Abducens Nerve: What It Is, Characteristics And Associated Disorders
Do you know what cranial nerves are? There are twelve pairs of nerves that leave from the brain to other areas of the body. One of them is the abducens nerve, also called the VI cranial nerve or abducens nerve which we will talk about today.
First we will make a brief introduction about what cranial nerves are to get a bearing, and then we will explain the most relevant characteristics of the abducens nerve (its anatomy, functions, injuries, etc.).
Before talking about the abducens nerve, we are going to make a brief introduction, in which we will see what the cranial nerves are, since the abducens nerve is one of them.
The cranial nerves (also called “cranial pairs”) are twelve pairs of nerves that arise from the brain or brain stem, and are distributed through the different holes that we have at the base of the skull, in different areas: head , neck, thorax and abdomen. That is to say, Their “apparent” origin is found in the area from where they exit or enter the brain but each nerve has its real origin, according to its specific function (each nerve has a specific function for the organism).
The cranial nerves are also named by their numbering (for example nerve I, II, III, IV… of the cranial nerve). On the other hand, each nerve has its nucleus, which is involved in the neurophysiological processes that take place through these sets of nerve cells grouped in the form of a bundle. The nuclei of the nerves are places in which several neuronal cell bodies are piled up (that is, where their genetic material is located) and perform processing and coordination functions of nervous activity.
According to the specific function of each nerve, we find a classification that groups them according to it. Nerves can be: motor, sensory or mixed (this would be their general function). The abducens nerve is purely motor as we will see later, which means that its functions have to do with movement based on the activation of muscle fibers.
Let’s see this classification:
The abducens nerve
The abducens nerve is a nerve that It also has other names, such as abducent nerve, abducens nerve, or cranial nerve VI (sixth cranial nerve). It is a purely motor nerve, which innervates the lateral rectus muscle; This allows the eyeball (eye) to rotate laterally.
Thus, its main function is the movement of the lateral rectus muscle, which allows the abduction of the eye (abduction is the separation of a part of the body from the plane of medial symmetry). In simpler words: at the level of functions, the abducens nerve allows us to move the eye to the sides, in the direction of the temple.
Additionally, the abducens nerve controls eye movements, photomotor reflexes and accommodation Let us remember that photomotor reflexes allow us to control the entry of light into the eye, through the contraction of the pupil when we are faced with a light stimulus.
Anatomy
At an anatomical level, the nucleus of the abducens nerve is located below the top of the 4th ventricle of the brain. Specifically, near the facial colliculus. This core receives input from other muscles, also oculomotor, as well as vestibular nuclei
Its apparent origin is found in the sphenoid fissure, also called superior orbital fissure, which is an area through which many other nerves pass (for example, the frontal, lacrimal, oculomotor, etc.).
Specifically, the abducens nerve arises from a larger area that is the pons, also called the pons, located at the base of the brain. The pons, next to the medulla oblongata, form the brain stem
As we have seen, the origin of the abducens nerve is located in the pons. The pons is an area of the brain, located at its base, that controls motor movements and performs sensory analysis (of sensations)
The main nuclei of this structure are the locus coeruleus (full of noradrenergic neurons), the reticular nucleus (which regulates REM sleep) and the superior olivary complex (which regulates hearing). On the other hand, the pons contains the nuclei of cranial nerves V-VIII (among which is the nucleus of pair VI, that is, the abducens nerve).
Segments
On the other hand, continuing with the anatomy of the abducens nerve, we find that this has different segments (specifically, five). These segments are:
Injuries
The abducens nerve, like all nerves, can be injured in some way. This causes different symptoms, as we will see later (depending on the type of damage and injury). The most common causes of injury to this nerve are (in order from most common to least common): trauma, inflammation, vascular injuries (CVA) and tumors.
When the nerve is stretched, paralysis occurs, and its most common symptoms are: Horizontal binocular diplopia (double vision), muscle spasms and cervicalgia (cervical pain) On the other hand, lesions caused by a tumor generally cause unilateral involvement; Finally, injuries that are associated with other pairs of cranial nerves are not as frequent.
1. Paralysis
What happens when abducens nerve palsy occurs? His paralysis affects the lateral rectus muscle, and also impairs ocular abduction that allows this nerve. This means that the eye of the person affected by said paralysis may be slightly abducted when the person looks straight ahead.
The causes of abducens nerve paralysis can be several: Wernicke’s encephalopathy, nerve infarction, infection, trauma, intracranial hypertension…
Its cause, however, can also be idiopathic (unknown or spontaneous onset). In order to determine the cause, it is necessary to perform a Magnetic Resonance (MRI) in addition to other complementary tests.
2. Mononeuropathy of the abducens nerve
We have talked about abducens nerve paralysis, but there is another pathology of the same that entails different alterations for the individual.
One of them is mononeuropathy, a nerve disorder that affects the function of this nerve (i.e. the nerve is damaged). As a consequence, the person may see double (double vision).
Causes
The causes of abducens nerve mononeuropathy can be diverse (from diseases to syndromes, damage, accidents…). Some of the most frequent are:
Multiple sclerosis (MS)
Infections (for example meningitis)
Brain aneurysms
Cerebrovascular accidents (CVA)
Pregnancy
Trauma
Tumors (for example behind the eye)
Gradenigo syndrome
Diabetes (diabetic neuropathy)
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Figueroa, R., Meléndez, F. and Jiménez, L. (2015). Clinical case. Paralysis of the abducens nerve secondary to Dorello canal meningioma: Anatomical considerations by magnetic resonance imaging. | Abducens nerve palsy secondary to Dorello’s canal meningioma: MRI anatomical considerations. Colombian Neurological Act, 32(1): 61-66.
Kandel, E.R.; Schwartz, J.H.; Jessell, T. M. (2000). Principles of Neural Science. New York: McGraw-Hill.
Latarjet, M. & Ruiz, A. (2004). Clinical Neuroanatomy Nerve abducens (4th Ed.). Panamericana Medical Editorial.
MedlinePlus. (2019). Mononeuropathy of the sixth cranial nerve.
