If the nucleus of the RIGHT CN IV is damaged, downward rotation of the LEFT eye is impaired. It is also the only CN that has contralateral function. At a quick glance, CN IV appears to originate from the top of the brainstem, lateral to CN III. In fact, CN IV is the only CN that originates from the back of the brainstem (dorsal). Thus, both eyes gaze horizontally toward the RIGHT. The stimulus is then carries to the RIGHT CN VI, which activates the LEFT CN III. Turning the head toward the RIGHT stimulates the RIGHT CN VIII (vestibular) involuntarily. Horizontal gaze can be initiated voluntarily (intentionally) or involuntarily. Small or pinpoint pupils may also be present with a lesion in the pons (loss of sympathetic control of the pupil is located in the pons). Inability to move either eye horizontally may indicate injury in the region of the pons or lower brainstem. When the Right CN VI is stimulated, it sends a stimulus up the medial lateral fasciculus to activate the LEFT CN III. The RIGHT EYE rotates horizontally toward the RIGHT temple (Right CN VI) and the LEFT EYE rotates horizontally toward the RIGHT temple (CN III), producing rightward horizontal gaze. CN VI is also ipsilateral (controls the ability to rotate the eye horizontally toward the temple on the same side). Each CN III activates the medial rectus, superior rectus, inferior rectus and inferior oblique muscles to cause orbital rotation on the same side of the CN III nucleus (ipsilateral). The ability to move the eye in all other directions is controlled by CN III. Cranial Nerve IV (Trochlear) controls downward eye movement toward the nose, and Cranial Nerve VI (Abducens) controls horizontal eye movement toward the temple. Eye Movement The 3rd cranial nerve also controls eye muscle movement. The parasympathetic response of the pupil (or "return to normal") is constriction. Pupil Constriction Each one of the two 3rd cranial nerves controls the parasympathetic response of the pupil on the same side (ipsilateral). They control eye muscles on the same side of the body (ipsilateral). They are Lower Motor Neurons (LMN) (second order neurons). The 3rd cranial nerves are pure motor nerves. The pair of 3rd cranial nerves (oculomotor nerves) are located at the top of the brainstem - one to the right and one to the left.
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