Fourth cranial nerve

The trochlear nerve (CN IV) is the fourth cranial nerve and is a motor nerve that innervates the superior oblique muscle of the eye. It is unique among the cranial nerves in that it is the only one that emerges dorsally from the brainstem and it has the longest intracranial course.

Anatomy

The trochlear nerve originates from the trochlear nucleus in the midbrain, specifically at the level of the inferior colliculus. It decussates (crosses) within the midbrain before emerging from the dorsal aspect of the brainstem. The nerve then travels anteriorly around the brainstem and continues through the cavernous sinus before entering the orbit via the superior orbital fissure.

Function

The primary function of the trochlear nerve is to provide motor innervation to the superior oblique muscle, which is responsible for depressing, intorsion, and abduction of the eye. This allows for coordinated eye movements and is essential for binocular vision.

Clinical significance

Damage to the trochlear nerve can result in a condition known as superior oblique palsy, which is characterized by vertical diplopia (double vision) and difficulty in moving the eye downward, especially when the eye is adducted. Patients may compensate by tilting their head to the opposite side of the affected eye.

Testing

The function of the trochlear nerve can be tested by asking the patient to follow a moving object with their eyes. The examiner looks for any difficulty in moving the eye downward and inward, which would suggest a dysfunction of the superior oblique muscle.

See also

• Cranial nerves
• Oculomotor nerve
• Abducens nerve

References

Susan,

 Gray's Anatomy: The Anatomical Basis of Clinical Practice, 
 41st edition, 
 Elsevier, 
 2016, 
  
  
 ISBN 978-0-7020-5230-9,

External links

• Fourth cranial nerve on WikiMD