Anatomy and Physiology of Children and Adolescents - M. R. Sapin 2007

Sensory Organs. Analyzers
Organs of Hearing and Balance (Vestibulocochlear Organ)
Organ of Balance (Vestibular Apparatus) - Pathway of the Vestibular Analyzer (Organs of Balance)

Sensitive Hair Cells of the maculae perceive linear accelerations, gravity, and vibrational oscillations. In a customary, normal HEAD position, otoliths exert pressure on specific Hair cells. When the position changes, the otoliths exert pressure on other receptor cells, generating new nerve impulses that travel to the Brain, specifically to the central Divisions of the vestibular analyzer, signaling a disruption of the body's familiar equilibrium.

Sensitive hair cells in the ampullary crests generate nerve impulses during various rotational Head movements. These sensitive cells are excited by the movement of endolymph located within the membranous semicircular ducts. Because the semicircular ducts are oriented in three mutually perpendicular planes, any head turn will inevitably set the endolymph in motion within one or more ducts, and its inertial pressure will stimulate the receptor cells. The excitation originating in the receptor hair cells of the maculae and ampullary crests is transmitted to the Nerve Cells of the vestibular ganglion, located at the floor of the internal acoustic meatus. The axons of these cells form the vestibular part of the Vestibulocochlear nerve (cranial nerve VIII), which exits together with the cochlear part through the internal acoustic meatus into the cranial cavity. The nerve fibers enter the brain tissue and reach the vestibular nuclei located in the region of the vestibular area in the floor of the Rhomboid fossa. Axons of the vestibular nuclear cells extend to the fastigial nuclei of the Cerebellum via its inferior cerebellar peduncle, to the Spinal Cord, and also as part of the dorsal longitudinal fasciculus of the Brainstem. From the vestibular nuclear cells, some fibers cross over and proceed to the thalamus, from which impulses are directed to the cortex of the parietal and temporal lobes (the cortical centers of the statokinetic analyzer). In response to The stimulation of vestibular receptors, reflex reactions occur involving the vestibulospinal tract, which connects the vestibular nuclei in the brainstem to the anterior horns of the spinal cord. Nerve impulses arriving at the spinal cord and the motor nuclei of Cranial Nerves reflexively alter Muscle tone. To maintain and restore balance, THE POSITION OF the head and the entire body is adjusted accordingly.

It is well known that damage to the vestibular apparatus causes dizziness and a loss of balance. Increased excitability of the sensitive cells of the vestibular apparatus triggers symptoms of motion sickness and other disorders.