Long-lasting potentiation in the secondary somatosensory cortex affects motor control: assessment by H-reflex

We investigated descending projections from the secondary somatosensory cortex to the feline spinal cord and the effects of long-lasting potentiation in secondary somatosensory cortex on the activities of motoneurons of the cat. Electrophysiological examinations revealed that the low-intensity subthreshold secondary somatosensory cortex stimulation could change the H-Reflex induced by radial nerve stimulation. The H-wave amplitudes, recorded in wrist flexor muscles, were enhanced when the intervals from secondary somatosensory cortex to radial nerve stimuli were altered from 0 to 30 ms (initial excitation, 146 +/- 11% (mean +/- S.E.M.) of the control value). In contrast, the H-waves were suppressed with intervals longer than 30 ms (80 +/- 3%). The descending pathways from secondary somatosensory cortex to the spinal cord were assessed using an immunohistochemical technique. c-Fos and Zif268 proteins, induced by stimulation of the hand-represented secondary somatosensory cortex areas, could thus express in activated cervical neurons. The density of labeled cells was significantly higher in the seventh and eighth cervical segments than in other levels. The great majority of positive cells were distributed in the lateral part of the contralateral ventral horn and their somas ranged from 10 to 50 microns in size. Finally, we examined the effects of long-lasting potentiation, induced by high-frequency stimulation of the ventral posterolateral thalamic nucleus, on the activities of spinal motoneurons. Long-lasting potentiation altered the previously observed effects of secondary somatosensory cortex stimulation on the H-wave amplitude. The secondary somatosensory cortex-conditioned initial excitation of the H-reflex was enhanced (from 139 to 175%, P < 0.05), while late suppression was completely blocked (from 74 to 112%, P < 0.01). In conclusion, the descending pathways from secondary somatosensory cortex to the spinal cord modulated the H-reflex, and long-lasting potentiation in secondary somatosensory cortex affected this modulation. We have previously reported that corticocortical inputs from primary to secondary somatosensory cortex is required for induction of long-lasting potentiation in secondary somatosensory cortex. Taken together, the present study suggests that cortical plasticity in secondary somatosensory cortex amplifies somatic inputs from primary somatosensory cortex as a means of adaptive motor control by the sensory system.