Experimental studies on the afferent innervation of the cricothyroid muscle in dogs.

The cricothyroid muscle in dogs received branches from two independent nerves, namely the external ramus of the cranial laryngeal nerve and the pharyngeal branch of the vagus. Classical spindles are infrequent in the muscle. Atypical forms of sensory endings were identified. Two end-plates were frequently met with on a single extrafusal fibre. Sectioning of the external ramus of the cranial laryngeal nerve was followed by degeneration of spindles. Intact axons detected up to 6 months after operation are probably derived from the pharyngeal branch of the vagus. Chromatolytic changes occurred in the ipsilateral dorsal vagal nucleus and the capsulated ganglion at the entry of the nerve into the muscle. Chromatolysis occurred in the intramuscular ganglion cell rows and in neurons of the ipsilateral nodose ganglion. Morphological alterations were more pronounced in the ipsilateral medial column of the nucleus ambiguus. No changes were observed in the somata of the mesencephalic nucleus.