| Nerve: Trochlear nerve | |
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| Latin | nervus trochlearis |
| MeSH | Trochlear+Nerve |
The trochlear nerve (the fourth cranial nerve, also called the fourth nerve or simply IV) is a motor nerve (a “somatic efferent” nerve) that innervates a single muscle: the superior oblique muscle of the eye. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. Medical Subject Headings ( MeSH) is a huge Controlled vocabulary (or metadata system for the purpose of indexing journal articles and books An older name is pathetic nerve, which refers to the dejected appearance (head bent forward) that is characteristic of patients with fourth nerve palsies.
The trochlear nerve is unique among the cranial nerves in several respects. It is the smallest nerve in terms of the number of axons it contains. It has the greatest intracranial length. It is the only cranial nerve that decussates (crosses to the other side) before innervating its target[1]. Finally, it is the only cranial nerve that exits from the dorsal aspect of the brainstem.
Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks. [2]
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Peripheral anatomy
The trochlear nerve emerges from the dorsal aspect of the brainstem at the level of the caudal mesencephalon, just below the inferior colliculus. In biological anatomy the mesencephalon (or midbrain) comprises the Tectum (or corpora quadrigemini Tegmentum, the ventricular mesocoelia (or "iter" The inferior colliculi ( Latin, lower hills) together with the superior colliculi form the eminences of the Corpora quadrigemina, and also part It circles anteriorly around the brainstem and runs forward toward the eye in the subarachnoid space. In the Central nervous system, the subarachnoid cavity ( subarachnoid space) is the interval between the Arachnoid membrane and Pia mater It passes between the posterior cerebral artery and the superior cerebellar artery, and then pierces the dura just under free margin of the tentorium cerebelli, close to the crossing of the attached margin of the tentorium and within millimeters of the posterior clinoid process. In Human anatomy, the posterior cerebral artery is the Blood vessel that supplies oxygenated Blood to the posterior aspect of the brain ( Occipital The superior cerebellar artery (SCA arises near the termination of the Basilar artery The tentorium cerebelli or cerebellar tentorium ( Latin: "tent of the Cerebellum " is an extension of the Dura mater that separates In the Sphenoid bone, the anterior boundary of the Sella turcica is completed by two small eminences one on either side called the Anterior clinoid processes, while [3] It enters the cavernous sinus, where it is joined by the other two extraocular nerves (III and VI), the internal carotid artery, and portions of the trigeminal nerve (V). The cavernous sinus (or lateral sellar compartment) within the human head is a large collection of thin-walled Veins creating a cavity bordered by the Sphenoid In Human anatomy, the internal carotid artery is a major Artery of the head and neck that helps supply blood to the Brain. The trigeminal nerve (the fifth Cranial nerve, also called the fifth nerve or simply V) is responsible for sensation in the face Finally, it enters the orbit through the superior orbital fissure and innervates the superior oblique muscle. The superior orbital fissure is a Foramen in the skull although strictly it is more of a cleft lying between the lesser and greater wings of the For the abdominal muscle see External oblique muscle The superior oblique muscle, or obliquus oculi superior, is a fusiform Muscle in the
The superior oblique muscle ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. Trochlea means “pulley” in Latin; the fourth nerve is named after this structure.
Actions of the superior oblique muscle
In order to understand the actions of the superior oblique muscle, it is useful to imagine the eyeball as a sphere that is constrained – like the trackball of a computer mouse – in such a way that only certain rotational movements are possible. Allowable movements for the superior oblique are (1) rotation in a vertical plane – looking down and up (depression and elevation of the eyeball) and (2) rotation in the plane of the face (intorsion and extorsion of the eyeball).
The body of the superior oblique muscle is located behind the eyeball, but the tendon (which is redirected by the trochlea) approaches the eyeball from the front. The tendon attaches to the top (superior aspect) of the eyeball at an angle of 51 degrees with respect to the primary position 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 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 eye is adducted (looking toward the nose), the force of depression increases. When the eye is abducted (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), contraction of the superior oblique produces depression and intorsion in roughly equal amounts.
To summarize, the actions of the superior oblique muscle are (1) depression of the eyeball, especially when the eye is adducted; and (2) intorsion of the eyeball, especially when the eye is abducted. The clinical consequences of weakness in the superior oblique (caused, for example, by fourth nerve palsies) are discussed below.
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 superior oblique inserts behind the equator of the eyeball in the frontal plane, so contraction of the muscle also tends to abduct the eyeball (turn it outward). In fact, each of the six extraocular muscles exerts rotational forces in all three planes (elevation-depression, 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 direction of gaze changes. The central control of this process, which involves the continuous, precise adjustment of forces on twelve different tendons in order to point both eyes in exactly the same direction, is truly remarkable.
The recent discovery of soft tissue pulleys in the orbit – similar to the trochlea, but anatomically more subtle and previously missed – has completely changed (and greatly simplified) our understanding of the actions of the extraocular muscles[4]. Perhaps the most important finding is that a 2-dimensional representation of the visual field is sufficient for most purposes.
Central anatomy
The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. In biological anatomy the mesencephalon (or midbrain) comprises the Tectum (or corpora quadrigemini Tegmentum, the ventricular mesocoelia (or "iter" The mesencephalic duct, also known as the aqueductus mesencephali, aqueduct of Sylvius or the cerebral aqueduct, contains Cerebrospinal It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon. The oculomotor nerve is the third of twelve paired Cranial nerves.
The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem. Thus a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei affect the ipsilateral side.
Clinical syndromes
Vertical diplopia
Injury to the trochlear nerve cause weakness of downward eye movement with consequent vertical diplopia (double vision). Diplopia, commonly known as double vision, is the simultaneous Perception of two images of a single object The affected eye drifts upward relative to 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. To compensate 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. This 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 the nose – the contribution of the superior oblique muscle to downward gaze is greater in this position. Common activities requiring this type of convergent gaze are reading the newspaper and walking down stairs. Diplopia associated with these activities may be the initial symptom of a fourth nerve palsy.
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). Alfred Bielschowsky { December 11, 1871 - April 5, 1940) was a German Ophthalmologist.
Torsional diplopia
Trochlear nerve palsy also affects torsion (rotation of the eyeball in the plane of the face). Torsion is a normal response to tilting the head sideways. The eyes automatically rotate in an equal and opposite direction, so that the orientation of the environment remains unchanged – vertical things remain vertical.
Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time. To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.
The characteristic appearance of patients with fourth nerve palsies (head tilted to one side, chin tucked in) suggests the diagnosis, but other causes must be ruled out. For example, torticollis can produce a similar appearance. Torticollis or Requa neck, is a condition in which the Head is tilted toward one side and the Chin is elevated and turned toward the opposite
Causes
The clinical syndromes can originate from both peripheral and central lesions.
Peripheral lesions
A peripheral lesion is a damage to the bundle of nerves, in contrast to a central lesion, which is a damage to the trochlear nucleus. Acute symtoms are probably a result of a trauma or disease, while chronic symptoms probably are congenital.
Acute palsy
The most common cause of acute fourth nerve palsy is head trauma. [5] Even relatively minor trauma can transiently stretch the fourth nerve (by transiently displacing the brainstem relative to the posterior clinoid process). Patients with minor damage to the fourth nerve will complain of “blurry” vision. Patients with more extensive damage will notice frank diplopia and rotational (torsional) disturbances of the visual fields. The usual clinical course is complete recovery within weeks to months.
Isolated injury to the fourth nerve can be caused by any process that stretches or compresses the nerve. A generalized increase in intracranial pressure – hydrocephalus, pseudotumor cerebri, hemorrhage, edema – will affect the fourth nerve, but the abducens nerve (VI) is usually affected first (producing horizontal diplopia, not vertical diplopia). Hydrocephalus (pronunciation ˌhaɪˌdɹoʊˈsɛfələs is a term derived from the Greek words "hydro" meaning water and "cephalus" meaning head and this condition Idiopathic intracranial hypertension (IIH sometimes called benign intracranial hypertension (BIH or pseudotumor cerebri (PTC is a Neurological disorder Bleeding, technically known as hemorrhaging / haemorrhaging (see American and British spelling differences) is the loss of Blood from Oedema (or Edema in American English formerly known as dropsy or hydropsy, is the increase of Interstitial fluid in any organ &mdash swelling The abducens nerve (the sixth cranial nerve, also called the sixth nerve or simply VI) is a “somatic efferent” nerve that controls the movement of a Infections (meningitis, herpes zoster), demyelination (multiple sclerosis), diabetic neuropathy and cavernous sinus disease can affect the fourth nerve, as can orbital tumors and Tolosa-Hunt syndrome. Meningitis is Inflammation of the protective membranes covering the Brain and Spinal cord, known collectively as the Meninges. Herpes zoster (or simply zoster) commonly known as shingles, is a Viral disease characterized by a painful skin rash with Blisters in a limited Multiple sclerosis (abbreviated MS also known as disseminated sclerosis or encephalomyelitis disseminata) is an autoimmune condition in which the Diabetic neuropathies are neuropathic disorders that are associated with Diabetes mellitus. The cavernous sinus (or lateral sellar compartment) within the human head is a large collection of thin-walled Veins creating a cavity bordered by the Sphenoid Tolosa-Hunt syndrome (THS is a rare disorder characterized by severe and unilateral Headaches with extraocular palsies, usually involving the third fourth fifth In general, these diseases affect other cranial nerves as well. Isolated damage to the fourth nerve is uncommon in these settings.
Chronic palsy
The most common cause of chronic fourth nerve palsy is a congenital defect, in which the development of the fourth nerve (or its nucleus) is abnormal or incomplete. Congenital defects may be noticed in childhood, but minor defects may not become evident until adult life, when compensatory mechanisms begin to fail. Congenital fourth nerve palsies are amenable to surgical treatment.
Central lesions
Central damage is a damage to the trochlear nucleus. It affects the contralateral eye. The nuclei of all other cranial nerves affect ipsilateral structures.
The trochlear nucleus and its axons within the brainstem can be damaged by infarctions, hemorrhage, arteriovenous malformations, tumors and demyelination. Bleeding, technically known as hemorrhaging / haemorrhaging (see American and British spelling differences) is the loss of Blood from Cerebral arteriovenous malformation Arteriovenous malformation or AVM in the majority of cases is a Congenital disorder consisting of a connection between A demyelinating disease is any Disease of the Nervous system in which the Myelin sheath of Neurons is damaged Collateral damage to other structures will usually dominate the clinical picture.
The fourth nerve is one of the final common pathways for cortical systems that control eye movement in general. Cortical control of eye movement (saccades, smooth pursuit, accommodation) involves conjugate gaze, not unilateral eye movement. A saccade is a fast movement of an eye, head or other part of an animal's body or device Disorders of conjugate gaze are discussed elsewhere in Wikipedia.
References
- ^ A technical exception to this rule occurs in the nucleus of the third cranial nerve. The oculomotor nuclear complex contains subnuclei for each of the ocular muscles that it innervates. Axons from one of these subnuclei, the medial nucleus, decussate before exiting the nucleus itself. The medial nucleus of the oculomotor nuclear complex controls the contralateral superior rectus muscle. Cf. Aktekin M, Aldur MM, Bayramoglu A, Atasever A, Ozturk AH, Basar R. The organization of the somatic cell nuclei within the oculomotor nuclear complex in rats. Neuroanatomy 1:22-25, 2002
- ^ Maisey JG. Morphology of the Braincase in the Broadnose Sevengill Shark Notorynchus (Elasombranchii, Hexanchiformes), Based on CT Scanning. American Museum Novitates, Number 3429. New York: American Museum of Natural History, 2004
- ^ Bisaria KK. Cavernous portion of the trochlear nerve with special reference to its site of entrance. J. Anat. 159:29-35, 1988
- ^ Demer JL. Pivotal Role of Orbital Connective Tissues in Binocular Alignment and Strabismus. Investigative Ophthalmology and Visual Science. 2004;45:729-738
- ^ Hoya K, Kirino T. Traumatic Trochlear Nerve Palsy Following Minor Occipital Impact. Neurol Med Chir 40:358-360, 2000
Bibliography
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Brodal A. Neurological Anatomy in Relation to Clinical Medicine, 3rd ed. Oxford University Press, 1981
Brodal P. The Central Nervous System, 3rded. Oxford University Press, 2004
Butler AB, Hodos W. Comparative Vertebrate Neuroanatomy, 2nd ed. Wiley-Interscience, 2005
Carpenter MB. Core Text of Neuroanatomy, 4th ed. Williams & Wilkins, 1991
Kandel ER, Schwartz JH, Jessell TM. Principles of Neural Science, 4th ed. McGraw-Hill, 2000
Martin JH. Neuroanatomy Text and Atlas, 3rd ed. McGraw-Hill, 2003
Patten J. Neurological Differential Diagnosis, 2nd ed. Springer, 1996
Ropper, AH, Brown RH. Victor’s Principles of Neurology, 8th ed. McGraw-Hill, 2005
Standring S (ed. ) Gray’s Anatomy, 39th edition. Elsevier Churchill Livingstone, 2005
Wilson-Pauwels L, Akesson EJ, Stewart PA. Cranial Nerves: Anatomy and Clinical Comments. Decker, 1998
Additional images
 Dura mater and its processes exposed by removing part of the right half of the skull, and the brain. |
 Hind- and mid-brains; postero-lateral view. |
 Dissection showing origins of right ocular muscles, and nerves entering by the superior orbital fissure. |
 Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ. |
See also
External links
- NeuroNames hier-449
- NEU59 at FPnotebook
- oph/697 at eMedicine - "Trochlear Nerve Palsy"
- MedEd at Loyola Grossanatomy/h_n/cn/cn1/cn4.htm
- Norman/Georgetown lesson3 (orbit2)
- Norman/Georgetown cranialnerves (IV)
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