Three cranial nerves innervate the extrinsic muscles of the eye. The most severe loss of eye movement will result from damage to which of these cranial nerves. Why?
The extrinsic muscles of the eye are made up of six muscles which control the movement of the eyeballs. They are described as extrinsic because they originate from points outside the eyeball and are attached to the outer surface of the eyeball.
They are very quick and precise in controlling eye movements and they are able to do so by the actions of three cranial nerves which control their activities.
The six extrinsic muscles of the eye can be sub-divided into three groups according to the role they play in the movement of the eyeball.
- The superior and inferior rectus muscles which control the up and down movement of the eyeball.
- The lateral and medial rectus muscles which control the left and right movement of the eyeball.
- The superior and inferior oblique muscles which keep the eyeball focused while the head moves.
The cranial nerves which control the actions of these muscles are: Abducens, Trochlear and the Oculomotor nerves. The abducens nerve controls the lateral rectus muscle while the trochlear nerve controls the superior oblique muscle. The oculomotor nerve controls the four other extrinsic muscles.
Apart from controlling more of the extrinsic muscles than the two other cranial nerves, the oculomotor nerve exerts control on the muscles of the eyelid and pupillary constriction. Therefore, damage to the oculomotor nerve will lead to a severe loss of eye movement and in addition, it will cause drooping of the eyelid and dilation of the pupil.