Overview of anatomy and physiology of the ocular motor system
Introduction
The shared goal of all components of the ocular motor system is to maintain clear, single vision by placing and maintaining an object of visual interest on the fovea, the retinal region with the highest density of photoreceptors and the best visual acuity. Several functional classes of eye movements coexist to meet this shared goal. These include saccades, smooth pursuit, vergence, optokinetic responses, and vestibular reflexes. Anatomically and physiologically, separate premotor or supranuclear command networks exist for initiation and modulation of each functional class of eye movements. These premotor networks converge upon a “final common pathway” that includes the ocular motoneuron, neuromuscular junction, and the final effector organ of eye movements – the extraocular muscle. It has long been held true that all motoneurons and extraocular muscle fibers participate in all types of eye movements (Scott and Collins, 1973), though some may be more important for certain types of eye movements (Büttner-Ennever et al., 2001, Büttner-Ennever, 2005).
Modern biologic, anatomic, and physiologic techniques such as gene expression profiling, single cell recordings to determine cell electrophysiologic properties, lesional inactivation with observation of behavioral changes, and tracer methodologies to determine neural networks have greatly advanced understanding of the ocular motor system – to the point of challenging some classically held truisms such as the concept of a definitive “final common pathway” and absolute conjugacy of the ocular motor system (Mays et al., 1986, Zhou and King, 1998, Miller et al., 2002, Miller, 2003, Sylvestre et al., 2003). The complexity and variety of demands that the ocular motor system must meet in order to maintain stable vision require complex anatomy and physiology at every level – from extraocular muscle to cortical ocular motor regions.
Section snippets
Saccades
Saccades are rapid, conjugate eye movements with which we explore a visual scene or shift gaze to point the fovea at pertinent details in the visual world (Robinson, 1964). Because of the small foveal size, a high degree of accuracy is required. Saccades may be voluntary or reflexive and generated to actual targets or to memory for target location. They are fast eye movements, with most ranging between 300 and 500 °/s; and they are brief, most lasting less than 100 ms so as not to disrupt vision.
Overview and muscle actions
Six extraocular muscles control the movements of each eye: medial rectus, lateral rectus, superior rectus, inferior rectus, superior oblique, and inferior oblique. The medial rectus, superior rectus, inferior rectus, and inferior oblique are innervated by the oculomotor nerve (cranial nerve III). The lateral rectus is innervated by the abducens nerve (cranial nerve VI). The superior oblique is innervated by the trochlear nerve (cranial nerve IV).
Coordinated extraocular muscle action facilitates
Neuromuscular junction
In the classic skeletal muscle neuromuscular junction (NMJ) or motor end-plate, a nerve axon terminates on the mid-belly of a muscle fiber in a large synaptic expansion. The pre-synaptic terminal consists of enlargements of the terminal nerve fibers called synaptic boutons. These are separated from the post-synaptic muscle end-plate by the synaptic cleft, through which acetylcholine (ACh) passes after active vesicle release from the pre-synaptic terminal. The post-synaptic end-plate contains
Ocular motoneurons (cranial nerves and nuclei)
The ocular motoneurons for horizontal eye movements are located in the abducens and oculomotor nuclei. These motoneurons supply the lateral rectus and the medial rectus, respectively. For vertical eye movements, the motoneurons are in the trochlear and oculomotor nuclei. The trochlear motoneurons supply the superior oblique and the oculomotor motoneurons supply the superior and inferior rectus muscles and the inferior oblique.
Internuclear
The medial longitudinal fasciculus (MLF) carries signals from the abducens nucleus to the contralateral medial rectus portion of the oculomotor nucleus (Fig. 12). These signals allow conjugate horizontal eye movements with co-contraction of the ipsilateral lateral rectus and contralateral medial rectus muscles. The MLF also carries signals for vertical gaze from the medullary vestibular nuclei to the midbrain vertical gaze control centers. These signals are most important for vertical smooth
Burst and pause neurons
A combination of factors including the initial force to overcome the elastic inertia of the extraocular orbital tissues, high saccadic velocity, long saccadic duration, and the requirement for a high degree of accuracy to place the small fovea on target make saccades a very demanding task for the brain. Many of these demands are met directly by brainstem burst neurons that carry the immediate premotor or supranuclear saccadic command and that project monosynaptically to ocular motoneurons (Horn
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