Ocular Motor Nerves
It is often said, “the eyes are the windows to the soul.” This beautiful literary quote is not exactly a scientifically provable statement... but science has shown us over the years that, our eyes are certainly the windows of our body, allowing us to view the world in which we live and everyone and everything contained within it.
The ability to see is accomplished using a combination of both sensory input from the eye (more specifically from the specialized cells of the retina), and motor output from the brain to the muscles around and attached to the eyes. The muscles surrounding the eye are aptly named the extraocular muscles, and they allow the eyes to move, giving more control over what is seen and where. Without these muscles and the nerves that innervate them, the process of observation would be completely different: rather than just shifting one’s gaze, one would have to turn the whole head to look in any direction. Instead, these muscles allow us to quickly adapt without the necessity of having to engage in significantly effortful, obvious movements. The following article focuses on these muscles, their functions, and the nerves that innervate them.
The Cranial Nerves
The nerves that innervate the extraocular muscles are among a group of nerves called the cranial nerves, which are so called because they arise in the brain and supply structures of the head and neck. There are a total of 12 cranial nerves (CN):
- the olfactory nerve (CN I)
- the optic nerve (CN II)
- the oculomotor nerve (CN III)
- the trochlear nerve (CN IV)
- the trigeminal nerve (CN V)
- the abducens nerve (CN VI)
- the facial nerve (CN VII)
- the vestibulocochlear nerve (CN VIII)
- the glossopharyngeal nerve (CN IX)
- the vagus nerve (CN X)
- the spinal accessory nerve (CN XI)
- the hypoglossal nerve (CN XII)
Of these, CN I, CN II, CN VII, CN VIII, CN IX and CN X play roles in special sensory functions (i.e. olfaction, vision, gustation, audition, and balance); CN V (all three branches, the ophthalmic, maxillary, and mandibular) and CN IX play roles in somatic sensory functions; and CN III, CN IV, CN V (the mandibular branch, V3, only), CN VI, CN VII, CN IX, CN X, CN XI, and CN XII are responsible for motor functions.
Of the cranial nerves with motor functions, CN III, CN IV, and CN VI are the ocular motor nerves, which provide innervation to the extraocular muscles.
The Oculomotor Nerve (CN III)
The oculomotor nerve originates in the midbrain, in the oculomotor nuclear complex. This complex is located at the level of superior colliculus near the midline. The oculomotor nerve fibers contain both somatic efferent fibers and special visceral efferent fibers, specifically autonomic parasympathetic fibers. These parasympathetic fibers originate from a group of neuron cell bodies in the midbrain called the Edinger-Westphal nucleus.
Somatic Efferent Component
Trajectory & Innervation
These axons pass through the red nucleus and emerge from the ventral midbrain medial to the crus cerebri. The nerve fibers travel through the lateral wall of the cavernous sinus and split into small superior and large inferior divisions; then enter the orbit via the superior orbital fissure along with the trochlear nerve, the ophthalmic division of the trigeminal nerve (CN V1), and the abducens nerve.
The oculomotor nerve innervates the majority of extraocular muscles: the superior rectus and levator palpebrae superioris are innervated by superior division while inferior division innervate , medial, and inferior recti and the inferior oblique.
The superior rectus muscle originates at the superior part of the common tendinous ring above and lateral to the optic canal, and inserts on the superior surface of sclera approximately 8 mm from the limbus. Contraction of the superior rectus elevates, adducts, and medially rotates the eye, and as such is the main muscle responsible for upward gaze.
The inferior rectus muscle originates at the inferior part of the common tendinous ring below the optic canal, and inserts on the inferior surface of the sclera approximately 6 mm from limbus. Contraction of the inferior rectus depresses, adducts, and laterally rotates the eye, and as such it is the main muscle responsible for downward gaze.
The medial rectus muscle originates at the medial part of the common tendinous ring, medial to and below the optic canal, and inserts on the medial surface of the sclera. Contraction of the medial rectus adducts the eye, both medial recti act together for convergence.
The inferior oblique muscle originates on the medial floor of the orbit just behind the orbital rim, and attaches on the orbital surface of the maxilla lateral to the nasolacrimal groove. It inserts on the outer inferior surface of the eye. Contraction of the inferior oblique elevates, abducts, and laterally rotates the eye, allowing for upward, outward gaze.
The levator palpebrae superioris originates on the lesser wing of the sphenoid bone anterior to the optic canal and inserts on the anterior surface of the tarsal plate, with a few fibers attaching to the skin and superior conjunctival fornix. Contraction of the levator palpebrae superioris elevates the upper eyelid, opening the eye.
Visceral Efferent Component (Parasympathetic)
The parasympathetic fibers originate from the Edinger-Westphal nucleus which is located in the midbrain. Preganglionic visceral motor axons leave the nucleus and course through the midbrain with somatic efferents both of which constitutes CNIII. Parasympathetic axons branch from the nerve to inferior oblique and terminate in the ciliary ganglion. Postganglionic axons leave the ciliary ganglion in the form of 6-10 short ciliary nerves which perforate the sclera and supply the cornea, choroid, iris, ciliary body and sclera.
Pupillary Light Reflex
The pretectal region of midbrain contributes to the circuit for the pupillary light reflex. This reflex has two main components, an afferent (sensory) component involving the optic nerve and an efferent (motor) component involving the oculomotor nerve.
The sequence begins with light-induced activation of retinal fibers (the afferent component of the reflex). These fibers synapse with neurons in pretectal region, which in turn project to the oculomotor nucleus. The preganglionic parasympathetic neurons from the Edinger-Westphal nucleus send axons via the oculomotor nerve to synapse with postganglionic neurons in the ciliary ganglion, which innervates the sphincter pupillae, constricting the pupil (the efferent component of the reflex).
Accommodation is adaptation of the eyes for near vision. It is obtained by increase in the curvature of the lens, pupillary constriction and convergence of eyes.
The parasympathetic fibers originating from the Edinger-Westphal nucleus also innervate the ciliary muscle. When this muscle contracts, it releases the suspensory ligament of the lens, which allows the lens to relax and increase its degree of curvature. The signals of Edinger-Westphal nucleus to sphincter like pupillary muscles result in the smaller pupil to sharpen the image on retina. Convergence of the eyes is brought about by both medial recti muscles supplied by somatic efferent component of the oculomotor nerve.
The Trochlear Nerve (CN IV)
The trochlear nerve originates from the trochlear nucleus which lies at the level of inferior colliculus in the tegmentum of the midbrain. It has only a somatic efferent component.
Axons arising from the nucleus course dorsally around the periaqueductal grey matter and cerebral aqueduct and cross the midline. The trochlear nerve is unique in that the axons of the cells within this nucleus cross over to the contralateral side before emerging from the dorsal surface of the caudal midbrain, just below the inferior colliculi. The nerve fibers travel through the cavernous sinus along with CN III,V1,V2 & VI. Within the sinus the trochlear nerve is lateral to the internal carotid artery. It leaves the sinus and enters the orbit via the superior orbital fissure, along with the oculomotor nerve, the ophthalmic division of the trigeminal nerve, and the abducens nerve. The nerve courses medially close to the roof of the orbit and reaches the superior oblique muscle to innervate it.
The superior oblique muscle originates on the body of the sphenoid superior and medial to the optic canal, follows the medial border of the roof of the orbit, and passes through a fibrocartilaginous structure called the trochlea. It inserts on the outer superior surface of the eye. Contraction of the superior oblique depresses, abducts, and medially rotates the eye, allowing for downward, outward gaze.
The Abducens Nerve (CN VI)
Abducens nerve contains only somatic efferent fibers to innervate only one muscle of the orbit, lateral rectus. The abducens nerve originates from cells in the abducens nucleus, which is located in the dorsomedial part of the posterior pons, pontine tegmentum, just ventral to the fourth ventricle.
The axons of the facial nerve loop around the abducens nucleus and form bulge in the floor of fourth ventricle called facial colliculus. The axons of the cells within this nucleus travel ventrally and emerge from the brainstem at the pontine-medullary border (junction of the pons & pyramid of medulla). It runs ventro-laterally in the subarachnoid space of posterior cranial fossa and penetrates the dura lateral to dorsum sellae of sphenoid bone. The nerve fibers travel forward between the dura and apex of the petrous temporal bone to enter he cavernous sinus. The nerve enters the orbit via the superior orbital fissure along with the oculomotor nerve, the trochlear nerve, and the ophthalmic division of the trigeminal nerve. The abducens nerve innervates the lateral rectus muscle.
The lateral rectus muscle originates at the lateral part of the common tendinous ring where it bridges the superior orbital fissure, and inserts on the lateral surface of the anterior aspect of the sclera. Contraction of the lateral rectus abducts the eye.
The Ocular Motor Nerves as Part of the Corticobulbar Tract
The corticobulbar tract (otherwise known as the corticonulcear tract) is responsible for influencing the motor nuclei of a number of cranial nerves, including the:
- oculomotor (III)
- trochlear (IV)
- mandibular component of the trigeminal (V3)
- abducens (VI)
- facial (VII)
- glossopharyngeal (IX)
- vagus (X)
- spinal accessory (XI)
- hypoglossal (XII) nerves
The tract operates as a two-neuron sequence: upper motor neurons (UMNs) descending from the cortex to the CN nuclei are considered part of the corticobulbar tract, and synapse on the cell bodies of lower motor neurons (LMNs) which are located in the CN nuclei. The axons of LMNs are considered as part of the cranial nerves themselves, with their axons projecting via the cranial nerves to the muscles of the face, head, and neck.
Corticobulbar motor fibers arise from the frontal eye fields (a region in the caudal portion of middle frontal gyrus), motor cortex (the precentral gyrus), and somatosensory cortex (the postcentral gyrus) where they travel from the cortex through the internal capsule and to the CN nuclei in the brainstem.
Fibers originating from the frontal eye fields project to two regions: the rostral interstitial nucleus of the MLF (riMLF), known as the vertical gaze center; and in the paramedian pontine reticular formation (PPRF), the pontine horizontal gaze center. Fibers from these two gaze centers then project to the motor nuclei of the oculomotor, trochlear, and abducens cranial nerves. The frontal eye fields and parietal eye field also provide cortical input to the superior colliculus, which also provides input to the riMLF and PPRF. The riMLF is part of the medial longitudinal fasciculus (MLF), a bundle fibers that originate from the medial vestibular nucleus, reticular formation, and superior colliculus.
An important function of the MLF is to connect the abducens nucleus on one side of the brain with the oculomotor nucleus on the opposite side of the brain. This connection allows it to play a crucial role in synchronizing horizontal gaze. When the right abducens nucleus sends signals to the right lateral rectus muscle to contract, which pulls the right eye laterally to look right, it also sends signals via the MLF to the left oculomotor nucleus. These signals instruct the left oculomotor nucleus to innervate and thus simultaneously contract the left medial rectus muscle, which pulls the left eye medially so that it also points right at the same time the right eye does.