Abducens nerve: what it is, characteristics and associated disorders

Do you know what the cranial nerves are? There are twelve pairs of nerves that leave the brain to other areas of the body. One of them is the abducens nerve, also called the sixth cranial nerve or oculomotor nerve, which we will talk about today.

First we will make a brief introduction about what the cranial nerves are to situate ourselves, and later we will explain the most relevant characteristics of the abducens nerve (its anatomy, functions, injuries, etc).

• Related article: "Cranial nerves: the 12 nerves that exit the brain"

cranial nerves

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 nerves”) are twelve pairs of nerves that arise from the brain or brainstem, and that 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 in the area from which they exit or enter the brain

, but each nerve has its real origin, according to its specific function (each nerve has a certain function for the organism).

The cranial nerves are also named through their numbering (for example nerve I, II, III, IV... of the cranial nerve). On the other hand, each nerve has its nucleus, which intervenes in the neurophysiological processes that take place through these sets of nerve cells grouped in the form of a bundle. Nerve nuclei are places where various neuronal cell bodies are clustered together (i.e., where finds their genetic material) and perform functions of processing and coordinating the activity nervous.

Classification

According to the specific function of each nerve, we find a classification that groups them accordingly. The nerves can be: motor, sensitive 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 from the activation of muscle fibers.

Let's see this classification:

• Special sensory afferents: the pairs I, II and VIII are in charge.
• Control of eye movements, photomotor reflexes and accommodation: pairs III, IV and VI (where we find the abducens nerve, which is pair VI).
• “Pure” motor function): pairs XI and XII.
• Mixed function (sensory and motor): pairs V, VII, IX and X.
• Function of carrying parasympathetic fibers: pairs III, VII, IX and X.

the abducens nerve

The abducens nerve is a nerve that It is also called by other names, such as the abducens nerve, oculomotor 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 abduction of the eye (abduction is the separation of a part of the body from the medial plane of symmetry). In simpler words: at the functional level, the abducens nerve allows us to move the eye to the sides, in the direction of the temple.

Also, the abducens nerve controls eye movements, light reflexes, and accommodation. 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

Anatomically, the nucleus of the abducens nerve is located below the upper part of the 4th ventricle of the brain. Specifically, near the facial colliculus. this core receives outputs from other muscles, also oculomotor, as well as from vestibular nuclei.

Its apparent origin is in the sphenoidal fissure, also called superior orbital fissure, which is an area through which many other nerves pass (for example the frontal, the lacrimal, the oculomotor…).

Specifically, the abducens nerve arises from a larger area that is the pons, also called the Varolian bridge, located at the base of the brain. The pons, next to the medulla oblongata, form the brain stem.

Boss

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 a sensory analysis (of sensations).

The main nuclei of this structure are the locus coeruleus (full of noradrenergic neurons), the reticular nucleus (regulating REM sleep) and the superior olivary complex (regulating hearing). On the other hand, the pons contains the nuclei of the V-VIII cranial nerves (between which is the nucleus of the VI pair, that is, of the abducens nerve).

• You may be interested in: "Varolio Bridge: structures, characteristics and functions"

segments

On the other hand, continuing with the anatomy of the abducens nerve, we found that this has different segments (specifically, five). These segments are:

• intraaxial segment
• cisternal segment
• Dorello channel
• cavernous segment
• extracranial segment

injuries

The abducens nerve, like all, 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 frequent causes of injury to this nerve are (in order from most frequent to least frequent): trauma, inflammation, vascular lesions (ACV) and tumors.

When the nerve is stretched, a nerve paralysis occurs, and its most common symptoms are: horizontal binocular diplopia (double vision), muscle spasms and cervicalgia (neck pain). On the other hand, the lesions caused by a tumor generally cause a unilateral compromise; Finally, the lesions associated with other pairs of cranial nerves are not as frequent.

1. Paralysis

What happens when abducens nerve palsy occurs? Its 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.

Causes

The causes of abducens nerve palsy can be several: Wernicke's encephalopathy, a nerve infarction, an infection, a trauma, intracranial hypertension...

Its cause, however, can also be idiopathic (unknown or of spontaneous onset). In order to determine the cause it is necessary to perform a Magnetic Resonance Imaging (MR), in addition to other complementary tests.

• You may be interested in: "Magnetic resonance imaging: what is it and how is this test performed?"

2. Abducens nerve mononeuropathy

We have talked about abducens nerve palsy, 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 (ie, the nerve is damaged). As a consequence, it can happen that the person sees 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)

Bibliographic references:

• Blumenfeld, H. (2002). Neuroanatomy Through Clinical Cases. Sunderland: Sinauer Associates.
• Figueroa, R., Meléndez, F. and Jimenez, L. (2015). Clinical case. Abducens nerve palsy 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). Abducens Nerve Clinical Neuroanatomy (4th Ed.). Panamerican Medical Editorial.
• MedlinePlus. (2019). Mononeuropathy of the VI cranial nerve.
• Rubin, M. & Safdieh, J.E. (2008). Netter. Essential neuroanatomy. Abducens nerve. Elsevier Masson. Spain.