Mechanisms for the excitation of ‘free nerve endings’

Abstract

AMONG the sensory nerve endings of mechanoreceptors there are two principal ways in which excitation can occur. In primary endings like the Pacinian corpuscle, the nerve membrane is directly excited by mechanical deformation conveyed by the surrounding non-nervous cells. In secondary endings like those in the ear, modified non-nervous cells are excited by the stimulus. They then excite the sensory nerve membrane either by current flow from cell to cell, or by extracellular field potentials, or finally by some secretory action which is often by way of a chemical synapse but could be of a more diffuse nature^1–3. For fine, unmyelinated sensory nerve fibres, however, the dendrites of which branch in tissues to form ‘free nerve endings’, it is not clear whether excitation is direct or indirect, or whether the endings are primary or secondary. Here I describe a preparation in which the activity of unmyelinated sensory neurones can be recorded simply and where the question is asked: are ‘free nerve endings’ excited directly by mechanical stimuli or is their excitation dependent on an electrical depolarisation of the cells that they innervate?