Spinocerebellar projections in the pigeon with special reference to the neck region of the body.

Neck sensory information is important for control of head and body movements in all vertebrates. Neuroanatomic tracing methods were used to study the pathways of neck afferent systems. Both the projection of primary afferent fibers and of secondary afferent pathways to brainstem and cerebellum were investigated with the anterograde transport of dextran amines as tracers (biotinylated dextran amine and tetramethyl rhodamine dextran amine). For comparison, the projections of spinocerebellar systems of wing and leg were studied also. Complementary experiments using retrograde tracers (Fast Blue, tetramethyl rhodamine dextran amine, rhodamine isothiocyanate) injected into the cerebellum served to corroborate the results of the anterograde tracing experiments. Primary neck afferent fibers terminated in the spinal gray substance with dense terminal fields in laminae I to V of the dorsal horn and lamina IX of the ventral horn as well as in the marginal nuclei located at the lateral border of the spinal cord. In the brainstem, dense terminal fields were seen in deep layers of the medullary dorsal horn, in the external cuneate nucleus, and in group x. Secondary neck afferents arising from ventral horn cells showed a significant projection to the descending and medial vestibular nuclei and to the medial cerebellar nucleus. Terminals were found both in the anterior and the posterior cerebellum. A quantitative evaluation disclosed that most terminals of neck afferents distributed in lobules II-IV of the anterior cerebellum and lobule IX of the posterior cerebellum. With injections aimed at spinocerebellar neurons located into the cervical and lumbosacral enlargements, no projections were found in the vestibular or deep cerebellar nuclei. Projections from the cervical enlargement were concentrated in lobules III-V and those from the lumbosacral enlargement in lobules III-VI. This points to a rostrocaudal somatotopic representation of neck, wing, and leg in the anterior cerebellum. The results of the retrograde tracing experiments support such a somatotopic organization.