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U N I T 1 0
Nervous System
The Vestibular System and
Disorders of Equilibrium
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
SUMMARY CONCEPTS
■■
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.
