SENSORIMOTOR CONTROL OVER FUSIMOTOR NEURONS OF THE TENUISSIMUS MUSCLE IN THE A NESTHETIZED CAT: A QUALITATIVE PRIMARY AFFERENT RECORDING

Cortical control of the sensory output of muscle spindles was studied in thirteen anesthetized cats in the present experiment. Gamma motoneuron activity was monitored during electrical stimulation of the sensorimotor cortex while recording from single primary afferents from the tenuissimus muscle. Findings are as follows: 1. The state of anesthesia is crucial in obtaining reproducible results and variation in the state of anesthesia can alter the fusimotor effect from static to dynamic or even from excitation to inhibition, a finding consistent with those of Vedel and Mouillac-Baudevin.30The anesthetic agent used was also important in determining the magnitude and types of responses to corticaI stimulation. the initial burst of the primary afferent in response to passive stretch was by far greater with chloralose than with barbiturate anesthesia in the tenuissimus muscle, suggesting that there may be a tonic low-level dynamic gamma excitation in chloralose anesthesia. 2. The state of the sensorimotor cortex is another determinant factor. Prevention of CO2 escape from the surface of the cortex in the present experiments, by covering the cortex with 1cm of mineral oil, is thought to be the sole factor which made these results different from those obtained by Gladden and McWilliam.ll•12 3. Different types of static gamma motoneurons could be recruited from the sensorimotor cortex independently. 4. The topographical mapping of the sensorimotor cortex in relation to the type of recruited gamma motoneurons, static or dynamic, was as follows: a) A"dynamic area" was identified from which dynamic effects were clearly elicited during stimulation. b) Static effects were elicited following stimulation of a much wider area across the sensorimotor cortex, the postcruciate dimple being almost at the center. 5. The sensorimotor cortex was not only capable of controlling static gamma motoneurons independently from dynamic ones, but also capable of simultaneously inhibiting static gamma motoneurons and exciting others, lending support to the idea put forward by others.6