Histology of the Human Ear I. External ear (Figure ... - Faculty.rmc.edu

Histology of the Human Ear
I. External ear (Figure 25.1)
A. Pinna
1. core of elastic cartilage and fibrocartilage
2. covering of "thin" skin (epidermis=stratified squamous epithelium and dermis=dense
irrecular FECT)
B. External auditory meatus (ear canal) (Figure 25.4)
1. surrounded by bone of skull (deepest layer)
2. dense collagenous connective tissue (periosteum of bone)
3. moderately dense FECT; the secretory units of Ceruminous Glands (simple tubular
glands which secrete ear wax) are in this layer (connected to the stratified squamous
epithelium by ducts)
4. thin layer of loose FECT
5. covering of stratified squamous epithelium (lines canal)
C. Tympanic membrane (eardrum) (Figure 25.4)
1. core of collagenous connective tissue
2. outer covering of stratified squamous epithelium
3. inner (facing middle ear cavity) covering of simple cuboidal epithelium
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II. Middle Ear (Figure 25.1, 25.3, 25.4, 25.5)
A. Cavity
1. surrounded by bone of skull
2. dense collagenous connective tissue of skull periosteum
3. thin layer of loose FECT
4. covering of simple cuboidal epithelium
B. Ossicles
1. core of bone
2. same wall composition as cavity
C. Eustacean tube
1. same wall structure as cavity
2. covering converts to pseudostratified columnar epithelium with goblet cells as the
tube approaches the pharynx
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III. Inner Ear (Figure 25.1, 25.3, 25.7)
A. Osseus Labyrinth
1. cavity in temporal bone
2. contains membranous labyrinth
3. also contains perilymphatic fluid space between bone and membranous labyrinth which
forms the vestibule and the outer channels of the cochlea (vestibular duct and tympanic
duct)
4. areas between bone and fluid spaces filled with loose FECT
5. fluid spaces are lined with simple squamous epithelium
B. Membranous Labyrinth
1. lined with simple squamous epithelium in most areas
2. lined with simple columnar epithelium with "hair" cells in sensory areas
3. lined with simple cuboidal or columnar epithelium in fluid production/resorption areas
4. epithelium underlain by loose FECT in most areas
C. Sensory Receptor Structures in the Inner Ear (Figure 25.8, 25.9)
1. Common features of receptors
a. Sensory receptors in all cases are "hair" cells.
(1) "Hair" cells are columnar epithelial cells in the wall of the membranous
labyrinth.
(2) "Hair" cells have an apical V-shaped array (viewed from the surface) of
microvilli.
(3) "Hair" cells have either a kinocilium (sensory cilium, found in hair cells in the
maculae and the ampullae) or the basal body of a kinocilium (in the hair cells of
the Organ of Corti in the cochlea) is located at the apex of the "V" shaped array
of microvilli.
b. The processes (microvillae) of the hair cells are embedded in gelatinous accessory
structures which modify the stimuli (gravity, movement, sound) into mechanical
motion of the hair cell processes.
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2. Receptor structures in the inner ear
a. Maculae (Figure 25.1, 25.4, 25.11)
(1) Structure
(a) Two maculae (oriented at right angles to each other) occur in each ear.
- maculus utriculus in the utricular chamber
- maculus sacculus in the saccular chamber
(b) "Hair" cells of the maculae are embedded in gelatinous material containing
otoliths.
(2) Function
- otoliths are crystals of calcium carbonate and protein
- otoliths make the gelatinous material more dense than the fluid around it
(a) The dense otoliths cause the the gelatinous material to be pulled down by
gravity.
(b) The movement of the gelatinous material bends the hair cell processes.
(c) Bending of the hair cell processes increases or decreases the rate of nerve
impulses going to the CNS.
(d) The receptors are actually sensing gravity, but the organism's CNS
interprets the signals as body orientation (relative to gravity).
b. Ampullae (and semicircular canals) (Figure 25.1, 25.4, 25.10, Plate 104)
(1) Structure
(a) One ampulla is located in the enlarged chamber near one end of each of
the semicircular canals of the inner ear.
(b) The 3 semicircular canals in each ear are oriented at right angles to each
other.
(c) The processes of the hair cells of each ampulla are embedded in
gelatinous material which forms a dam-like structure (the cupula) across
the semicircular canal.
(d) Each semicircular canal is filled with endolymphatic fluid.
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(2) Function
(a) When the organism's head moves, the fluid in the semicircular canal tends
to stay in place due to inertia.
(b) The head movement therefore results in the wall of the canal moving past
the fluid.
(c) Movement of the wall relative to the fluid causes the fluid to press on the
"dam" in the ampulla (the cupula) formed by the gelatinous mass attached
to the hair cells.
(d) Bending of the cupula in turn moves the processes of the hair cells.
(e) Movement of the processes on the hair cells changes the frequency of
impulses in the sensory neurons which receive inputs from the hair cells.
(f) The organism experiences stimulation of the ampullae as motion, but the
stimulus is actually acceleration (a change in the rate of motion).
c. Cochlea (Figure 25.1, 25.4, 25.13, 25.14, 25.17, 25.21, Plate 104, Plate 105)
(1) Structure
(a) The cochlea is a tubular structure which is coiled into a snail shell-like
spiral.
(b) Two outer canals run the full length of the cochlea.
- the vestibular canal (scala vestibuli)
- the tympanic canal (scala tympani)
(c) The outer canals are continuous at the tip of the cochlea.
(d) A central canal (the cochlear duct) is located between the two outer
canals and does not connect directly to the two outer canals.
(e) The sensory cells (hair cells) are embedded in the columnar epithelium that
forms the "floor" of the cochlear duct. The complex of hair cells and
supporting cells is called the Organ of Corti.
(f) Two arrays of hair cells (inner and outer) run longitudinally within the organ
of Corti.
(g) The microvilli of the hair cells are embedded in a gelatinous structure (the
tectorial membrane) which runs longitudinally in the organ of Corti.
(h) One edge of the organ of Corti (both tectorial membrane and sensory
epithelium) is anchored to bone (the osseus spiral lamina). The other edge
is much more flexible.
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(2) Function
(a) Sound waves in the air enter the outer ear and cause vibrations of the
tympanic membrane (eardrum).
(b) Vibration of the tympanic membrane causes vibrations of the middle ear
ossicles.
(c) Vibration of the stapes causes vibration of the oval window in the surface of
the vestibular canal at the base of the cochlea.
(d) Vibration of the oval window causes vibration (pressure waves) in the fluid
of the vestibular canal and (through its communication with the tympanic
canal at the apex of the cochlea) the tympanic canal.
(e) Where the pressure waves in the outer canals (vestibular and tympanic)
are out of phase, they result in vertical vibration of the organ of Corti.
(f) Vertical movement of the organ of Corti causes the tectorial membrane to
move laterally on the surface of the hair cells. The conversion of vertical
movement into lateral movement is due to the lack of movement of the
attachment of the tectorial membrane and the sensory epithelium with the
osseus spiral lamina.
(g) This system converts both frequency (determines the location along the
organ of Corti at which maximum vibration occurs) and intensity
(determines the extent of the vibrating area) of sound into mechanical
movement of hair cell processes. Further processing of the signal in the
spiral ganglion and in the auditory centers in the brain appear necessary to
produce the ability to discriminate among sounds that is demonstrated by
musicians.
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