-none spike that travels along the axon.

The soma and dendrites are where inputs are received and integrated, generating graded potentials that may or may not reach threshold. The myelin sheath, on the other hand, speeds signal conduction by insulating the axon and reducing leakage, but it does not initiate the action potential itself; initiation still occurs at the axon hillock, with regeneration of the spike typically occurring at subsequent nodes of Ranvier along the myelinated segments.