Shapes and projections of tertiary plexus neurons of the guinea-pig small intestine.

The axons of neurons that innervate the longitudinal muscle of the small intestine in small mammals such as rabbit, rat, guinea pig and mouse form a network, the tertiary plexus, against the inner surface of the muscle. In general, because of their substantial overlap, it has not been possible to follow the ramifications of individual axons in the tertiary plexus. In the present work, the longitudinal muscle motor neurons were filled with marker dyes through an intracellular microelectrode, and their morphologies and projections were examined in whole-mount preparations of longitudinal muscle and myenteric plexus. Most neurons that were examined were in the small intestine (ileum and duodenum), but a few were examined in the distal colon. Neurons in all regions had similar morphologies and projections. The cell bodies were amongst the smallest in myenteric ganglia, with major and minor axes of 14 microns and 25 microns (mean, n = 40) in the plane of the myenteric plexus. Each neuron had a single axon that branched extensively in the tertiary plexus, most had multiple lamellar dendrites and a few had filamentous dendrites or a mixture of filamentous and lamellar dendrites. The mean area of muscle covered by an axon and its branches extended 1.6 mm orally to anally and 1.7 mm circumferentially. The area covered was 2.8 +/- 1.9 mm2 (mean +/- SD, n = 23). From the density of occurrence of cell bodies, it can be calculated that each point in the longitudinal muscle is innervated by the processes of about 100 motor neurons and is influenced by electrotonic conduction of signals through the muscle by about 300 motor neurons.