When pressure is first applied to the corpuscle, it initiates a volley of impulses in its sensory neuron. However, with continuous pressure, the frequency of action potentials decreases quickly and soon stops. This is the phenomenon of adaptation. Adaptation occurs in most sense receptors. It is useful because it prevents the nervous system from being bombarded with information about insignificant matters like the touch and pressure of our clothing.
Stimuli represent changes in the environment.
If there is no change, the sense receptors soon adapt. But note that if we quickly remove the pressure from an adapted Pacinian corpuscle, a fresh volley of impulses will be generated. The speed of adaptation varies among different kinds of receptors. The knee jerk is a stretch reflex. Your physician taps you just below the knee with a rubber-headed hammer.