The abducens nerve is the VIth cranial nerve. It is responsible for the movement of the eyeball. It is composed primarily of motor fibers and innervates the lateral straight muscle.
What is the abducens nerve?
The abducens nerve is the VIth of the total XII. Cranial Nerves. Like most other cranial nerves, it supplies areas of the head. The abducens nerve is a purely somatomotor nerve. This means that it consists of motor fibers and its main function involves a motor function. Through the abducens nerve, the abduction movement of the eye is made possible. This is the outward movement of the eye. Its path goes from the brain stem to the orbit. The abducens nerve innervates only one muscle. This is responsible for the movement of the eyeball. The abducens nerve exists in both hemispheres. Thus, damage to one side does not automatically lead to a complete loss of eye movements in the other hemisphere. The abducens nerve is part of the central nervous system. The VI cranial nerve is unusually long compared to others. Of all the existing eye muscle nerves, it has the longest extradural course. Its path runs along the base of the skull, among other places, where it is also the most susceptible to lesions at the same time. Impairment of the functional activity of the abducens nerve leads to a malposition of the affected eye. It is directed slightly inward and causes the perception of double images. Sufferers then suffer from so-called silver vision. This refers to strabismus to a slight degree.
Anatomy and structure
The VI cranial nerve originates in the pons of the brain stem. This is bridge that the abducens nerve leaves medially at the inferior border. It continues to the clivus. The clivus separates the middle from the posterior fossa. At the clivus of the occipital bone, it passes under the dura mater. The dura mater is the meninges that separate the brain from the skull. Subsequently, the abducens nerve runs in the cavernous sinus. The cavernous sinus is a venous blood conductor of the brain. In it, the abducens nerve passes forward to the superior orbital fissure. This is a fissure in the human skull located between the greater and lesser wings of the sphenoid bone. The superior orbital fissure connects the middle cranial fossa to the orbit. The abducens nerve enters the orbit through the superior orbital fissure together with the oculomotor nerve, the trochlear nerve, and the three branches of the ophthalmic nerve. From there, it pulls laterally to the rectus lateralis muscle. This muscle is the only muscle supplied by the abducens nerve. It is responsible for the movement of the eyeball.
Function and tasks
The abducens nerve is responsible for regulating the movement of the eyeball. The VI cranial nerve innervates the rectus lateralis muscle. The rectus lateralis muscle is also called the lateral straight muscle. It is responsible for abduction of the bulbus oculi. The bulbus oculi is a part of the visual organ. This is spherical in shape and is located in the orbit, the eye socket. The structures necessary for seeing with the eye are located in the bulbus oculi. They include the iris, the lens and the retina. An abduction is basically understood as a lateral displacement of a body part. This process can also be described as the spreading of an extremity away from the body, for example. An abduction causes the corresponding body part to move from the center of the body or the longitudinal axis of the body in a lateral direction. In the area of the eye, an abduction refers to the movement of the eye to the outer edge of the eye. Thus, when the rectus lateralis muscle is contracted by the abducens nerve, the rotational movement of the eyes takes place outward, consequently to the lateral side. The abducens nerve is therefore responsible for visual perception in the entire lateral visual field. To it falls the task that the eye axis can be moved to the side or abducens according to its name.
Diseases
Lesions of the abducens nerve always result in ocular misalignment. It is diagnosed as mild strabismus. Damage to the abducens nerve can occur very easily in the cavernous sinus region. In this region of the head, the VI cranial nerve is very vulnerable. The reason for this is that it is the only cranial nerve that passes through the middle of the lumen of the venous blood duct. In addition, the abducens nerve is susceptible to lesions of the skull base.Its long course at the base of the skull can cause damage in basal skull fractures. The same happens in diseases such as basal meningitis. As soon as the functional activity of the abducens nerve is restricted, the rectus lateralis muscle can no longer be supplied. This means that the gaze deviation of the affected eye no longer functions. As a result, the two visual axes of the eyes no longer coincide. This inevitably leads to the perception of double images that are next to each other. When the patient’s eye looks in the direction of the damage, the double images become stronger. When the eye looks in the direction of the healthy eye, they become weaker. The last direction of gaze implies that the rectus lateralis muscle is inactive even under normal conditions. Damage to the abducens nerve leads to inward misalignment of the eyeball. This is the medial side of the visual field.
