Trochlear Nerve – Cranial Nerve IV/4 (Easiest Cranial Nerve)

Introduction

• Its only job is to supply one eye muscle. Therefore it has only motor fibers.
• Before it synapses in the midbrain, the neurons are UMN*. After it synapses in its nucleus in the midbrain, the fibers carried to the muscles are LMN**.

* UMN = Upper Motor Neuron

**LMN = Lower Motor Neuron

Functions in General:

• GSE (general somatic efferent AKA general somatic motor): innervates superior oblique muscle

Nucleus of Trochlear Nerve in Brainstem

• Lies at level of inferior colliculus of the midbrain.
• In anterior part of gray matter.
• Near the cerebral aqueduct.
• Nerve fiber leaving the nucleus pass posteriorly to reach posterior surface of midbrain.
• The nucleus receives corticonuclear fibers from both cerebral hemispheres.
• It receives the tectobulbar tract from the superior colliculus (to connect the trochlear nucleus with the visual cortex)
• Receives fibers from medial longitudinal fasciculus by which it is connected to the nuclei of 3^rd (oculomotor), 6^th (abducent) and 8^th (vestibulocochlear) cranial nerves

Figure 1: Cross section of midbrain at level of inferior colliculus showing the nucleus of trochlear nerve. Also showing is the decussation of both trochlear nerves and their exit from the nuclei and brainstem

Figure 2: Posterior aspect of brainstem showing the trochlear nucleus at the level of the inferior colliculus

Figure 3: Anterior aspect of brainstem showing the
appearance of the trochlear nerve anteriorly

Emerges from brainstem

• Emerges from the posterior surface of the midbrain and immediately decussates with the nerve of the opposite side.

Figure 4: Posterolateral aspect of the brainstem showing the origin of the trochlear nerve.
Red line shows the level of the section of Figure 1

Figure 5: Interior aspect of the skull showing the position of 
the cavernous sinuses

Route

• After emerging from the midbrain it continues into the middle cranial fossa.
• It passes into the edge of the tentorium cerebelli and then enters into the lateral wall of the cavernous sinus below the oculomotor nerve.

Figure 6: Coronal section of the skull showing the cavernous sinuses 
on both sides of the pituitary

Exit from skull

Nerve	Exits	Enters
Trochlear nerve (CN IV)	Superior orbital fissure	Orbit

Figure 7: Interior aspect of the skull showing the brainstem from a postero-superolateral
view and the emergence of the trochlear nerve and its exit from the skull through
the superior orbital fissure

Figure 8: Nerves passing through the superior orbital
fissure

Figure 9: Superior orbital fissure

Branches
It only supplies the Superior Oblique muscle of the eye. No other branches.

Lesions

• LMNL of the trochlear nerve causes paralysis of the contralateral eye’s superior oblique muscle.
• Trochlear nerve palsy and resulting contralateral superior oblique paralysis has varying causes and presentations. 
• The most common presentation required in medical school exams is:
• Normally the superior oblique muscle causes intorsion of the eye. Its paralysis results in extorsion of the affected eyeball.
• This leads to vertical diplopia. The diplopia increases when looking down. Why? Recall that the superior oblique has a role in downward movements of the eyeball. Therefore not only will the affected eyeball be extorted but also can't look downward properly.
• Therefore, the patient will tilt his head to compensate for the diplopia.

Figure 10

References

Drake, R., Vogl, A., & Mitchell, A. (2010). Chapter 8: Head and neck - orbit. Gray's anatomy for students (2nd ed., pp. 878-902). Philadelphia: Elsevier.
Fix, J. (2008). Chapter 11: Cranial Nerves. High-Yield Neuroanatomy (4th ed., pp. 74-87). Philadelphia: Lippincott Williams and Wilkins.

Snell, R. (2010). The cranial nerve nuclei and their central connections and distribution. Clinical neuroanatomy (7th ed., pp. 331-370). Philadelphia: Lippincott Williams and Wilkins.

Snell, R. (2007). Chapter 18: The eye and the ear. Clinical anatomy by systems (pp. 657-685). Philadelphia: Lippincott Williams & Wilkins.

Picture references

1. Done by me
2. By Gray696.png: User:mcstrother derivative work: Mcstrother (Gray696.png) [Public domain], via Wikimedia Commons / retouched from original
3. Done by me
4. "Posterolateral view of brainstem" from Netter's Atlas of Human Anatomy 5e / retouched from original
5. By Anatomist90 (Own work) [CC-BY-SA-3.0 (http://creativecommons.org/licenses/by-sa/3.0)], via Wikimedia Commons  / retouched from original
6. Drake: Gray's Anatomy for Students 2e
7. Unknown (if you know source please message me so I can mention it)
8. "Innervation of orbit and eyeball, anterior view" Drake: Gray's Anatomy for Students 2e
9. "Formation of the lacrimal sac" by Drake: Gray's Anatomy for Students 2e
10. "Figure 11-2" by High-Yield Neuroanatomy series