Organization of tectospinal neurons in the cat and rat superior colliculus

Neurons in the superior colliculus which send axons to the spinal cord were identified by the retrograde axonal transport method. Injections of the retrograde axonal tracer, horseradish peroxidase, made into different levels of the cervical spinal cord indicate that the spinally projecting neurons in the superior colliculus are topographically organized. Tectospinal neurons projecting to the rostral segments of the cervical spinal cord are found over a larger extent of the contralateral superior colliculus than those terminating in the cervical enlargement. The tectospinal projection to the cervical enlargement in both rats and cats arises almost exclusively from the caudolateral quadrant of the contralateral superior colliculus, whereas the tectal projection to the rostral (upper) cervical spinal cord originates, in cats, from almost the entire extent of the colliculus and, in rats, from its greater part. No evidence for tectospinal projections to the thoracic or lumbar levels of the spinal cord was found. In the context of the hypothesis that, in these species, head and body movements which orient the animal toward stimuli in its environment may be mediated by the superior colliculus, these data are consistent with the view that direct tectospinal connections may play a role in such movements.