Extraocular motoneuron behavior in synergistic action

Abstract

In several aspects, the oculomotor system is unique and different from all skeletomotor systems. It is a two-ball-joint system equipped with a set of 12 muscles, which over a limited range operate in six rotatory degrees of freedom in the configuration space of two rotators. Although there is no mechanical coupling between the two eyes, the central innervation reduces the number of degrees of freedom. Because of the small inertia of the almost spherical eye in its well-protected enclosure, eye movements can be very rapid and very precise. The force needed to execute the same eye movements at different times remains constant, within small limits. This led to the creation of models that can be checked against actual measurements. To investigate how eye muscles interact to move and hold the eyes in tertiary position, motoneuron activity was recorded and results were presented as iso-frequency curves indicating all possible eye positions for a given frequency in a motoneuron. This gives a picture of how motoneurons interact in a synergistic fashion to hold the eyes in tertiary positions.