The trochlear nerve is the fourth cranial nerve (CN IV) and one of the ocular motor nerves that control eye movement. The trochlear nerve, while the smallest of the cranial nerves, has the longest intracranial course as it is the only nerve to have a dorsal exit from the brainstem. It originates in the midbrain and extends laterally and anteriorly to the superior oblique muscle.[rx]
The trochlear nerve also called the fourth cranial nerve or CN IV is a motor nerve (a somatic efferent nerve) that innervates only a single muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea.
The trochlear nerve is unique among the cranial nerves in several respects:
- It is the smallest nerve in terms of the number of axons it contains.
- It has the greatest intracranial length.
- It is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem.
- It innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus. The trochlear nerve decussates within the brainstem before emerging on the contralateral side of the brainstem (at the level of the inferior colliculus). An injury to the trochlear nucleus in the brainstem will result in a contralateral superior oblique muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral superior oblique muscle palsy.
| Type | General somatic efferent (GSE) |
| Target muscle | Trochlear nerve (CN IV) to the superior oblique muscle Abducens nerve (CN VI) to the lateral rectus muscle |
| Extraocular muscles | Four rectus muscles: lateral, medial, superior, and inferior Two oblique muscles: inferior and superior |
| Medial longitudinal fasciculus | Relay axons between the oculomotor (CN III), trochlear (CN IV), and the abducens (CN VI) nerves |
| Location of nuclei | Trochlear nerve (CN IV) – Level of the inferior colliculus Abducens nerve (CN VI) – Dorsal pons, the floor of 4th ventricle |
| Parts of nerve | Nucleus Intraparenchymal Cisternal Cavernous Intraocular |
| Disorders of the trochlear nerve (CN IV) | Torsional and Vertical Diplopia Fourth Nerve Palsy (Acute & Chronic) Nuclear Lesions |
| Disorders of the abducens nerve (CN VI) | Esotropia & Diplopia Wernicke-Korsakoff and Tolusa-Hunt Syndromes Nuclear and Supranuclear Lesions |
| Mnemonic for innervation of extraocular muscles | SO4 – Superior oblique by CN IV LR6 – Lateral rectus by CN VI AO3 – All others by CN III |
Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.[rx]
Structure
The trochlea is Latin for pulley, which appropriately describes the sling of connective tissue that houses the tendon of the superior oblique. Moreover, the trochlear nerve is a somatic efferent (motor) nerve, and along with oculomotor (III) and abducens (VI) nuclei, it is responsible for eye movement. Through its innervation of the superior oblique, the trochlear nerve controls the abduction and intorsion of the eye.[rx]
The trochlear nerve emerges from the back (dorsal) brainstem,[rx] just below the inferior colliculus. It circles from behind around the brainstem and runs forward toward the eye in the subarachnoid space. It passes between the posterior cerebral artery and the superior cerebellar artery and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process.[rx] It runs on the outer wall of the cavernous sinus.[rx] Finally, it enters the orbit through the superior orbital fissure and supplies the superior oblique muscle.[rx]
Blood Supply and Lymphatics
The midbrain is supplied by the posterior cerebral artery, superior cerebellar artery, and the basilar artery. As cranial nerve IV has a motor nucleus, it is located near the midline along the medial longitudinal fasciculus. A disruption in any of the aforementioned arterial structures could affect the medial midbrain, and thus, the trochlear nucleus.
Nerves
The trochlear nerve pair originates from a pair of symmetrical trochlear nuclei within the medial midbrain at the level of the inferior colliculus. The left and right nerves then travel dorsally surrounded by the periaqueductal gray matter, decussating before their exit in the dorsal midbrain. The two nerves run on contralateral sides, extend laterally and then anteriorly around the pons, before penetrating the dura above the trigeminal nerve.
It enters the cavernous sinus where it runs anteriorly above the abducens nerve and ophthalmic branch of the trigeminal nerve. Here in the cavernous sinus, a few sympathetic fibers join the trochlear nerve with the possibility of some sensory fibers from the trigeminal nerve. The nerve then enters the orbit through the superior orbital fissure and continues to extend anteriorly to the superior oblique muscle.[rx][rx] The superior orbital fissure is also the nerve pathway for cranial nerves III, VI, and V and is vulnerable to shearing forces in the setting of trauma.
Muscles
The only muscle the trochlear nerve innervates, the superior oblique muscle, is the longest and thinnest muscle among the extraocular muscles. The muscle belly originates from the back of the roof of the orbit near the common tendinous ring, but it takes an unusual course to reach the eye. The tendon extends between the orbital roof and passes through a fibrous loop (known as the trochlea) located on the frontal bone. The tendon then reaches laterally and posteriorly prior to its insertion point on the posterior half of the eye.
This “pulley” system afforded by the trochlea makes the superior oblique unique among the extraocular muscles and allows for its muscular functions of depression, abduction, and intorsion of the eye. Because of the muscle’s placement at the posterior portion of the eye, the muscle elevates the posterior of the eye, causing the front of the eye to become depressed. The muscle also causes abduction of the eye, moving the pupil away from the nose, and intorsion, rotating the eye such that the top of the eye moves toward the nose.
The superior oblique muscle is the only extraocular muscle that can lower the pupil with the eye adducted. Thus, to isolate the function of the superior oblique muscle from the other extraocular muscles, the muscle can be tested by requesting the patient to adduct the eye and then ask to depress the eye. Failure to depress the eye during adduction indicates a problem with the superior oblique muscle or the trochlear nerve.[rx] In addition, a general rule of thumb is that “obliques go opposite”; the left superior oblique is tested by having the patient look right, while the right superior oblique is tested with the patient looking left.
Anatomical Course
The trochlear nerve arises from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain (it is the only cranial nerve to exit from the posterior midbrain).
It runs anteriorly and inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone.
The nerve then moves along the lateral wall of the cavernous sinus (along with the oculomotor nerve, the abducens nerve, the ophthalmic and maxillary branches of the trigeminal nerve and the internal carotid artery) before entering the orbit of the eye via the superior orbital fissure.
Nucleus
The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.
The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem posteriorly. Thus a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei affect the ipsilateral side.
Functions
The trochlear nerve provides motor supply to the superior oblique muscle of the eye,[rx] The trochlear nerve carries axons of type GSE, general somatic efferent, which innervate skeletal muscle of the superior oblique muscle.
The superior oblique muscle ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. Trochlea means “pulley” in Latin; the fourth nerve is named after this structure.
Actions of the superior oblique muscle
In order to understand the actions of the superior oblique muscle, it is useful to imagine the eyeball as a sphere that is constrained—like the trackball of a computer mouse—in such a way that only certain rotational movements are possible. Allowable movements for the superior oblique are
- (1) rotation in a vertical plane—looking down and up (depression and elevation of the eyeball) and
- (2) rotation in the plane of the face (intorsion and extorsion of the eyeball).
The body of the superior oblique muscle is located behind the eyeball, but the tendon (which is redirected by the trochlea) approaches the eyeball from the front. The tendon attaches to the top (superior aspect) of the eyeball at an angle of 51 degrees with respect to the primary position of the eye (looking straight forward). The force of the tendon’s pull, therefore, has two components: a forward component that tends to pull the eyeball downward (depression), and a medial component that tends to rotate the top of the eyeball toward the nose (intorsion).
This pure motor nerve is responsible for delivering efferent stimuli to the superior oblique muscle. Recall that the superior oblique muscle is attached to the upper outer quadrant on the posterolateral surface of the globe of the eye. It then passes through a sling, or trochlea, located on the roof of the orbit, before coursing posteriorly to insert in the common tendinous ring. Therefore, when the muscle contracts, the eyeball moves downwards and outwards. Therefore, the trochlear nerve (CN VI) is responsible for inward rotation, depression, and abduction of the eyeball contralateral to the nucleus.
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