Trochlear nerve

Trochlear nerve is the fourth cranial nerve (CN IV) of the twelve pairs of cranial nerves in the human body. It is the smallest nerve in terms of the number of axons it contains. It is unique among the cranial nerves in several respects: it is the only nerve that emerges dorsally from the brain (all others emerge from its sides), and it is the only cranial nerve that decussates (crosses to the other side) before emerging from the brain.

Anatomy

The trochlear nerve carries a somatic efferent supply to the superior oblique muscle of the eye, which operates to depress, abduct, and internally rotate the eye. It is derived from the basal plate of the embryonic midbrain.

Nucleus

The nucleus of the trochlear nerve is located in the midbrain at the level of the inferior colliculus. The axons from the neurons of the nucleus run dorsally and caudally and decussate in the superior medullary velum, a thin layer of white matter covering the fourth ventricle. After decussation, they emerge from the dorsal aspect of the brainstem (the only cranial nerve to do so).

Clinical significance

Damage to the trochlear nerve results in a condition known as fourth nerve palsy. This condition is characterized by the eye drifting upward and slightly outward, and the patient may tilt their head to compensate. The most common cause is congenital, but it can also result from head injury or from conditions that cause generalized neuropathy.

See also

• Cranial nerves
• Superior oblique muscle
• Fourth nerve palsy

References

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