THE EXTRAOCULAR MUSCLES & EYE MOVEMENTS

The six extraocular muscles (four rectus and two oblique) move the eye in different directions. The nerves that go to these muscles are the IIIrd, IVth and VIth cranial nerves - the oculomotor (to the superior, medial and inferior rectus and the inferior oblique) , the trochlear (to the superior oblique) and abducens (to the lateral rectus). Their actions are highly co-ordinated bilaterally, so that the two eyes always have matching (conjugate) images of the external world on their retinae. They are also "wired" to compensate for head and body movements, through the mediation of the vestibular apparatus and stretch receptors in the neck, in order to maintain a stable image of the external world on the retina, in spite of head and body movements.

Perhaps because of the stringent demands placed on the visual system, to maintain stable, matching images on the retinae, the extra-ocular muscles can move only in very circumscribed ways. We can't simply move our eyes in whatever way we wish. Try to scan your closed eyes from extreme left to extreme right, with your fingers placed lightly against the outer corners of the eye ball - so that you can feel the movement. You will notice that your eyes do not scan smoothly, but make a series of quick jumps called saccadic jumps or saccades. This is because continuous movement of the eyes would produce a blurred image on the retina and so impair vision. The eyes therefore are forced to jump quickly from one fixation point to another, so that a series of clear images are formed - which is presumably better than having no clear image at all. It is not simply that the extraocular muscles are unable to move smoothly.

Hold up a finger at arms' length to the left, with your right index finger against the corner of your right eye. Keep looking at the finger and move it from extreme left to extreme right. You will observe that in this case the eyes do move smoothly, following the moving finger. Thus, if the eye fixates a moving object, it is able to follow it smoothly, keeping a stable image of the moving object on the retina. This type of movement is called smooth-pursuit movement.

It we are traveling on a train or bus looking at the scenery going by, our eyes also show smooth-pursuit movements. We pick up an object, fixate and follow it until our eyes can go no further in the orbit. This triggers a reflex jump back to the opposite extreme of the range, and fixation and following of a new object. The result is a series of to-and-fro movements of the eyes with a smooth movement in one direction, and a rapid jump back in the other. This movement is called nystagmic movement or nystagmus. In this case, the nystagmus is caused by a moving optical stimulus and is referred to as optokinetic nystagmus. If you rotate continuously with your fingers at the corners of your closed eyes, you will also detect nystagmic movements. The eyes move slowly in the direction opposite to that of your rotation, then when they reach the end of the range of movement, they jump forward in the direction of rotation, then scan once more. This is controlled by the vestibular apparatus - in this case, the lateral semicircular canals - and is termed vestibular nystagmus.

When we move our gaze from a distant to a near object, our eyes converge in order that the image of the object will fall on the fovea of both retinae. At the same time, the lenses have to accommodate for near vision, and the pupils constrict in order to provide greater depth of field and resolution for near vision, and to compensate for the greater light flux which would be entering the eye from a close object. These three actions are obligatorily linked as the accommodation triad. The convergence and divergence movements of the eyes as the gaze is shifted from far to near objects and vice versa, are known as vergence movements.

If the extraocular muscles are weak or malformed, or if for some reason, the muscles fail to work properly together, then the retinal images in both eyes will not correspond. This is termed strabismus, and diplopia (double vision) may result if the tendency is not over-ridden by active compensation designed to co-ordinate the images in both eyes. Damage to the vestibular apparatus (or we will see later to the cerebellum - which works closely with the vestibular nuclei) can lead to abnormal nystagmus. Abnormal nystagmus can also result from impaired fixation reflexes, which might be caused, for example, by very poor visual acuity.

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