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ANAT D502 – Basic Histology
The Ear
Revised 11.13.15
Reading assignment: Chapter 25: Ear; pay particular attention to Boxes 25.1, 25.2 and 25.3 (Clinical
Correlations).
Outline:
I.
II.
III.
IV.
V.
VI.
Introduction
The air cell
Internal ear
Vestibular system
Cochlear system
External and middle ear
I. Introduction
The vestibulocochlear (or more broadly, the vestibuloacoustical) apparatus comprises the internal ear
(inner ear) and is located within petrous portion of the temporal bone. It provides two functions:
1. vestibular function for equilibrium (balance)
2. auditory function for hearing
The vestibular portion contains five neuroepithelial sensory receptors: Three cristae ampullaris and two
maculae (one utricular, one saccular). These receptors detect rotational and linear acceleration as well
as gravity and are used to maintain equilibrium. The auditory portion contains a single neuroepithelial
sensory receptor, the spiral organ. The auditory portion in humans has two adnexa (look it up), the
external ear and middle ear (tympanic cavity). Collectively the external ear, middle ear and cochlea
(which contains the spiral organ) are used to detect sound waves.
The primary sensory cell in all of the vestibuloacoustical organs is the hair cell. These cells are derived
from the lateral line system of our aquatic ancestors and thus detect water (fluid) displacement. The
hair cells of vestibulocochlear apparatus have been internalized but their function remains the same, i.e.,
detecting fluid displacement. Transduction in all of the vestibuloacoustical organs involves converting the
stimulus to be detected [sound waves, linear acceleration, etc.) to fluid (endolymph) displacement and
using hair cells to detect this motion.
As in all sensory systems, the sensory organs of the vestibulocochlear apparatus transduce physical
stimuli into electrical impulses. These neural signals are then processed by the brain to produce the
conscious sensations of sound and balance.
II. The hair cell
Hair cells along with support cells form a neuroepithelium in each of the inner ear sensory organs. Hair
cells are mechanoreceptors that convert mechanical energy (movement) to electrical energy (changes in
cell membrane potential). Each hair cell possesses numerous stereocilia (more properly termed long
microvilli, i.e., non-motile cytoplasmic extensions) called sensory hairs that project from its apical surface
into a fluid-filled chamber (the fluid being endolymph). In the vestibular organs only, each hair cell also
posses a single true cilium (containing microtubules) called the kinocilium.
The base of each hair cell synapses with afferent and efferent nerve endings. The efferent nerve endings
are thought to exert an inhibitory effect on hair cell function. Displacement (bending) of the stereocilia
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causes the hair cell to change its membrane potential and this change is conveyed to the CNS via the
afferent fibers. In hair cells with kinocilium, bending of the stereocilia away from the kinocilium produces
hyperpolarization of the hair cell membrane resulting in decreased firing activity in the associated afferent
fiber. Conversely, bending towards the kinocilium causes depolarization of the hair cell membrane and
increased firing activity in the associated afferent fiber. The different organs of the inner ear are
positioned so that the fluid flow of endolymph responds to a specific stimulus assisted by special hair cell
capping structures.
III. Inner ear
Within the petrous portion of the temporal bone is a series of interconnected chambers called the bony
labyrinth. The largest of these chambers is the vestibule which has two membrane-covered openings,
the oval and round windows that function in audition. Additional chambers extending from the vestibule
are the three, orthogonally arranged semicircular canals (anterior, posterior, and lateral) and the cochlea.
Within these bony chambers lies a series of interconnected ducts called the membranous labyrinth that
contains the sensory receptors of the vestibuloacoustical apparatus. The ducts are the three semicircular
ducts, utricle and saccule (both within the vestibule), and the cochlea duct. The vestibular sensory
receptors are the maculae (pl.) of the utricle and saccule and the three cristae ampullaris of the
semicircular ducts. The auditory sensory receptor is the spiral organ (organ of Corti) lying within the
cochlear duct.
Both labyrinths are filled with a fluid. As their name suggests, endolymph is contained within the
membranous labyrinth and perilymph surrounds the membranous labyrinth filling the bony labyrinth. The
composition of the fluids differs slightly; perilymph resembles extracellular fluid (low K, high Na) whereas
endolymph resembles cytoplasmic (intracellular) fluid (high K, low Na).
IV. Vestibular system
A. Semi-circular ducts
The semi-circular ducts are orthogonallly arranged in three planes to permit detection of angular
acceleration. At their junction with the utricle the ducts (and canals) expand to form an ampulla. Within
the ampulla of each of the three ducts is a ridge covered with a neuroepithelium containing support and
hair cells called the crista ampullaris. The sensory hairs (kinocilium and stereocilia) are embedded in a
gelatinous mass called the cupula that projects into the lumen of the duct. Rotational movement of the
head in any plane produces a relative flow of the endolymph that moves the cupula displacing the
stereocilia and altering the hair cells’ membrane potential [recalling that displacement of the stereocilia
towards the kinocilium causes depolarization and displacement away from the kinocilium produces
hyperpolarization].
B. Saccule and utricle
Within the utricle and saccule are two patches of neuroepithelium (support and hair cells) called the
maculae that detect linear acceleration and gravitational pull. The macula of the saccule is oriented
vertically whereas that of the utricle is horizontally aligned. [Note that this dual, orthogonal arrangement
permits detection of gravity and linear acceleration in all body positions). The sensory hairs of the hair
cells are embedded in a gelatinous otolithic membrane that contains calcium carbonate concretions
called (such alliteration) otoliths; the otoliths serve to increase the mass of the detector, thus rendering it
sensitive to gravity. The otolithic membrane and its otoliths project into the lumen of the duct and are
suspended within endolymph. When stationary, gravity acting on the otoliths causes the displacement of
the sterocilia altering the cell membrane potential. Linear movement causes fluid flow within the duct to
push against the otolithic membrane causing the stereocilia to further displace (again altering the cell
membrane potential). Thus, these sensory organs can detect both static orientation (standing up or lying
down) and dynamic changes in linear acceleration.
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C. CN VIII (Vestibulocochlear nerve)
The nerve cell bodies of the [sensory] afferent fibers of the vestibular nerve are found in the vestibular
ganglion within the internal acoustic meatus. The nerve cells bodies for the afferent fibers of the cochlear
(acoustical) nerve are found in the spiral ganglion within the modiolus (the center bony core of the spiral
cochlea). These ganglion cells are bipolar neurons with the peripheral process contacting the hair cells
and their central process forming the vestibular and cochlear nerves. The vestibular and cochlear nerves
join together within the internal acoustic meatus to form cranial nerve VIII (vestibulocochlear or
vestibuloacoustical)
V. Cochlear system
Situated within the spiral cochlear canal is the cochlear duct containing the spiral organ that indirectly
detects sounds waves. The cochlear duct divides the cochlear canal into three parallel, fluid-filled
compartments or scalae:
1. scala vestibuli
2. scala media (= cochlear duct)
3. scala tympani
The cochlear duct (scala media) is filled with endolymph and is triangular in cross section. Its external
wall is formed by the spiral ligament and stria vascularis; the latter tissue is thought to produce
endolymph. The roof is formed by the vestibular membrane and the floor by the basilar membrane.
Resting atop the basilar membrane is the spiral organ (organ of Corti).
The perilymph-filled compartment above the vestibular membrane is the scala vestibuli that terminates at
the oval window of the vestibule. The perilymph filled compartment below the basilar membrane is the
scala tympani that terminates at the round window of the vestibule. A communication between the scala
vestibuli and scala tympani is found at the distal tip of the cochlear canal and is called, logically enough,
the helicotrema.
A. Spiral organ (organ of Corti)
The spiral organ is a complex neuroepithelia resting atop the flexible basilar membrane. It is comprised
of hair cells and large number of different types of support cells. The hair cells are arranged in two
groups: a single row of inner hair cells and 3-5 rows of outer hair cells; both groups extend along the
length of the duct. Each hair cells sits atop a support cell called a phalangeal cell that attaches to the
basilar membrane. Hair cells in the spiral organ lack a kinocilium and their stereocilia are attached
apically to the tectorial membrane. The tectorial membrane is a stiff, immobile keratinous-like structure
rigidly attached to the modiolus.
B. Wave transduction
Excursions of the stapes (see below) at the oval window produce pressure waves within the perilymph of
the scala vestibuli. These pressure waves pass across vestibular membrane into the endolymph of the
cochlear duct and then across the basilar membrane into perilymph of scala tympani where they travel
the length of the cochlea to ultimately dissipate at the membrane-covered round window.
As the pressure waves cross the basilar membrane they cause it to oscillate resulting in displacement of
the sterocilia of the hair cells relative to the rigid tectorial membrane. This displacement of the sterocilia
results in changes in the membrane potential of the hair cell.
The pressure waves produced by the movement of the stapes vary in frequency and amplitude. These
modalities are converted by the spiral organ into sound qualities. The loudness of a sound corresponds
to the amplitude of the pressure wave; the greater the amplitude, the greater the displacement of the
basilar membrane, and the louder the sound. The pitch of the sound corresponds to the frequency of the
waves; the basilar membrane is mechanically tuned such that high frequency sounds are sensed at the
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base of the cochlea whereas low frequency sounds are detected at the apex. Of course, these modalities
are simply represented by discharges of the hair cells and these neural signals are processed in the
brain to produce the perception of sound. [The same is true of our perception of balance; the vestibular
organs simply produce neural signals which are processed by the brain to give the sensation of balance.]
VI.
External and middle ear
A. External ear
The external ear is comprised of the auricle (pinna) and external acoustic meatus. The auricle is an
irregularly concave appendage located on the lateral side of the head. It is supported internally by an
elaborate elastic cartilage and is covered with thin skin. It is connected to the surrounding regions by
ligaments and muscles.
The external acoustic meatus is a short tube leading from the auricle to the tympanic membrane
(eardrum); it is divided into a lateral cartilaginous and medial bony portion. The thin pileous skin lining the
tube contains hair follicles, sebaceous glands and ceruminous glands. Ceruminous glands are modified
apocrine sweat glands. Thus, like apocrine sweat glands they are simple, coiled tubular glands found in
association with hair follicles. In histological section they appear as simple cuboidal tubules with empty
lumen. Cerumen or earwax is a mixture of ceruminous and sebaceous glands secretions mixed with
desquamated keratinocytes (yum!) that traps particulate matter entering the canal.
Functionally, the auricle (pinna) and external acoustic meatus act together to collect and slightly amply
the sound pressure reaching the tympanum (tympanic membrane).
B. Middle ear (tympanic cavity)
The tympanic membrane separates the tympanic cavity from the external acoustic meatus. Suspended
from the tympanic annulus of the temporal bone, its core is formed by a disc of radially and circularly
arranged collagen fibers. Covering the external aspect of the tympanum is a layer of thin skin
(continuous with that of the external acoustic meatus) and medially the tympanum is covered by the
mucosa of tympanic cavity.
The tympanic cavity lies medial to the tympanic membrane within the temporal bone. The cavity connects
to the pharynx via the misnamed auditory tube (more properly the pharyngotympanic tube). It is lined by
a mucous membrane consisting of a simple cuboidal epithelium with underlying lamina propria.
Within the cavity and situated between the tympanic membrane and the oval window of the vestibule is a
chain of three, articulating ossicles: the malleus, incus and stapes. The malleus has an attachment to the
tympanic membrane and the footplate of the stapes covers the oval window. Air vibrations (sound
waves) striking the tympanic membrane cause the ossicles to move and thus transmit the air vibrations of
the tympanic membrane to the oval window where they are converted to hydraulic pressure waves.
In this process the middle ear ossicles also amplify the sound waves (through a couple of mechanisms )
and thus overcome the impendence mis-match between the air-born sounds waves and the liquid-born
pressure waves transmitted at the oval window. Impedenace is the opposition by a system to the flow of
energy from a source, in this case from low-impendence air to high impedance liquid. Without this
impedance matching, air-born sound waves would suffer a 30 db loss in intensity at the oval window.
Two skeletal muscles attach to the ossicles, the tensor tympani and stapedius. These muscles function
in the attenuation reflex; i.e., in response to loud sounds (e.g., iPods, rock music, jet engines, etc) both
muscles contract to make the chain of ossicles more rigid, thus reducing the transmission of air vibrations
to the internal ear.
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