| Transcript |
"So today, I want to talk to you about the role of the flocculus in the development of downbeat nystagmus, but you could use the same information for other abnormal forms of movement that are intrusions where the fovea is moved off the target and you have to make a saccade back. So, this is just one example of cerebellar eye movements and we'll just use the flocculus as an example. So, under normal conditions, the paramedian tract in your brainstem is where the signal is going to originate from for control of the fine movements of your eyes, and that signal is going to be mediated by the flocculus in the cerebellum, and there are specific cell types within the flocculus of the cerebellum that help mediate this signal. The granular cell receives the information from the paramedian tract in the brainstem, and is modulated through the flocculus in the cerebellum to the Purkinje cell, "Purkinje," in the cerebellum. And that Purkinje cell is talking to the vestibular nuclei-- two input points on the superior vestibular nuclei-- you've got the peripheral vestibular system which is coming from your ear, that's the semicircular canals in your ear, and those semicircular canals in your ear have to talk to your eye muscles to mediate the vestibular ocular reflex. So, the vestibular system-- not the hearing part of your ear but the vestibular part of your ear-- is there to control the movement in response to head movement, so when your head moves, your eyes have to roll. So, if you tilt your head, for example, to the right, we have to have counter-rolling to the left in order to maintain the vertical orientation of our world. Otherwise, every time we tilted our head, the whole world would tilt with us. This peripheral vestibular response is mediated by the movement of those hair cells in your ear and is responsible for a reflex called the vestibular ocular reflex, "VOR," which means you don't have to think about it. Which is good, because when you move your head, you don't want to be thinking about rolling your eyes the opposite direction, it just has to happen automatically. And so, that is mediated here at the superior vestibular nuclei, and the superior vestibular nuclei are going to talk to the eye now, the superior rectus muscle and the inferior rectus muscle. So, we have to make these muscles move from input from our ear that is modulated centrally and controlled at the fine level by the flocculus and the cerebellum. The fine motor control of your eyes is dependent on your brainstem, your cerebellum, and your ear. That's an amazing coordination of effort. However, if we have a lesion that disrupts anywhere in this pathway, it will let a signal come to here, and that signal will be interpreted the same way as an ear stimulus. And so, if you disinhibit this, then this superior rectus will receive the message to fire, the eye will drift up away from the target, and the inferior rectus muscle will now have to fire to compensate for the drift and will be moving the eye down. So, this is what will happen: a slow pathologic drift up and a corrective cut down. This we call downbeat nystagmus. So even though it's named downbeat, the pathologic part is actually the slow updrift of the eye that was mediated by inhibition of the signal from the Purkinje cell, mediated by the granular cell, from the paramedian tract in the brain stem at the level of the flocculus and cerebellum. We could also generate the same type of nystagmus from a peripheral lesion, but because the semicircular canals are not oriented completely vertically, they're oriented slightly at an angle, so they have to do double duty, which means they have to do both vertical, and horizontal, and rotary. And so, the nystagmus of peripheral vestibulopathies cannot be purely vertical and it cannot be purely down, it has to be a combination. And so, when we're dealing with a downbeat nystagmus, we really are thinking about lesions at the level of the flocculus in the cerebellum, or the brainstem, and in particular the cervical medullary junction. It will manifest as a slow drift up and a fast saccadic nystagmus corrective motion down, and that will look like downbeat nystagmus." |
