Trigeminal projections to the superior colliculus of the rat.

The deep layers of the rodent superior colliculus contain a vibrissae-related organization that is in "spatial register" with the overlying visuotopic organization (Dräger and Hubel, '76). The distribution of vibrissae-related afferents and their cells of origin were determined with a number of anatomical techniques. The brainstem trigeminal complex afferents to the superior colliculus terminate in the lateral portions of the strata album intermediate and griseum profundum and, to a lesser degree, in deep portions of the stratum griseum intermediate. The cells giving rise to these afferents are located mainly in the ventral portions of the contralateral principal sensory nucleus, subnucleus oralis, and subnucleus interpolaris. The majority of tectal projection cells are found in subnucleus interpolaris, and the fewest in the principal sensory nucleus. Further, the density of projection cells in the three components of the brainstem trigeminal complex can be correlated with the density of their projections to the superior colliculus. The afferents from the somatosensory cortex terminate in a continuous band in the strata album intermediate and griseum intermediate. The cells of origin of this projection are located in layer Vb of the agranular zones of the ipsilateral somatosensory cortex. The present results suggest that the organization of trigeminal afferents to the deep portion of the superior colliculus is similar to that of the visual afferents to the superficial laminae. Further, the results suggest that observations on the nature of afferent termination pattern should be made with care, considering both the techniques employed and the idiosyncrasies of the local neuropil.