Trochlear Nerve

Origin, Innervation, Function and Features

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Origin of Trochlear Nerve

Trochlear nerve is fourth cranial nerve. It originates from lower part of the mid brain. Its nucleus is present in inferior colliculus of mid brain, when we see the section of mid brain at the level of inferior colliculus there is cerebral aqueduct, around this cerebral aqueduct there is Central Gray matter (periaqueductal gray matter).

So exact location of nucleus of fourth cranial nerve is anterolateral side of Central Gray matter (periaqueductal gray matter) at the level of inferior colliculus in lower part of mid brain.

There are group of nucleus that are present in one vertical line, they are basically Somatic motor fibers, also called General Somatic Efferent fibers and eventually innervate the skeletal muscle, its include nucleus of III, IV, VI, XII   cranial nerves from rostral to caudal end, III cranial nerve nucleus is present in superior colliculus and fourth cranial nerve in inferior colliculus. In pons there is VI cranial nerve nucleus and in medulla oblongata there is XII cranial nerve nucleus. These Somatic motor fibers are actually continuation of gray matter (Anterior Gray Horn) of spinal cord and when it enters in to brain stem it discontinuous but exist as a vertical Column called General Somatic efferent fibers.

Unique Features of Trochlear Nerve

  • fourth cranial nerve is only nerve which emerges from the back (dorsal part of mid brain), its fibers move backward and then forward

  • It has least number of fibers as compare to others.

  • It dissociates so right nucleus supply to the left superior oblique muscle and left nucleus supply to the right superior oblique muscle.

  • Longest intracranial course.

Course of fourth Cranial Nerve

Its fibers moves backward and dissociates then they emerges on superior medullary velum and comes out  and present on  contra lateral side of cerebral peduncle, moves around the mid brain and anteriorly passes below the posterior cerebral artery and above the superior cerebellar artery  and reaches lateral wall of cavernous sinus passing through lateral wall of cavernous sinus not passing through the cavity of cavernous sinus,( fourth cranial nerve passes through the cavity) as it moves forward it moves upward and enter in to the orbit through lateral part of Superior Orbital Fissure outside the tendenous ring and medial to frontal nerve and going to innervate Superior Oblique muscle,

In lateral wall of cavernous sinus there are fibers of III, IV, ophthalmic and maxillary division of trigeminal nerve.

Innervation of fourth Cranial Nerve

fourth cranial nerve innervates superior oblique muscle

There is a belly of superior oblique muscle from where tendon passes under the superior rectus and eventually attach to the sclera. Below the superior oblique muscle there are 2 more muscles

  1. Levator palpebrae superioris muscle

  2. Superior rectus

Function Superior Oblique Muscle

When this muscle will contract, back of the eye ball will move upward and front will rotate downward.  Basically it pushes the eye ball down and outside.

Trochlear Nerve Injury

 Causes of Injury:

  • Congenital (rare).

  • Thrombosis in cavernous sinus.

  • Aneurysm in internal carotid artery.

Due to trochlear nerve injury superior oblique muscle will become weak,

  1. Weakness of depression.

  2. Weakness of abduction.

  3. Failure of intorsion.

If nerve is damaged, downward vision of affected eye will be altered and will not look down especially in adducted position because depression of eye ball is due to superior oblique muscle.

Superior oblique muscle is major intorting muscle and inferior oblique is extorting muscle. IV nerve injury may cause extortion of eye, in this case binocular vision will be affected and patient will present with diplopia.

If you turn your head, both eyes will turn accordingly. One eye will be in extort position and other will be in intort position to keep their visual field axis parallel.

If right sided trochlear nerve is impaired, patient may see tilt objects. Vertical diplopia may occur upon seeing downward. For correction of visual field patient will turn his head on left side that will bring right eye extort while left eye intort that will help patient to see proper downward.

When IV cranial nerve is damaged chin is also tucked in because they want to keep the eye above the horizontal axis to manage vertical diplopia.

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