Functional anatomy of motor control system of the cortico-basal ganglial-thalamo loop without passing through the cerebral cortex

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The main purpose of the present study was to determine the characteristics of ascending projection pattern of neurons in the deep layers of the SC constituting the tecto-reticulospinal tract. We studied ascending axonal trajectories of single neurons in the deep layers of the cat superior colliculus(SC) by labeling a small pool of neurons with biotinylated dextran amine(BDA). All of the SC axons forming the predorsal bundle emitted 1-3 ascending branches before decussation. Ipsilateral ascending branches ran rostrally and gave rise to several collaterals to the central gray and mesencephalic reticular formation before entering the thalamus. At the meso-diencephalic junction, these branches began to run dorsally and/or ventrally towards the thalamus. Based on the distribution of axon collaterals and terminals in the thalamus, 28 SC neurons reconstructed on serial sections were classified into 3 types : 1) neurons projecting to the ventral thalamus (the fields of Forel and zona incerta)(n=10), 2)neurons projecting to the dorsal thalamus (mainly to the intralaminar nuclei) (n=4), and 3)neurons projecting to both the ventral and dorsal thalamus (n=14). Our data indicate that SC neurons convey the same information to the spinal cord, brainstem, and the thalamus. These results suggest that single SC neurons simultaneously control eyes, head and body movements, and also send their information to the cerebral cortex and the basal ganglia via the thalamus as a collorary discharge. Anatomical studies have indicated that predorsal bundle neurons could be influenced by extensive nigrotectal projections. The present study has revealed that predorsal bundle cells projected to the CM-pf complex, which is known as a major source of thalamostriatal projection. Therefore, outputs of the basal ganglia from the substantia nigra could be transmitted by way of the nigrotectal-tectothalamic-thalamostriatal projection back to the striatum.