Trochlear nerve

What is the trochlea?

Fibrocartilaginous pulley used by the superior oblique muscle to redirect its line of action

The superior oblique muscle is the only extra-ocular muscle to operate through what?

Fibrocartilaginous pulley, the trochlea

What does the trochlear nerve carry?

General somatic efferent, GSE, fibres to innervate the superior oblique muscle

Is it common for the trochlear nerve to be paralysed alone?

No

Which cranial nerve has the longest intracranial length?

Trochlear nerve

Which cranial nerve is the smallest, in terms of the number of axons it contains?

Trochlear nerve

Where do the general somatic efferent, GSE, fibres of the trochlear nerve that innervate the superior oblique muscle, originate from?

Trochlear nucleus Ventral part of the midbrain periaqueductal grey, level with the upper part of the inferior colliculus

How would the eye appear in trochlear nerve lesions?

Adducted and elevated

What is the superior oblique muscle?

An extra-ocular muscle responsible for depression, abduction and intorsion of the eyeball

What would lesions of the trochlear nerve result in?

An inability to look inferiorly when the eye is adducted (down and in) Diplopia

Origin of the superior oblique muscle

Body of the sphenoid bone

Describe the course of the general somatic efferent, GSE, fibres of the trochlear nerve

1. Pass dorsally, around the periaqueductal grey, and decussate in the midline. 2. They then emerge from the dorsal surface of the brainstem immediately caudal to the inferior colliculus. 3. The nerve courses round the cerebral peduncle to gain the ventral aspect of the brain, passing between the posterior cerebral and superior cerebellar arteries. 4. After curving around the midbrain, it enters the inferior surface of the free edge of the tentorium cerebelli, continues in an anterior direction in the lateral wall of the cavernous sinus, and enters the orbit through the superior orbital fissure.

Example of a cause of a trochlear nerve lesion

Severe head injuries Orbital fracture

3 reasons as to why the trochlear nerve is unique

Smallest cranial nerve in terms of the number of axons it contains Longest intracranial course of all the cranial nerves Only cranial nerve to emerge from the dorsal aspect of the brainstem

Insertion of the superior oblique muscle

Superior portion of the posterolateral sclera