Internuclear ophthalmoplegia occurs due to lesions in the medial longitudinal fasciculus (MLF), which is located in the paramedian area of the midbrain and the pons.
In order to generate conjugate eye movements, a signal is sent from the paramedian pontine reticular formation (PPRF) to the pontine abducens nucleus. A signal is then sent from the abducens nucleus to the ipsilateral lateral rectus muscle via the abducens nerve and to the contralateral oculomotor nucleus to control the contralateral medial rectus muscle. Fibres from the abducens nucleus cross the midline of the brainstem and form a highly myelinated tract, the MLF, which travels up the contralateral side of the brainstem to the oculomotor nucleus. The contralateral medial rectus muscle is then innervated from the oculomotor nucleus in the midbrain. The eye which fails to adduct is ipsilateral to the lesion in the MLF as the oculomotor nucleus does not receive a signal from the contralateral abducens nucleus to adduct. This results in a failure of conjugate gaze and diplopia.
The horizontal nystagmus of the right eye above is explained by Hering's law of equal innervation, whereby the 'weak, medial rectus muscle prompts the cortex to increase innervation to the oculomotor nucleus. This prompts an equally increased, and in this case, excessive innervation to the contralateral abducens nucleus, resulting in horizontal nystagmus of the right eye.