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The vestibulo-ocular reflexes help to stabilize the visual image on the retina, and the vestibulocollic (vestibular neck) reflexes play a role in restoring the head position in space during head movements. The vestibular nucleus neurons play a crucial role in both reflex pathways. It was observed that individual vestibular relay neurons receive inputs from two to three semicircular canal pairs, or from canals and otolith organs, when natural stimuli were applied in alert cats. However, according to observations in anesthetized cats, it has been considered that the primary afferent fibers from each semicircular canal have their own target neurons in the vestibular nuclei.

The ultimate objective in studying the vestibulocollic (VCR) and cervicocollic (CCR) reflexes is to understand their dynamic and spatial properties at three levels: the kinetics of movement of the head, neck vertebrae, and neck muscles following perturbation of the body in any direction; the dynamics of activation of each class of receptor and each neck muscle; and spatial and dynamic properties of neuronal circuits that transform receptor input to muscle output. Current progress and future prospects for understanding each of these aspects are summarized. Three-dimensional power spectral characteristics of movements of the head-neck system have been measured during functional activities. Significant power was observed in the pitch and yaw planes, with the wider bandwidth for pitch occurring for walking in place.

In recent years, the vestibular system has been implicated not only in the reflex movements produced by labyrinthine inputs but also in the control of active motor behaviors, especially eye and head movements during gaze. This chapter discusses the experimental proof pertaining to the functional organization of vestibulo-ocular and vestibulo-oculo-collic pathways, with focus on recent researches of the signals carried by secondary vestibular neurons on the vertical canal systems. The synaptic organization of the vestibulo-ocular reflex (VOR) and vestibulocollic reflex (VCR) pathways stemming from the semicircular canals have been extensively studied and the fundamental patterns of excitatory and inhibitory connections with specific canal-muscles relationships have been well established. Although excitatory connections from the anterior involve the pathways through the deep reticular formations in the cat, the major part of the excitatory pathways ascend in the contralateral medial longitudinal fasciculus (MLF), and the inhibitory pathways in the ipsilateral MLF.

This chapter studies the properties and the functional coupling of four sensory-motor subsystems that contribute to the head and eye stabilization during linear motion in the vertical plane. The problems investigated in this chapter include the way the reflexes interact and are functionally coupled and the solutions relating to the central nervous system (CNS) when the there is an impaired eye-head coordination. The dynamics of the vestibulocollic reflex, oculocollic coupling, and the compensatory eye movements are discussed here. It is concluded that tight coupling between the visual and vestibular systems in head postural control and gaze stabilization also operates during linear motion in the vertical plane.

Some vestibular nerve afferents have a regular spacing of action potentials; in others, the space is irregular. Fibers first classified as regularly or irregularly discharging also differ in their response dynamics to rotational stimuli. Among canal afferents, the responses of regular fibers parallel angular head velocity, whereas those of irregular afferents are more phasic and show a high-frequency gain enhancement together with a phase advance. Consider the vestibulo-ocular (VOR) and vestibulocollic (VCR) reflexes. The VOR is an open-loop reflex controlling a predominantly viscous load. The VCR is closed loop, the load is largely inertial, and neck muscles are more sluggish than are extraocular muscles.