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Projections from substantia nigra and zona incerta to the cat's nucleus of Darkschewitsch.

The goal of the present experiments was to examine the relationships of the nucleus of Darkschewitsch (ND) with the substantia nigra pars reticulata (SNr), the zona incerta (ZI), and the oculomotor nuclei by using wheat germ agglutinin-horseradish peroxidase (WGA-HRP) as a retrograde and anterograde neuronal tracer injected into various sites of the cat's brain. To eliminate the possibility that fibres of passage from the motor cortex passing through the SNr and ZI were responsible for the ND label, WGA-HRP also was injected into the SNr or the ZI after a large area of the frontal cortex, including the motor area, was destroyed. Retrograde axonal transport demonstrated that many cells of the rostromedial part of the ZI project to the ND, with the ipsilateral projections being dominant. Some cells of the caudomedial part of the SNr project to the ND, again, with the ipsilateral projection being dominant. A few small cells in the ND project bilaterally to the oculomotor nucleus. Anterograde tracer demonstrated that the SNr-ND terminal fields are less dense than the ZI-ND terminal fields. A few fine terminal fibres were observed bilaterally in the oculomotor, trochlear, and abducens nuclei. Electron microscopic examination demonstrated that these fine, labelled terminals contain pleomorphic vesicles and have symmetrical synaptic contacts with dendrites. These results indicate that the ND, a structure that is known to be important for the control of axial muscles (i.e., eye, head, and body muscles), is the target of projections from restricted areas of the SNr and ZI: areas that, during saccadic eye movement, may lead to disinhibition of the ND-oculomotor projection. Accordingly, the ND may function to inhibit the activity of extraocular muscles during saccades.