A study of an ascending nigro-caudate pathway

Abstract

1. 1. Former electrophysiological investigations have shown that, following substantia nigra (SN) stimulation, responses could be recorded in the caudate nucleus (n Cd). In order to analyse the characteristics of these responses, macro- and micro-electrode records were taken from 116 cats, 46 of them being maintained under local analgesia after an operating phase with volatile anaesthetics; in 47 volatile anaesthesia was prolonged, and 23 were anaesthetized with chloralose or barbiturate. 2. 2. It was shown that such n Cd responses persist after degeneration of the descending cortical pathways following a broad anterior cortical ablation performed 15 days earlier. 3. 3. Micro-electrodes in the head of the n Cd reveal two types of unit. They exhibit opposite reactions when SN is stimulated. Those detected through their spontaneous firing most often have their activity depressed. The other units can only be detected through their excitatory responses to nigral stimulation. The characteristics of 419 of these latter neurones were studied in detail. 4. 4. These excited neurones respond after a relatively stable long latency (16.8±5 msec, in cats under local analgesia). They follow each stimulus of short trains of shocks (100–320/sec). These responses are suppressed by barbiturate anaesthesia. Their orthodromic origin was demonstrated by the collision technique, extra spikes having been provoked by repetitive stimulation or by electrophoretic application of excitatory amino acids. 5. 5. A study of the organization of the nigro-caudate inflow has shown that responsive units are driven by stimulating electrodes placed in the posterior as well as the anterior parts of SN. The responsive units are predominantly found in the medial two-thirds of the head of caudate nucleus. No significant difference in latency can be correlated with differences in unit localization. Given the mean value of the latencies and the distance between the stimulating electrode and the active units, the average conduction velocity is 0.90±0.27 m/sec. 6. 6. Stimuli applied to the periphery and to different central regions were mostly unable to drive the cells responding to SN stimulation. Internal capsule, ventro-medial tegmental areas and n Cd were the only ones to be efficacious.