The properties and connections of supernumerary sensory and motor nerve cells in the central nervous system of an abnormal leech

The characteristic features of individual neurons in leech ganglia are highly consistent from animal to animal. Hy chance a leech was dis‐ covered that had more cells to be seen than usual in many of its ganglia. In some segmental ganglia of the abnormal animal there were four or even five sensory cells responding independently to touch (T cells) instead of the usual complement of three on each side. Ganglia also contained three or four pressure sensitive cells (P cells) and three or four nociceptive cells (N cells) instead of two of each. The membrane properties, as well as the shapes, sizes, and positions, of all these cells were normal. Their axons reached the skin by the appropriate nerve bundles. Two sensory neurons of one modality, often independently innervated, overlay regions of skin that normally would be supplied by one cell. On the other hand, the areas innervated by some supernumerary cells were abnormal in their shape and position. In some ganglia instead of one motor neuron that raised the skin into bumps (the annulus erector, or AE cell) there were two, each of which could independently erect annuli. The regions innervated by the extra AE cells appeared normal and overlapped extensively. Normal synaptic interactions were found between supernumerary sensory cells and motor cells. The results with this animal indicate that two or more nerve cells can form effective connections with other neurons and with skin that normally would be innervated by a single cell.