Trochlear Nerve

INTRODUCTION:
  • The trochlear nerve,also called the fourth cranial nerve or cranial nerve IV, is a motor nerve (a somatic efferent nerve) that 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.
  1. It arises from dorsum of brainstem
  2. The trochlear nerve is unique among the cranial nerves in several respects:
  3. It is the smallest nerve in terms of the number of axons it contains.
  4. It has the greatest intracranial length.
  5. It is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem.
  6. It innervates a muscle, Superior Oblique muscle(SO), on the opposite side (contralateral) from its origin.
  7. This is the only cranial nerve to decussate prior to exiting the brainstem.
FUNCTIONAL COMPONENTS:
  • SOMATIC EFFERENT-concerned with movement of eye ball through SO.
  • GENERAL SOMATIC AFFERENT-carries proprioceptive impulses from SO which are relayed in the mesencephalic nucleus of 5th nerve.
COURSE AND DISTRIBUTION:
  • Fascicular part:-
  1. Axons leave the nucleus
  2. Curve posteriorly around the aquiduct in the central greymatter
  3. Decussate in the anterior medullary velum.
1.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

3. 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:
  1. Enters the orbit through lateral part of Superior orbital fissure
  2. Frontal & lacrimal nerves laterally, ophthalmic vein inferiorly.
  3. Divides in to fan shaped manner into 3 or 4 branches
  4. Ends by supplying SO on its orbital surface near lateral border.
SITUATION:
  1. It is located at the level of inferior colliculus in the ventromedial part of central gray metter of midbrain dorsal to medial longitudinal bundle.
  2. Caudal & continous with 3rd nucleus complex.
CONNECTIONS:
  • Cerebral cortex
  • Motor cortex - Cortico nuclear tracts
  • Visual cortex - Supeiorcolliculus & tactobulbar tracts . Frontal eye field
2. Nuclei of 3rd ,6th & 8th - Medial longitudinal fasciculus
3. Superior colliculi -Descending predorsal bundle
4. Vertical & torsional gaze centres
5. Cerebellum - Vestibular nuclei

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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