презентация Trochlearis

1. TheThe trochlear nervetrochlear nerve (the(the fourth cranial nervefourth cranial nerve , also called the, also called the
fourth nervefourth nerve or simplyor simply IVIV) is a motor nerve that innervates a) is a motor nerve that innervates a
single muscle: thesingle muscle: the superior obliquesuperior oblique muscle of the eye.muscle of the eye.
The trochlear nerve is unique among the cranial nerves in severalThe trochlear nerve is unique among the cranial nerves in several
respects. It is the smallest nerve in terms of the number of axons itrespects. It is the smallest nerve in terms of the number of axons it
contains. It has the greatest intracranial length. It is the only cranialcontains. It has the greatest intracranial length. It is the only cranial
nerve that decussates (crosses to the other side) beforenerve that decussates (crosses to the other side) before
innervating its targe.Finally, it is the only cranial nerve that exitsinnervating its targe.Finally, it is the only cranial nerve that exits
from the dorsal aspect of the brainstem.from the dorsal aspect of the brainstem.

2. The trochlear nerve emerges from the dorsal aspect of the brainstem atThe trochlear nerve emerges from the dorsal aspect of the brainstem at
the level of the caudalthe level of the caudal mesencephalonmesencephalon, just below the, just below the inferiorinferior
colliculuscolliculus. It circles anteriorly around the brainstem and runs. It circles anteriorly around the brainstem and runs
forward toward the eye in theforward toward the eye in the subarachnoidsubarachnoid spacespace. It passes. It passes
between thebetween the posterior cerebral arteryposterior cerebral artery and theand the superiorsuperior cerebellarcerebellar
arteryartery, and then pierces the, and then pierces the duradura just under free margin of thejust under free margin of the
tentorium cerebellitentorium cerebelli, close to the crossing of the attached margin of, close to the crossing of the attached margin of
the tentorium and within millimeters of thethe tentorium and within millimeters of the posteriorposterior clinoidclinoid processprocess..[3][3]
It enters theIt enters the cavernous sinuscavernous sinus, where it is joined by the other two, where it is joined by the other two
extraocular nerves (III and VI), theextraocular nerves (III and VI), the internal carotid arteryinternal carotid artery, and, and
portions of theportions of the trigeminaltrigeminal nervenerve (V). Finally, it enters the orbit(V). Finally, it enters the orbit
through thethrough the superior orbital fissuresuperior orbital fissure and innervates theand innervates the
superior oblique musclesuperior oblique muscle..
The superior oblique muscle ends in a tendon that passes through aThe superior oblique muscle ends in a tendon that passes through a
fibrous loop, thefibrous loop, the trochleatrochlea, located anteriorly on the medial aspect, located anteriorly on the medial aspect
of the orbit.of the orbit.

3. Actions of the superior oblique muscleActions of the superior oblique muscle
Allowable movements for the superior oblique areAllowable 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 andand
elevationelevation of the eyeball) andof the eyeball) and
(2)(2) (2) rotation in the plane of the face ((2) rotation in the plane of the face (intorsionintorsion andand extorsionextorsion of theof the
eyeball).eyeball).

4.  The nucleus of the trochlear nerve is located inThe nucleus of the trochlear nerve is located in
the caudalthe caudal mesencephalonmesencephalon beneath the cerebralbeneath the cerebral
aqueduct. It is immediately below the nucleus ofaqueduct. It is immediately below the nucleus of
the oculomotor nerve (III) in the rostralthe oculomotor nerve (III) in the rostral
mesencephalon.mesencephalon.
 The trochlear nucleus is unique in that its axons runThe trochlear nucleus is unique in that its axons run
dorsally and cross the midline before emerging fromdorsally and cross the midline before emerging from
the brainstem. Thus a lesion of the trochlear nucleusthe brainstem. Thus a lesion of the trochlear nucleus
affects theaffects the contralateralcontralateral eye. Lesions of all othereye. Lesions of all other
cranial nuclei affect thecranial nuclei affect the ipsilateralipsilateral sideside

5.  The body of the superior oblique muscle is locatedThe body of the superior oblique muscle is located behindbehind the eyeball, but the tendon (which isthe eyeball, but the tendon (which is
redirected by the trochlea) approaches the eyeball from theredirected by the trochlea) approaches the eyeball from the frontfront. The tendon attaches to the top. The tendon attaches to the top
(superior aspect) of the eyeball at an angle of 51 degrees with respect to the(superior aspect) of the eyeball at an angle of 51 degrees with respect to the primary positionprimary position ofof
the eye (looking straight forward). The force of the tendon’s pull therefore has two components: athe 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 a medialforward 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).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 eyeThe relative strength of these two forces depends on which way the eye is looking. When the eye
isis 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 amounts.contraction of the superior oblique produces depression and intorsion in roughly equal 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 isof the eyeball, especially when the eye is
abducted. The clinical consequences of weakness in the superior oblique (caused, for example,abducted. The clinical consequences of weakness in the superior oblique (caused, for example,
by fourth nerve palsies) are discussed below.by fourth nerve palsies) are discussed below.
 This summary of the superior oblique muscle describes its most important functions. However, itThis 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 superior obliqueis an oversimplification of the actual situation. For example, the tendon of the superior oblique
insertsinserts behindbehind the equator of the eyeball in the frontal plane, so contraction of the muscle alsothe equator of the eyeball in the frontal plane, so contraction of the muscle also
tends totends to abductabduct the eyeball (turn it outward). In fact, each of the six extraocular muscles exertsthe eyeball (turn it outward). In fact, each of the six extraocular muscles exerts
rotational forces inrotational forces in all three planesall three planes (elevation-depression, adduction-abduction, intorsion-(elevation-depression, adduction-abduction, intorsion-
extorsion) to varying degrees, depending on which way the eye is looking. The relative forcesextorsion) to varying degrees, depending on which way the eye is looking. The relative forces
change every time the eyeball moves – every time the direction of gaze changes. The centralchange every time the eyeball moves – every time the direction of gaze changes. The central
control of this process, which involves the continuous, precise adjustment of forcescontrol of this process, which involves the continuous, precise adjustment of forces onon twelvetwelve
different tendons in order to point both eyes in exactly the same direction, is truly remarkable.different tendons in order to point both eyes in exactly the same direction, is truly remarkable.
 The recent discovery of soft tissueThe recent discovery of soft tissue pulleyspulleys in the orbit – similar to the trochlea, but anatomicallyin the orbit – similar to the trochlea, but anatomically
more subtle and previously missed – has completely changed (and greatly simplified) ourmore subtle and previously missed – has completely changed (and greatly simplified) our
understanding of the actions of the extraocular musclesunderstanding of the actions of the extraocular muscles[4][4]
. Perhaps the most important finding is. Perhaps the most important finding is
that a 2-dimensional representation of the visual field is sufficient for most purposes.that a 2-dimensional representation of the visual field is sufficient for most purposes.

6. Clinical syndromesClinical syndromes
Vertical diplopiaVertical diplopia
Injury to the trochlear nerve cause weakness of downward eye movement withInjury to the trochlear nerve cause weakness of downward eye movement with
consequent vertical diplopia (double vision). The affected eye drifts upward relative toconsequent vertical diplopia (double vision). The affected eye drifts upward relative to
the normal eye, due to the unopposed actions of the remaining extraocular muscles.the normal eye, due to the unopposed actions of the remaining extraocular muscles.
The patient sees two visual fields (one from each eye), separated vertically. ToThe patient sees two visual fields (one from each eye), separated vertically. To
compensate for this, patients learn to tilt the head forward (tuck the chin in) in order tocompensate for this, patients learn to tilt the head forward (tuck the chin in) in order to
bring the fields back together – to fuse the two images into a single visual field. Thisbring the fields back together – to fuse the two images into a single visual field. This
accounts for the “dejected” appearance of patients with “pathetic nerve” palsies.accounts for the “dejected” appearance of patients with “pathetic nerve” palsies.
As would be expected, the diplopia gets worse when the affected eye looks toward theAs would be expected, the diplopia gets worse when the affected eye looks toward the
nose – the contribution of the superior oblique muscle to downward gaze is greater innose – the contribution of the superior oblique muscle to downward gaze is greater in
this position. Common activities requiring this type of convergent gaze are readingthis position. Common activities requiring this type of convergent gaze are reading
the newspaper and walking down stairs. Diplopia associated with these activities maythe newspaper and walking down stairs. Diplopia associated with these activities may
be the initial symptom of a fourth nerve palsy.be the initial symptom of a fourth nerve palsy.
Alfred Bielschowsky's head tilt test is a test for palsy of the superior oblique muscleAlfred Bielschowsky's head tilt test is a test for palsy of the superior oblique muscle
caused by damage to cranial nerve IV (trochlear nerve).caused by damage to cranial nerve IV (trochlear nerve).

7. Torsional diplopiaTorsional diplopia
 TheThe superiorsuperior salivatorysalivatory nucleusnucleus contains the cell bodies of parasympatheticcontains the cell bodies of parasympathetic
axons within the nervus intermedius. These fibers reach the geniculateaxons within the nervus intermedius. These fibers reach the geniculate
ganglion but do not synapse. Some of these preganglionic parasympatheticganglion but do not synapse. Some of these preganglionic parasympathetic
fibers persist within thefibers persist within the greatergreater petrosalpetrosal nervenerve as they exit the geniculateas they exit the geniculate
ganglion and subsequently synapse with neurons in theganglion and subsequently synapse with neurons in the pterygopalatinepterygopalatine
ganglionganglion. These postganglionic neurons send. These postganglionic neurons send axonsaxons that providethat provide
parasympathetic innervation to theparasympathetic innervation to the lacrimallacrimal glandgland..
 The remaining preganglionic fibers continue as the mixed facial nerveThe remaining preganglionic fibers continue as the mixed facial nerve
proper as it extends through the facial canal. Before the nerve exits theproper as it extends through the facial canal. Before the nerve exits the
skull via theskull via the stylomastoidstylomastoid foramenforamen and after theand after the nerve to thenerve to the stapediusstapedius
musclemuscle has branched off, the facial nerve gives off thehas branched off, the facial nerve gives off the chordachorda
tympani nervetympani nerve. This nerve exits the skull through the. This nerve exits the skull through the pterygotympanicpterygotympanic
fissurefissure and merges with theand merges with the lingual nervelingual nerve, after which it synapses with, after which it synapses with
neurons in theneurons in the submandibularsubmandibular ganglionganglion. These postganglionic neurons. These postganglionic neurons
provide parasympathetic innervation to theprovide parasympathetic innervation to the submandibularsubmandibular andand
sublingual glandssublingual glands