Neuromuscular Control of Primate Eye Movements

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Analyzing the visual world requires the meticulous coordination of both eyes, achieved by six muscles that surround each eye. Controlled by motoneurons in the brainstem, these muscles move the eyes rapidly to acquire targets (using saccades) or track them (smooth pursuit). The muscles also make slower adjustments to maintain proper binocular alignment in depth (vergence) and keep the eyes still for target inspection (fixation). Common visuomotor disorders such as strabismus result from abnormalities in this neuromuscular system. Current treatments mitigate symptoms, but do not fix the underlying problems. To progress toward cures for the disorders, we need to learn more about the details of the neuromuscular circuits. A longstanding curiosity about extraocular muscle fibers has been that they come in two major types: multiply-innervated fibers (MIFs) that receive numerous neuromuscular junctions along their entire length, and singly-innervated fibers (SIFs) that receive a single band of neuromuscular junctions in their middle region. It was discovered recently in primates that these two types of muscle fibers are supplied by distinct groups of motoneurons, revealing that the MIF vs. SIF distinction extends to full motor units. Anatomical characteristics of MIFs and their motoneurons suggest they control slow, binocular alignment (vergence and fixation) whereas SIFs and their motoneurons control faster, targeting movements (saccades and smooth pursuit). The overall goal is to test this hypothesis of dual- motor control of the eyes. This will be accomplished using linear array recordings and optogenetics to study MIF and SIF motoneurons in behaving macaques. Pilot work showed that macaque extraocular motoneurons can be virally transduced to express exogenous genes, setting the stage for the optogenetic approach. The first aim is to achieve reliable, robust viral transduction and opsin expression in macaque motoneurons. Three viral vectors will be injected into orbital muscles at varying volumes and titers. Consequent transgene expression in motoneurons, along with any evidence of neurotoxicity, will be analyzed histologically up to 1 year post-injection to determine optimal parameters. The second aim is to distinguish the functional roles of MIF and SIF motoneurons. Exploiting the separate, colinear locations of MIF and SIF motoneurons in the primate oculomotor nucleus, we will angle linear array electrodes to sample both populations simultaneously. Motoneuron activity will be analyzed in relation to vergence, saccade, and smooth pursuit movements made by the macaques. MIF motoneurons will be identified with in vivo optogenetic phototagging and histological analyses. The outcome of the study will be to resolve whether MIF and SIF motor units constitute a dual-motor system for controlling the eyes. If the MIF subsystem is specialized for binocular alignment, as predicted, it would be implicated as a locus of disruption in strabismus, providing a specific target for treatment. More generally, the work will provide new viral and neurophysiological techniques for studying primate motoneurons, establish a novel testbed for primate optogenetics, and inform the safety and efficacy of gene therapy for neuromuscular disorders.