Physiological properties and receptive fields of mechanosensory neurones in the head ganglion of the leech: comparison with homologous cells in segmental ganglia.

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Abstract
The properties of specific sensory cells in the cerebral ganglion of the leech [Hirudo medicinalis] were compared with those of homologous neurons in the segmental ganglia. In the cerebral ganglion, cells were identified that had electrical properties, sensory modalities and adaptation properties similar to those of touch (T), pressure (P) and nociceptive (N) cells in the segmental ganglia. The cell bodies of these neurons were situated in characteristic positions that could be correlated with those in the segmental ganglia. Several lines of evidence suggested that they were primary sensory neurons. Fewer T, P and N neurons were identified in the cerebral ganglion than would be expected from its 6 constituent segmental ganglia. The receptive fields of identified T, P and N cells were situated on the external surface of the head and the interior of the mouth with considerable overlap. They were generally smaller in size than those situated on the main part of the body. The receptive fields were displaced anteriorly so that some of them were situated in segments anterior to those of the innervating cells. The morphology of the sensory cells in the head ganglion was studied by intracellular injection of horseradish peroxidase. The general branching characteristics of the cells and the structural appearance of their processes resembled those of homologous cells in the segmental ganglia. The routes taken to the periphery by some of the cells showed individual variation. This variability was confirmed by electrophysiological evidence, and differed from the constancy seen in segmental sensory cells. Sensory cells in the cerebral ganglion seem to resemble homologous cells in the segmental ganglia with respect to their organization and physiological properties.

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