Do you know what are cranial pairs? These are twelve pairs of nerves that run from the brain to other areas of the body. One of them is the external ocular motor nerve, also called the cranial VI nerve or the external ocular motor nerve, What are we going to talk about today.
We will first do a brief introduction on what cranial nerves are for positioning, then we will explain the most relevant features of the external ocular motor nerve (its anatomy, functions, injuries, etc.).
Before talking about the external ocular motor nerve, we will do a brief introduction, in which we will see what cranial nerves are, since the external ocular motor nerve is one of them.
Cranial nerves (also called “cranial pairs”) are twelve pairs of nerves that originate from the brain or brainstem, and are distributed through the different holes that we have at the base of the skull., In different areas: head, neck , chest and abdomen. In other words, that is to say their “apparent” origin is in the area from which they exit or enter the brainBut each nerve has its true origin, according to its specific function (each nerve has a function determined by the organism).
The cranial nerves are also called by their numbering (for example nerve I, II, III, IV … of the cranial pair). On the other hand, each nerve has its own nucleus, which is involved in the neurophysiological processes that take place through these sets of nerve cells grouped together in a bundle. Nerve nuclei are places where several neural sums are stacked (i.e. where their genetic material is) and perform functions of processing and coordinating nerve activity.
Depending on the specific function of each nerve, we find a classification that groups them together according to this. The nerves can be: motor, sensitive or mixed (this would be their general function). The external ocular motor nerve is purely motor, As we will see later, which means that their functions are related to the movement of the activation of muscle fibers.
Let’s look at this classification:
- Special sensitive 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 external ocular motor nerve, which is pair VI).
- “Pure” motor function): pairs XI and XII.
- Mixed function (sensitive and motor): pairs V, VII, IX and X.
- Carrying function of parasympathetic fibers: pairs III, VII, IX and X.
The external ocular motor nerve
The external ocular motor nerve is a nerve that it also receives other names, such as abductor nerve, external ocular motor nerve, or cranial pair VI nerve. (Sixth cranial nerve). It is a purely motor nerve, which innervates the lateral rectus muscle; this allows the eyeball (eye) to rotate sideways.
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 medial plane of symmetry). In simpler terms: Functionally, the external ocular motor nerve moves the eye sideways toward the temple.
In addition, the external ocular motor nerve controls eye movements, light reflexes and accommodation. Remember that photomotor reflexes allow us to control the entry of light into the interior of the eye, by the contraction of the pupil when we are in front of a light stimulus.
Anatomically, the nucleus of the external ocular motor nerve is located below the upper part of the 4th ventricle of the brain. Specifically, near facial colic. this core it receives efferents from other muscles, also oculomotor, as well as from the vestibular nuclei.
Its apparent origin is in the sphenoid fissure, also called the superior orbital fissure, which is an area crossed by many other nerves (for example, the frontal, the lacrimal, the oculomotor …).
Specifically, the external ocular motor nerve originates from a larger area which is the bulge, also called the Varolio bridge, located at the base of the brain. The bulge, next to the medulla oblongata, they form the brainstem.
As we have seen, the origin of the external ocular motor nerve lies in the protuberance. The bulge is an area of the brain, located at the base, which controls motor movements and performs sensory (sensation) analysis.
The main nuclei in 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). In contrast, the protuberance contains the nuclei of the cranial pairs V-VIII (among which is the nucleus of the VI pair, that is, the external ocular motor nerve).
On the other hand, according to the anatomy of the external ocular motor nerve, it is observed that this it has different segments (Specifically, five). These segments are:
- Intraaxial segment
- Tank segment
- Dorello Canal
- Cavernous segment
- extracranial segment
The external ocular motor nerve, like all, can injure in one way or another. this causes different symptoms, As we will see later (depending on the type of damage and injury). The most common causes of damage to this nerve are (in order of most frequent to least frequent): trauma, inflammation, vascular damage (stroke) and tumors.
When the nerve is stretched it becomes paralyzed and its most common symptoms are: horizontal binocular diplopia (double vision), muscle spasms and neck pain (neck pain). In contrast, injury caused by a tumor usually results in a one-sided compromise; finally, injuries associated with other pairs of cranial nerves are not that common.
What happens in case of external ocular motor nerve palsy? His paralysis affects the lateral rectus muscle and also impairs eye abduction. which helps this nerve. This results in the eye of the person affected by this paralysis, can be slightly removed when the person is looking straight ahead.
The causes of paralysis of the external ocular motor nerve can be multiple: Wernicke’s encephalopathy, nervous infarction, infection, trauma, intracranial hypertension …
However, its cause can also be idiopathic (unknown or spontaneous onset). In order to determine the cause, it is necessary to perform an MRI (MRI), In addition to other additional tests.
2.mononeuropathy of the external ocular motor nerve
We have spoken of paralysis of the external ocular motor nerve, but there is another pathology of the same which involves different alterations for the individual.
One of them is mononeuropathy, a nerve disorder that affects the function of this nerve (In other words, the nerve is damaged). As a result, the person can see double vision (double vision).
The causes of mononeuropathy of the external ocular motor nerve can be diverse (from diseases to syndromes, including injuries, accidents, etc.). Some of the most common are:
- Multiple sclerosis (MS)
- Infections (eg Meningitis)
- brain aneurysms
- Stroke (LCA)
- Tumors (for example behind the eye)
- Gradenigo syndrome
- Diabetes (diabetic neuropathy)
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