Medullary Structures Relevant to Upbeat Nystagmus

This is an axial section of the medulla, slightly more caudal as compared to (please refer to figure "medullary structures relevant to the ocular motor and vestibular consequences of the lateral medullary (Wallenberg) syndrome). Again seen are the inferior cerebellar peduncle (ICP) and caudal aspect of the vestibular nucleus (medial vestibular nucleus [MVN]), in addition to the nucleus of Roller and nucleus intercalatus. These nuclei normally have an inhibitory influence over the flocculus, and when there is a lesion of Roller/intercalatus, there is less inhibition of the flocculus. The Purkinje cells of the flocculus normally inhibit the anti-gravity/anterior semicircular canal (SCC) pathways. With a lesion of Roller/intercalatus, the flocculus will over-inhibit the anterior SCC pathways, causing relative activation of the posterior SCC pathways which will generate a downward slow phase. The fast/position-reset phase will be upward, and these alternating slow (downward) and fast (upward) phases are responsible for upbeat nystagmus.