Descending and ascending somaesthetic pathways, and their interdigitations in the middle tectal layers of the midbrain

ABSTRACT

The relationship between the ascending and descending somaesthetic collicular afferents in the middle superior collicular layers is investigated at light microscopic level. The study is based on albino rats surviving forty-eight to seventy-two hours following the combined stereotaxic placement of electrolytic lesions in the homolateral somaesthetic cerebral cortex and in the heterolateral subnucleus interpolaris of the trigeminal nuclear complex in the same animal. The method reveals interdigitating degenerated terminals that form bands occupying most of the mediolateral collicular extent as determined by the collicular topography of the particular parts of the body that are represented in the ablated or lesioned trigeminal nuclear areas. Correlation of the degenerating terminals with directions of the representation lines of the optical meridia in the superficial collicular layers is discussed on the light of the present findings and an explanation for the patchy appearance of degeneration of the various collicular afferents is proposed