The structure and source of lingual proprioceptors in the monkey

The proprioceptive innervation of the tounge has been investigated in the Cynamolgus monkey by silver impregnation methods following unilateral section of lingual, hypoglossal, and cervical nerves. Muscle spindles were constantly present in the intrinsic and extrinsic muscles. They varied greatly in number, averaged half the length of lumbrical spindles, and showed an unusual arrangement of chain fibre nuclei. Other, inconstant proprioceptors included tendon endings, Ruffini endings, Pacinian corpuscles, paciniform and lamellated endings. Topologically, the endings other than spindles were extra-muscular, so that the overall pattern of proprioceptive innervation resembled that of skeletal muscle in general. Lingual nerve section was without apparent effect on the proprioceptors. Section of the hypoglossal nerve at its point of entry into the tongue caused severe depletiion of ipsilateral proprioceptors and of fusimotor nerves. In the anterior tongue there was evidence of transmedian overlap by efferent and afferent axons contained in the hypoglossal nerve. Hypoglossal section at the skull base caused degeneration of fusimotor nerves but not of proprioceptors. Section of (a), the connexion of C2-C3 ventral rami with the hypoglossal, together with section of (b), the ramus descendens hypoglossi, coused depletion of lingual proprioceptors; again there was evidence of transmedian overlap. Procedures (a) or (b) alone had a lesser effect. It was concluded that lingual proprioceptive afferent fibres occupy the distal hypoglossal nerve, leaving it in the ramus descendens and in the C2-C3 connexion to enter the spinal cord via nerves C2 and C3.