Somatosensory neurons projecting from the superior colliculus to the intralaminar thalamus in the rat

Neurons of the rat superior colliculus projecting to the intralaminar thalamus were tested for their responses to somatosensory stimulation. They were identified by antidromic stimulation of the parafascicular nucleus and central lateral nucleus. To establish the existence of descending as well as ascending axon collaterals antidromic stimulation was applied to the upper cervical spinal cord in some cases. Somatosensory receptive fields were delineated and their laminar location in the superior colliculus was noted. Units were distributed throughout the intermediate and deep tectal layers, none were located in the superficial layers. Units with somatosensory receptive fields could be classified as low threshold, high threshold, wide dynamic range or complex. The majority of the peripherally responsive units (52%) were low threshold somatosensory units with contralateral receptive fields. All units were distributed throughout the intermediate and deep layers. Their distribution reflected the typical somatotopic organization of the superior colliculus. These results indicate that the intralaminar thalamus receives some sensory information by way of the tectum. In turn, the basal ganglia may gain direct access to this information by way of the thalamoneostriatal projection.