Activation of the primary motor cortex by somatosensory stimulation in adult rats is mediated mainly by associational connections from the somatosensory cortex

In anaesthetized adult rats, facial nerve injury causes a disinhibition of the interhemispheric connections between homotopic representation fields in the primary motor cortex with a latency of 4 min (Toldi et al., 1996, Neur osci. Lett. 203, 179-182). One possible explanation for the induction of su ch rapid changes is an alteration of the somatosensory input to the motor c ortex. To test this hypothesis, unit activity in primary motor cortex was r ecorded during electrical stimulation of trigeminal afferents in the contra lateral whisker-pad. About one-third of all recorded primary motor cortex n eurons responded with latencies shorter than in the ventrolateral and poste rior nuclei of the thalamus. Responses failed at stimulation frequencies gr eater than or equal to 10 Hz and after elimination or inactivation of the s omatosensory cortex. Within primary motor cortex, the activatable neurons d isplayed a bilaminar distribution and were identified as pyramidal neurons by neurobiotin labelling. The results suggest that trigeminal afferents participate in modulation of the activity of primary motor cortex output neurons via primary somatosenso ry cortex-to-primary motor cortex associational connections, even under ana esthesia. (C) 1999 IBRO. Published by Elsevier Science Ltd.