Physiological responses, receptive fields and terminal arborizations of nociceptive cells in the leech

The physiological responses, receptive fields and morphology of individual nociceptor (N) neurones have been studied in the leech. In each of the midbody ganglia there are four N cells (two on either side). Each N cell has a distinctive territory that it supplies in the periphery, on the surface or internally. 1. Both N cells respond selectively to noxious mechanical stimuli applied to the skin but not to touch, light, pressure or stretch. The receptive field of each cell is well defined and covers roughly the same area, extending from the dorsal midline to the ventral midline, with considerable overlap. 2. One of the N cells, situated more medially in the ganglion, also fires at high frequencies in response to mechanical stimulation, such as pinching or squeezing, of the connective tissue lining the viscera. In contrast, the other N cell (situated laterally in the ganglion) is activated by pressure or pinches applied to the opening of the excretory duct but not the gut. 3. Following injection of horseradish peroxidase into the soma, axons of N cells appear as unspecialized fine processes about 1 micrometer in diameter, in the dermis of the leech, deep to the layer of epidermal cells. In addition, at specific sites in the skin, the N cell situated laterally in the ganglion makes distinctive coiled terminals in association with the expanded dendrites of large neurones in the periphery, the functions of which are unknown. This finding raises the possibility that lateral N cells may perform some additional role as yet not understood.