Anatomy and physiology of peripheral and central vestibular system

The vestibular system includes different anatomical structures and physiological pathways. In the first part of this chapter, we will briefly introduce the main anatomical constituents, i.e., vestibule, semicircular canals, vestibular ganglia and nerve, vestibular nuclei, flocculo-nodular lobe of cerebellum and other structures in the brain stem. In the second parts, we will discuss the pathways generated by signals transduced by vestibular structures. Briefly, the bony labyrinth is lined with the endolymph-containing membranous labyrinth, showing two chambers (utricle and saccule) and three semicircular canals (horizontal, anterior, and posterior). Each canal contains a receptor organ called crista ampullaris, whereas receptor organs of both utriculus and sacculus are called otolithic macula. All the information transduced by receptors travels through the vestibular nerve fibers and reachs the vestibular nuclei in the pons and medulla oblongata. The axons originated here supply various CNS areas, e.g., the spinal cord, oculogyric nuclei, the reticular formation, and the cerebellum. Vestibular receptors respond to accelerations of the head in space, providing direct information about its rotations (receptors of semicircular canals), orientation with respect to gravity and linear movements (receptors of otolithic maculae). Semicircular canals are excited by rotational accelerations, converted into rotational velocity signals by the mechanical structure of the canal. Each labyrinth is excited by ipsilateral head rotation, both labyrinths are excited by purely forward or backward rotation. The utricular macula lies horizontally, whereas the saccular macula is placed vertically. When the head is still and held erect with respect to gravity, the utricular macula is not stimulated, and tilts from the erect posture will excite utricular afferents. Conversely, the saccular macula is strongly stimulated when the head is erect, and little or not when the head is lying on either side. Vestibular signals (ipsilateral rotational head velocity, head tilt, or equivalent linear acceleration) assist balance: the vestibulocervical reflex excites neck muscles via the medial vestibulospinal tract to oppose head tilt, and vestibulospinal reflexes excite ipsilateral limb extensors via the lateral vestibulospinal tract to oppose body tilt.