Trochlear (iv) nerve
TROCHLEAR NERVE
- Also called the fourth cranial nerve, pure motor nerve (a somatic efferent nerve).
- Trochlear nerve enters the orbit through superior orbital fissure ouiside the annulus of Zinn.
- Innervates only a single muscle: the superior oblique muscle of the eye, which operates through the pulley-like trochlea.
- It is the smallest (thinnest) of the cranial nerves (largest is trigeminal)
- It is the only cranial nerve whose fibers originate totally from the contralateral nucleus.
UNIQUE FEATURES OF TROCHLEAR NERVE:
- Most slender cranial nerve.
- Only cranial nerve to emerge on the dorsal aspect of brain.
- Only cranial nerve to undergo complete internal decussation before emerging i.e. right trochlear nerves arises from left trochlear nucleus and vice versa.
- Has longest intracranial course.
- Thinnest cranial nerve (smallest nerve in terms of the number of axons it contains).
FUNCTIONAL COMPONENTS:
- SOMATIC SOMATIC EFFERENT- Through trochlear nucleus for contraction of superior oblique.
- GENERAL SOMATIC AFFERENT- carries proprioceptive impulses from superior oblique (SO) to mesencephalic nucleus of 5th nerve.
COURSE AND DISTRIBUTION:
Fascicular part:-
- Axons leave the nucleus
- Curve posteriorly around the aquiduct in the central greymatter
- Decussate in the anterior medullary velum.
Pre cavernous part:-
Leaves the brainstem on the dorsal surface( just caudal to inferior colliculus)
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Winds around brainstem
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Runs forwards beneath the free edge of tentorium
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Pierces the dura on the posterior corner of the roof of cavernous sinus to enter in to it
Intra cavernous part:-
Runs forwards in the lateral wall of the sinus(lying below 3rd nerve and above the 1st division of 5th nerve)
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In the anterior part of the sinus(it rises)
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Crosses over the 3rd nerve
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Passes through the superior orbital fissure, above and lateral to annulus of zinn.
4. Intra orbital part:
- Enters the orbit through lateral part of Superior orbital fissure
- Frontal & lacrimal nerves laterally, ophthalmic vein inferiorly.
- Divides in to fan shaped manner into 3 or 4 branches
- Ends by supplying SO on its orbital surface near lateral border.
SITUATION:
- It is located at the level of inferior colliculus in the ventromedial part of central gray metter of midbrain dorsal to medial longitudinal bundle.
- Caudal & continous with 3rd nucleus complex.
Exam Question
- Trochlear nerve lies in lateral wall of cavernous sinus.
- Trochlear nerve has the longest intracranial course.
- Trochlear nerve arises from dorsum of brainstem.
- Trochlear nerve enters the orbit through lateral part of Superior orbital fissure.
- Trochlear nerve have 2 functional components somatic efferent & General somatic afferent.
- Trochlear nerve is the only cranial nerve to decussate prior to exiting the brainstem.
- Superior Oblique muscle is innervated by trochlear nerve.
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