The Ear
Objectives
At the end of this lecture, student will be able to
• Describe the anatomy of ear
• Identify the receptor organs for equilibrium
• Describe the function of receptor organs for equilibrium
• Describe the auditory pathway
• Explain the major events in the physiology of hearing
Content
• Auditory Pathway
• Physiology of hearing
The Ear
• Organ of hearing
• Supplied by the 8th cranial nerve, stimulated by
vibrations caused by sound waves
• Entire structure encased within the petrous portion of
temporal lobe, except pinna
Anatomy of
the ear
Ear is divided into three main regions
1. External (Outer)
Ear
– Auricle
(Pinna)
– Eardrum
– External
auditory canal
2. Middle Ear
– Auditory
ossicles
– Auditory
(eustachian) tube
3. Internal (inner)
ear
– Cochlea
– Vestibular
apparatus
– Semicircular
ducts
– Utricle
– Saccule
External
(Outer) Ear
Auricle (Pinna)
• Flap of elastic cartilage
• Rim of the auricle is the helix
• The inferior portion is the lobule
• Ligaments and muscles attach the auricle to the head
External auditory canal
• A curved tube, 2.5 cm long
• Lies in the temporal bone and leads to the eardrum
Eardrum/ Tympanic membrane
• A thin, semitransparent partition between the external auditory
canal and middle ear
• Covered by epidermis; lined by simple cuboidal epithelium
• Tearing of the tympanic membrane – perforated eardrum
Ceruminous glands –
specialized sweat glands in the external auditory canal; secrete wax called
cerumen
• Hairs + cerumen à prevent entry of dust and foreign objects
• Prevents damage
Middle Ear
• Small, air-filled cavity in the petrous portion of the
temporal bone, lined by epithelium
• Separated from the external ear by the tympanic membrane
• From the internal ear by a thin bony partition
• Contains two small
membrane-covered openings:
– The oval window
– The round window
Auditory ossicles
• Three smallest bones in the body
• Extending across the middle ear and attached to it by
ligaments
• Connected by synovial joints
The bones, named for their shapes
• The malleus (Hammer)
• Incus (Anvil)
• Stapes (Stirrup)
• Incus, the middle bone in the series, articulates with the
head of the stapes
• Base or footplate of the stapes fits into the oval window
• Round window
– Opening below the oval window
– Enclosed by a membrane, the secondary tympanic membrane
Two tiny skeletal muscles attach to the ossicles
|
Tensor |
Stapedius |
|
• Supplied by the trigeminal (V) nerve |
• Supplied by the facial (VII) nerve |
|
• Limits movement |
• Smallest skeletal muscle in the human body |
|
• Increases tension on the eardrum |
• Protects the oval window |
|
• Prevent damage to the inner ear from loud noises |
• decreases sensitivity of hearing |
The Auditory ossicles
Auditory (eustachian)
tube
• Opening in the anterior wall of the middle ear
• Consists of both bone and elastic cartilage
• Connects the middle ear with the nasopharynx
• Normally closed at its medial (pharyngeal) end
• During swallowing and yawning, it opens
• Allows air to enter or leave the middle ear until the
pressure in the middle ear equals the atmospheric pressure
Internal
(inner) ear
• Also called the labyrinth
• Because of its complicated series of canals
Consists of two main divisions:
• An outer bony labyrinth encloses an inner membranous
labyrinth
Bony labryinth –
A series of cavities in the petrous portion of the temporal bone
Divided into three areas:
(1) The semicircular canals, contain receptors for
equilibrium
(2) The vestibule, contain receptors for equilibrium
(3) The cochlea, contains receptors for hearing
• Bony labyrinth, lined with periosteum and contains
perilymph
• Epithelial membranous labyrinth contains endolymph
• Potassium ions in endolymph helps in generation of
auditory signals
• Vestibule, the oval central portion of the bony labyrinth
• Membranous labyrinth in the vestibule consists of two
sacs, the utricle and the saccule
• Semicircular canals
– 3 superior and posterior bony projections from the vestibule
• Ampulla –
swollen enlargement at one end of each canal
• Semicircular ducts
– Portions of the membranous labyrinth inside the bony semicircular canals
Cochlea
• A bony spiral canal, anterior to the vestibule
• Resembles a snail’s shell
• Makes almost three turns around a central bony core called
the modiolus
Divided into three channels
• Cochlear duct
• Scala vestibuli
• Scala tympani
Cochlear duct (Scala
media)
– A continuation of the membranous labyrinth into the
cochlea
– Filled with endolymph
• Channel above the cochlear duct, Scala vestibuli; ends at
the oval window
• Channel below, Scala tympani; end at the round window
• Vestibular membrane separates the cochlear duct and Scala
vestibuli
• Basilar membrane separates the cochlear duct from the
Scala tympani
Section through one turn of the cochlea
• Resting on the basilar membrane is the spiral organ or
organ of Corti
• Spiral organ, coiled sheet of epithelial cells
• Consists of supporting cells and about 16,000 hair cells
• Hair cells – receptors for hearing
• Tectorial membrane, a flexible gelatinous membrane, covers
the hair cells of the spiral organ
• Sensory neurons + motor neurons à cochlear branch of the
vestibulocochlear (VIII) nerve
• Cell bodies of the sensory neurons are located in the
spiral ganglion
Enlargement of spiral organ (organ of Corti)
Physiology of
hearing: Events are involved in hearing
Hair cells of the spiral organ convert a mechanical
vibration (stimulus) into an electrical signal (receptor potential)
The auditory pathway
Physiology of
equilibrium
Two types of equilibrium (balance)
Static equilibrium- maintenance of the position of the body
(mainly the head) relative to the force of gravity
• The maculae of the utricle and saccule are the sense
organs
• Body movements like tilting the head and linear
acceleration or deceleration stimulate the receptors
Dynamic equilibrium –
maintenance of body position (mainly the head) in response to rotational
acceleration/ deceleration
Position of a cupula with the head in the still position (left) and
when the head rotates
• Cristae in the semicircular ducts, main sense organs
• Any change of position of the head causes movement in the
perilymph and endolymph, bends the hair cells, stimulates sensory receptor in
utricles, saccule and ampullae
• The nerve impulse passed to cerebellum through vestibulo
cochlear nerve
Summary
• Ear is the organ for hearing
• External (outer) ear – auricle, external auditory canal,
and tympanic membrane (eardrum)
• Middle ear – auditory tube, ossicles, oval window, and
round window
• The internal (inner) ear consists of the bony labyrinth
and membranous labyrinth
• The internal ear contains the spiral organ (organ of
Corti), the organ of hearing
• Hair cells of the spiral organ convert a mechanical
vibration (stimulus) into an electrical signal (receptor potential)
• Static equilibrium is the orientation of the body relative
to the pull of gravity; utricle and saccule are the sense organs of static
equilibrium
• Dynamic equilibrium is the maintenance of body position in
response to rotational acceleration or deceleration; ristae in the semicircular
ducts are the main sense organs of dynamic equilibrium