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Somatosensory Response Properties of Excitatory and Inhibitory Neurons in Rat 1 Motor Cortex 2 3

17 18 In sensory cortical networks, peripheral inputs differentially activate excitatory and inhibitory 19 neurons. Inhibitory neurons typically have larger responses and broader receptive field tuning 20 compared to excitatory neurons. These differences are thought to underlie the powerful 21 feed-forward inhibition that occurs in response to sensory input. In the motor cortex, as in the 22 somatosensory cortex, cutaneous and proprioceptive somatosensory inputs, generated before 23 and during movement, strongly and dynamically modulate the activity of motor neurons 24 involved in a movement, and ultimately shape cortical command. Human studies suggest that 25 somatosensory inputs modulate motor cortical activity in a center excitation, surround 26 inhibition manner such that input from the activated muscle excites motor cortical neurons that 27 project to it, while somatosensory input from nearby, non-activated muscles inhibit these 28 neurons. A key prediction of this hypothesis is that inhibitory and excitatory motor cortical 29 neurons respond differently to somatosensory inputs. We tested this prediction with the use of 30 multi-site extracellular recordings in anesthetized rats. We found that fast-spiking (presumably 31 inhibitory) neurons respond to tactile and proprioceptive inputs at shorter latencies and larger 32 response magnitudes compared to regular-spiking (presumably excitatory) neurons. In contrast, 33 we found no differences in the receptive field size of these neuronal populations. Strikingly, all 34 fast-spiking neuron pairs analyzed with crosscorrelation analysis displayed common excitation, 35 which was significantly more prevalent than common excitation for regular-spiking neuron 36 pairs. These findings suggest that somatosensory inputs preferentially evoke feed-forward 37 inhibition in the motor cortex. We suggest that this provides a mechanism for dynamic selection 38 of motor cortical modules during voluntary movements. 39 40 4