Similarities between some properties of the soma and sensory receptors of primary afferent neurones.

The electrical properties of the somata of neurones associated with low-threshold, cutaneous mechanoreceptors were studied in young, anaesthetized rats. The adaptation properties of slowly and rapidly adapting afferents were compared by recording from the somata during mechanical stimulation of the skin and during the injection of depolarizing current pulses through the recording electrode. Membrane potential and action potential conformation were recorded from each cell, together with the axonal conduction velocity. Axonal conduction velocities were in the ranges 62-18 m/s (A alpha/beta) and 7-4 m/s (A delta). Somata with A alpha/beta-axons which were associated with slowly adapting receptors gave sustained discharges when depolarized by injected current, whereas those associated with rapidly adapting receptors gave a brief discharge only (median response: five impulses). Somata with A delta-axons had similar characteristics to fast-conducting, rapidly adapting afferents. In a separate study it was shown that, for neurones which gave an adapting response to intracellular current injection, action potentials recorded from the somata were able to follow electrical stimulation of their axons at frequencies up to 100 Hz, indicating that the adaptation of responses to mechanical stimulation in the other experiments was not due to failure of impulses to invade the somata. The results suggest that the adaptation properties of a mechanoreceptor may be due to the presence of a particular type of ionic channel in the terminals of the nerve which is also present in the membrane of its soma.