Prepositus

Prepositus typically refers to the nucleus prepositus hypoglossi (also called the prepositus nucleus), a brainstem structure essential for controlling eye movements. The term is used in neuroanatomy to designate this nucleus that lies in the dorsal medulla near the floor of the fourth ventricle, closely associated with the nucleus of the hypoglossal nerve and the abducens nucleus. In many classifications it is considered part of the horizontal vestibulo-ocular pathway as a key component of the neural integrator that sustains gaze.

Functionally, the prepositus nucleus serves as a neural integrator for horizontal eye position. It converts velocity signals into position signals, enabling a stable gaze after saccades or following vestibular input. The nucleus receives input from vestibular nuclei and cerebellar structures, particularly the flocculus, and projects to the ocular motor nuclei that control the medial and lateral eye muscles. By providing persistent signals, it helps maintain eye position in the absence of ongoing movement, supporting both smooth pursuit and fixation tasks.

Clinical significance arises when the prepositus nucleus is damaged or its connections are disrupted. Lesions can impair gaze holding, producing gaze-evoked nystagmus or horizontal gaze palsy, and may contribute to broader brainstem or cerebellar dysfunction. Diagnosis typically relies on clinical observation and imaging studies, with attention to symptoms related to persistent eye position and stability.

Etymology: prepositus is Latin for “placed before” or “in front,” a descriptor reflecting historic naming and anatomy conventions for this and related brainstem structures.