An oculomotor nerve is the third cranial nerve, which stimulates motor functions. There are 12 cranial nerves that control motor and sensory processes of the head and neck. The oculomotor nerve controls the muscles that regulate all eye movements except moving the eyeball down or outward. This nerve’s nucleus is made up of many small nuclei that are divided into anterior and posterior groups. These groups can be further divided into cell groups that control individual muscles.
The oculomotor nerve is connected to two nuclei in the midbrain: the oculomotor nucleus and the Edinger-Westphal nucleus. The oculomotor nucleus is the source of somatic fibers to the recti muscles, the inferior oblique muscle and the levator palpebrae superious muscle. These muscle groups control various eye movements. The Edinger-Westphal nucleus projects fibers to the ciliary ganglion, the structure located in the posterior orbit where sensory processes, pupil constriction, dilation and blood vessel activity occurs.
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From the brainstem, the oculomotor nerve enters the dura mater, the tough outermost layer of three membranes that surround the brain and spinal cord. It lies on the wall of the cavernous sinus, a compartment of veins located in the head, and enters the orbit through the orbital fissure, the space between the orbit’s floor and wall, which serves as a movement channel for nerves and blood vessels. The oculomotor nerve carries parasympathetic fibers, which slow the heart rate and allow for actions like digestion. These fibers are carried to the pupil and the muscle that is responsible for raising the eyelid.
Oculomotor nerve fibers arise from the nucleus in the cerebral aqueduct floor. Their journey includes passing through the floor of the midbrain known as the tegmentum, the red nucleus and the substantia nigra, a structure located in the midbrain that controls reward and addiction. The fibers eventually end up in the middle of the cerebral peduncle, which is one half of the midbrain, after emerging from the lateral wall of the midbrain known as the oculomotor sulcus.
Oculomotor nerves consist of two components: the somatic motor and the visceral motor. The somatic motor components are responsible for exact eye movements, such as tracking and fixating on a specific object. The visceral motor components handle light and accommodation reflexes, mostly having to do with the pupil. Four of the six extraocular muscles, those which control eye movement, are supplied by the somatic motor components, as well as the levator palpebrae superioris muscle, that which controls the upper eyelid movement. Visceral motor components control the nerve supply of the pupil muscles and those of the lens of the eye.