7. Ocular Motor Systems
Part 1 of 5

Valentin Dragoi, Ph.D.
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The simplicity of the motor systems involved in controlling eye musculature make them ideal for illustrating the mechanisms and principals you have been studying in the preceding material on motor systems.  They involve the action of few muscles and of well defined neural circuits.  

We use our eyes to monitor our external environment and depend on our ocular motor systems to protect and guide our eyes.  The ocular motor systems control eye lid closure, the amount of light that enters the eye, the refractive properties of the eye, and eye movements.  The visual system provides afferent input to ocular motor circuits that use visual stimuli to initiate and guide the motor responses. Neuromuscular systems control the muscles within the eye (intraocular muscles); the muscles attached to the eye (extraocular muscles) and the muscles in the eyelid.  Ocular motor responses include ocular reflexes and voluntary motor responses to visual and other stimuli. The complexity of the circuitry (the chain or network of neurons) controlling a ocular motor response increases with the level of processing involved in initiating, monitoring, and guiding the response.

In this chapter we will start at the level of reflex responses and move onto more complex voluntary responses in the following lecture.  The eye blink reflex is the simplest response and does not require the involvement of cortical structures.  In contrast, voluntary eye movements (i.e., visual tracking of a moving object) involve multiple areas of the cerebral cortex as well as basal ganglion, brain stem and cerebellar structures. 

The ocular reflexes are the simplest ocular motor responses.   Ocular reflexes compensate for the condition of the cornea and for changes in the visual stimulus.  For example, the eye blink reflex protects the cornea from drying out and from contact with foreign objects.  The pupillary light reflex compensates for changes in illumination level, whereas the accommodation responses compensate for changes in eye-to-object-viewed distance.  Note that reflex responses are initiated by sensory stimuli that activate afferent neurons (e.g., somatosensory stimuli for the eye blink reflex and visual stimuli for the pupillary light reflex and accommodation responses). 

In general, ocular reflexes are consensual (i.e., the response is bilateral involving both eyes).  Consequently, a light directed in one eye elicits responses, pupillary constriction, in both eyes.  In this chapter you will learn of the structures normally involved in performing these ocular responses and the disorders that result from damage to components of neural circuit controlling these responses.

A. The Eye Blink Reflex

Tactile stimulation of the cornea results in an irritating sensation that normally evokes eyelid closure (an eye blink).  The response is consensual (i.e., bilateral) - involving automatic eyelid closure at both eyes. 

The corneal eye blink reflex neural circuit: This neural circuit (Figure 7.1) is relatively simple, consisting of the

• trigeminal1° afferent (free nerve endings in the cornea, trigeminal nerve, ganglion, root, and spinal trigeminal tract), which end on
• trigeminal 2° afferent in the spinal trigeminal nucleus, some of which send their axons to
• reticular formation interneurons, which send their axons bilaterally to
• facial motor neurons in the facial nucleus, which send their axons in the facial nerve to 
• orbicularis oculi, which functions to lower the eyelid

Figure 7.1

The corneal eye blink reflex is initiated by the free nerve endings in the cornea and involves the trigeminal nerve and ganglion, the spinal trigeminal tract and nucleus, interneurons in the reticular formation, motor neurons in the facial nucleus and nerve, and the orbicularis oculi. As the afferent information from each cornea is distributed bilaterally to facial motor neurons by the reticular formation interneurons, the eye blink response is consensual, that is, both eye lids will close to stimulation of the cornea of either eye.

B.  Pupillary Light Reflex

The pupillary light reflex involves adjustments in pupil size with changes in light levels.

• The reflex is consensual: Normally light that is directed in one eye produces pupil constriction in both eyes.
• The direct response is the change in pupil size in the eye to which the light is directed (e.g., if the light is shone in the right eye, the right pupil constricts).
• The consensual response is the change in pupil size in the eye opposite to the eye to which the light is directed (e.g., if the light is shone in the right eye, the left pupil also constricts consensually).

The pupillary light reflex allows the eye to adjust the amount of light reaching the retina and protects the photoreceptors from bright lights. The iris contains two sets of smooth muscles that control the size of the pupil (Figure 7.2).

• The sphincter muscle fibers form a ring at the pupil margin so that when the sphincter contracts, it decreases (constricts) pupil size.
• The dilator muscle fibers radiate from the pupil aperture so that when the dilator contracts, it increases (dilates) pupil size.

Both muscles act to control the amount of light entering the eye and the depth of field of the eye¹.

• The iris sphincter is controlled by the parasympathetic system, whereas the iris dilator is controlled by the sympathetic system.
• The action of the dilator is antagonistic to that of the sphincter and the dilator must relax to allow the sphincter to decrease pupil size.

Normally the sphincter action dominates during the pupillary light reflex.

The pupillary light reflex neural circuit:  The pathway controlling pupillary light reflex (Figure 7.3) involves the

• retina, optic nerve, optic chiasm, and the optic tract fibers that join the
• brachium of the superior colliculus, which terminate in the
• pretectal area of the midbrain, which sends most of its axons bilaterally in the posterior commissure to terminate in  the
• Edinger-Westphal nucleus of the oculomotor complex, which contains parasympathetic preganglionic neurons and sends its axons in the oculomotor nerve to terminate in the
• ciliary ganglion, which sends its parasympathetic postganglionic axons in the
• short ciliary nerve, which ends on the
• iris sphincter

Figure 7.3

The pupillary light reflex pathway.  The lines ending with an arrow indicate axons terminating in the structure at the tip of the arrow.  The lines beginning with a dot indicate axons originating in the structure containing the dot.  Bilateral damage to pretectal area neurons (e.g., in neurosyphilis) will produce Argyll-Robertson pupils (non-reactive to light, active during accommodation).

Recall that the optic tract carries visual information from both eyes and the pretectal area projects bilaterally to both Edinger-Westphal nuclei: Consequently, the normal pupillary response to light is consensual.  That is, a light directed in one eye results in constriction of the pupils of both eyes.

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