The structure of the brainstem and cervical spinal cord in lungless salamanders (family plethodontidae) and its relation to feeding

Abstract

We present an HRP study of the sensory tracts and motor nuclei associated with feeding (especially use of the tongue) in plethodontid salamanders (mainly Batrachoseps attenuatus, Bolitoglossa subpalmata, Desmognathus ochrophaeus, Eurycea bislineata, and Plethodon jordani). The nerves studied are VII (ramus hyomandibularis only), IX, X, XI, the first spinal nerve (hypoglossus), and the second spinal nerve. Two types of sensory projections are universally found in the brainstem: superficial somatosensory projections of VII, IX, and X, and deeper visceral sensory projections of IX and X to the fasciculus soltarius. The first spinal nerve and the spinal accessory nerve (XI) have no sensory projections, but the second spinal nerve has typical projections along the dorsal funiculus of the spinal cord. The motor nuclei of VII ramus hyomandibularis, IX, and X form a combined nucleus situated at the level of the IX/X root complex. The nucleus of the first spinal nerve is well separated from the combined nucleus and is situated rostral and caudal to the obex. The rostral part of the motor nucleus of the second spinal modestly overlaps that of the first. The motor nucleus of the spinal accessory nerve is more or less restricted to the region of the second spinal nerve. Its fibers leave the brain through the last root of the IX/X complex and the related ganglion. Bolitoglossine and nonbolitoglossine differ in the architecture of the spinal nuclei. Two distinct types of motor neurons occur in spinal nuclei of nonbolitoglossine species--some of those with tongue projection--but only one type is found among the tongue-projecting bolitoglossine group.