Porth's Essentials of Pathophysiology, 4e

984

Nervous System

U N I T 1 0

The Vestibular System and Disorders of Equilibrium

SUMMARY CONCEPTS

The vestibular apparatus, which is part of the inner ear, serves to maintain a sense of equilibrium and orientation in space. The equilibrium sense, which is also dependent on vision and input from stretch receptors in muscles and tendons, serves to maintain and assist recovery of a stable body and head position through control of postural reflexes, and to maintain a stable visual field despite marked changes in head position. The Vestibular System The structures of vestibular system, collectively referred to as the vestibular apparatus, are located in the bony labyrinth of the inner ear next to and continuous with the cochlea of the auditory system. Like the cochlea, the vestibular apparatus consists of two fluid-filled compartments—an outer bony labyrinth that is filled with perilymph and an inner membranous labyrinth that is filled with endolymph. 2,16,22 The bony membranous labyrinth is divided into three semicircular ducts and two large chambers known as the utricle and saccule (Fig. 38-21A). The semicircular ducts sense angular and rotary movements of the head, and the utricle and saccule sense forward and backward movement of the head. The three semicircular ducts (anterior, posterior, and lateral) are arranged at right angles to each other and represent all three planes of space. Each of these ducts has an enlarged swelling at one end called an ampulla . The ampulla of each of the semicircular ducts contains a ridge that is covered by a sensory epithelium with tufts of hair cells that are covered by a flexible gelatinous cap called the cupula (see Fig. 38-21B). The ampulla of the three semicircular ductls, the lateral, anterior, and posterior ducts, are oriented in one of three planes of space. The lateral (horizontal) ducts are in the same plane, whereas the anterior (superior) duct of one side is parallel with the posterior (inferior) duct on the other side, and the two function as a pair. Thus, regardless of which plane one moves in, there will be receptors to detect movement. Impulses from the semicircular ducts are particularly important in reflex movement of the eyes. During head rotation the eyes slowly drift in the opposite direction and then jump rapidly back toward the direction of rotation to establish a new fixation point. Located on the inside surface of each utricle and saccule is a small sensory area about 2 mm in diameter called the macula that responds to our sense of static equilibrium (see Fig. 38-21C). Each macula is a flat epithelial patch containing supporting cells and sensory hair cells, which synapse with sensory endings of the vestibular nerve. These hair cells are embedded in a gelatinous mass, the otolithic membrane, which is studded with tiny stones (calcium carbonate crystals) called otoliths . Although they are small, the density of the otoliths increases the membrane’s weight and its resistance to change in

■■ The outer ear collects sound vibrations and channels them to the tympanic membrane, which separates the outer ear from the middle ear. The middle ear is an air-filled cavity in the temporal bone that amplifies the sound waves and transmits them to the fluid-filled inner ear. ■■ The middle ear is connected to the nasopharynx by the eustachian tube, which opens briefly during swallowing to allow for equalization of air pressures on either side of the tympanic membrane.The eustachian tube is lined with a mucous membrane that is continuous with the nasopharynx, allowing infections from the nasopharynx to travel along the eustachian tube to the middle ear. ■■ Otitis media (OM) refers to inflammation of the middle ear. It can represent an acute otitis media (AOM) that has an abrupt onset and is usually related to bacterial infection, or otitis media with effusion (OME) that is associated with fluid in the middle ear without the manifestations of infection and which does not usually require treatment with antimicrobial agents. ■■ The inner ear houses two separate sensory systems: the auditory and vestibular systems. The auditory system contains the cochlea whose receptors convert sound waves to nerve impulses that are transmitted via the cochlear nerve to the auditory cortex. ■■ Hearing is a special sensory function that incorporates the sound-transmitting properties of the external ear canal, the eardrum that separates the external and middle ear, the bony ossicles of the middle ear, the sensory receptors of the cochlea in the inner ear, the neural pathways of the vestibulocochlear or auditory nerve, and the primary auditory and auditory association cortices. ■■ Hearing loss or deafness can be caused by conductive disorders, in which auditory stimuli are not transmitted through the structures of the outer and middle ears to the sensory receptors in the inner ear; by sensorineural disorders that affect the inner ear, auditory nerve, or auditory pathways; or by a combination of conductive and sensorineural disorders.