Chapter 53 - The sense of hearing
Tympanic membrane and the ossicular system
From left to right (picture).
Sound travels through the auditory canal to
the tympanic membrane (commonly referred
to as eardrum). The tympanic membrane is
attached to the handle of the malleus
(indicated top left). The malleus itself is
attached to the incus by small ligaments,
meaning when the malleus moves, the incus
also moves. The end of the incus joins with the
stem of the stapes (shape commonly
compared to horseshoe). In the oval window,
the ‘faceplate’ of the stapes lies against the
membranous labyrinth of the cochlea. The
Source: Hall, J., 2020. Guyton And Hall Textbook
three structures (malleus, incus and stapes) are jointly Of Medical Physiology
referred to as the ossicles or ossicular system.
The tip end of the handle of the malleus is attached to the center of the tympanic membrane, this point
of attachment is constantly pulled by the tensor tympani muscles, which keeps the tympanic membrane
tensed, it would not function the same if the membrane were flaccid. The connection of the incus with
the stapes (horseshoe shaped structure) causes the stapes to firstly push forward on the oval window
(see picture) and subsequently on the cochlear fluid on the others side of the oval window every time
the tympanic membrane moves according to a specific vibration/sound. Secondly, the stapes pulls
backward on the cochlear fluid every time the malleus moves outward. Fluid has a greater inertia than
air does, therefore increased amounts of force are necessary to cause vibration in the fluid. This increase
of force is generated by the lever system of the ossicular system. The tympanic membrane and ossicular
system provide impedance matching between the sound waves in air and the sound vibrations in the
fluid of the cochlea.
When loud sounds are transmitted through the ossicular system and from there into the central nervous
system, a reflex occurs to cause contraction of the stapedius muscle and, to a lesser extent, the tensor
tympani muscle. This attenuation reflex can reduce the intensity of lower-frequency sound transmission
by 30-40 decibels and is believed to protect the cochlea from damaging vibrations and to mask low-
frequency sounds in loud environments.
Transmission of sound through bone
Because the inner ear, the cochlea, is embedded in a bony cavity in the temporal bone, called the bony
labyrinth, vibrations of the entire skull can cause fluid vibrations in the cochlea. This can subsequently
be used in diagnostic testing by means of a tuning fork.
, Cochlea
It consists of three tubes coiled side by
side
1. The scala vestibuli
2. Scala media
3. Scala tympani
The scala vestibuli (top right on
picture) and scala media are separated
from each other by Reissner's
membrane (vestibular membrane); the
scala tympani and scala media are
separated by the basilar membrane.
On the surface of the basilar
membrane lies the organ of Corti,
Source: Hall, J., 2020. Guyton And Hall Textbook Of
which contains a series of electromechanically sensitive Medical Physiology
cells, the hair cells. At the oval window sound vibrations
enter the scala vestibule from the faceplate of the stapes. Inward movement causes the fluid to move
forward in the scala vestibuli and scala media, and outward movement causes the fluid to move
backward. The basilar membrane is a fibrous membrane that contains basilar fibers (20.000-30.000) that
project from the bony center of the cochlea, the modiolus, toward the outer wall. The lengths of the
basilar fibers increase progressively. The diameter of the fibers decreases from the oval window to the
helicotrema (where scala tympani and scala vestibuli meet). Thus, high-frequency resonance of the
basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval
window. Low-frequency resonance occurs near the helicotrema.
The organ of Corti is the receptor
organ that generates nerve
impulses in response to vibration
of the basilar membrane. The
actual sensory receptors in the
organ of corti are two specialized
types of nerve cells called hair
cells:
- Single row of internal hair
cells
- Outer hair cells.
The bases and sides of the hair cell
synapse with a network of
cochlear nerve endings. The nerve
fibers stimulated by the hair cells
lead to spiral ganglion of corti, Source: Hall, J., 2020. Guyton And Hall Textbook
which lies in the modiolus of the Of Medical Physiology
cochlea. The spiral ganglion neuronal cells send axons
into the cochlear nerve and then into the central nervous system at the level of the upper medulla.
The outer ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the
reticular lamina, supported by triangular rods of Corti, which are attached tightly to the basilar fibers.
Tympanic membrane and the ossicular system
From left to right (picture).
Sound travels through the auditory canal to
the tympanic membrane (commonly referred
to as eardrum). The tympanic membrane is
attached to the handle of the malleus
(indicated top left). The malleus itself is
attached to the incus by small ligaments,
meaning when the malleus moves, the incus
also moves. The end of the incus joins with the
stem of the stapes (shape commonly
compared to horseshoe). In the oval window,
the ‘faceplate’ of the stapes lies against the
membranous labyrinth of the cochlea. The
Source: Hall, J., 2020. Guyton And Hall Textbook
three structures (malleus, incus and stapes) are jointly Of Medical Physiology
referred to as the ossicles or ossicular system.
The tip end of the handle of the malleus is attached to the center of the tympanic membrane, this point
of attachment is constantly pulled by the tensor tympani muscles, which keeps the tympanic membrane
tensed, it would not function the same if the membrane were flaccid. The connection of the incus with
the stapes (horseshoe shaped structure) causes the stapes to firstly push forward on the oval window
(see picture) and subsequently on the cochlear fluid on the others side of the oval window every time
the tympanic membrane moves according to a specific vibration/sound. Secondly, the stapes pulls
backward on the cochlear fluid every time the malleus moves outward. Fluid has a greater inertia than
air does, therefore increased amounts of force are necessary to cause vibration in the fluid. This increase
of force is generated by the lever system of the ossicular system. The tympanic membrane and ossicular
system provide impedance matching between the sound waves in air and the sound vibrations in the
fluid of the cochlea.
When loud sounds are transmitted through the ossicular system and from there into the central nervous
system, a reflex occurs to cause contraction of the stapedius muscle and, to a lesser extent, the tensor
tympani muscle. This attenuation reflex can reduce the intensity of lower-frequency sound transmission
by 30-40 decibels and is believed to protect the cochlea from damaging vibrations and to mask low-
frequency sounds in loud environments.
Transmission of sound through bone
Because the inner ear, the cochlea, is embedded in a bony cavity in the temporal bone, called the bony
labyrinth, vibrations of the entire skull can cause fluid vibrations in the cochlea. This can subsequently
be used in diagnostic testing by means of a tuning fork.
, Cochlea
It consists of three tubes coiled side by
side
1. The scala vestibuli
2. Scala media
3. Scala tympani
The scala vestibuli (top right on
picture) and scala media are separated
from each other by Reissner's
membrane (vestibular membrane); the
scala tympani and scala media are
separated by the basilar membrane.
On the surface of the basilar
membrane lies the organ of Corti,
Source: Hall, J., 2020. Guyton And Hall Textbook Of
which contains a series of electromechanically sensitive Medical Physiology
cells, the hair cells. At the oval window sound vibrations
enter the scala vestibule from the faceplate of the stapes. Inward movement causes the fluid to move
forward in the scala vestibuli and scala media, and outward movement causes the fluid to move
backward. The basilar membrane is a fibrous membrane that contains basilar fibers (20.000-30.000) that
project from the bony center of the cochlea, the modiolus, toward the outer wall. The lengths of the
basilar fibers increase progressively. The diameter of the fibers decreases from the oval window to the
helicotrema (where scala tympani and scala vestibuli meet). Thus, high-frequency resonance of the
basilar membrane occurs near the base, where the sound waves enter the cochlea through the oval
window. Low-frequency resonance occurs near the helicotrema.
The organ of Corti is the receptor
organ that generates nerve
impulses in response to vibration
of the basilar membrane. The
actual sensory receptors in the
organ of corti are two specialized
types of nerve cells called hair
cells:
- Single row of internal hair
cells
- Outer hair cells.
The bases and sides of the hair cell
synapse with a network of
cochlear nerve endings. The nerve
fibers stimulated by the hair cells
lead to spiral ganglion of corti, Source: Hall, J., 2020. Guyton And Hall Textbook
which lies in the modiolus of the Of Medical Physiology
cochlea. The spiral ganglion neuronal cells send axons
into the cochlear nerve and then into the central nervous system at the level of the upper medulla.
The outer ends of the hair cells are fixed tightly in a rigid structure composed of a flat plate, called the
reticular lamina, supported by triangular rods of Corti, which are attached tightly to the basilar fibers.
