Trochlear and abducens nerves
Roughly at the age of two months, an individual begins to fix and follow items with their eyes. Even before that, they can sometimes be seen gazing around the room as they explore this new environment. The ability to follow an object by moving the eyes and without shifting the position of the head or body is facilitated by the extraocular muscles (i.e. the muscles that are attached externally to the eyeball). These six muscles (four recti and two obliques) are innervated by cranial nerves III, IV, and VI. This article, however, focuses on the embryology, anatomy, function, and clinical examination of the trochlear (CN IV) and abducens (CN VI) nerves. It is not uncommon to encounter an alternative spelling for CN VI (abducent nerve). Both are recognized in the medical literature.
These two nerves, along with the oculomotor (CN III) nerve, interact with each other via the medial longitudinal fasciculus. Therefore, the nerves indirectly facilitate a combination of eye movements so that the individual can gather as much visual input as possible. Both the trochlear and abducens nerves provide general somatic efferent (motor) innervation to their respective target muscles. Of note, general somatic efferent fibers are those that arise from lower motor neurons arising from either the ventral horn of the grey matter or an analogous area in the brainstem.
|Type||General somatic efferent (GSE)|
Trochlear nerve (CN IV) to the superior oblique muscle
Abducens nerve (CN VI) to the lateral rectus muscle
Four rectus muscles: lateral, medial, superior, and inferior
Two oblique muscles: inferior and superior
|Medial longitudinal fasciculus||Relay axons between the oculomotor (CN III), trochlear (CN IV), and the abducens (CN VI) nerves|
|Location of nuclei||
Trochlear nerve (CN IV) – Level of the inferior colliculus
Abducens nerve (CN VI) – Dorsal pons, the floor of 4th ventricle
|Parts of nerve||Nucleus
|Disorders of the trochlear nerve (CN IV)||Torsional and Vertical Diplopia
Fourth Nerve Palsy (Acute & Chronic)
|Disorders of the abducens nerve (CN VI)||Esotropia & Diplopia
Wernicke-Korsakoff and Tolusa-Hunt Syndromes
Nuclear and Supranuclear Lesions
|Mnemonic for innervation of extraocular muscles||
SO4 – Superior oblique by CN IV
LR6 – Lateral rectus by CN VI
AO3 – All others by CN III
To better understand the discussion of these nerves that regulate visual movement, this article will also briefly review the extraocular muscles. There will also be a discussion regarding different types of eye movement and clinical examination of both nerves.
- Embryology of the trochlear and abducens nerves
- Anatomy of the trochlear nerve
- Anatomy of the abducens nerve
- The extraocular muscles
- Types of eye movements
- Clinical application
Embryology of the trochlear and abducens nerves
The trochlear (CN IV) and abducens (CN VI) nerves are derivatives of the somatic efferent column of the basal plates of the brainstem. As such, they are pure motor nerves that are responsible for carrying general somatic efferent impulses to their end target organs. Therefore they are similar to the nerves arising from the ventral spinal roots, which are also spinal basal plate derivatives. Both nerves will eventually travel towards the preoptic myotomes (embryonic origins of the extraocular muscles), where they will innervate their respective extraocular muscle.
The cells of the trochlear nerve develop in the dorsal and most inferior aspect of the midbrain. They can be identified in older fetuses (around 20 weeks) as large multipolar neurons. While there is no clear discontinuity in the somatic efferent column to clearly separate the superiorly related oculomotor nerve (CN III) nucleus from CN IV, there is an area of reduced cellular density that marks the transition from one nucleus to the other. More often than not, the CN IV nucleus occurs as a solitary mass of cells. However, there may be an additional, caudally related secondary cluster of cells that accompany the CN IV nucleus. When the additional caudal group is present, it may be bilateral or unilateral. In the latter scenario, there is no predilection for whether or not it will be on the right or the left. It is still unclear as to whether or not these additional cell bodies provide added function to the trochlear nerve. This may occur as a solitary group (on either the left or right) or as a bilateral pair.
The cell bodies that form the abducens nerve are mesencephalic derivatives. They also arise from the basal plates of the brain stem in the caudal aspect of the pons. Each nucleus eventually becomes surrounded by the genu of the facial nerve (CN VII).
Anatomy of the trochlear nerve
The trochlear nerve (CN IV) is a midbrain structure located at the level of the inferior colliculus. It is caudally related to another cranial nerve that innervates the remaining extraocular muscles called the oculomotor nerve (CN III). While most other motor nerves emerge from the midbrain and brainstem anterolaterally, CN IV first projects dorsally decussate within the midbrain, and then leaves the brainstem dorsally. The nerve eventually takes an anterior route to reach its target muscle (superior oblique muscle), which is derived from the second preoptic myotome. Take a look at the next article and video tutorial to find out everything about the trochlear nerve's place of origin - the brainstem.
Trochlear nerve nucleus and intraparenchymal portion
The nucleus of the trochlear nerve (CN IV) is located within the ventral aspect of the periaqueductal grey matter of the midbrain. The cells are small and oval-shaped and are partially embedded within the dorsal aspect of the medial longitudinal fasciculus.
The axons arising from these cell bodies coalesce at the dorsolateral boundary of the CN IV nucleus, near its inferior surface. The fibers continue obliquely through the periaqueductal grey matter toward the lateral boundary of the region. They continue in a downward course, traveling nearly parallel to the mesencephalic division of the trigeminal nerve (CN V). Once the nerve bundles begin to approach the superior medullary velum, they begin to converge.
The fibers subsequently decussate prior to emerging from the dorsal aspect of the brainstem at the superior medullary velum. Therefore each superior oblique muscle is innervated by the contralateral CN IV nucleus.
Furthermore, the CN IV nucleus receives bilateral corticobulbar innervation that regulates its activity. It also interacts with the tectobulbar tract that is responsible for integrating the information processed in the visual cortex with the superior colliculus. The medial longitudinal fasciculus acts as a communication pathway and the other two visual motor nerves (oculomotor nerve (CN III) and abducens nerve (CN VI)) and the vestibulocochlear nerve (CN VIII).
Cisternal portion of trochlear nerve
Recall that a cistern is a communicating subarachnoid dilatation within the brain that contains cerebrospinal fluid. The quadrigeminal cistern (also known as the superior or ambient cistern) is dorsal to the midbrain and is divided into supratentorial and infratentorial parts.
The trochlear nerve (CN IV) is the only cranial nerve to exit the brainstem from its posterior surface. As the nerve bundle emerges from the brainstem, it enters the infratentorial part of the quadrigeminal cistern. Other structures in this cistern include the great cerebral vein of Galen and the superior cerebellar and posterior cerebral arteries.
The nerve then continues laterally as it protrudes from the base of the inferior colliculus and wraps around the crus cerebri. It then travels anteriorly and superiorly, where it pierces the dura mater at its junction with the free edge of the tentorium cerebelli.
Along its course in the quadrigeminal cistern, CN IV passes between the posterior cerebral and superior cerebellar arteries prior to entering the cavernous sinus with its fellow visual motor nerve, the oculomotor nerve (CN III).
Cavernous portion of trochlear nerve
Within the middle cranial fossa the trochlear nerve (CN IV) is inferiorly related to the oculomotor nerve (CN III), and superiorly related to the trigeminal (CN V) and abducens (CN VI) nerves. CN IV pierces the lateral wall of the cavernous sinus as it travels anteriorly toward the orbit.
In this position, it maintains the inferior relationship to CN III and superior relationship to the ophthalmic branch of the trigeminal nerve (CN V1). Medial to CN IV is the cavernous part of the internal carotid artery, CN VI, and the pituitary gland. CN IV leaves the cavernous sinus via the superior orbital fissure. As it approaches this bony tunnel, it moves superiorly and is now superiorly related to CN III.
Intraorbital portion of trochlear nerve
As the trochlear nerve (CN IV) enters the orbit of the eye, it is found outside the common tendinous ring (i.e. the annular tendon or annulus of Zinn). It continues along an anterosuperior course toward the superior oblique muscle.
As it travels along this path, the nerve is superiorly related to the ophthalmic branch of the trigeminal nerve (CN V1), and medially related to the superior ophthalmic vein. The nerve then moves superomedially as it passes above the levator palpebrae superioris and the superior rectus muscles to reach the lateral border of the superior oblique muscle.
Learn more about the nerves of the orbit in the following study unit!
Function of the Trochlear nerve
This pure motor nerve is responsible for delivering efferent stimuli to the superior oblique muscle. Recall that the superior oblique muscle is attached to the upper outer quadrant on the posterolateral surface of the globe of the eye. It then passes through a sling, or trochlea, located on the roof of the orbit, before coursing posteriorly to insert in the common tendinous ring. Therefore, when the muscle contracts, the eyeball moves downwards and outwards. Therefore, the trochlear nerve (CN VI) is responsible for inward rotation, depression, and abduction of the eyeball contralateral to the nucleus.
|Embryology||The somatic efferent column in the inferior midbrain|
|Nucleus||Inferior to oculomotor (CN III) nucleus
Within the periaqueductal grey matter
Embedded in the medial longitudinal fasciculus
|Intraparenchymal part||Oblique course through the periaqueductal grey matter
Travels around the peripheral boundary of periaqueductal grey matter
Decussates beneath the superior medullary velum
Emerges from the posterior surface of the brainstem below the inferior colliculus
|Cisternal part||Enters quadrigeminal cistern
Wraps around crus cerebri
Ascends as it travels anteriorly
Pierces the dura mater to enter the cavernous sinus
|Cavernous part||The lateral wall of the cavernous sinus
Inferior to oculomotor nerve (CN III) and superior to ophthalmic branch of the trigeminal nerve (CN V1)
Exits skull through superior orbital fissure
|Intraorbital part||Travels lateral to common tendinous ring
Superior to oculomotor nerve (CN III)
|Target muscle||Superior oblique muscle|
Inward rotation of the eye
Depression of eye
Abduction of eye
Anatomy of the abducens nerve
The abducens (also called abducent) nerve (CN VI) is the last of the three visual motor nerves. It is a brainstem structure that is located in the dorsal aspect of the pons, deep to the facial colliculus in the 4th ventricle (rhomboid fossa). As the fibers of CN VI emerge from the pons, they travel ventrally, to leave the brain parenchyma. It eventually arrives at the preoptic region where it will innervate the lateral rectus muscle, which arises from the third preoptic myotome.
Abducens nucleus and intraparenchymal portion
The nucleus of the abducens nerve (CN VI) is composed of spherical primary motor neurons that are partially circumscribed by the genu of the facial nerve (CN VII). Additionally, fibers of the paramedian pontine reticular formation (PPRF) and the medial longitudinal fasciculus, also surround the CN VI nucleus. In addition to the primary motor neurons, there are also interneurons located within the substance of the nucleus that facilitate communication between CN VI nucleus and the contralateral oculomotor nerve (CN III) via the medial longitudinal fasciculus.
CN VI is caudally related to both the sensory and motor nuclei of the trigeminal nerve (CN V). The nucleus tractus solitarius (solitary nucleus and tract) is ventral to CN VI nucleus, while the vestibular and facial nuclei are laterally related to CN VI nucleus.
The axons arising from the motor neurons of CN VI coalesce near the inferior border of the nucleus. They then continue inferiorly, anteriorly, and laterally as it continues its intraparenchymal journey. The nerve becomes medially related to the superior olivary nucleus. They also travel alongside the spinal tract of CN V and through the substance of the corticobulbar fibers. The nerve then approaches the pontomedullary junction (i.e. the inferior pontine sulcus) where it will emerge from the ventral surface of the brainstem.
Much like the trochlear nerve (CN IV) nucleus, the CN VI nucleus receives bilateral corticobulbar innervation to regulate its activity. It is also regulated by the tectobulbar tract originating from the superior colliculus in order to coordinate visual input with ocular motion. Another similarity between CN IV and CN VI is that they – along with the nuclei of CN III and vestibulocochlear nerve (CN VIII) – are connected by the fibers of the medial longitudinal fasciculus.
Cisternal portion of the abducens nerve
As the abducens nerve (CN VI) leaves the brainstem at the inferior pontine sulcus, it is superiorly related to the medullary pyramids, medially related to facial nerve (CN VII), and inferolaterally related to the basilar groove of the pons. CN VI enters the pontine cistern, where it shares the space with the basilar artery and the cerebellar vessels.
It continues anteriorly for a short distance until it meets the clivus. Here it begins the superior part of its journey as it ascends along the contour of the clivus. Near the apex of the petrous part of the temporal bone, the nerve fibers pierce the dura mater in order to gain access to the canal of Dorello (i.e. beneath the petroclinoid ligament of Gruber); here it is accompanied by the inferior petrosal sinus.
Cavernous portion of the abducens nerve
The abducens (CN VI) nerve fibers leave the canal of Dorello and enter the cavernous sinus. Unlike the other cranial nerves within this sinus, CN VI is the only one to travel in the middle of the sinus. The nerve continues anteriorly through the sinus, being inferolaterally related to the horizontal segment of the cavernous part of the internal carotid artery.
Sympathetic branches that originally accompanied the internal carotid artery also join CN VI for a brief part of the journey. Of note, the other cranial nerves – oculomotor nerve (CN III), trochlear nerve (CN IV), opthalmic branch of the trigeminal nerve (CN V1), and maxillary branch of the trigeminal nerve (CN V2) – are laterally related to CN VI within the cavernous sinus.
Intraorbital portion of the abducens nerve
The abducens nerve (CN VI) leaves the cavernous sinus by way of the superior orbital fissure. It passes through the common tendinous ring (i.e. annular tendon or annulus of Zinn) below the inferior division of the oculomotor nerve (CN III). Therefore it is the most inferior structure that passes through the common tendinous ring. As the nerve enters the orbit, it continues to the medial aspect of the lateral rectus muscle, which it pierces and innervates.
Function of the abducens nerve
The abducens nerve (CN VI) is responsible for the motor innervation of the lateral rectus muscle. Therefore, the nerve’s primary function is to abduct or move the eye towards the temporal field in the horizontal plane. However, CN VI also facilitates a phenomenon known as conjugate eye movement. This process ensures that both eyes move in the same direction on the horizontal plane (i.e. to the left or right). Note that the lateral rectus of the left eye would turn the eye to the left, while the same muscle of the right eye would shift that eye to the right.
Therefore, in the absence of conjugate gaze, the eyes would diverge and the ability to focus on an image would be challenging. Therefore, CN VI not only supplies the ipsilateral lateral rectus muscle but also influences the contralateral medial rectus muscle. This is made possible by the internuclear neurons found in the CN VI nucleus. They form synapses between the motor neurons of CN VI nucleus with the fibers of the medial longitudinal fasciculus. The medial longitudinal fasciculus then synapses with oculomotor (CN III) nucleus, which innervates the medial rectus muscle.
|Embryology||The somatic efferent column in the inferior pons|
|Nucleus||Beneath the facial colliculus of the 4th ventricle
Encircled by the genu of the facial nerve (CN VII)
Partly embedded in the paramedian pontine reticular formation (PPRF) and the medial longitudinal fasciculus (MLF)
|Intraparenchymal part||Travels anteriorly, inferiorly, and slightly laterally
Passes the superior olivary nucleus
Adjacent to the spinal tract of the trigeminal nerve (CN V) and corticobulbar tracts
Emerges at the pontomedullary junction
Medial to facial nerve (CN VII) and vestibulocochlear nerve (CN VIII)
Superior to the medullary pyramids
Inferolateral to basilar groove
Enters pontine cistern
Ascends the clivus
Traverses Dorello’s canal
Pierces the dura mater
|Cavernous part||Enters the middle of the cavernous sinus
Adjacent to the horizontal part of the cavernous internal carotid artery
Related to sympathetic fibers from the artery
Medial to oculomotor nerve (CN III), trochlear nerve (CN IV), the ophthalmic branch of trigeminal nerve (CN V1), and maxillary branch of trigeminal nerve (CN V2)
Exits skull through the superior orbital fissure
|Intraorbital part||Enters the common tendinous ring (annulus of Zinn)
Most inferior structure within the common tendinous ring
|Target muscle||Lateral rectus muscle|
|Function||Abduction of the eye|
The extraocular muscles
Movement of the eyes in response to an auditory or visual stimulus is activated by intricate pathways between the visual and auditory centers. However, the actual movement of the eyes is made possible by the action of the extraocular muscles.
There are six such muscles found within the orbit; four are unidirectional muscles known as the recti muscles, while the remaining two are the oblique muscles that can perform multi-directional activities. All extraocular muscles work in various combinations to provide a relatively wide variety of eye movements. These muscles, along with their primary function are listed below:
- The lateral rectus muscle is responsible for moving the eye horizontally towards the temporal side of the face. In other words, it abducts the eye.
- In contrast, the medial rectus muscle moves the eye towards the nasal visual field in the horizontal plane. Therefore, it adducts the eye.
- The superior rectus moves the eye up in the vertical plane. Hence, it elevates the eye.
- The inferior rectus does the opposite and pulls the eye downward in the vertical plane. So it depresses the eye.
- The superior oblique causes inward rotation (intorsion), abduction, and depression of the eye.
- On the other hand, the inferior oblique results in an outward rotation (extorsion), abduction, and elevation of the eye.
Types of eye movements
While each ocular motor cranial nerve is responsible for particular extraocular muscles, there are numerous central polysynaptic connections that regulate their activities. Eye movements can be voluntary (as is the case for saccadic and smooth pursuit movements) and involuntary (e.g. vergence eye and vestibular movements).
The frontal eye fields, which are located anterior to the primary motor cortex, play an important part in the regulation of voluntary eye movements. Structurally, it is considered as a part of Brodmann area 8. Each frontal eye field also participates in the initiation of conjugate gaze (described earlier in Functions of the Abducens Nerve) to the contralateral side. Voluntary eye movement depends heavily on the integration of multiple pathways. There are a wide variety of eye movements, however, the four basic kinds of ocular motions are discussed below.
Saccadic eye movements
At times it is necessary to rapidly capture the overall picture of the environment in a single glance. The eyes rapidly scan the area using saccadic eye movements, or saccades. Essentially, saccades are rapidly alternating eye movements (where the eyes are moving anywhere between 200 to 700 degrees per second) from one point to the other.
The eyes stop only for a short period of time during which the fovea will concentrate on these points. These same movements are also engaged while reading as well. These bursts of horizontal movements of the eye often result in sudden changes in fixed visual points. Although this is largely a voluntary reflex, this visual response can also be activated involuntarily as is the case during REM sleep.Since we've mentioned reading, you have probably heard about the speed reading and thought how great would it be if you could read hundreds of pages within dozens of minutes. Take a look at this interesting article about this technique and find out which part is a myth, and which part you could actually use.
Smooth pursuit movements
The ability to fix and follow an object in the field of vision is referred to as smooth pursuit. It is the kind of eye movement that occurs when an individual sees something or someone that is aesthetically appealing. Not only does this activity maintain focus on an item of interest, but it also requires proprioceptive input regarding the head. In smooth pursuit, the eyes move at a much slower rate (30 to 100 degrees per second).
Information about the direction in which the object of interest is traveling and its speed is conveyed to neurons located in the midtemporal and lateral parietal cortices. There is an intricate relay system established among the frontal eye field, midtemporal, and lateral parietal parts of the cortex, the flocculus, and paraflocculus of the cerebellum, the dorsolateral pons, and the vestibular nuclei in order to accurately determine head and eye position in relation to the moving object.
Furthermore, smooth pursuit contributes to establishing and maintaining the balance of the body. Most individuals require a moving target in order to perform a smooth pursuit, otherwise they end up executing a relatively slow saccade. We know that the CNS is a large unit, so if you need to recall the anatomy of some of the mentioned structures in order to get a full picture - the cortex, cerebellum, pons, vestibular nuclei - then go through these articles and watch these great video tutorials.
Clinically, smooth pursuit can be assessed by asking the patient to sit and observe a projection of vertical bars moving in the horizontal plane (a part of the optokinetic test). The patient picks a bar, fixes and follows it until it arrives at the end of the screen. Reflexively, the eyes stop and move horizontally in the opposite direction to fix on the bar at the beginning of the screen. Following this saccadic activity, the eyes return to smooth pursuit as they follow the bar throughout its course.
The rapid switch between slow smooth pursuit and rapid saccadic movements is referred to as optokinetic nystagmus. This is a normal physiological response and should not be misinterpreted as a pathological nystagmus.
Vergence is a form of disjunctive (or disconjugate) movement of the eyes that allows each eye to fixate on objects at different special points from the patient. This is significantly different from the other types of ocular movements where both eyes move in the same direction (i.e. conjugate movements).
Vergence movements can be either convergence (where the eyes attempt to fix on a nearby object) or divergence (where the eyes are focusing on a distant object). Convergence, along with pupillary constriction and lens accommodation, make up the three entities that facilitate refocusing on a nearby object after looking at something far away.
Vestibular eye movements
Vestibular eye (also called vestibulo-ocular) movements are responsible for keeping the eyes stable with respect to the outside environment. This allows the eyes to remain relatively still irrespective of changes in head position. Furthermore, it allows images to remain fixed on the fovea even though there may be changes in the position of the head. Whenever the eyes are fixed and the head moves, the extraocular muscles accommodate for this change by shifting the eyes equidistance in the opposite direction.
Recall that the vestibular part of the vestibulo-ocular reflex is mitigated by the vestibular division of the vestibulocochlear nerve (CN VIII). Changes in the contents of the semicircular canals of the inner ear that result from positional changes of the head are transmitted via the vestibular part of CN VIII. This information is then relayed to the medial vestibular nucleus in the upper part of the medulla oblongata. Some of the second-order neurons that leave the nucleus decussate and transmit excitatory signals to the contralateral abducens nucleus and the rostral interstitial nucleus of the medial longitudinal fasciculus, and inhibitory signals to the ipsilateral nuclei.
The neurons of the rostral interstitial nucleus of the medial longitudinal fasciculus subsequently decussate and synapses on the contralateral CN III nucleus. Therefore, each medial vestibular nucleus causes excitation of the ipsilateral CN III and contralateral CN VI but inhibits the ipsilateral CN VI and contralateral CN III.
In a practical sense, when an individual turns their head to the left, this activates the sensory receptors of the left horizontal canal. The impulse is then transmitted to the left medial vestibular nucleus. Excitatory signals are sent to the contralateral CN VI (which activates the right lateral rectus muscle) and the ipsilateral CN III (which activates the left medial rectus muscle). Therefore, the eyes are turned to the right to compensate for the head tilt to the left. The opposite is true if the head is tilted to the right.
|Saccadic eye movement||Brisk horizontal motions to rapidly scan|
|Smooth pursuit||Slow horizontal motions to fix and follow an object|
|Vergence||Diverging from or converging on an object|
|Vestibulo-ocular movement||Repositioning of the eyes in response to head movement|
A clear understanding of how the extraocular muscles work aids a clinician in detecting deviations from the norm. Disorders of the trochlear (CN IV) and abducens (CN VI) nerves can arise from the level of the nucleus (nuclear lesions), above the level of the nucleus (supranuclear lesions), or in the periphery.
There are also syndromic disorders, nutritional imbalances, and toxic entities that may also disrupt the normal functioning of these nerves. Patients with defective CN IV or CN VI may complain of diplopia (double vision), or symptoms of a cerebrovascular accident (stroke). The clinician may elicit signs in-keeping with central or peripheral lesions (i.e. nystagmus, strabismus, pathological saccades, loss of the doll’s-eye reflex, etc.). A detailed discussion regarding these and other disorders can be found in a subsequent article.
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