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 in testine in small mammals such as rabbit, rat, guinea pig and mouse form a n etwork, the tertiary plexus, against the inner surface of the muscle. In ge neral, because of their substantial overlap, it has not been possible to fo llow the ramifications of individual axone in the tertiary plexus. In the p resent work, the longitudinal muscle motor neurons were filled with marker dyes through an intracellular microelectrode, and their morphologies and pr ojections were examined in whole-mount preparations of longitudinal muscle and myenteric plexus. Most neurons that were examined were in the small int estine (ileum and duodenum), but a few were examined in the distal colon. N eurons in all regions had similar morphologies and projections. The cell bo dies were amongst the smallest in myenteric ganglia, with major and minor a xes of 14 mu m and 25 mu m (mean, n=40) in the plane of the myenteric plexu s. Each neuron had a single axon that branched extensively in the tertiary plexus, most had multiple lamellar dendrites and a few had filamentous dend rites 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 mm(2) (mean /- SD, n=23). From the density of occurrence of cell bodies, it can be calc ulated that each point in the longitudinal muscle is innervated by the proc esses of about 100 motor neurons and is influenced by electrotonic conducti on of signals through the muscle by about 300 motor neurons.