Afferent projections from the rat longus capitis muscle studied by transganglionic transport of HRP

Using the method of transganglionic transport of horseradish peroxidase (HRP) according to Mesulam (1978), the course and central terminations of the afferents from the longus capitis muscle were studied in the rat. Application of HRP to the cut muscle nerves was followed by heavy labeling of a considerable number of cell bodies of different sizes in the spinal ganglia C2 and C3. The labeled central processes follow two distinct main routes: one to the external cuneate nucleus, which is known to project ipsilaterally to the cerebellum, the other to the central cervical nucleus (CCN) of the spinal cord. The latter nucleus contains dense arborizations and terminals of muscle afferents in close relationship to medium sized cells which project contralaterally to the cerebellum. This could be shown in double labeling experiments with HRP as a tracer for primary afferents and Nuclear yellow for the cerebellar pathway. The labeled area of CCN extends from the fourth cervical segment up to the medulla oblongata where it lies laterally adjacent to the hypoglossal nucleus, though clearly separated from the latter. In the cervical part of CCN, dendrites of neck muscle motoneurons arborize within the area of afferent terminals. Besides the external cuneate and the central cervical nuclei, afferent projections were seen in lamina X, partly in close contact with the central canal, and in more lateral areas of lamina VII. In contrast to results from studies on other neck muscles, no reaction product was observed in dorsal horn laminae I-VI.