LINKED AND INDEPENDENT RESPONSES OF TONIC ALPHA AND GAMMA HIND-LIMB MOTONEURONS TO DEEP CEREBELLAR STIMULATION

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Abstract
Deep cerebellar structures (in and around the nuclei) of precollicular decerebrate were stimulated with repetitive low-voltage electric pulses. The effects were observed on stretch-induced tonic discharges of hindlimb extensor alpha motoneurons and on the concomitant gamma (fusimotor) activity. The gamma loop was essentially intact in most experiments. Facilitory effects were seen in 43% of the alpha reactions, but in 85% of the gamma reactions. The remaining observations showed inhibitory or diphasic (inhibitory/facilitory) reactions. Simultaneous recordings of alpha and gamma motoneuron reactions to the same cerebellar stimulation revealed the following combined patterns: alpha facilitation + gamma activation in 42%; alpha inhibition + gamma activation in 33%; alpha inhibition + gamma inhibition (including diphasic gamma effects) in 12%, The combination, alpha facilitation + gamma inhibition, was never observed. It is concluded that the cerebellum exerts dual spinal motor influences, predominantly facilitory for the fusimotor neurons, but bidirectional (facilitory or inhibitory) for the alpha motoneurons, which are independent in nature, but linked in purpose.