PHYSIOLOGY FOR NURSES

1. Physiology of Ear

2. Functional anatomy of ear
 The ear consists of 3 parts
 External ear
 Middle ear
 Internal ear

4. External ear
 Formed by 2 parts
Auricle or pinna
 It projects from side of the head.
 It is composed of a thin piece of elastic
fibrocartilage,covered by skin.
 It funnels sound waves towards the external
auditory meatus.

5.  The skin covering the pinna contains many
fine hairs and sebaceous glands.
 The depression of auricle,which forms the
orifice of external auditory meatus is called
concha.
 External auditory meatus
 Starts from concha and extends inside as a
slightly curved canal with a length of about
55 mm

6.  Meatus consist of 2 parts
 The outer cartilaginous part
 Initial part of external auditory meatus which made up
of cartilage.
 It is covered by thick skin,which contains stiff
hairs(prevents the entry of foreign particles)
 Large sebaceous glands and cereminous glands are also
present in this skin.
 The secretions from these glands form the ear wax.

7.  Inner bony part
 Inner part is made up of bone and covered by skin
 Only sebaceous glands are present here.
 Small hairs are present on the superior wall of the
canal
Middle ear
 The middle ear or tympanic cavity is a
small,narrow,irregular chamber situated within the
temporal bone.
 It is separated from external auditory meatus by
tympanic membrane.

9.  Tympanic membrane
It is a thin,semi transparent membrane,which
separates the middle ear from auditory
meatus.
Middle ear consists of
 Auditory ossicles
 Auditory muscles
 Eustachian tube.

10. 1. Auditory ossicles
The auditory ossicles are the 3 miniature bones,which are
arranged in the form of a chain extending across the
middle ear from the tympanic membrane to oval window.
 Malleushammer
It has a handle(manubrium),head,and neck.
The head articulates with the body of next bone incus.
 Incusanvil
It has a body,one long process,and one short process.
Anterior surface of the body articulates with head of
malleus and the tip of long process articulates with next
bone stapes

12.  Stapesstirrup
It is the smallest bone present in the body.
It has a head,neck,anterior crus,posterior crus and a foot plate.
Head articulates with the incus and foot plate fits into the
oval window.
2.Auditory muscles
1.Tensor tympani
2.Stapedius
 1.tensor tympani
Arises from cartilaginous portion of Eustachian tube.the tensor
tympani is attached to tympanic membrane through malleus.

14.  Function
It pulls and keeps the tympanic membrane stretched or
tensed constantly.this constant stretching of tympanic
membrane is essential for transmission of sound waves,which
may reach any part of the tympanic membrane.
2.Stapedius
 It is the smallest skeletal muscle in human body.
 It lies on the posterior wall of the tympanic cavity and is
inserted into posterior surface of neck of stapes.
 Function
 Prevents excess movements of stapes.

15. Tympanic reflex
 It is a attenuation reflex characterized by involuntary
contraction of tensor tympani and stapedius muscle in
response to a loud noise.
 When both the muscles contract,manubrium of malleus
moves inward and stapes is pulled outward.these 2 actions
result in stiffness of auditory ossicles.so the transmission
of sound is decreased.
 Significance
 Protects the tympanic membrane from being ruptured by
loud sound.
 Prevents fixation of footplate of stapes.
 Protect the cochlea from damaging effects of loud sounds.

17.  Eustachian tubeauditory tube
 It is a flattened canal extending from the anterior
wall of the middle ear to the nasopharynx.
 It forms the passage of air between middle ear and
atmosphere.thus the pressure on both sides of
tympanic membrane is equalized.

18. Internal earlabyrinth
 Consists of 2 structures
 1.bony labyrinth
 2.membranous labyrinth
 1.Bony labyrinth
It is a series of cavities or channels present in the petrous
part of temporal bone.
 2.membranous labyrinth
Membranous labyrinth is situated inside bony labyrinth
which is formed by membranous tubules and sacs.
 The space between bony and membranous labyrinth is
filled with a fluid called perilymph or periotic fluid which
contain large amount of sodium.

22.  The membranous labyrinth consist of
 1.cochlea concerned with sensation of hearing
 2.vestibular apparatus concerned with posture and
equilibrium.
 The ML is filled with a fluid called endolymph or otic fluid
containing large amount of potassium ions
 Vestibular apparatus
 Formed by 3 semicircular canals and otolith organ.
 Semicircular canals are the tubular structures placed at right
angles to each other
 The semicircular canals are
 Anterior or superior canal

24.  Posterior canal
 Lateral or horizontal or external canal
 Ampulla
 There are two ends for each semicircular canal
.one end is narrow and the other end is
enlarged.this enlarged end is called ampulla.
 This ampulla contains the receptor organ of
semicircular canals known as crista ampullaris.
 All the 3 semicircular canals open into
utricle.utricle opens into saccule.

26. Otolith organ or vestibule
 It is formed by utricle and saccule.
 Utricle communicates with saccule through utriculo saccular duct.
 Saccule communicates with cochlear duct through ductus reuniens.
 Another duct called endolymphatic duct arises from utriculosaccular
duct.it ends like a bag like structure called endolymphatic sac.
 The receptor organ in otolith organ is called macula.
 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 central nervous
system through the fibers of vestibular division of vestibulocochlear
nerve.

28. Functions of vestibular apparatus
 The vestibular apparatus is responsible for detecting the position of
head during different movements.it also causes the reflex
adjustments in the position of eyeball,head and body during
postural changes.
 Semicircular canals
 These canals are concerned with angular(rotatory) acceleration.
 They sense the rotational movements.
 Each semicircular canal is sensitive to rotation in a particular plane.
 Superior canal- gives response to rotation in anteroposterior plane.(
(eg ;yes yes)
 Horizontal canal- gives response to rotation in horizontal plane.
(no no)
 Posterior canal- gives response to rotation in vertical plane.shoulder
to shoulder

29.  Otolith organ
Utricle responds during horizontal acceleration
and saccule responds during vertical
acceleration.

30. cochlea
 Cochlea is a coiled structure like a snail’s shell.it
consists of two structures
 1.central conical axis formed by spongy bone called
modiolus
 2.Bony canal or tube,which winds around the modiolus.
 From the modiolus,a bony ridge called osseous spiral lamina
projects into canal,winding around the modiolus like the
thread of screw.
 Compartments of cochlea
 2 membranes divide the spiral canal of cochlea into 3
compartments.
 1.basilar membrane
 2.vestibular membrane

34.  The compartments of spiral canal of cochlea are
 1.scala vestibule
 It lies above the scala media.
 It arises from oval window(fenestra vestibule) which is closed
by footplate of stapes
 It follows the osseous canal up to its apex and at the apex,it
communicates with scala tympani by a small opening called
helicotrema
 2.scala tympani
 It lies below the scala media.
 It is parallel to scala vestibule and ends at the round
window.
 The round window is closed by a strong thin membrane

36.  Scala mediacochlear ductmembranous cochleaotic cochlea
 It is a triangular compartment enclosed by basilar and
vestibular membranes.
 Scala media is formed by upper,outer and lower walls.
 The sense organ of hearing ,organ of corti is situated on the
upper surface of the basilar membrane.
 .

37. Organ of corti
 Organ of corti is the receptor organ for
hearing.
 It rests upon the basilar membrane.
 The receptors for hearing are the hair cells in
the organ of corti.
 The hair cells are arranged on either sides
of pillar like cells called rods of corti.
 The triangular space enclosed by the rods of
corti is called tunnel of corti.

38.  There are also supporting cells in the organ of
corti
 The roof of the organ of corti is formed by
tectorial membrane.
 The tip of each hair cells consists of numerous
stereocillia that extend into the endolymph in
the scala media.
 The sensory nerve fibers of the cochlear division
of VIII cranial nerve arise from the base of the
hair cells.

41.  All the cells of organ of corti are arranged in order
from center towards periphery of the cochlea
 1.border cells
 2.inner haircells
 3.inner phalangeal cells
 4.inner pillar cells
 5.outer pillar cells.
 6.outer phalangeal cells
 7.outer hair cells
 8.cells of hensen
 9.cells of Claudius
 10.tectorial membrane and lamina reticularis

42. Mechanism of hearing
1.Role of external ear
External ear directs the sound waves towards the
tympanic membrane.the sound waves produce pressure
changes over the surface of tympanic membrane.
2.Role of middle ear
Tympanic membrane:
Due to the pressure changes produced by sound
waves,the tympanic membrane vibrates.

43.  Auditory ossicles
The vibrations set up in tympanic membrane are
transmitted through the malleus,incus and reach the
stapes,causing to and fro movement of stapes against
oval window and against the perilymph present in
scala vestibule of cochlea.
Impedance matching
It is the process,in which the tympanic membrane and
auditory ossicles convert the sound energy into the
mechanical vibrations in the fluid of internal ear with
minimum loss of energy by matching the
impedance(means any obstruction to the passage of
sound waves) offered by the fluid.

44.  Significance of impedance matching:
It is the most important function of middle ear. Because of IM ,the
sound waves are transmitted to cochlea with minimum loss of
intensity.
 Conduction of sound
Conduction of sound from external ear to internal ear through
middle ear occurs by 3 routes:
1. Ossicular conduction:conduction of sound waves through auditory
ossicles
2. Air conduction :through air in the middle ear
3. Bone conduction : the sound waves are transmitted to cochlear
fluid by the vibrations set up in the skull bones.it is tested by
placing the vibrating tuning fork directly on the skull.

45.  Eustachian tube.
It is resposnsible for equalizing the pressure on either side of
tympanic membrane.
3.Role of inner ear
The movement of footplate of stapes against oval window
causes movement of perilymph in scala vestibuli.
The movement of fluid in scala vestibuli causes displacement of
fluid in scala media,as the vestibular membrane is flexible.
This causes bulging of basal portion of basilar membrane
towards round window.this in turn moves the fluid in scala
tympani towards round window and bulging of round window into
middle ear.

47.  The travelling wave causes movement of stereocillia
leading to excitement of hair cells and generation of
receptor potential.
 This produces action potential in the auditory nerve
fibers which is conducted to the auditory cortex.
 When stereocillia of the haircells of the organ of corti
bend in one direction ,it produces stimulation and
when it bends in the opposite direction ,there will be
inhibition of impulse transmission.

49. Auditory pathway

50. Properties of sound
 Pitch
Pitch of a sound depends on its frequency.
Greater the frequency,higher will be the pitch.
Frequency of sound is expressed in
hertz(Hz)orcycles/sec.
The frequency of sound human ear can hear lies
between 20 Hz and 20,000 Hz.

52.  Loudness or intensity of sound
It depends on the amplitude of sound waves.
It is expressed in decibels(dB).
Continued exposure of ear to high intensity
sounds is one of the cause of deafness.

53. Auditory defects
 Impairment of hearing is called deafness.
 2 types
1.Conduction deafness.
2.Nerve deafness.

54. Conduction deafness
 Occurs due to defects in the transmission of sound
waves through the external ear or middle ear.
 Causes are
 Obstruction of external auditory canal with wax or
foreign bodies
 Damage to the tympanic membrane
 Infection of middle ear(otitis media)
 Otosclerosis (fixation of footplate of stapes against
oval window)

55. Nerve deafness
 It occurs when there is damage to the hair
cells,auditory nerve or auditory cortex.
 Causes
Very loud sound can damage the hair cells
Tumor of the auditory nerve
Drugs causing degeneration of hair cells.

56. Tests for hearing
 Whispering test-the examiner stands about 60 cm away from
the subject at his side and whispers some words.
 Tickling of watch test
Wrist watch with tickling sound is kept near the ear of the
subject.
 Rinne’s test
 Weber’s test
 Audiometry—it is the technique used to determine the nature
and the severity of auditory defect.An instrument called
audiometer is used.this instrument is capable of generating
sound waves of different frequencies from lowest to highest

57. Theories of hearing
 Mechanism of appreciation of frequency (pitch) of sound reaching the
ear.
Travelling wave theory.
When the stapes vibrates,it setup pressure waves in the cochlear fluid.the
pressure wave in the scala vestibuli causes displacement of vestibular
membrane and basilar membrane.A wave is produced at the base of the
basilar membrane and this wave is transmitted to the tip of the cochlea
through the basilar membrane.high frequency sounds produces waves that
reach maximum at the base of cochlea.low frequency sounds generate
waves that reach maximum at the apex of cochlea.
 Mechanism of appreciation of loudness of sound.

59.  A loud sound cause greater displacement of
basilar membrane and more number of
afferent nerve fibers will be activated.the
number of impulses reaching the auditory
cortex is increased and the cortex appreciates
it as a high intensity sound.