Friday, October 28, 2005

Cranial Nerve III

The third cranial nerve or oculomotor nerve controls most of the eye movements and also provides fibers for constriction of the pupil, accomodation, and elevating the eyelid .
The oculomotor nerve arises the ocular motor nucleus in the midbrain on the anterior aspect in floor of the cerebral aqueduct. The third nerve passes forward thru the tegmentum, red nucleus, substantia nigra and emerge on the medial aspect of the cerebral peductal. Figure 1 :
The nucleus of the oculomotor nerve is composed of subnuclei, which are arranged into anterior and posterior groups. The posterior group numbers six. The anterior group consists of two subnuclei, an antero-medial and an antero-lateral . Fibers for pupillary constriction and accomodation arise from the Edinger-Westfall nucleus.

The course of the third nerve. It is critical to review the circle of Willis (Figure below). The third nerve passes (#3 in figure) between posterior cerebral arteries (#11 in figure) and superior cerebellar (#12 in Figure) and runs parallel to the posterior communicating artery (#10 in Figure), which joins the internal carotid artery (#9 in Figure) and the posterior cerebral artery. In fact the 3rd nerve is fed by branches of the posterior communicating artery. Of extreme clinical significance is the fact that most aneurysms producing a pupil involving 3rd nerve palsy affect the posterior communicating artery at this point. The 3rd nerve penetrates the dura mater in front of and lateral to the posterior clinoid process, runs along the lateral wall of the cavernous sinus. After division into two branches, the nerve enters 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 inferior oblique muscle, which enters the muscle at its posterior border.
Reference: Gray's Anatomy


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