The Ear – Human Anatomy and Physiology B. Pharma 1st Semester

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

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