Final answer:
The action potential has a lower threshold at the axon hillock due to an increased density of sodium channels, leading to rapid depolarization and initiation of the action potential.
Step-by-step explanation:
The AP (action potential) at the axon hillock (trigger zone) has a lower threshold due to the increased density of sodium channels in that region. When a stimulus depolarizes the neuron to its threshold potential, these sodium channels open, allowing positive ions to enter the cell and leading to a rapid depolarization that initiates the action potential. This process is the "all-or-nothing" event necessary for a neuron to transmit a signal along the axon towards the synapse.
The depolarization of the membrane by Na+ ions influx is followed by the repolarization phase during which potassium channels open, allowing K+ ions to leave the cell and restore the negative membrane potential.
Mechanically gated channels respond to physical stimuli such as stretch or pressure, which can be the initiating factor for an action potential if they lead to sufficient membrane depolarization.