Do you know what cranial pairs are? They are twelve pairs of nerves that come 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 nerve , which we will talk about today.

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

Cranial nerves

So, before we talk 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 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, chest and abdomen. That is to say, its “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 specific function for the organism).

The cranial nerves are also called by their number (e.g. cranial nerve I, II, III, IV…). On the other hand, each nerve has its own nucleus.

Classification

According to the specific function of each nerve, we find a classification that groups them accordingly. The nerves can be: motor, sensory or mixed (this would be their general function). The abducens nerve is purely motor , as we will see later.

Let’s see that ranking:

  • Special sensory input: pairs I, II and VIII are ordered.
  • Control of eye movements, photomotor reflexes and accommodation: pairs III, IV and VI (where we find the nerve abducens, 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 fibres: pairs III, VII, IX and X.

The nerve abducens

The abducens nerve is a nerve that is also called by other names, such as the abducens nerve, the external ocular motor nerve or the cranial nerve VI (the sixth cranial nerve). This is a purely motor nerve, which innervates the lateral rectus muscle, allowing 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: on the level of functions, the abducens nerve allows us to move the eye sideways, towards the temple.

In addition, the nerve abducens 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 faced with a light stimulus.

Anatomy

Anatomically, the nucleus of the abducens nerve is located below the top of the 4th ventricle of the brain. Specifically, near the facial colliculus. This nucleus receives efferences from other muscles, also oculomotor, as well as from vestibular nuclei .

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

Specifically, the nerve abducens originates from a larger area that is the bump, also called the pons, located at the base of the brain. The bulge, together with the spinal cord , forms the brain stem .

Lump

As we have seen, the origin of the nerve abducens is located in the protrusion. The bump is an area of the brain, located at its base, which controls motor movements and performs sensory analysis (of sensations) .

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The main nuclei of this structure are the locus coeruleus (full of noradrenergic neurons), the reticular nucleus (which regulates REM sleep) and the upper olive complex (which regulates hearing). On the other hand, the protuberance contains the nuclei of the cranial pairs V-VIII (among which is the nucleus of the pair VI, i.e. the abducens nerve).

Segments

On the other hand, following the anatomy of the abducens nerve, we find that this one has different segments (specifically, five). These segments are:

  • Intraaxial segment
  • Cistern segment
  • Dorello Canal
  • Cavernous segment
  • Extracranial segment

Injuries

The nerve abducens, like everyone else, can be injured in some way. This causes different symptoms , as 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 (stroke) and tumours.

When the nerve is stretched, it becomes paralyzed, and the most common symptoms are binocular horizontal diplopia (double vision), muscle spasms and cervicalgia (cervical pain) . On the other hand, lesions caused by a tumour generally originate unilateral involvement; finally, lesions associated with other pairs of cranial nerves are not so frequent.

1. Paralysis

What happens when nerve abducens paralysis occurs? Your paralysis affects the lateral rectus muscle, and also impairs the 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 paralysis of the nerve abducens can be several: a Wernicke’s encephalopathy, a nerve infarction, an infection, a trauma, intracranial hypertension…

Its cause, however, may also be idiopathic (unknown or spontaneously occurring). In order to determine the cause it is necessary to perform a Magnetic Resonance Imaging (MRI) , in addition to other complementary tests.

2. Mononeuropathy of the nerve abducens

We have talked about paralysis of the abducens nerve, but there is another pathology of the same one that entails different alterations for the individual.

One of these is mononeuropathy, a nerve disorder that affects the function of this nerve (i.e. the nerve is damaged). As a result, the person may see double (double vision).

Causes

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

  • Multiple Sclerosis (MS)
  • Infections (e.g. meningitis)
  • Brain Aneurysms
  • Strokes
  • Pregnancy
  • Trauma
  • Tumors (e.g., behind the eye)
  • Gradenigo Syndrome
  • Diabetes (diabetic neuropathy)

Bibliographic references:

  • Figueroa, R., Meléndez, F. and Jiménez, L. (2015). Clinical case. Abducens nerve paralysis secondary to Dorello’s canal meningioma: Anatomical considerations by magnetic resonance. | Abducens nerve palsy secondary to Dorello’s canal meningioma: Anatomical MRI considerations. Colombian Neurological Record, 32(1): 61-66.
  • Latarjet, M. & Ruiz, A. (2004). Clinical Neuroanatomy Nerve Abducens (4th Ed.). Editorial Médica Panamericana.
  • MedlinePlus. (2019). Cranial mononeuropathy VI.
  • Rubin, M. & Safdieh, J.E. (2008). Netter. Essential neuroanatomy. Nerve abducens. Elsevier Masson. Spain.