Oculomotor nerve
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The oculomotor nerve (N. Oculomotorius) is the third of twelve paired cranial nerves. It controls most of the eye movements (cranical nerves IV and VI also do some), constriction of the pupil, and holding the eyelid open.
The oculomotor nerve arises from the anterior aspect of mesencephalon (midbrain). The muscles it controls are the ciliary muscle (affecting accommodation), and all extrinsic eye muscles except for the superior oblique muscle and the lateral rectus muscle. In addition, it supplies parasympathetic fibres to the eye via the ciliary ganglion, and thus controls pupil constriction.
Figure 1 : Plan of oculomotor nerve.
Nucleus of the oculomotor nerve
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Section through superior colliculus showing path of oculomotor nerve.
The fibers of the oculomotor nerve arise from a nucleus in the midbrain, which lies in the gray substance of the floor of the cerebral aqueduct and extends in front of the aqueduct for a short distance into the floor of the third ventricle. From this nucleus the fibers pass forward through the tegmentum, the red nucleus, and the medial part of the substantia nigra, forming a series of curves with a lateral convexity, and emerge from the oculomotor sulcus on the medial side of the cerebral peduncle. The nucleus of the oculomotor nerve does not consist of a continuous column of cells, but is broken up into a number of smaller nuclei, which are arranged in two groups, anterior and posterior. Those of the posterior group are six in number, five of which are symmetrical on the two sides of the middle line, while the sixth is centrally placed and is common to the nerves of both sides. The anterior group consists of two nuclei, an antero-medial and an antero-lateral .
The nucleus of the oculomotor nerve, considered from a physiological standpoint, can be subdivided into several smaller groups of cells, each group controlling a particular muscle.
A nearby nucleus, the Edinger-Westphal nucleus, is responsible for the autonomic functions of the oculomotor nerve, including pupillary constriction and lens accommodation.
Anatomy of the oculomotor nerve
On emerging from the brain, the nerve is invested with a sheath of pia mater, and enclosed in a prolongation from the arachnoid. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater in front of and lateral to the posterior clinoid process, passing between the free and attached borders of the tentorium cerebelli. It runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic, and a communicating branch from the ophthalmic division of the trigeminal. It then divides into two branches, which enter the orbit through the superior orbital fissure, between the two heads of the Rectus lateralis. Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami.
The superior ramus, the smaller, passes medialward over the optic nerve, and supplies the Rectus superior and Levator palpebræ superioris. The inferior ramus, the larger, divides into three branches. One passes beneath the optic nerve to the Rectus medialis; another, to the Rectus inferior; the third and longest runs forward between the Recti inferior and lateralis to the Obliquus inferior. From the last a short thick branch is given off to the lower part of the ciliary ganglion, and forms its short root. All these branches enter the muscles on their ocular surfaces, with the exception of the nerve to the Obliquus inferior, which enters the muscle at its posterior border.
Testing the oculomotor nerve
Cranial nerves III, IV and VI are usually tested together. This is done by getting a person to follow the path of your finger with their eyes (without moving their head). This is commonly shown in movies and on hospital programs on television. A path that tests all the eyes movements (such as drawing a large H with your finger) should be used.
Since the oculomotor controls most of the eye muscles, it may be easier to detect damage to it. The eye will be looking downward, and looking onwards on the affected side. There will also be strabismus.
They may show a degree of ptosis, which is where the eyelid droops.
The oculomotor nerve controls the pupils. This can be tested in two main ways. By moving a finger towards a person's face, as well as them going cross-eyed, their pupils should constrict. This is called accommodation.
Shining a light into their eyes should also make their pupils constrict. Both pupils should constrict at the same time, independent of what eye the light is actually shone on.