Page 377 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

Conduction of Sound, Sound Sensors tines of the tuning fork are placed in front of the ear
(air conduction). Individuals with normal hearing or
Sound waves are transmitted to the organ of sensorineural deafness can hear the turning fork in
hearing via the external ear and the auditory the latter position anew (positive test result),
canal, which terminates at the tympanic mem- whereas those with conduction deafness cannot
(test negative).
brane or eardrum. The sound waves are con- The internal ear consists of the equilibrium organ
ducted through the air (air conduction) and set (! p. 342) and the cochlea, a spiraling bony tube
the eardrum in vibration. These are trans-
that is 3–4 cm in length. Inside the cochlea is an en-
Central Nervous System and Senses ear (labyrinth) begins. equilibrium organ. The scala media is accompanied
dolymph-filled duct called the scala media (cochlear
mitted via the auditory ossicles of the tympanic
duct); the ductus reuniens connects the base of the
cavity (middle ear) to the membrane of the oval
cochlear duct to the endolymph-filled part of the
window (! A 1,2), where the internal or inner
on either side by two perilymph-filled cavities: the
In the middle ear, the malleus, incus and
scala vestibuli and scala tympani. These cavities
stapes conduct the vibrations of the tympanic
merge at the apex of the cochlea to form the heli-
membrane to the oval window. Their job is to
cotrema. The scala vestibuli arises from the oval win-
conduct the sound from the low wave re-
dow, and the scala tympani terminates on the mem-
brane of the round window (! A2). The composition
sistance/impedance in air to the high re-
of perilymph is similar to that of plasma water
sistance in fluid with as little loss of energy as
similar to that of the cytosol (see below). Perilymph
curs at f ! 2400 Hz and is based on a 22-fold
circulates in Corti’s tunnel and Nuel’s spaces (! A4).
pressure amplification (tympanic membrane
Organ of Corti. The (secondary) sensory cells
area/oval window area is 17 : 1, and leverage
12 possible. This impedance transformation oc- (! p. 93 C), and the composition of endolymph is
of the hearing organ consist of approximately
arm action of the auditory ossicles amplifies
force by a factor of 1.3). Impairment of im- 10 000–12 000 external hair cells (HCs) and
pedance transforming capacity due, e.g., to de- 3500 internal hair cells that sit upon the
struction of the ossicles, causes roughly 20 dB basilar membrane ( ! A4). Their structure is
of hearing loss (conduction deafness). very similar to that of the vestibular organ
(! p. 342) with the main difference being that
Muscles of the middle ear. The middle ear contains the kinocilia are absent or rudimentary.
two small muscles—the tensor tympani (insertion:
manubrium of malleus) and the stapedius (insertion: There are three rows of slender, cylindrical outer
stapes)—that can slightly attenuate low-frequency hair cells, each of which contains approximately 100
sound. The main functions of the inner ear muscles cilia (actually microvilli) which touch the tectorial
are to maintain a constant sound intensity level, pro- membrane. The bases of the hair cells are firmly at-
tect the ear from loud sounds, and to reduce dis- tached to the basilar membrane by supporting cells,
tracting noises produced by the listener. and their cell bodies float in perilymph of Nuel’s
Bone conduction. Sound sets the skull in vibra- spaces (! A4). The outer hair cells are principally in-
tion, and these bone-borne vibrations are conducted nervated by efferent, mostly cholinergic neurons
directly to the cochlea. Bone conduction is fairly in- from the spiral ganglion (N M-cholinoceptors;
significant for physiological function, but is useful for ! p. 82). The inner hair cells are pear-shaped and
testing the hearing. In Weber’s test, a vibrating completely surrounded by supporting cells. Their
1
tuning fork (a ) is placed in the middle of the head. A cilia project freely into the endolymph. The inner hair
person with normal hearing can determine the loca- cells are arranged in a single row and synapse with
tion of the tuning fork because of the symmetrical over 90% of the afferent fibers of the spiral ganglion.
conduction of sound waves. A patient with unilateral Efferent axons from the nucleus olivaris superior
conduction deafness will perceive the sound as com- lateralis synapse with the afferent endings.
ing from the affected side (lateralization) because of Sound conduction in the inner ear. The stapes
the lack of masking of environmental noises in that
ear (bone conduction). A person with sensorineural moves against the membrane of the oval win-
deafness, on the other hand, will perceive the sound dow membrane, causing it to vibrate. These are
as coming from the healthy ear because of sound at- transmitted via the perilymph to the mem-
tenuation in the affected internal ear. In Rinne’s test, brane of the round window (! A2). The walls
the handle of a tuning fork is placed on one mastoid of the endolymph-filled cochlear duct, i.e.
364 process (bony process behind the ear) of the patient Reissner’s membrane and the basilar mem-
(bone conduction). If the tone is no longer heard, the
!
Despopoulos, Color Atlas of Physiology © 2003 Thieme
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