Motor cortex modulation of exteroceptive information at bulbar and thalamic lemniscal relays in the cat

We have studied the control of the primary motor cortex on the activity of lemniscal neurons in semi-chronic implanted cats. In each experiment, two to three foci in the primary motor cortex were identified by intracortical microstimulation at low threshold (up to 20 microA) for their capacity to evoke movements of contralateral single limb joints. Neurons belonging to the dorsal column nuclei (main cuneate nucleus and gracile nucleus), or to the ventral posterolateral nucleus, were sampled for their response to stimulation of the peripheral cutaneous fields, as well as the antidromic response to stimulation of the contralateral medial lemniscus and ipsilateral somatosensory cortex, respectively. These neurons were then tested for stimulation of the cortical foci using a current intensity equal to the threshold needed to evoke motor effects, although we reduced the duration of the stimulating trains; thus, we avoided evoking movements which could elicit afferent volleys along the somatosensory paths. It was found that the primary motor cortex was able to modulate the transmission of exteroceptive signals at the level of both dorsal column nuclei and ventral posterolateral nucleus with analogous modalities. In particular: (i) a high percentage of responses, with a prevalence of excitatory effects, was observed when the receptive field of the neurons topographically corresponded to, or was very close to, the joint controlled by a given cortical focus; (ii) in these cases, higher percentages of excitations were observed in tests which concerned the distal segments of limbs than the proximal segments; (iii) the percentage of responses became lower as the neuronal receptive field was located further from the cortical motor target, the pattern being more frequently inhibitory in nature. From a functional point of view, the motor cortex control appears to be organized in a very precise manner. Its excitatory nature might subserve integrative mechanisms by which exteroceptive information arising in a given limb segment would be enhanced by a motor command inducing movements of the same body part. Moreover, a better definition of the afferent input could be obtained by a simultaneous depression of neurons, which send towards the cortex signals from adjacent or more distant cutaneous regions. It can be hypothesized that such an organization of the cortical control could improve the discriminative somatosensory aspects during the execution of explorative movements, besides supplying a sharper cutaneous feedback to the motor cortex.