The trochlear nerve (), ( lit. ''pulley-like'' nerve) also known as the fourth cranial nerve, cranial nerve IV, or CN IV, is a

cranial nerve Cranial nerves are the nerves that emerge directly from the brain (including the brainstem), of which there are conventionally considered twelve pairs. Cranial nerves relay information between the brain and parts of the body, primarily to and f ...

that innervates a single muscle - the

superior oblique muscle The superior oblique muscle or obliquus oculi superior is a fusiform muscle originating in the upper, medial side of the orbit (anatomy), orbit (i.e. from beside the nose) which abducts, depresses and internally rotates the eye. It is the only e ...

of the eye (which operates through the

pulley Sheave without a rope A pulley is a wheel on an axle or shaft enabling a taut cable or belt passing over the wheel to move and change direction, or transfer power between itself and a shaft. A pulley may have a groove or grooves between flan ...

-like trochlea). Unlike most other cranial nerves, the trochlear nerve is exclusively a

motor nerve A motor nerve, or efferent nerve, is a nerve that contains exclusively efferent nerve fibers and transmits motor signals from the central nervous system (CNS) to the effector organs (muscles and glands), as opposed to sensory nerves, which transf ...

( somatic efferent nerve). 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 exits from the dorsal (rear) aspect of the

brainstem The brainstem (or brain stem) is the posterior stalk-like part of the brain that connects the cerebrum with the spinal cord. In the human brain the brainstem is composed of the midbrain, the pons, and the medulla oblongata. The midbrain is conti ...

. * It innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus. The trochlear nerve

decussates Decussation is used in biological contexts to describe a crossing (due to the shape of the Roman numeral for ten, an uppercase 'X' (), ). In Latin anatomical terms, the form is used, e.g. . Similarly, the anatomical term chiasma is named aft ...

within the brainstem before emerging on the contralateral side of the brainstem (at the level of the

inferior colliculus The inferior colliculus (IC) (Latin for ''lower hill'') is the principal midbrain nucleus of the Auditory system, auditory pathway and receives input from several peripheral brainstem nuclei in the auditory pathway, as well as inputs from the aud ...

). An injury to the trochlear nucleus in the brainstem will result in an contralateral superior oblique muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral superior oblique muscle palsy. The superior oblique muscle which the trochlear nerve innervates 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 thus also named after this structure. The words ''trochlea'' and ''trochlear'' (, ) come from

Ancient Greek Ancient Greek (, ; ) includes the forms of the Greek language used in ancient Greece and the classical antiquity, ancient world from around 1500 BC to 300 BC. It is often roughly divided into the following periods: Mycenaean Greek (), Greek ...

''trokhiléa'', “pulley; block-and-tackle equipment”.

Structure

The trochlear nerve provides motor innervation to the

of the eye, a skeletal muscle; the trochlear nerve thus carries axons of general somatic efferent type.

Course

Each trochlear nerve originates from a trochlear nucleus in the medial

midbrain The midbrain or mesencephalon is the uppermost portion of the brainstem connecting the diencephalon and cerebrum with the pons. It consists of the cerebral peduncles, tegmentum, and tectum. It is functionally associated with vision, hearing, mo ...

. From their respective nuclei, the two trochlear nerves then travel dorsal-ward through the substance of the midbrain surrounded by the

periaqueductal gray The periaqueductal gray (PAG), also known as the central gray, is a brain region that plays a critical role in autonomic function, motivated behavior and behavioural responses to threatening stimuli. PAG is also the primary control center for ...

, crossing over (decussating) within the midbrain before emerging from the dorsal midbrain just inferior to the

. Each trochlear nerve thus comes to course on the contralateral side, first passing laterally (to the side) and then anteriorly around the

pons The pons (from Latin , "bridge") is part of the brainstem that in humans and other mammals, lies inferior to the midbrain, superior to the medulla oblongata and anterior to the cerebellum. The pons is also called the pons Varolii ("bridge of ...

, then running forward toward the eye in the subarachnoid space. It passes between the

posterior cerebral artery The posterior cerebral artery (PCA) is one of a pair of cerebral arteries that supply oxygenated blood to the occipital lobe, as well as the medial and inferior aspects of the temporal lobe of the human brain. The two arteries originate from the d ...

and the

superior cerebellar artery The superior cerebellar artery (SCA) is an artery of the head. It arises near the end of the basilar artery. It is a branch of the basilar artery. It supplies parts of the cerebellum, the midbrain, and other nearby structures. It is the cause of t ...

. It then pierces the dura just under free margin of the

tentorium cerebelli The cerebellar tentorium or tentorium cerebelli (Latin for "tent of the cerebellum") is one of four dural folds that separate the cranial cavity into four (incomplete) compartments. The cerebellar tentorium separates the cerebellum from the cere ...

, close to the crossing of the attached margin of the tentorium and within millimeters of the

posterior clinoid process The posterior clinoid processes are the tubercles of the sphenoid bone situated at the superior angles of the dorsum sellae (one on each angle) which represents the posterior boundary of the sella turcica. They vary considerably in size and form. ...

.Bisaria KK. "Cavernous portion of the trochlear nerve with special reference to its site of entrance". ''J. Anat.'' 159:29–35, 1988 It runs on the outer wall of the

cavernous sinus The cavernous sinus within the human head is one of the dural venous sinuses creating a cavity called the lateral sellar compartment bordered by the temporal bone of the skull and the sphenoid bone, lateral to the sella turcica. Structure The ...

. Finally, it enters the orbit through the superior orbital fissure and to innervate the

Development

The human trochlear nerve is derived from the basal plate of the

embryo An embryo ( ) is the initial stage of development for a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sp ...

nic

Clinical significance

Vertical diplopia

Injury to the trochlear nerve cause weakness of downward eye movement with consequent vertical

diplopia Diplopia is the simultaneous perception of two images of a single object that may be displaced in relation to each other. Also called double vision, it is a loss of visual focus under regular conditions, and is often voluntary. However, when occ ...

(double vision). 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.

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.

Causes

The clinical syndromes can originate from both peripheral and central lesions.

Peripheral lesion

A peripheral lesion is damage to the bundle of nerves, in contrast to a central lesion, which is damage to the trochlear nucleus. Acute symptoms are probably a result of trauma or disease, while chronic symptoms probably are congenital.

=Acute palsy

= The most common cause of ''acute'' fourth nerve palsy is head trauma. 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 Hydrocephalus is a condition in which cerebrospinal fluid (CSF) builds up within the brain, which can cause pressure to increase in the skull. Symptoms may vary according to age. Headaches and double vision are common. Elderly adults with n ...

, pseudotumor cerebri,

hemorrhage Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, ...

edema Edema (American English), also spelled oedema (British English), and also known as fluid retention, swelling, dropsy and hydropsy, is the build-up of fluid in the body's tissue (biology), tissue. Most commonly, the legs or arms are affected. S ...

—will affect the fourth nerve, but the

abducens nerve The abducens nerve or abducent nerve, also known as the sixth cranial nerve, cranial nerve VI, or simply CN VI, is a cranial nerve in humans and various other animals that controls the movement of the lateral rectus muscle, one of the extraocula ...

(VI) is usually affected first (producing horizontal diplopia, not vertical diplopia). Infections (

meningitis Meningitis is acute or chronic inflammation of the protective membranes covering the brain and spinal cord, collectively called the meninges. The most common symptoms are fever, intense headache, vomiting and neck stiffness and occasion ...

, herpes zoster), demyelination (

multiple sclerosis Multiple sclerosis (MS) is an autoimmune disease resulting in damage to myelinthe insulating covers of nerve cellsin the brain and spinal cord. As a demyelinating disease, MS disrupts the nervous system's ability to Action potential, transmit ...

diabetic neuropathy Diabetic neuropathy includes various types of nerve damage associated with diabetes mellitus. The most common form, diabetic peripheral neuropathy, affects 30% of all diabetic patients. Studies suggests that cutaneous nerve branches, such as the s ...

and

disease can affect the fourth nerve, as can orbital tumors and Tolosa–Hunt syndrome. 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 lesion

Central damage is damage to the trochlear nucleus. It affects the ''contralateral'' eye. The nuclei of other cranial nerves generally affect ''ipsilateral'' structures (for example, the optic nerves - cranial nerves II - innervate both eyes). The trochlear nucleus and its axons within the brainstem can be damaged by infarctions,

arteriovenous malformation An arteriovenous malformation (AVM) is an abnormal connection between arteries and veins, bypassing the capillary system. Usually congenital, this vascular anomaly is widely known because of its occurrence in the central nervous system (usually ...

s, tumors and demyelination. 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.

Clinical assessment

The trochlear nerve is tested by examining the action of its muscle, the superior oblique. When acting on its own this muscle depresses and abducts the eyeball. However, movements of the eye by the extraocular muscles are

synergistic Synergy is an interaction or cooperation giving rise to a whole that is greater than the simple sum of its parts (i.e., a non-linear addition of force, energy, or effect). The term ''synergy'' comes from the Attic Greek word συνεργία ' f ...

(working together). Therefore, the trochlear nerve is tested by asking the patient to look 'down and in' as the contribution of the superior oblique is greatest in this motion. 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).

Other animals

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.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

References

Bibliography

*Blumenfeld H. Neuroanatomy Through Clinical Cases. Sinauer Associates, 2002 *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

Image:Gray567.png, Dura mater and its processes exposed by removing part of the right half of the skull, and the brain. Image:Gray719.png, Hind- and mid-brains; postero-lateral view. Image:Gray787.png, Dissection showing origins of right ocular muscles, and nerves entering by the superior orbital fissure. Image:Gray792.png, Upper part of medulla spinalis and hind- and mid-brains; posterior aspect, exposed in situ. File:Slide2ior.JPG, Trochlear nerve.Deep dissection.Superior view.

External links

* * - "Trochlear Nerve Palsy" * * () * ()
Animations of extraocular cranial nerve and muscle function and damage (University of Liverpool)

Trochlear nerve
at Neurolex {{DEFAULTSORT:Trochlear Nerve Cranial nerves Human head and neck Nervous system Neurology Nerves of the head and neck Ophthalmology