Ascending projections of the substantia nigra in the rat,

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Abstract
The Nauta method was used to study the fiber degeneration resulting from electrolytic lesions in the midbrain of 34 albino rats. Lesions were placed in the substantia nigra alone (pars compacta, 3; pars reticularis, 8; both parts, 3); in the substantia nigra and surrounding structures (6); immediately caudal (3), dorsocaudal (3), dorsal (4) and ventral (1) to the substantia nigra; and in the hemisphere and lateral midbrain along the track of the electrode (3). All the degeneration resulting from damage by the electrode to the cerebrum and lateral midbrain was accounted for in terms of known fiber pathways. Lesions involving the substantia nigra (pars reticularis and/or pars compacta) with or without damage to surrounding structures produced fine fiber degeneration which passed rostrally in the pars compacta and the dorsal capsule of the subthalamic nucleus, turned dorsomedially and entered the ventrolateral border of nucleus ventralis medialis thalami (VM) where it terminated. This nigrothalamic pathway did not degenerate when the nigra was not involved. Lesions involving the superior cerebellar peduncle, red nucleus or prerubral radiation also produced terminal degeneration in VM. However, the fibers of this cerebellorubrothalamic pathway, which were not interrupted by lesions confined to the substantia nigra, entered the caudal pole of VM. There was no evidence of a direct nigrostriatal pathway. The demonstration of projections from the cerebellum and substantia nigra to VM establishes this nucleus as the homologue in the rat of nuclei ventralis anterior and ventralis lateralis thalami in primates.