Trochlear (iv) 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.
- 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).
- 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:
- 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)
Winds around brainstem
Runs forwards beneath the free edge of tentorium
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)
In the anterior part of the sinus(it rises)
Crosses over the 3rd nerve
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.
- 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.
- 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.