• Two important functions of the ear
• Enables us to hear
• Sensory organ of balance or equilibrium
• Location of one ear on each side of head produces
• Hearing from both sides
• External ear
• Visible portion, not contained within the head
• Auricle or pinna
• Cartilaginous flap or ear lobe
• External auditory canal
• Tube leading from auricle to the middle ear
• Lined with tiny hairs called cilia to aid in
transmitting sound waves inward
• Tympanic membrane (eardrum)
• Separates external ear from middle ear
Structures of the Ear
• Middle ear
• Three tiny bones known as auditory ossicles
• Resembles shape of a hammer
• Connected to tympanic membrane and transmits
sound vibrations to second auditory ossicle
• Resembles shape of an anvil
• Transmits sound vibrations from malleus to third
Structures of the Ear
• Middle ear
• Shaped like a tiny stirrup
• Transmits sound vibrations from incus to inner ear
• Eustachian tube
• Connects middle ear to pharynx
• Auditory tube
• Oval window
• Separates middle ear from inner ear
• Base of stapes fits into oval window
Structures of the Ear
• Inner ear
• Central portion of inner ear
• Located next to stapes and between cochlea and
• Contains utricle and saccule-membranous
pouches or sacs that aid in maintaining balance
• Snail-shaped bony structure
• Contains endolymph and perilymph
• Auditory fluids that aid in transmission of sound
Structures of the Ear
Introduction to vestibular appratus
It is part of inner ear or labyrinth
It is a sensory organ that detects sensation of
Vestibular apparatus plays an important role in
maintaining posture and equilibrium
The other part of labyrinth is the cochlea, which
is concerned with sensation of hearing
Labyrinth or inner ear consists of two structure:
It is composed of a system of bony tubes and
chambers located in the petrous portion of the
temporal bone, called the bony labyrinth,
And within this, is a system of membranous
tubes and chambers called the membranous
The space between bony and membranous
labyrinth filled with a fluid called perilymph or
periotic fluid. This fluid is similar ECF in
composition with large amount of sodium ions.
It is formed by membraneous tubules and sacs.
Tubules are semicicular canals & sacs are
vestibular and cochlear duct.
The vestibular duct consists of otolith organ.
Otolith organ is formed is formed by utricle and
saccule. The membranous labyrinth is filled with
a fluid called endolymph or otic fluid. The
endolymph is similar to ICF in composition.This
has more quantity of potassium ions.
STRUCTURE OF SEMICIRCULAR CANAL
Vestibular apparatus is formed by three
semicircular ducts and otolith organ
The three semi-circular canals in each vestibular
apparatus, are placed at right angles to each
Because of this type of arrangement, the
semicircular canals represent the three axes of
rotation: vertical, anteroposterior and transverse
The semicircular canals are named according to
the situation as follows:
1.Anterior or superior canal
3.Lateral or horizontal or external canal
The anterior and posterior canals are situated
vertically and the lateral canal is situated in
The lateral canal of vestibular apparatus on both
sides are placed horizontally in the same plane
with the convexities directed outwards and a little
When the head is bent forward about 30 degrees,
the lateral semicircular canals are then
approximately horizontal with respect to the
surface of the earth
The anterior canals are in vertical planes that
project forward and 45 degrees outward, whereas
the posterior canals are also in vertical planes but
project backward and 45 degrees outward.
Among the two ends of semicircular canal , one
and is narrow and other end is enlarged.
The enlarged end is called ampulla.
The ampulla contains the receptor organ known as
The ampulla of all the 3 canals & narrow end of
horizontal canal open directly into the utricle .
The narrow end of anterior & posterior canals
open into the utricle jointly , by forming a common
Thus, semicircular canals open into the utricle by
means of five openings.
OTOLITH ORGAN OR VESTIBULE
Otolith organ or vestibule is formed by utricle and
Often utricle and saccule are together called
Utricle communicates with saccule through
Saccule communicates with cochlear duct through
Another duct called endolymphatic duct arises
from utriculosaccular duct.
It ends in a bag like structure called
endolymphatic sac, which lies on the cranial
surface of petrous bone.
RECEPTOR ORGAN IN SEMICIRCULAR
CANAL CRISTA AMPULLARIS
It is a crest like structure situated inside the ampulla
of semicircular canals
The crest is formed by a receptor epithelium
(neuroepithelium) which consists if hair cells,
supporting cells and secreting epithelial cells
The secreting cells secrete the ground substance,
These cells are arranged in planum semilunatum
(group of epithelial cells) around hair cells
• Hair cells are the receptor cells of crista ampullaris. There are
2 types of hair cells, type I and type II hair cells. Hair cells of
semicircular canals, utricle and saccule receive both afferent
and efferent nerve terminals.
TYPE I HAIR CELLS
Type I hair cells are flask shaped. The afferent nerve
terminates in the form of calyx that surrounds the cell body.
The efferent nerve terminal ends on the surface of the calyx.
TYPE II HAIR CELLS
These cells have a cylindrical or test tube shape. Both
afferent and efferent nerve fibres terminate on the surface cell
without forming calyx.
From crista ampullaris, a dome-shaped
gelatinous structure extends up to the roof of
the ampulla. It is known as cupula. The
cupula encloses the cilia of hair cells. The
cilia of hair cells are projected in the cupula.
The receptor organs in utricle and saccule are
Like crista ampullaris it is also formed by
neuroepithelium and supporting cells
The neuroepithelial of this also has two types of
hair cells, the type 1 and 2 hair cells.
Situation of macula is different in utricle and
In utricle , the macula is situated in horizontal
plane, so that the cilia from hair cells are in
verticle plane .
In saccule , the macula is in verticle plane and
the cilia are in horizontal.
BLOOD SUPPLY TO THE VESTIBULAR END
The main blood supply is through the internal auditory
(labyrinthine artery), which usually arises from the anterior
cerebellar artery, superior cerebellar artery, or basilar artery
The labyrinthine artery divides into two branches- anterior
vestibular artery and the common cochlear artery
Anterior vestibular artery provides the blood supply to most
of the utricle, to superior and horizontal ampullae’ and to
small portion of the saccule
The common carotid artery supply posterior ampulla, the
major part of the saccule, parts of the body of the utricle and
the horizontal and superior ampullae
NERVE SUPPLY TO VESTIBULAR APPARATUS
The impulses from the hair cells of crista ampullaris and
maculae are transmitted to medulla oblongata and other
parts of CNS through the fibres of vestibular division of
vestibulocochlear (VIII cranial) nerve.
FIRST ORDER NEURON
The first order neurons of the sensory pathway are bipolar in
nature. The soma of the bipolar cells is situated in vestibular
or Scarpa’s ganglion ( situated in internal auditory meatus) .
The dendrites of the bipolar cells reach the receptor organs,
i.e. crista ampullaris and maculae in vestibular apparatus.
The axon form the vestibular division of
vestibulo cochlear nerve.
The fibres from bipolar reach medulla oblongata
and terminate in the vestibular nuclei.The nerve
fibres are called primary vestibular fibres.
There are four vestibular nuclei in the medulla oblongata,
viz. superior, inferior, lateral and medial nuclei.
Most of the primary vestibular fibres reaching superior and
medial nuclei come from crista ampullaris of semicircular
Lateral vestibular nucleus receives the fibres mostly from
maculae of otolith organ, and, the inferior vestibular nucleus
receives fibres from both crista ampullaris and maculae.
It is believed that the efferent fibres to hair cells provide tonic
inhibition of hair cells.
The fibres from some bipolar cells reach cerebellum directly
and terminate in the flocculonodular lobe or the fastigial
nucleus in the cerebellum.
Connections to the vestibular nucleus from
The Process of Hearing
Pathway of sound
SECOND ORDER NEURON
The second order neurons of this pathway are located in the
four vestibular nuclei which forms secondary vestibular fibres
which further forms four tracts :
1. Vestibulo-ocular tract
This tract is concerned with movements of eyeballs in
relation to the position of the head.
2. Vestibulo-spinal tract
The fibres of this tract are involved in reflex movements
of head and body during postural changes.
3. Vestibulo-recticular tract
These fibres are concerned with facilitation of muscle
4. Vestibulo-cerebellar tract
Involved in coordination of movement acc. to body position.
FUNCTIONS OF VESTIBULAR APPRATUS
It is responsible for dectecting the position of
head during different movements.
It causes the reflex adjustments in the position of
eyeball, head and body during postural changes.
FUNCTION OF SEMICIRCULAR CANALS
Semicircular canals sense angular acceleration
FUNCTION OF OTOLITH ORGAN
The otolith organs sense linear acceleration
The saccule senses acceleration in the sagittal
vertical plane: up and down (so it senses
gravity) and forward and backward.
The utricle senses acceleration in the horizontal
APPLIED PHYSIOLOGY – EFFECT OF
Removal of labyrinthine apparatus on both sides leads to
complete loss of equilibrium
The equilibrium could be maintained only by visual
The postural reflexes are severely affected.
There is loss of hearing sensation .
• Removal of labyrinthine apparatus on one side causes
less effects on postural reflexes.
• Severe autonomic symtoms occur like nausea , vomiting
o It is defined as syndrome of physiological response
during movement to which the person is not adapted .
o Motion sickness is due to excessive and repeated
stimulation of vestibular apparatus.
o Nausea Discomfort
o Vomiting Headache
o Sweating Disorientation
o It can be done by avioding greasy and bulky
food before travel and by taking antiemic drugs