Nerve abducens: what it is, characteristics and associated disorders.

A summary of the characteristics of the abducens nerve, one of the cranial nerves.

Do you know what the cranial nerves are? They are twelve pairs of nerves that go from the brain to other areas of the body. One of them is the abducens nerve, also called the VI cranial nerve or external ocular motor nervewhich we will talk about today.

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

The cranial nerves

Before talking about the abducens nerve, we will 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 are distributed through the different holes 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 where they exit or enter the brain.However, 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 denominated through their numbering (for example nerve I, II, III, IV... of the cranial pair). On the other hand, each nerve has its nucleus, which intervenes in the neurophysiological processes that take place through these bundles of nerve cells grouped in the form of a bundle. Nerve nuclei are places in which several neuronal somas are stacked (i.e. where their genetic material is located) and perform functions of processing and coordinating nerve activity.

Classification

According to the specific function of each nerve, we find a classification that groups them accordingly. Nerves can be: motor, sensory or mixed (this would be their general function). The abducens nerve is purely motor, as we will see below.as we will see later, which means that its functions have to do with the movement from the activation of Muscle fibers.

Let's look at this classification:

• Special sensory afferents: 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 is also called by other names also receives other names, such as abducens nerve, external ocular motor nerve or nerve VI of the cranial nerve pair (sixth cranial nerve). (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 body part from the medial plane of symmetry). In simpler words: at a functional level, the abducens nerve allows us to move the eye sideways, in the direction of the temple.

In addition, the abducens nerve controls eye movements, photomotor reflexes and accommodation.. Let us remember that photomotor reflexes allow us to control the entrance of light into the eye, through the contraction of the pupil when we are facing a luminous stimulus.

Anatomy

Anatomically, the nucleus of the abducens nerve is located under the upper part of the 4th ventricle of the brain. Specifically, near the facial colliculus. This nucleus receives efferents 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 (e.g. frontal, lacrimal, oculomotor...).

Specifically, the abducens nerve arises from a wider area which is the pons, also called the pons, located at the base of the brain. The pons, together with the medulla oblongata, form the brainstem.

Protuberance

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 the base of the brain, which controls motor movements and performs sensory analysis (sensations). motor movements and which performs sensory analysis (of sensations)..

The main nuclei of this structure are the locus coeruleus (filled with 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 (including the nucleus of cranial nerve VI, i.e. of the abducens nerve).

Segments

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

• Intraaxial segment
• Cisternal segment
• Dorello's canal
• Cavernous segment
• Extracranial segment

Injuries

The abducens nerve, like all nerves, can be injured in some way. This causes different symptomsas we will see below (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 (CVA) and tumors.

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

1. Paralysis

What happens when the abducens nerve is paralyzed? Its paralysis affects the lateral rectus muscle, and also impairs ocular abduction. that this nerve allows. This means that the eye of the person affected by such paralysis may be slightly abducted when the person looks straight ahead.

Causes

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

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

2. Mononeuropathy of the abducens nerve

We have talked about abducens nerve palsy, but there is another pathology of the abducens nerve 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). (i.e. the nerve is damaged). As a result, double vision (double vision) may occur.

Causes

The causes of mononeuropathy of the abducens nerve can be diverse (from diseases to syndromes, damage, accidents...). Some of the most frequent are:

• Multiple sclerosis (MS)
• Infections (e.g. meningitis)
• Cerebral aneurysms
• Cerebrovascular accidents (CVA)
• Pregnancy
• Trauma
• Tumors (e.g. behind the eye)
• Gradenigus syndrome
• Diabetes (diabetic neuropathy)

Bibliographic references:

• Blumenfeld, H. (2002). Neuroanatomy Through Clinical Cases. Sunderland: Sinauer Associates.
• Figueroa, R., Meléndez, F. and Jiménez, L. (2015). Clinical case. Abducens nerve palsy secondary to Dorello's canal meningioma: Anatomical considerations by magnetic resonance imaging. | Abducens nerve palsy secondary to Dorello's canal meningioma: MRI anatomical considerations. Acta Neurológica Colombiana, 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). Neuroanatomía Clínica Nervio abducens (4th Ed.). Editorial Médica Panamericana.
• MedlinePlus. (2019). Cranial nerve VI mononeuropathy.
• Rubin, M. & Safdieh, J.E. (2008). Netter. Essential neuroanatomy. Nervus abducens. Elsevier Masson. Spain.