Neck muscle afferent projections to the brainstem of the monkey: implications for the neural control of gaze.

Brainstem projections of first-order afferent neurons that innervate the suboccipital muscles of the monkey have been determined by using the technique of transganglionic transport of wheat-germ-agglutinin-conjugated horseradish peroxidase (WGA/HRP) and HRP. Neck muscle afferents distribute to several distinct regions located within the caudal brainstem and rostral spinal cord. Terminal labeling was heaviest within the ventral portion of the ipsilateral lateral cuneate nucleus. Muscle afferent terminals also distributed to ventrolateral portions of the pars triangularis division of the cuneate nucleus. Projections were consistent with the known somatotopic (i.e., both place and modality) organization of the cuneate nucleus. Moreover, neck muscle projections to the cuneate nucleus were, in part, coincident with those previously demonstrated for the extraocular muscles (Porter: J. Comp. Neurol. 247:133-143, '86). Sparse terminal projections were noted in the central cervical nucleus. In addition, light terminal labeling was present in group x of the vestibular complex and in an ill-defined region along the lateral margin of the brainstem. Present observations, which provide the first complete description of the central distribution of neck muscle afferent neurons in the primate, may contribute to the known substrate for eye/head coordination.