The effects of cerebellar stimulation on alpha motoneuron excitability and the stretch reflex in the cat

Abstract

The effects of cerebellar surface stimulation on electrically evoked monosynaptic and polysynaptic reflexes and the stretch reflex were investigated in cats anesthetized with alpha-chloralose. The electrically evoked monosynaptic and polysynaptic reflexes were recorded from the ventral root following stimulation of nerves innervating the anterior tibial and gastrocnemius-soleus muscles. These responses were averaged at various times after the initiation of cerebellar stimulation and compared with the amplitude of the control response. The effects of cerebellar surface stimulation on the stretch reflex were assessed while recording the multiunit EMG activity evoked by a passive sinusoidal stretch of the gastrocnemius-soleus muscle. Cycle histograms of this EMG activity were constructed during control periods and during cerebellar stimulation. The stimuli were applied to the lateral vermis of the anterior lobe and crus I and crus II of the posterior lobe. The effects of stimulating at different frequencies and charge densities were evaluated in each animal. The magnitude of the effects produced by cerebellar stimulation on these reflexes was highly dependent on the charge density and intensity of the cerebellar stimulus. In general, higher intensities and frequencies of surface stimulation produced greater reductions in the excitability of alpha motoneurons and the amplitude of the stretch reflex. Occasionally low frequency stimulation produced an increase in alpha-motoneuron excitability, particularly when the cerebellar stimuli were applied to the hemisphere. The results of this study indicate that the maximal reduction in motoneuron excitability and the amplitude of the stretch reflex is produced at different stimulus parameters in different animals. However, in each animal the effects evoked by a single set of stimulus parameters were consistent throughout the experiment. These results demonstrate that in an animal with an intact neuraxis cerebellar stimulation applied at relatively safe charge densities can produce dramatic reductions in the amplitudes of segmental reflexes. The inter-animal variability observed emphasizes the importance of establishing the optimal set of stimulus parameters in order to maximize the reduction in motoneuron excitability and stretch reflexes in each animal.