Activités évoquées chez le chat dans la région du nucleus ventralis lateralis par diverses stimulations sensorielles. II. étude micro-physiologique

Abstract

Resume 1. 1. In cats under chloralose anaesthesia, a region including the nucleus ventralis lateralis (VL), ventralis anterior and the anterior part of the nucleus ventralis posterior has been explored. In this region, somatic, auditory or visual stimulation induces a complex response which is composed of different elements: first, a short latency excitatory response, then an inhibitory phase and finally a late excitatory response. 2. 2. The evoked response which appears in the VL after sensory stimulation has characteristics in common with those of associative structures like the centrum medianum. Thus, its amplitude is greatly reduced when the stimulation frequency is higher than 1/sec. For frequencies of 1 to 5/sec, the reduction of the response has its origin in the VL or in an afferent relay to the VL where the sensory convergence has already occurred: the same reduction can be obtained by applying the stimulation to one leg or by applying it alternately to two different legs. On the contrary, when the stimulation frequency is higher than 5/sec, there is a complete disappearance of the response and this takes place at a spinal level where sensory convergence has not yet occurred: during repetitive stimulation of one leg a single shock applied to another leg or even to a different part of the same leg still evokes the characteristic response of the VL. 3. 3. The various elements of the evoked response in the VL are reduced but not suppressed after cerebellectomy. They also persist after extensive decortication. 4. 4. The different elements of the evoked response in the VL appear after stimulation of the most rapidly conducting fibres (group II) of a cutaneous nerve. The stimulation of group I fibres in muscle nerves from the anterior and posterior limbs was ineffective in evoking responses in the explored regions of the VL. To be effective the stimulation should excite fibres from group II (forelimbs) or from group III (hindlimbs). 5. 5. The somatic afferent pathway to the VL is at the spinal level (Th5-Th12), composed of 2 main pathways: one is contralateral to the stimulated leg and located in the ventral quadrant, the other is ipsilateral and located in the dorsal part of the lateral fascicle. The afferents following each of these pathways have, at the thalamic level, a bilateral distribution. 6. 6. In the discussion the characteristics of the VL responses are compared with those of convergent (CM, posterior group) and specific (VPl) structures.