Segmental variation in the arborization of identified neurons in the leech central nervous system

Mechanosensory and motor neurons in the central nervous system of the leech have been examined by intracellular injection of horseradish peroxidase and electrophysiological mapping of their peripheral fields to determine how the arborizations of homologous cells are influenced by their segmental position. The branching patterns of annulus erector (AE) motoneurons in ganglia near the head and tail were found to be more extensive than those of cells in midbody ganglia. As in midbody ganglia, the peripheral fields of AE motoneurons in adjacent ganglia near the head and tail overlapped extensively, but the subfields innervated by individual branches of a single AE motoneuron showed little or no overlap. No AE motoneurons were found in the head ganglion or in the 20th and 21st free segmental ganglia. The branching pattern of touch-sensitive mechanosensory cells showed a similar segmental variation; touch cells in ganglia near the head and tail had more extensive arborizations than those in midbody ganglia. The rostrocaudal position along the cord at which the branching pattern changed from that characteristic of midbody ganglia to one with a more extensive arborization differed for different types of neurons. These findings demonstrate that a cell's pattern of arborization is not determined by a simple segmental difference between ganglia and suggest that during development neurons respond individually to cues that vary along the length of the cord.