Understanding the anatomy and physiology of the central vestibular system is indispensable for understanding the pathophysiology of vertigo and body disequilibrium, and for proper diagnosis of patients with vestibular symptoms. This article first describes the location and classification of the vestibular nuclear complex, together with the principle of the arterial blood supply in the brainstem. Then, the characteristics of the eye movements caused by the vestibuloocular reflex (VOR) are summarized, and the neural circuits underlying the VOR are briefly explained. Finally, the neural mechanism underlying the generation of the slow phase of vestibular nystagmus is discussed in relation to the pathophysiology of vertigo caused by peripheral vestibular diseases.

　This article deals with the neural circuits involved in the generation of horizontal and vertical saccades. Following an explanation about the horizontal saccade system, the vertical saccade system, which has not been described in textbooks, is discussed in detail. Comparing the results with the well-known vestibuloocular pathways, we propose that the saccade system uses the same frame of reference as the vestibuloocular system. The last part of this article explains that the neural circuits for saccades and the quick phase of vestibular nystagmus share a common pathway.

　It has generally been accepted that voluntary eye movements are organized in horizontal and vertical systems, based on the results of clinical studies in humans and lesion studies in animals. However, this issue is still under debate, because the neural circuits for vertical saccades are yet to be identified. The riMLF was identified as a premotor center for vertical gaze by Büttner-Ennever, although the exact pathways from the superior colliculus to the vertical ocular motoneurons are unknown. Using intracellular recording and staining techniques, we found that both systems use the same 3D cartesian coordinates as the semicircular canal coordinates. The oldest phylogenetic system, the vestibular system, is used as a common coordinate system by the newer system, the voluntary eye movement subsystem, which constantly captures visual information in relation to the gravitational axis, and the sensory and motor coordinate systems that form the basis of visual cognition in the brain are unified using the common vestibular coordinate system. Use Assumption of the common coordinate system conflicts with Listing's law, but the excitatory commissural connection between the superior colliculi that we discovered is considered to provide the neural substrate for Listing's law.

　Mumps virus infection may cause acute sensorineural hearing loss, usually unilateral. It is sometimes accompanied by dizziness, and the vestibular dysfunction caused by mumps virus infection has not yet been well documented. We encountered two cases of mumps virus-induced hearing loss with vertigo, and conducted a series of vestibular function tests.

　Case 1, a 47-year-old man, presented with fever and bilateral subaural swelling. Two days later, he complained of left-sided hearing loss. Then, on day 7, he developed vertigo. Examination revealed total deafness on the left side. Serology revealed a positive result for anti-mumps IgM antibody, indicating that he did not have prior mumps infection or vaccination.

　Case 2, a 25-year-old man, presented with bilateral tinnitus and subaural swelling. On day 3, he visited our hospital, and audiometry showed total deafness on the right side. On day 6, he developed vertigo. Serology revealed a positive result for anti-mumps IgM antibody.

　We performed an infrared nystagmus test, caloric test, cervical and ocular Vestibular Evoked Myogenic Potential (VEMP), and video-Head Impulse Test (vHIT) in both cases.

　Both cases showed Canal Paresis (CP) on the affected side (Case 1, 94.5%; Case 2, 71.9%). vHIT showed decreased Vestibulo-Ocular Reflex (VOR) gain and Catch Up Saccade (CUS) in the affected semicircular canal in both. VEMP showed no response of c-VEMP on the affected side in Case 1, and no bilateral differences in either c-VEMP or o-VEMP in Case 2. The symptoms of vertigo disappeared about one month after the onset, but the findings of right CP on vHIT persisted, and in fact, became worse in Case 2.

　It is generally accepted that voluntary quick eye movements (saccades) are organized in the horizontal and vertical eye movement systems. Therefore, oblique saccades are considered to be generated by the vector sum of the outputs from the coordinated horizontal and vertical saccade systems, but the precise neural mechanism of generation of oblique saccades remains unresolved. The superior colliculus (SC) is known to be the center for saccades. Our previous studies show that many efferent neurons in the SC project to the midbrain and the pontine reticular formation, but their branching patterns and exact terminal areas in the brainstem are not known. In this study, we used an electrophysiological method for investigating branching patterns of single tectofugal neurons that project to various last-order premotor neurons for horizontal and vertical saccades in the brainstem of the cat. We recorded antidromic spikes and examined the effects of stimulation of the vertical and horizontal last-order premotor neuron areas (Forel's field H, FFH; excitatory burst neuron (EBN) region and inhibitory burst neuron (IBN) region for horizontal saccades). The results showed that there are four types of branching patterns of single tectofugal neurons that project to the last-order horizontal and vertical premotor neuron areas, and the upper cervical spinal cord. Among these, two types of the tectofugal neurons (about 69%) have axonal branches that project to the ipsilateral FFH and the contralateral EBN region, and furthermore, about 69% of them have another axonal branch that extends to the spinal cord. This finding indicates that single tectofugal neurons innervating both horizontal and vertical saccade generators with their axonal collaterals can easily synchronize the onsets of the horizontal and vertical saccade generating systems, and provides evidence that the branching patterns of single tectofugal neurons determine the functional synergies for coordinated oblique eye and head movements.