The The trochlear nervetrochlear nerve (the (the fourth cranial nervefourth cranial nerve, also , also called the called the fourth nervefourth nerve or simply or simply IVIV) is a motor nerve ) is a motor nerve that innervates a single muscle: the that innervates a single muscle: the superior obliquesuperior oblique

muscle of the eye. muscle of the eye. The trochlear nerve is unique among the cranial nerves in The trochlear nerve is unique among the cranial nerves in several respects. It is the smallest nerve in terms of the several respects. It is the smallest nerve in terms of the

number of axons it contains. It has the greatest number of axons it contains. It has the greatest intracranial length. It is the only cranial nerve that intracranial length. It is the only cranial nerve that

decussates (crosses to the other side) before innervating decussates (crosses to the other side) before innervating its targe.Finally, it is the only cranial nerve that exits from its targe.Finally, it is the only cranial nerve that exits from

the dorsal aspect of the brainstem.the dorsal aspect of the brainstem.

The trochlear nerve emerges from the dorsal aspect of the The trochlear nerve emerges from the dorsal aspect of the brainstem at the level of the caudal brainstem at the level of the caudal mesencephalonmesencephalon, just below , just below the the inferiorinferior colliculus colliculus. It circles anteriorly around the brainstem . It circles anteriorly around the brainstem and runs forward toward the eye in the and runs forward toward the eye in the subarachnoidsubarachnoid space space. It . It passes between the passes between the posterior cerebral arteryposterior cerebral artery and the and the superiorsuperior cerebellar cerebellar artery artery, and then pierces the , and then pierces the duradura just under free just under free margin of the margin of the tentorium cerebellitentorium cerebelli, close to the crossing of the , close to the crossing of the attached margin of the tentorium and within millimeters of the attached margin of the tentorium and within millimeters of the posteriorposterior clinoid clinoid process process..[3][3] It enters the It enters the cavernous sinuscavernous sinus, where , where it is joined by the other two extraocular nerves (III and VI), the it is joined by the other two extraocular nerves (III and VI), the internal carotid arteryinternal carotid artery, and portions of the , and portions of the trigeminaltrigeminal nerve nerve (V). (V). Finally, it enters the orbit through the Finally, it enters the orbit through the superior orbital fissuresuperior orbital fissure and innervates the and innervates the superior oblique musclesuperior oblique muscle..

The superior oblique muscle ends in a tendon that passes through The superior oblique muscle ends in a tendon that passes through a fibrous loop, the a fibrous loop, the trochleatrochlea, located anteriorly on the medial , located anteriorly on the medial aspect of the orbit. aspect of the orbit.

Actions of the superior oblique muscleActions of the superior oblique muscle

Allowable movements for the superior oblique are Allowable movements for the superior oblique are (1)(1) rotation in a vertical plane – looking down and up rotation in a vertical plane – looking down and up

((depressiondepression and and elevationelevation of the eyeball) and of the eyeball) and (2)(2) (2) rotation in the plane of the face ((2) rotation in the plane of the face (intorsionintorsion and and

extorsionextorsion of the eyeball). of the eyeball).

The nucleus of the trochlear nerve is located The nucleus of the trochlear nerve is located in the caudal in the caudal mesencephalonmesencephalon beneath the beneath the cerebral aqueduct. It is immediately below cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.the rostral mesencephalon.

The trochlear nucleus is unique in that its axons run The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the dorsally and cross the midline before emerging from the brainstem. Thus a lesion of the trochlear nucleus affects brainstem. Thus a lesion of the trochlear nucleus affects the the contralateralcontralateral eye. Lesions of all other cranial nuclei eye. Lesions of all other cranial nuclei affect the affect the ipsilateralipsilateral side side

The body of the superior oblique muscle is located The body of the superior oblique muscle is located behindbehind the eyeball, but the tendon (which the eyeball, but the tendon (which is redirected by the trochlea) approaches the eyeball from the is redirected by the trochlea) approaches the eyeball from the frontfront. The tendon attaches to . The tendon attaches to the top (superior aspect) of the eyeball at an angle of 51 degrees with respect to the the top (superior aspect) of the eyeball at an angle of 51 degrees with respect to the primary primary positionposition of the eye (looking straight forward). The force of the tendon’s pull therefore has two of the eye (looking straight forward). The force of the tendon’s pull therefore has two components: a forward component that tends to pull the eyeball downward (depression), and components: a forward component that tends to pull the eyeball downward (depression), and a medial component that tends to rotate the top of the eyeball toward the nose (intorsion).a medial component that tends to rotate the top of the eyeball toward the nose (intorsion).

The relative strength of these two forces depends on which way the eye is looking. When the The relative strength of these two forces depends on which way the eye is looking. When the eye is eye is adductedadducted (looking toward the nose), the force of depression increases. When the eye is (looking toward the nose), the force of depression increases. When the eye is abductedabducted (looking away from the nose), the force of intorsion increases, while the force of (looking away from the nose), the force of intorsion increases, while the force of depression decreases. When the eye is in the primary position (looking straight ahead), depression decreases. When the eye is in the primary position (looking straight ahead), contraction of the superior oblique produces depression and intorsion in roughly equal contraction of the superior oblique produces depression and intorsion in roughly equal amounts.amounts.

To summarize, the actions of the superior oblique muscle are (1) To summarize, the actions of the superior oblique muscle are (1) depressiondepression of the eyeball, of the eyeball, especially when the eye is adducted; and (2) especially when the eye is adducted; and (2) intorsionintorsion of the eyeball, especially when the eye of the eyeball, especially when the eye is abducted. The clinical consequences of weakness in the superior oblique (caused, for is abducted. The clinical consequences of weakness in the superior oblique (caused, for example, by fourth nerve palsies) are discussed below.example, by fourth nerve palsies) are discussed below.

This summary of the superior oblique muscle describes its most important functions. This summary of the superior oblique muscle describes its most important functions. However, it is an oversimplification of the actual situation. For example, the tendon of the However, it is an oversimplification of the actual situation. For example, the tendon of the superior oblique inserts superior oblique inserts behindbehind the equator of the eyeball in the frontal plane, so contraction the equator of the eyeball in the frontal plane, so contraction of the muscle also tends to of the muscle also tends to abductabduct the eyeball (turn it outward). In fact, each of the six the eyeball (turn it outward). In fact, each of the six extraocular muscles exerts rotational forces in extraocular muscles exerts rotational forces in all three planesall three planes (elevation-depression, (elevation-depression, adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the adduction-abduction, intorsion-extorsion) to varying degrees, depending on which way the eye is looking. The relative forces change every time the eyeball moves – every time the eye is looking. The relative forces change every time the eyeball moves – every time the direction of gaze changes. The central control of this process, which involves the continuous, direction of gaze changes. The central control of this process, which involves the continuous, precise adjustment of forcesprecise adjustment of forces on on twelve different tendons in order to point both eyes in twelve different tendons in order to point both eyes in exactly the same direction, is truly remarkable.exactly the same direction, is truly remarkable.

The recent discovery of soft tissue The recent discovery of soft tissue pulleyspulleys in the orbit – similar to the trochlea, but in the orbit – similar to the trochlea, but anatomically more subtle and previously missed – has completely changed (and greatly anatomically more subtle and previously missed – has completely changed (and greatly simplified) our understanding of the actions of the extraocular musclessimplified) our understanding of the actions of the extraocular muscles[4][4]. Perhaps the most . Perhaps the most important finding is that a 2-dimensional representation of the visual field is sufficient for important finding is that a 2-dimensional representation of the visual field is sufficient for most purposes.most purposes.

Clinical syndromesClinical syndromes Vertical diplopiaVertical diplopiaInjury to the trochlear nerve cause weakness of downward eye movement with Injury to the trochlear nerve cause weakness of downward eye movement with

consequent vertical diplopia (double vision). The affected eye drifts upward consequent vertical diplopia (double vision). The affected eye drifts upward relative to the normal eye, due to the unopposed actions of the remaining relative to the normal eye, due to the unopposed actions of the remaining extraocular muscles. The patient sees two visual fields (one from each extraocular muscles. The patient sees two visual fields (one from each eye), separated vertically. To compensate for this, patients learn to tilt the eye), separated vertically. To compensate for this, patients learn to tilt the head forward (tuck the chin in) in order to bring the fields back together – head forward (tuck the chin in) in order to bring the fields back together – to fuse the two images into a single visual field. This accounts for the to fuse the two images into a single visual field. This accounts for the “dejected” appearance of patients with “pathetic nerve” palsies.“dejected” appearance of patients with “pathetic nerve” palsies.

As would be expected, the diplopia gets worse when the affected eye looks As would be expected, the diplopia gets worse when the affected eye looks toward the nose – the contribution of the superior oblique muscle to toward the nose – the contribution of the superior oblique muscle to downward gaze is greater in this position. Common activities requiring this downward gaze is greater in this position. Common activities requiring this type of convergent gaze are reading the newspaper and walking down type of convergent gaze are reading the newspaper and walking down stairs. Diplopia associated with these activities may be the initial symptom stairs. Diplopia associated with these activities may be the initial symptom of a fourth nerve palsy.of a fourth nerve palsy.

Alfred Bielschowsky's head tilt test is a test for palsy of the superior oblique Alfred Bielschowsky's head tilt test is a test for palsy of the superior oblique muscle caused by damage to cranial nerve IV (trochlear nerve).muscle caused by damage to cranial nerve IV (trochlear nerve).

Torsional diplopiaTorsional diplopia

The The superior superior salivatorysalivatory nucleus nucleus contains the cell bodies of parasympathetic contains the cell bodies of parasympathetic axons within the nervus intermedius. These fibers reach the geniculate axons within the nervus intermedius. These fibers reach the geniculate ganglion but do not synapse. Some of these preganglionic parasympathetic ganglion but do not synapse. Some of these preganglionic parasympathetic fibers persist within the fibers persist within the greater greater petrosalpetrosal nerve nerve as they exit the geniculate as they exit the geniculate ganglion and subsequently synapse with neurons in the ganglion and subsequently synapse with neurons in the pterygopalatinepterygopalatine ganglion ganglion. These postganglionic neurons send . These postganglionic neurons send axonsaxons that provide that provide parasympathetic innervation to the parasympathetic innervation to the lacrimallacrimal gland gland..

The remaining preganglionic fibers continue as the mixed facial nerve The remaining preganglionic fibers continue as the mixed facial nerve proper as it extends through the facial canal. Before the nerve exits the proper as it extends through the facial canal. Before the nerve exits the skull via the skull via the stylomastoidstylomastoid foramen foramen and after the and after the nerve to the nerve to the stapediusstapedius muscle muscle has branched off, the facial nerve gives off the has branched off, the facial nerve gives off the chordachorda tympani nerve tympani nerve. This nerve exits the skull through the . This nerve exits the skull through the pterygotympanicpterygotympanic fissure fissure and merges with the and merges with the lingual nervelingual nerve, after which it synapses with , after which it synapses with neurons in the neurons in the submandibularsubmandibular ganglion ganglion. These postganglionic neurons . These postganglionic neurons provide parasympathetic innervation to the provide parasympathetic innervation to the submandibularsubmandibular and and sublingual glandssublingual glands