Abducens Nucleus - Neurophysiology

Neurophysiology

Two primary neuron types are located in the abducens nucleus: motorneurons and interneurons. The former directly drive the contraction of the ipsilateral lateral rectus muscle via the abducens nerve (sixth cranial nerve); contraction of this muscle rotates the eye outward (abduction). The latter relay signals from the abducens nucleus to the contralateral oculomotor nucleus, where motoneurons drive the contraction of the ipsilateral medial rectus muscle (hence, contralateral to the abducens nucleus that issues the command) ; contraction of this muscle rotates the eye inward (adduction).

This "wiring" pattern suggests that the main function of the abducens nucleus is to generate coordinated movements of both eyes in the same direction. Indeed, electrical stimulation of the abducens nucleus has been shown to generate conjugate eye movements (i.e. both eyes rotate in the same direction, and by the same angle). Such eye movements occur whenever we look between targets located in the distance. Moreover, lesions to the axonal tract of interneurons (in the medial longitudinal fasciculus) have been shown to disrupt conjugate eye movements through the paralysis of the contralateral eye. Importantly, despite the lesions, this muscle remains functional during convergence eye movements. Finally, experiments where the electrical activity of single neurons in the abducens nucleus has been recorded during slow and fast conjugate eye movements have demonstrated very little differences in the discharge patterns of motoneurons and interneurons.

Altogether, it is now well accepted that the abducens nucleus is a key structure for the conjugated movements of both eyes.