Mechanosensory units in the hypoglossal nerve of the cat

Abstract

Sensory fibers were found along the extracranial course of the hypoglossal nerve of the cat: terminal branches, mid-portion of the trunk and ramus descendens hypoglossi. They were also present in a communicating branch between the hypoglossal nerve and the nodose ganglion of the vagus, here called the ‘hypogloss0-nodosal branch’. These fibers responded with slow adaptation to mechanical stimulation produced by longitudinal or lateral tensions or displacements of the tongue, but they did not respond to tactile stimuli applied to its surface. Most hypoglossal sensory units showed a basal rate of discharge in resting conditions, but some fibers were silent at rest. Their rates of discharge during mechanical stimulation depended on the intensity of the stimulus and its acceleration during application or interruption. Those fibers having a base-line discharge showed a silent pause at off, when the stimuli were abruptly interrupted. The rates of discharge of some hypoglosso-nodosal sensory units were accelerated or reduced by tongue movements elicited through stimulation of some terminal branches of the contralateral hypoglossus. However, other patterns of tongue movement evoked by stimulation of other terminal branches were ineffective. It is suggested that sensory fibers present along most of the extracranial course of the hypoglossus have their perikarya in the nodose ganglion and enter the brain stem through the dorsal roots of the vagus nerve.