8. Ocular Motor Control
Part 1 of 5

Valentin Dragoi, Ph.D.
.

Introduction

Normal visual perception requires the proper functioning of ocular motor systems that control the position and movement of the eyes to focus the image of the object-of-interest (i.e., the visual target) on corresponding areas of the retinas of the two eyes.  For example, in addition to producing adjustments in pupil size and lens refraction, accommodation involves the convergence of the two eyes to direct onto the foveae the images of near objects.  Eye movements are also controlled to direct the eyes towards a visual target and to follow the movements of the visual target.  Such eye movements are controlled by gaze systems. They coordinate the movement of the two eyes to ensure that the images on the two retinas fall on corresponding areas of the binocular field.  When this fails, diplopia (double vision) results. 

The extraocular muscles execute eye movements and are innervated by three cranial nerves.  The muscles are attached to the sclera of the eye at one end and are anchored to the bony orbit of the eye at their opposite ends.  Contraction of the muscles produce movement of the eyes within the orbit.  The cranial lower motor neurons innervate these muscles and thereby control their contractions.

A. The Extraocular Muscles

For each eye, six muscles work together to control eye position and movement.  Two extraocular muscles, the medial rectus and lateral rectus, work together to control horizontal eye movements (Figure 8.1, left). 

• Contraction of the medial rectus pulls the eye towards the nose (adduction or medial movement).
• Contraction of the lateral rectus pulls the eye away from the nose (abduction or lateral movement).

Figure 8.1 The extraocular muscles of the right eye and their actions.  Antagonistic actions pull the eye in opposite directions whereas synergistic actions pull the eye in the same direction.

The actions of these two muscles are antagonistic: one muscle must relax while the other contracts to execute horizontal eye movements. Four other extraocular muscles working together control vertical eye movements and eye rotation around the mid-orbital axis (Figure 8.1, right).  Contraction of the 

• superior rectus produces
  • eye elevation
  • minor movements: medial rotation and adduction
    
    
• superior oblique produces
  • eye depression
  • other movements: medial rotation and abduction

• inferior rectus produces
  • eye depression
  • minor movements: lateral rotation and adduction
    
    
• inferior oblique produces
  • eye elevation 
  • other movements: lateral rotation and abduction 

To direct the eye upward or downward, two muscles contract synergistically as the two antagonist muscles relax.  For example, to elevate the eye while looking straight ahead, the superior rectus and inferior oblique contract together as the inferior rectus and superior oblique relax.  The superior rectus and inferior oblique muscles working together pull the eye upward without rotating the eye. To depress the eye while looking straight ahead, the inferior rectus and superior oblique contract together as the superior rectus and inferior oblique relax.  The inferior rectus and superior oblique working together pull the eye downward without rotating the eye.

B. Extraocular Muscle Efferents

Three cranial motor nuclei provide efferent control of the extraocular muscles.  Activation of the motor neurons produces contraction of the innervated muscle. 

• The abducens nucleus
  • sends its axons in the abducens (VI cranial) nerve
  • controls the lateral rectus of the ipsilateral eye.
• The trochlear nucleus
  • sends its axons in the trochlear (IV cranial) nerve
  • controls the superior oblique of the contralateral eye.
• The oculomotor complex contains nuclei that
  • send axons in the oculomotor (III cranial) nerve
  • control
    • the superior levator in the eyelid of both eyes
    • extraocular muscles, which include the
      • medial rectus of the ipsilateral eye,
      • inferior oblique of the ipsilateral eye
      • inferior rectus of the ipsilateral eye
      • superior rectus of the contralateral eye¹.

C. Upper Motor Neuron Control

Figure 8.2

The axons of the abducens interneurons decussate and travel in the medial longitudinal fasciculus to the contralateral oculomotor nucleus to excite the motor neurons controlling the medial rectus of the eye contralateral to the abducens nucleus.

Interconnections between the trochlear nucleus and oculomotor nuclear complex coordinate their activity to allow the upward and downward movement of the eyes.  These interconnecting axons appear to travel along with the fibers of the tectospinal tract (that is, they do not travel in the medial longitudinal fasciculus).

There are two functional classes of eye movements (Table I): those that stabilize the eye when the head moves or appears to move (gaze stabilization) and those that keep the image of a visual target focused on the fovea (a.k.a., foveation) when the visual target changes or moves (gaze shifting).  Two gaze stabilization systems operate during head movement: the vestibulo-ocular and the optokinetic systems. Vestibulo-ocular and optokinetic movements are conjugate movements in which both eyes move in the same direction.

A. The Vestibulo-ocular Reflexes

Vestibulo-ocular reflexes produce eye movements that compensate for head movements detected by the vestibular system.  You have learned in earlier chapters how the vestibular system detects head movements and initiates the vestibulo-ocular responses.

B. Optokinetic Nystagmus

Optokinetic nystagmus is elicited

• by slow head movements undetected by the vestibular system,
• by moving objects that produce the illusion of head movement (e.g., alternating bands of light and dark lines rotated around the viewer's head)
• as corrections for small spontaneous eye movements 

Notice that optokinetic nystagmus is a visual-ocular response - driven by visual stimuli moving across the visual field.  Vestibular nystagmus is a vestibulo-ocular response - driven by a vestibular stimulus (i.e., accelerating head movement).  In humans, the smooth pursuit system predominates in producing eye movements that track moving visual targets.  As the optokinetic system is vestigial in humans, it will not be covered in this lecture. 

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