The spinomesencephalic tract in the cat: its cells of origin and termination pattern as demonstrated by the intraaxonal transport method.

The cells of origin and terminal areas of the feline spinomesencephalic tract were investigated by the intraaxonal transport method. Following injection of wheat germ agglutinin-horseradish peroxidase conjugate into the cervical and lumbar enlargements, anterograde labelling was observed in several regions of the dorsal midbrain. The main terminal areas were the periaqueductal gray matter, the intercollicular nucleus, the posterior pretectal nucleus and the nucleus of Darkschewitsch. In addition, there was a sparse projection to the cuneiform nucleus and the anterior pretectal nucleus. The superior colliculus was virtually devoid of labelling except for a weak termination in the caudal part of the deep layers. Although there was a considerable overlap, the projection from the cervical spinal cord to the periaqueductal gray matter terminated more rostrally than that from the lumbar segments, indicating the presence of a somatotopic organization. The retrograde labelling seen after tracer injection into the midbrain terminal areas revealed that the cells of origin were located mainly in the upper cervical segments and in the cervical and lumbar enlargements; in the latter parts of the cord an overwhelming majority were situated in lamina I, with smaller fractions in laminae IV and V, whereas in the upper cervical segments and in the less densely labelled thoracic and sacral segments a much larger proportion of the peroxidase-positive neurons were found in the deep laminae. About 75% of the labelled cells were located contralateral to the injection site. The functional implication of the present results are discussed in relation to somatosensory activity in the mesencephalon. It is suggested that several regions of the dorsal midbrain have an important somesthetic function including that of pain.