Distribution of motoneurons supplying dorsal suboccipital and intervertebral muscles in the cat neck.

Multiple labelling methods were used to examine the topographical organization of motoneurons supplying the suboccipital muscles, rectus capitis posterior (RCP) and obliquus capitis inferior (OCI); the intervertebral muscles, spinalis dorsi (SD) and semispinalis cervicis (SSC); and the dorsal extensor biventer cervicis (BC). Muscle nerves were isolated, cut, and exposed in the same animal to one of three different retrograde tracers, Fluorogold, Fast Blue, and horseradish peroxidase. Motoneurons supplying suboccipital muscles were found to be located in the rostral two cervical segments. The RCP motor nucleus was confined to C1 and the caudal brainstem, whereas the OCI nucleus usually originated in mid-C1 and extended caudally as far as mid-C2. Most motoneurons supplying OCI and SD were concentrated in the deepest part of the ventral horn, but some cells were found in the commissural region, the contralateral ventral horn, the dorso-medial aspect of the ipsilateral ventral horn, and the ventromedial white matter. Cells supplying the functional extensor RCP were intermingled with those supplying the turning muscle OCI, although a gradient in mediolateral distribution was detected. Motoneurons supplying the intervertebral muscles SD and SSC were found caudal to C3. Within the ventral horn, they occupied locations similar to those occupied by suboccipital motoneurons in C1 and C2. The cell column for intervertebral motoneurons was just medial to that of BC and some cells supplying SD and SSC were found in the territory occupied by BC motoneurons. Motoneurons supplying intervertebral and suboccipital muscles had unimodal diameter spectra, and most cells had mean equivalent diameters smaller than 35 microns.