Overlapping representations of the neck and whiskers in the rat motor cortex revealed by mapping at different anaesthetic depths.

The primary motor cortex of mammals has an orderly representation of different body parts. Within the representation of each body part the organization is more complex, with groups of neurons representing movements of a muscle or a group of muscles. In rats, uncertainties continue to exist regarding organization of the primary motor cortex in the whisker and the neck region. Using intracortical microstimulation (ICMS) we show that movements evoked in the whisker and the neck region of the rat motor cortex are highly sensitive to the depth of anaesthesia. At light anaesthetic depth, whisker movements are readily evoked from a large medial region of the motor cortex. Lateral to this is a small region where movements of the neck are evoked. However, in animals under deep anaesthesia whisker movements cannot be evoked. Instead, neck movements are evoked from this region. The neck movement region thus becomes greatly expanded. An analysis of the threshold currents required to evoke movements at different anaesthetic depths reveals that the caudal portion of the whisker region has dual representation, of both the whisker and the neck movements. The results also underline the importance of carefully controlling the depth of anaesthesia during ICMS experiments.