Axonal branching patterns and location of nigrothalamic and nigrocollicular neurons in the cat

1. Antidromic activation from the thalamus and the superior colliculus was used to identify the axonal projections of neurons in the substantia nigra. 2. Up to 43% of the nigral neurons studied in individual animals had branching axons that could be activated antidromically from both the ipsilateral ventromedial thalamus and the ipsilateral superior colliculus. 3. For several neurons, the conduction time spent in the unbranched axonal segment could be calculated using measurable collision and antidromic latency times. Time in the unbranched segment was less than 20% of the antidromic latency for most neurons studied. 4. Latencies for antidromic activation from the thalamus were measured and would be appropriate if these axons were to mediate the monosynaptic nigrothalamic inhibition reported earlier. 5. Stimulation of the caudate nucleus produced inhibition of spontaneous activity of both collicular- and thalamic-projecting neurons.