Final answer:
Excitability is the ability to reach threshold without stimulation. It refers to the ability of a neuron to generate an action potential without external stimulation.
Step-by-step explanation:
The ability to reach threshold without stimulation is called excitability. It refers to the ability of a neuron to generate an electrical signal, known as an action potential, without any external stimulation. This occurs when the cell membrane of a sensory receptor reaches a certain voltage, known as the threshold, which triggers the initiation of an action potential.