Final answer:
In an OKN test with a drum spinning to the left, two eye movements, slow tracking and a fast saccade, are utilized involving the PPRF for saccades and oculomotor nuclei for tracking, coordinated by the brain stem.
Step-by-step explanation:
When an optokinetic nystagmus (OKN) drum spins to the left from a patient's perspective, two types of eye movements are being utilized: slow tracking or pursuit movements to the left followed by a fast reset movement, called a saccade, to the right. The paramedian pontine reticular formation (PPRF) initiates the saccade, quickly bringing the eyes back to the point of focus. The slow tracking movement to follow the drum is controlled by the oculomotor nuclei coordinating the lateral and medial rectus muscles. These conjugate gaze eye movements ensure that visual stimuli are kept centered on the fovea of the retina, despite head and object movement within the visual field.
Proper function of these movements is crucial, and their coordination is achieved through the processing of sensory and motor components by various parts of the brain stem, including the PPRF, superior colliculus, and oculomotor nuclei connected by the medial longitudinal fasciculus (MLF). Any damage or disease affecting these areas or the MLF can lead to conditions like internuclear ophthalmoplegia, resulting in diplopia, where the eyes temporarily point at different stimuli.
The vestibulo-ocular reflex (VOR) coordinates eye movements during head rotation. The abducens nuclei and oculomotor nuclei control the lateral and medial rectus muscles of the eyes, respectively, for opposite rotation of the eyes to compensate for head movement. The midbrain's superior colliculus and the paramedian pontine reticular formation (PPRF) initiate eye movements, while the medial longitudinal fasciculus (MLF) connects these areas to the oculomotor, trochlear, and abducens nuclei.