Relations between extraocular muscle contraction and extension times in each phase of nystagmus

Abstract

The present study was undertaken to relate studies on motor nerve-muscle fibers to the time courses of muscle contraction and extension during tonic and phasic movements, and to determine the active and passive components of the decay and extension times during vestibular nystagmus. In encéphale isolé cat preparations, isometric muscle responses of antagonist extraocular muscles were measured for rise and relaxation times of slow and fast phases and their half times during vestibular nystagmus. The rise phase shows essentially linear contraction. The slow phase is directly related to the nystagmus frequency. The decay during the fast phase to the opposite side is nonlinear and is due to the passive properties of the muscle; it is similar in time course to the decay following cessation of electrical stimulation to the distal portion of a sectioned abducens nerve. The half decay time is not related to the frequency of the nystagmus. On the other hand, the extension time during the slow phase to the opposite side is due to a combination of passive muscle factors and graded exponential decrease in the neural outflow from the brain stem motor nuclei which is directly related to the frequency of the nystagmus. The results obtained in these mechanical studies support those obtained previously by an analysis of motor nerve impulses in the abducens nerve during nystagmus.