Download Hearing

Hearing
The Ear
• The ear is designed to capture disturbances in
the air in the form of moving waves of energy
known as sound waves
• The ear consists of three portions, the outer
ear, the middle ear, and the inner ear
• Each of these portions contain structures that
help direct sound waves from the exterior to
the receptors in the inner ear
The Outer Ear
• The outer ear is made
up of the auricle and
the external auditory
canal
• The external auditory
canal (EAC) penetrates
through the skull at the
external auditory
meatus and leads to
the middle ear
The Outer Ear
• The EAC contains glands in its lining of skin
that secrete a waxy substance called cerumen
which, along with tiny hairs, help to keep dirt
from entering the ear
• The auricle collects sound waves and directs
them down the EAC to the eardrum, which is
the entrance to the middle ear
The Middle Ear
• The middle ear contains an air-filled space
inside the temporal bone called the tympanic
cavity
• It also includes the eardrum, or tympanic
membrane, and three small bones called the
auditory ossicles
• The tympanic cavity connects to the throat by
way of the eustachian tubes
The Middle Ear
The Middle Ear
• The eustachian tubes allow air pressure to be
equalized on both sides of the tympanic
membrane so that it can vibrate freely
• Unfortunately, infections can travel from the
mouth to the throat to the middle ear by way
of mucous membranes
• The tympanic membrane, or eardrum,
vibrates in response to sound waves it
receives from the EAC
The Middle Ear
• It is connected to the auditory ossicles
(malleus, incus, and stapes or “hammer, anvil,
and stirrup”) which amplify and transmit
those vibrations to the oval window
• Within the inner ear, vibrations from the
stapes cause fluid to move, stimulating the
receptors for hearing
The Inner Ear
• The inner ear is also known as the labyrinth
because it is made up of a complicated series
of passageways; there is a bony portion and a
membranous portion
• The bony labyrinth is a series of canals within
the temporal bone
• It has three regions; the semicircular canals,
the vestibule, and the cochlea
Bony Labyrinth
The Inner Ear
• Within each of these bony regions is a
membranous region which parallels its outline
• Between the outer walls of the bony labyrinth
and the walls of the membranous labyrinth is
a fluid called perilymph
• Within the membranous labyrinth is a fluid
called endolymph
The Inner Ear
• The cochlea resembles a snail shell in that it
coils and spirals; generation of nerve impulses
occurs inside of this structure at the organ of
Corti
• Stimulating the organ of Corti is a round-about
process and requires the transmission of
waves through many different structures
The Inner Ear
• The cochlea is separated into two
compartments by a thin shelf of bone lined
with membrane
• The upper compartment is the scala vestibuli,
which extends from the oval window to the
end of the cochlea
• The lower compartment is the scala tympani,
extending in the opposite direction from the
end of the cochlea to the round window
located inferior to the oval window
From Vibration to
Nerve Impulse Transmission
The Inner Ear
• Within the membranous labyrinth is a portion
called the cochlear duct
• The cochlear duct passes between the two
tubular compartments (scala) to end as a
closed sac
• The two sides of the cochlear duct have
different names: the vestibular membrane
separates it from the scala vestibuli, the
basilar membrane separates it from the scala
tympani
Cross-sections of Cochlea
The Inner Ear
• On top of the basilar membrane is the organ
of Corti with is made up of hair cells with cilia
that extend into the endolymph of the
cochlear duct
• They contact a delicate, gelatinous membrane
that lies over them called the tectorial
membrane
• The basal ends of the hair cells are in contact
with the nerve fibers of the cochlear nerve
that connects to the brain
Organ of Corti
Cochlear Sensitivities
The Inner Ear
• Movement of the basilar membrane causes
the hair cells of the organ of Corti to bend
against the tectorial membrane
• The movement of the hairs causes the hair cell
membranes to release neurotransmitters
• If the movement is great enough, an action
potential is generated
From Vibration to
Nerve Impulse Transmission
Sense of Equilibrium
• Detected by receptor cells in the inner ear
• Has two aspects: static equilibrium and
dynamic equilibrium
• Static equilibrium refers to the sensation of
body position
• Dynamic equilibrium refers to the sensation of
rapid movements
Static Equilibrium
• Comes from receptors in the vestibule of the
inner ear
• Gives information regarding the exact position
of the head
• The membranous labyrinth contains two sacs
called the utricle and the sacule which are
connected to each other by a small duct
• Within their walls is a small, flat region of
special cells known as the macula
Macula
Static Equilibrium
• The macula contains two types of cells:
supporting epithelial cells and hair cells
• The hair cells are receptors and each contain a
long strand that extends into a thick, jelly-like
mass
• Lying in this mass is a layer of calcium
carbonate crystals, called otoliths
• The position of the head is monitored by the
movement of otoliths in the macula
Static Equilibrium
• When you tilt your head to one side, the
otoliths shift their position because of gravity
• Their movement causes the jelly-like mass to
move and pull on the hair cells
• The bending causes release of
neurotransmitters
Dynamic Equilibrium
• Detects rapid movements, mostly of the head
• The receptors that sense this aid in balancing
and are located within the fluid-filled
semicircular canals of the inner ear
• Within the membranous labyrinth of each of
these canals is an expanded region, called the
ampulla
• The ampulla contains the sensory organs,
called the cristae
Cupula
Dynamic Equilibrium
• Each crista contains groups of supporting cells
and a group of hair cells
• Hair cells are the receptors and embedded in
a jelly-like mass known as the cupula
• When the head shifts position rapidly, the
cupula moves, bending the hair cells which
release neurotransmitters in response
Last Slide
• I feel that it’s rude for you to pack up your
things before I’m finished talking so I put in a
“Dummy Note” to make you think there was
more ; )