Frontal projections of dorsal neck and extraocular muscles.

We investigated the projection sites of dorsal neck muscle afferents in cats anesthetized with chloralose urethane. Electrical stimulation of a nerve branch to the biventer cervicis and complexus, and nerve branches of the suboccipital nerve to the rectus capitis dorsalis major and to the obliquus capitis caudalis muscles evoked field and single-cell responses in frontal brain regions corresponding with frontal eye field regions in cats, at latencies of 6 to 15 ms. By recording afferent volleys from dorsal rootlets of the first and second cervical segments, it was shown that neuronal activity in frontal brain regions could be elicited by signals from fibers electrophysiologically characterized as belonging to group I. Electrical stimulation of the nerve of the superior rectus muscle also elicited short-latency (6 to 20 ms) field and single-cell activity in the frontal eye field regions. Moreover, a high degree of convergence was observed at the single-cell level between dorsal neck and extraocular muscle afferents. This convergence was significantly higher than the convergence observed between the rectus capitis dorsalis major and obliquus capitis caudalis (deep muscles) with the biventer cervicis and complexus muscles. The convergence of extraocular and dorsal neck muscle afferents onto frontal eye field regions isuggests an involvement of these cortical regions in mechanisms related with coordination of sequential eye-head movements.