Final answer:
The stimulus voltage needed for the voltage-gated ion channels to reach threshold is the membrane depolarization to -55 mV, initiated by action potentials. Spatial and temporal summation at the axon hillock or the initial segment of a sensory neuron help to achieve threshold, allowing the propagation of action potentials along the axon.
Step-by-step explanation:
The stimulus voltage required for the voltage-gated ion channels to reach threshold in the portions of the axon after the initial segment is the depolarization that occurs due to the action potentials propagated along the axon. This depolarization is initiated when the membrane potential reaches around -55 mV, which is the threshold for triggering the opening of voltage-gated Na+ channels. In unmyelinated axons, this sequence of depolarization and subsequent action potentials occurs along the entire length of the axon.
However, in myelinated axons, propagation is facilitated by nodes of Ranvier, where the density of voltage-gated Na+ channels is high, allowing the action potential to jump from node to node in a process called saltatory conduction.
Spatial and temporal summation contribute to reaching the threshold at the initial segment, which could be the axon hillock or the part of the sensory neuron directly adjacent to the dendritic endings. Once the threshold is reached, an all-or-nothing action potential is generated and can be propagated along the axon. This electrochemical process is critical for the fast transmission of neural signals across the nervous system.