Vestibular Reflexes Coordinate Head and Eye Movements to MaximizeVisuaI Acuity During Movements of the Head
Vestibular reflex control of the extraocular muscles of the eye, known as the vestibuloocular reflex (VOR), coordinates eye and head movements so that as the head turns (rotates), the eyes remain fixed on the original field of vision for as long as possible.
Imagine that a dog is seated on a piano stool and you rotated her clockwise to the right. As you rotate her slowly to the right, her eyes rotate in her head slowly to the left so that the eyes remain fixed on the same field of vision as long as possible. As the eyes reach the limit of their leftward excursion, they swiftly move to the right, in the direction of the head movement, until they fix on a new field of vision. If the head continues to rotate, the cycle is repeated until constant velocity is achieved. This allows the animal time to interpret a field of vision despite rotary acceleration of the head. When this pattern of eye movement occurs during the VOR, it is referred to as normal nystagmus or physiological nystagmus; the slow drift opposite head rotation followed by a fast flick back in the direction of head rotation. A transient postrotatory nystagmus, with the converse eye movement pattern to that just noted, can be seen if a spinning animal or person is suddenly stopped. Inertia of the endolymph causes it to continue rotating in the semicircular duct, pushing on the crista ampullaris, even though the head and duct have stopped moving.These reflex eye movement patterns require normal sensory input from the semicircular ducts, an intact MLF in the brainstem, and normally functioning motor units of the extraocular muscles (as well as an intact cerebellum). The VOR occurs on rotation of the head in the horizontal plane or the vertical plane, or on clockwise or counterclockwise torsional rotation of the head. Voluntary control of the eyes is independent of vestibular reflexes and is controlled by the cerebral cortex.
Nystagmus may appear occasionally under pathological conditions of the vestibular system, even when the head is straight and at rest. This condition is known as spontaneous nystagmus. A persisting head tilt, falling, and compulsive Circlingor rolling often accompany spontaneous nystagmus in animals with acute vestibular disease. These actions often tend to be oriented in a consistent pattern with respect to the side of a peripheral lesion of the vestibular system. This is presumably a result of abnormal, asymmetric action potential inputs to the brainstem from the vestibular apparatus on the two sides of the head.