ABDUCENS NERVE: HISTORY, ANATOMICAL RELATIONSHIPS, INJURY PARALYSIS - ANATOMY AND PHYSIOLOGY - 2025

The abducens nerve, also called the external ocular motor nerve or cranial nerve VI, is a motor nerve that is part of the peripheral nervous system. It enters the orbit through the sphenoid cleft, located in the posterior part of the lateral aspect of the orbit, which separates it from the roof or superior orbit.

The nucleus of origin of the external ocular motor nerve is located on the floor of the IV cerebral ventricle in the pons. Just inside a small depression called the "superior fovea" is a rounded prominence called "eminence teres" that covers the nucleus of the sixth cranial nerve.

Nerve abducens (VI)

The abducens nerve exits the groove between the pyramid and the pons, runs upward and outward, and then travels forward over the curve of the temporal bone rock. From there it passes through the cavernous sinus and enters the orbit through the sphenoid cleft.

This nerve supplies the external rectus muscle of the eye. The lateral rectus muscle of the eye is one of the six muscles that control the movement of the eyeball. In this case, this muscle abducts the eyeball, that is, it turns the eye outward in the opposite direction to the nose.

History

Some theorists were collecting certain information until that obtained today:

- Claudius Galenus, better known as Galen (129-216 AD), described seven cranial nerves and described the sixth cranial nerve as being part of the common ocular motor nerve.

- Jacob Winslow (1669-1760) to whom the distinction of the spinal and cranial nerves is owed, was the one who defined the sixth cranial nerve as a totally independent nerve, giving it the name of external ocular motor.

- Samuel Thomas von Sömmerring (1755-1830) gave it the name of “nerve abducens” that is still present today.

Organization of the nervous system

The nervous system can be described as a communication network that allows the organism to relate and interact with its environment. It has sensory elements, integrating elements of sensory signals, memory and motor components.

Sensory elements detect stimuli from the external environment, integrating components process sensory data and information from stored memory. Meanwhile, the motor components generate the movements that allow us to speak, communicate, wander and many other activities in the environment.

Central Nervous System

The nervous system is divided into the central nervous system and the peripheral nervous system. The central nervous system includes the encephalon or brain and the spinal cord. In turn, the brain, from the embryonic point of view, is divided into five regions:

- Mielencephalon: constituted in the adult by the medulla oblongata.

- Metancephalon: which includes the bridge or pons and the cerebellum.

- Midbrain: which is the midbrain.

- Telencephalon: which includes the different cortical lobes.

- Diencephalon: which includes the thalamus and the basal ganglia.

Nerve abducens in the peripheral nervous system

The peripheral nervous system includes the anatomical structures that allow the central nervous system to relate to the environment, including as such the external environment and the body itself.

The peripheral nervous system has a sensory component that includes sensory receptors and primary afferent neurons, as well as a motor effector component that connects with the effectors, which are the muscles and glands; through which it orders the performance of movements and glandular activity.

The motor component is made up of somatic motor fibers, autonomic motor fibers, and autonomic ganglia. The autonomic component is subdivided, in turn, into sympathetic neurons, parasympathetic neurons, and enteric neurons.

The somatic component has the spinal nerves and the cranial nerves. The external ocular motor nerve or sixth cranial nerve, is part of the motor component of the peripheral nervous system.

Motor system and abducens nerve

This name refers to the system that innervates and controls skeletal muscle, controlling the sequence and patterns of movement. It intervenes in posture, reflexes, rhythmic activities such as walking and voluntary movements.

Among the especially important motor activities are language, eye movement, and skilled, high-precision motor activities performed by the fingers.

Innervation of skeletal muscle cells or fibers is provided by α motor neurons. These motor neurons are distributed throughout the entire length of the spinal cord, in the anterior horns of the spinal cord and in the motor nuclei of the cranial nerves or pairs.

An α motor neuron, together with all the fibers it innervates, makes up what is called a “motor unit”. The motor unit is the functional unit of the motor nervous system. The only way the central nervous system can make a muscle cell contract is by stimulating an α-motor neuron.

Therefore, all motor nerve circuits must end up stimulating some α-motor neuron to elicit movement. This is the reason why α-motor neurons are called the common end pathway of the motor system.

The abducens nerve or sixth cranial nerve is an exclusively motor nerve; In its pontine nucleus, there are α motor neurons that emit axons that are packed in the nerve and that innervate the external rectus muscle of the eye.

This nerve is one of the components of the somatic peripheral nervous system.

Anatomical relationships of the sixth nerve

The nucleus of origin of this cranial nerve is located on the floor of the IV ventricle, in the protrusion below the eminence of teres and surrounded by the facial nerve (VII cranial nerve).

The abducens nerve has a fairly extensive intracranial course within the posterior fossa and in the cavernous sinus. When it enters the cavernous sinus, it is located between the external carotid and the ophthalmic nerve (1st cranial nerve).

Cranial nerves and their effectors (Source: Government via Wikimedia Commons)

At the level of the pons, it is related to the basilar vestibule vascular system that supplies the portion of the brain located in the posterior cranial fossa. The vertebral artery passes through the transverse foramen of the atlas and penetrates the atlanto-occipital membrane and the dura mater to pass through the foramen magnum into the cranial cavity.

The artery runs anteriorly laterally with respect to the medulla oblongata and then ventrally with respect to the hypoglossal (XII cranial nerve), reaches the medial bulbo-pontine line, passing between the two external ocular motor nerves (cranial nerve VI) and joining with the vertebral artery on the opposite side to form the basilar artery.

Due to its long intracranial path, this nerve is vulnerable to injuries, pathologies, traumas or increases in intracranial pressure that affect parts near its path, which produces characteristic alterations in eye movements.

Paralysis due to injury to the sixth nerve

Injury to the abducens nerve impairs abduction of the eyeball by causing paralysis of the lateral rectus muscle of the eyeball.

The paralysis can be secondary to a vascular injury to the nerve, which occurs frequently in diabetic neuropathic vasculitis, due to traumatic injuries, infections or intracranial hypertension, among others.

Symptoms of 6th cranial nerve palsy include binocular horizontal diplopia (double vision) when looking to the side of the affected eye. The eye is slightly adducted (deviated towards the nose) when looking straight ahead.

References

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