A 39-year-old neurosurgeon picks up a scalpel, which activates numerous sensory receptors in her hand. An increase in which of the following best describes the basis for transduction of the sensory stimuli into nerve impulses?

Viually all mechanical stimuli cause an increase in ion permeability (usually to sodium) in mechanoreceptors. If the membrane potential of the mechanoreceptor reaches a critical threshold value, an action potential is initiated. The G-protein "second messenger" system is typically involved. Transmitter release does not occur at the level of the mechanoreceptor, but if a mechanoreceptor is activated, afferent nerve impulses do stimulate transmitter release at the nerve terminal in the central nervous system.