THE TROCHLEAR NERVE (CN IV)

 
   
The Origin(s)

The GSE fibres arise from the trochlear nucleus near nucleus lies near the midline of the midbrain at the level of the inferior colliculus.


The Course

  • Fibres curve laterally and dorsally

  • Exit from the dorsal surface of the brain stem

  • Pass below the inferior colliculus lateral to the crus cerebri

  • Pierce the dura mater to enter the lateral wall of the cavernous venous sinus just below the oculomotor nerve.

  • Enters the orbit through the superior orbital fissure

  • Pass above the common tendinous ring.

  • Within the orbit the trochlear nerve crosses over the levator palpebrae superioris and superior rectus muscles.

Functional Components
  • Predominantly general somatic efferent (motor) to the superior oblique

  • A few afferent fibres from the muscle spindles

Distribution

Despite its size, this nerve supplies only one muscle - the superior oblique.


Probable cause and sites of injury

Due to its long complicated course, the trochlear nerve is quite vulnerable to injury. Causes include:

•  Midbrain vascular lesions

•  Hydrocephalus with dilatation of the cerebral aqueduct

•  Fracture base of skull

•  Meningeal tumors

•  Cavernous sinus thrombosis

•  Orbital tumors