Final answer:
Failure to reach the threshold voltage in a sensory neuron's axon can be due to a more negative resting membrane potential due to receptor potential, an insufficient depolarizing receptor potential, or the application of a stimulus of the wrong modality. The threshold must be reached for an action potential to be triggered, with inputs summing at the initial segment or axon hillock.
Step-by-step explanation:
The failure to reach the threshold voltage in the axon of a sensory neuron could be caused by several factors. These include 1) generation of a receptor potential that makes the axonal resting membrane potential more negative, 2) an insufficient depolarizing receptor potential, or 3) the application of the wrong stimulus modality. In the context of sensory neurons, receptor potentials can initiate action potentials if they are sufficient in magnitude to cause depolarization of the membrane potential up to the threshold.
When a stimulus is applied to a sensory receptor, such as a thermoreceptor, it alters the cell membrane's voltage. If the stimulus is strong enough (the graded potential is sufficient), the neuron is depolarized to its threshold potential, typically around -55 mV. A depolarizing graded potential must reach this threshold to open voltage-gated Na+ channels, leading to the generation of an action potential.
Furthermore, the initial segment or axon hillock acts as the crucial site for the summation of these graded potentials, which if sufficient, can result in an action potential that propagates along the axon. Severe hyperpolarization or inadequate stimulus strength can prevent the neuron from reaching the threshold, thereby failing to initiate an action potential.