Cranial nerve III is the oculomotornerve. It assists in moving the muscles outside and within the eye. It is a nerve that carries both motor and parasympathetic fibres to assist in our ability to see the environment around us. In this article, we take a closer look at the oculomotor nerve and its course to the eye.
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The intracranial oculomotor nerve
The oculomotor nucleus is a collection of neuronal somas (cell bodies) that is found anterior to the periaqueductal grey matter and cerebral aqueduct of the midbrain, at the level of the superior colliculus.
The oculomotor fibres exit anteriorly from this oblong-shaped nucleus and dive through the midbrain, emerging from the medial surface of the two cerebral peduncles.
There is a second important nucleus in the midbrain associated with the oculomotor nucleus and nerve – the Edinger-Westphal nucleus (EWN).
It forms the site of origin of the parasympathetic fibres that travel with the oculomotor nerve. The EWN lies immediately posterior and adjacent to the oculomotor nucleus, and its fibres encase the oculomotor fibres like a pipe.
CN II sends afferent light-sensing fibres to the pretectal nucleus of the midbrain. The pre-tectal nucleus then communicates to the EWN that there has been a change in light intensity, causing the pupils to constrict or dilate.
The oculomotor nerve complex
The fibres of the EWN and the oculomotor nucleus travel together in this cylindrical structure through the middle cranial fossa. They pierce a layer of dura mater and enter the lateral wall of the cavernous sinus.
The cavernous sinus is a venous structure that lies either side (both anterior/posterior and laterally) of the pituitary gland. Within it are part of the internal carotid artery and cranial nerve VI, the abducens nerve.
The lateral wall is formed by four nerves (top to bottom): cranial nerves III, IV, V1 and V2.
As the oculomotor nerve passes in the wall of the cavernous sinus, the internal carotid plexus contributes some sympathetic fibres to the sheath of the oculomotor nerve complex.
Upon leaving the cavernous sinus, the oculomotor nerve complex is free to travel through the superior orbital fissure.
The extracranial oculomotor nerve
As the oculomotor nerve complex enters the orbital cavity, it divides into two distinct branches, the superior and inferior branch.
Superior branch (smaller)
Motor fibres to the superior rectus and levator palpebrae superioris muscles. Sympathetic fibres to the superior tarsal and dilator pupillae muscles.
Inferior branch (larger)
Motor fibres to the medial rectus, and inferior rectus and oblique muscles.
Parasympathetic fibres to the ciliary ganglion for innervation of the ciliary and sphincter pupillae muscles.
Actions of the oculomotor nerve
Superior branch of the oculomotor nerve
The superior branch of the oculomotor nerve has several functions which are discussed below.
Motor functions include:
Levator palpebrae superioris: elevates the upper eyelid
Superior rectus: elevates the globe
Sympathetic functions include:
Superior tarsal muscle: fixate the eyelid in elevation after levator palpebrae superioris elevates it
Dilator pupillae muscle: pupillary dilation to increase the amount of light reaching the retina
Inferior branch of the oculomotor nerve
The inferior branch of the oculomotor nerve has several functions which are discussed below.
Motor functions include:
Medial rectus: adduct the globe
Inferior rectus: depress the globe
Inferior oblique: abduct, elevate and extort (laterally rotate) the globe
Parasympathetic functions include:
Ciliary muscles: contract, adapting to close-range vision with a more spherical lens
Constrictor pupillae: pupillary constriction to reduce the amount of light reaching the retina
Clinical relevance: diabetic ophthalmoplegia versus aneurysmal compression
With an increasing prevalence of metabolic disease and type II diabetes mellitus, strict adherence to both medical and non-medical treatment regimens is necessary to prevent a condition known as diabetic ophthalmoplegia.
The oculomotor nerve receives nutrients from two arterial circulations. The first is superficial and supplies the parasympathetic fibres forming the shell of the pipe. The second is deep within the oculomotor nerve – tiny blood vessels that nourish the more central motor fibres.
In diabetes, non-enzymatic glycosylation can lead to hyalinosis and occlusion of the small vessels in the middle of the oculomotor nerve. This pathologic process can cause an ischaemic injury to the motor fibres of the oculomotor nerve, but leave the superficial parasympathetic fibres with adequate circulation. This presents as “down and out eyes” with ptosis and normal-sized, reactive pupils (ie, the pupil is spared).
This is in contrast to an aneurysm from the internal carotid artery. Small aneurysms may impact upon the superficial parasympathetic fibres of the oculomotor nerve, but leave the motor fibres intact. This presents as dilated pupils with loss of the accommodation reflex, and in-tact extraocular muscles. As the aneurysm grows, the motor fibres will become involved and ptosis and ophthalmoplegia will accompany.
CN III is the oculomotor nerve
It provides general somatic efferent and general visceral efferent fibres to the extraocular muscles and pupillary constrictor muscles respectively
The muscles are the levator palpebrae superioris, inferior oblique, and superior, medial and inferior recti
CN III damage causes a ‘down and out’ eye
It connects to the midbrain
It is the efferent limb for the pupillary light reflex
It passes through the superior orbital fissure of the skull
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Patrick J. Lynch, medical illustrator License: [CC BY]. Modified by Dr Lewis Potter.