An ultrastructural study of the marginal nucleus, the intrinsic mechanoreceptor of the snake's spinal cord.

Previously reported anatomical and electrophysiological studies have shown that there are neurons in the lamprey's spinal cord that respond to stretching of the spinal cord. Neurons in similar locations are especially prominent in reptiles, where they form the marginal nuclei. These nuclei have been examined in snakes, and it has become apparent that the denticulate ligament is both structurally and functionally closely related to the marginal nuclei. The ligament loses collagen in a short segment of every intervertebral area, and the marginal nuclei are located only in this area. The marginal nuclei consist of a group of medium-sized neurons along the edge of the spinal cord, with a strip of neuropil separating them from the ligament; the neurons extend dendritic processes into this lateral neuropil area and give rise to long finger-like processes. In the present study, these processes were found to be longer than the ones that have been described for peripheral mechanoreceptors; they are thought to be important in sensory transduction. Closely associated with these processes were axon-like structures. They did not make any type of contact with the finger-like processes; however, an occasional synaptic-like contact, consisting of membrane specialization and a congregation of vesicles, was made with dendritic processes. The conclusion is that these finger-like processes are similar to those of peripheral mechanoreceptors, but that there is no equivalent process to the axon-like structure.