[Central afferent projections from the rat sternocleidomastoid and trapezius muscles. A study using transganglionic transport of horseradish peroxidase].

Primary afferent fibers from the sternocleidomastoid and trapezius muscles were transganglionically labeled with HRP, using rats anesthetized with urethane. All labelings were found ipsilaterally; retrogradely labeled cells were located in the C2 to C4 spinal ganglia, and transganglionic labelings in the C1 to the rostral C6 spinal segments and in the medulla oblongata. Labeled terminal fields were the lamina VI, the central cervical nucleus and the ventral horn in the cervical spinal cord, and in the medulla oblongata, many labeled fibers projected to the lateral (Cul) and medial cuneate nuclei (Cum), with a few of them projecting to the vestibular nucleus and the intermediate nucleus of Cajal. In the lateral cuneate nucleus, the terminal field was located in the medial portion at rostral levels, but it gradually shifted laterally as traced caudally; in the medial cuneate nucleus, the terminal field was located rostrally in the dorsolateral portion, but caudally in the ventrolateral portion. Although the sternocleidomastoid and trapezius muscles displayed a similar afferent projection pattern, the projection from the former muscle differed from that of the latter in the following respects: 1) Levels of projections to the spinal ganglia and the cervical spinal cord were located slightly more rostrally. 2) The terminal field in the lamina VI was located slightly more medially. 3) In the ventral horn, the sternocleidomastoid muscle afferents projected rostrally to the dorsomedial portion and caudally to the ventrolateral portion, but the trapezius muscle afferents projected solely to the ventrolateral portion at more caudal levels. 4) The terminal fields in the external cuneate nucleus were more extensive. 5) The projection area in the rostral medial cuneate nucleus was located slightly more medially, and projection to the ventromedial portion of the caudal medial cuneate nucleus and that to the intermediate nucleus were considerably fewer.