Cortical and spinal somatosensory input to the superior colliculus in the golden hamster: an anatomical and electrophysiological study.

The horseradish peroxidase technique was used to identify the sources of somatosensory afferent fibers to the hamster superior colliculus. These experiments demonstrated that the tectum receives axons from pyramidal cells in layer V of the ipsilateral sensorimotor cortex, contralateral lamina IV of all levels of the spinal cord, the contralateral dorsal column nuclei, lateral cervical nucleus, internal basilar nucleus, and nucleus of the spinal trigeminal tract. Electrical stimulation of the spinal cord coupled with extracellular single unit recordings concentrated, for the most part, in the posterior portion of the tectum revealed that such stimuli activated approximately 40% of the cells tested. Almost all of these units were isolated ventral to the stratum opticum and 86% were responsive only to somatosensory stimulation. Analysis of the latencies of collicular responses obtained with two point spinal stimulation in intact hamsters and in animals subjected to somatosensory cortical and/or spinal damage indicated that the initial impulse elicited from most collicular cells was mediated by a polysynaptic pathway(s) which probably synapses in the dorsal column, lateral cervical, and/or internal basilar nuclei. Damage to the dorsal spinal cord and/or somatosensory cortex altered neither the incidence nor the response characteristics of spinally driven collicular neurons. This indicated that most somatosensory collicular cells also received input from the spinotectal fibers which travel in the ventrolateral quadrant. Electrical stimulation of somatosensory cortex activated about 20% of the cells tested in the ipsilateral superior colliculus. If cortical and spinal stimulation were delivered with an interstimulus interval ranging between 50 and 80 msec the response of the tectal neuron to the latter stimulus was suppressed in most cases. This was true regardless of the order of the stimulus pairing. Concurrent somatosensory cortical shocks also suppressed responses to tactile stimuli for 21% of the cells tested.