Oculomotor System, Mechanisms

Abstract

The oculomotor system consists of extraocular muscles attached to the eye ball, motoneurons in the IIIrd (oculomotor), IVth (trochlear), and VIth (abducens) cranial nerve nuclei, and premotor systems. Three pairs of extraocular muscles (medial and lateral rectus, superior and inferior rectus, superior and inferior oblique), each subserving horizontal, vertical, or rotatory eye movement, respectively, conjointly produce an eye movement. Various types of oculomotor activities are distinguished, e.g., fixation, saccade, and slow movement. The same motoneurons participate in all these activities, none of which is the exclusive product of one type of motoneuron. Impulse discharge rates in each motoneuron are represented by the sum of two terms, one proportional to eye position and the other to eye velocity. However, values of the proportion coefficients vary widely among motoneurons, yielding five major types of responses (tonic, predominantly tonic, tonic-phasic, predominantly phasic, and phasic). Since direct application of currents to the motoneuronal membrane elicits tonic discharge with a frequency linearly related to injected currents, the varied responses of motoneurons related to eye movements should reflect mainly the properties of their input signals from premotor systems. Premotor systems provide command signals for execution of varied reflexive and voluntary eye movements.