The abducent nerve, also called the abducens nerve, is the sixth of the 12 cranial nerves. It controls the lateral rectus muscle of the eye, which moves the pupil away from the midline of the body. An abducent nerve can be found in nearly all vertebrates.
The abducent nerve is a somatic efferent nerve, meaning it is part of the somatic nervous system. This system controls voluntary muscles and receives sensory information. Efferent nerves in the somatic nervous system send signals about movement to voluntary muscles.
The abducent nerve originates between the pons and medulla, two structures in the brainstem. It travels upward then forward to reach the eye. Once the nerve leaves the brainstem, it enters the subarachnoid space on the surface of the brain below the arachnoid membrane. It then moves through the dura mater, the outermost covering of the brain, and runs between the dura mater and the skull. It returns to the eye area through the superior orbital fissure, a cleft at the base of the skull near the eye.
As the abducent nerve takes such a long path from the brainstem to the eye, it is susceptible to injury. Lesions, tumors, or anything else that presses downward on the brainstem can put pressure on the sixth cranial nerve, causing impaired function. This nerve is also the cranial nerve most commonly affected by tuberculosis. Diseases that damage the nervous system, such as diabetic neuropathy, can also damage this nerve. Infections like meningitis or demyelination of the axons are other possible causes of damage.
Complete disruption of the abducent nerve causes double vision, because the affected eye is pulled to the midline of the body. This happens because the medial rectus muscle, or the muscle that pulls the eye medially, pulls on the eye, but the muscle that typically opposes it, the lateral rectus muscle, is not enervated and so does not function. To see normally, affected people must rotate their heads so both eyes are looking to the side.
Damage to the nucleus where the abducent nerve originates affects both eyes. The abducent nucleus contains both neurons that affect same side vision and neurons that cross over to affect the other side of the body. Damage here results in a gaze palsy, or an inability to move both eyes in the same direction at the same time. The horizontal movement of the eyes is affected, but vertical movement, controlled by other pathways, remains intact.