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Somatosensory responses in the cat motor cortex. I. Identification and course of an afferent pathway.

1. The main aim of the present series of experiments was to demonstrate with electrophysiological methods that the spinothalamocortical system may send somesthetic information to the pyramidal and corticospinal tract cells in the motor cortex of the cat. 2. Experiments were carried out on acutely prepared cats anesthetized with alpha-chloralose. Extra- and intracellular recordings were made from the cells located in the pericruciate motor cortex (the lateral portion of area 4 gamma). They were identified by their antidromic responses to pyramidal stimulation and/or stimulation of the dorsolateral funiculus of the spinal cord. The animals were subjected to a set of nervous tissue lesions to prevent any transit of extereoceptive information to the motor cortex via the cerebellum and the somatosensory cortex. A lesion of the dorsal part of the spinal cord was also made, leaving intact only the afferent inflow ascending in the spinal ventral half, i.e., the spinothalamic system. 3. In this cat preparation it was observed that both electrical and natural stimulation of the limbs still efficiently activated the motor cortical efferent cells. 4. The pathway was mapped by applying microstimulation along its whole course in the spinal cord and brain stem. Stimulation of the primary afferent fibers running in the dorsal columns caudally to the spinal cord lesion produced activation and/or inhibition of the cortical cells. The existence of these responses may be attributable to the existence of collaterals from primary afferent fibers located in the dorsal columns, which activate the spinothalamic tract cells either mono- or polysynaptically. In the brain stem the fibers join the medial lemniscus. 5. In view of the short latency of the responses (mean latency 10.5 ms from the spinal cord) it is suggested that this component of the spinothalamic system may play an important role in the sensory regulation of ongoing movements.