The document discusses the physiology of hearing and balance. It describes how sound waves are converted into nerve impulses in the ear that are transmitted to the brain. The outer, middle, and inner ear structures are described along with how they work together to transmit sound vibrations. The basilar membrane vibrates based on sound frequency, stimulating hair cells which produce nerve impulses. For balance, the inner ear contains structures like the semicircular canals and maculae that detect head position and movements, signaling the brain.

1. CLARKE INTERNATIONAL
UNIVERSITY
E N T
THE PHYSIOLOGY OF HEARING AND BALANCE
GROUP MEMBERS
Dubi Joel
Imalany Moses
Korobe Emmanuel
Sserubona Ibrahim
Asiimwe Faiza

2. PHYSIOLOGY OF HEARING AND BALANCE
INTRODUCTION
Hearing is the process by which the ear transforms
sound vibrations in the external environment into nerve
impulses that are conveyed to the brain, where they are
interpreted as sounds.
Sounds are produced when vibrating objects, such as the
plucked string of a guitar, produce pressure pulses of
vibrating air molecules, better known as sound waves.

3. ACTION OF HUMAN EARS IS TO
INTERPRET SOUND INPUT BY
PERCEIVING:
INTENSITY = LOUDNESS
FREQUENCY= PITCH
TIMBRE = MUSICAL QUALITY
LOCALISATION = SPATIAL
POSITION
MASKING = ONE SOUND IS
SELECTIVELY HEARD IN
PREFERENCE TO OTHERS

4. SOUND INTENSITY IS NORMALLY
MEASURED IN N/M² OR PASCALS
The vast range of sound pressures perceived
by humans is more conveniently described by
a smaller range expressed in a logarithmic
scale as decibels as a ratio of a reference
intensity (2 x 10⁻⁵ N/m²)
Human range is:
0 db - 130 dB Spain threshold
“absolute”
threshold

5. Frequency is measured in Hz
•Frequency has the subjective correlate of
pitch which is how the ear perceives changes
in frequency
•Human ear can appreciate frequencies
between 12 Hz and 20,000Hz
•“Speech range” is 200Hz - 10,000Hz
•Music 50Hz - 20,000Hz

6. TIMBRE
• The fundamental frequency is the lowest
note in a complex sound
• The overtones or harmonics are simple
multiples of the fundamental frequency
and are responsible for the quality of the
sound

7. PHYSIOLOGY OF HEARING
The hearing system, also called also the
auditory system, consists of the outer ear,
middle ear, inner ear, and central auditory
nervous system. The overall function of the
hearing system is to sense the acoustic
environment thus allowing us to detect and
perceive sound.

9. EAR
• Conducting apparatus
- auricle - EAC - TM - ossicular chain - IE fluids -
• Perceiving apparatus
- end-organ (Corti) - VIII - cerebral cortex -

10. In order to facilitate perception of sound,
the hearing system needs to sense sound
energy and to convert the received acoustic
signals into the electrochemical signals
that are used by the nervous system

11. OUTER EAR
contains the following structures:
➔ Auricle
➔ External auditory meatus(EAC)
• The auricle directs sound waves into the external
auditory canal
• Waves strike the eardrum and the compression
and decompression of air causes the eardrum to
vibrate back and forth.
• The central area of the eardrum connects to the
malleus therefore it also starts to vibrate.

12. MIDDLE EAR
• Contains:
malleus - hammer
incus - anvil
stapes - stirrup
• Small “auditory muscles” stapedius and tensor
tympani

14. FUNCTIONS OF OSSICLES
Acoustic reflex protects ear from loud sounds.
-Dampening of middle ear mechanics: Loud sounds
(80 dB and above) cause contraction of stapedius that
limits stapes movement
-Gain control mechanism: Acoustic reflex keep
cochlear input more constant and expand dynamic
range.
Reduction in self generated noise: Stapedius muscle
contracts with chewing and vocalization

15. • Transfers maximum energy from air to
fluid
• Stapedius & tensor tympani muscles
reflexly contract above 90 dB
• This attenuates loud sound to protect IE
against acoustic trauma
Impact / explosion noise reaches cochlea
before reflexes can act – damage is worse
than with steady state noise

16. HEARING PROCESS
• The vibration is transmitted from
eardrum to the malleus to the incus
and then to the stapes.
• The stapes push the membrane of the
oval window in and out and it vibrates
20 times more than the eardrum

17. THE INNER EAR
• It consists of the the following
structures:
➔ cochlea
➔ semicircular canals

18. THEORIES OF HEARING
Helmholtz resonance theory/ place theory
According to this theory, sound waves
entering the internal ear set up
vibrations of particular fibres of basilar
membrane depending upon tones , lower
tones stimulate longer fibres at the apex
of the cochlear and higher toes stimulate
smaller fibres at the base of cochlear

19. Ruthterfold’s telephone theory
this theory says that the cochlear acts like
telephone transmitter hence , basilar membrane
gets stimulated to every frequency of sound
Von Bekesy’s travelling wave theory
It is a combination of the above theories and it is
most widely accepted as it explains the response of
whole cochlear to low frequencies and also explains
the cochlear analysis to higher frequencies above
500 HZ at the basal turn of the cochlea

20. ● Movement of the oval window sets up
fluid pressure waves in the perilymph
of the cochlea
● When the oval window bulges
inward,it pushes the on the perilymph
of the scala vestibuli and pressure
waves are transmitted from the scala
vestibuli to the scala tympani causing
the round window to burge outward
into the middle ear

22. • The pressure waves deform walls of the
scala vestibuli and scala tympani and
they also push the vestibular membrane
back and forth and as a result the
pressure of the endolymph inside the
cochlear duct increases and decreases.
• Pressure fluctuations of the endolymph
move to the basilar membrane and cause
the basilar membrane to vibrate and the
hair cells in the organ of Corti hit the
tectorial membrane causing hair to bend

23. The bending of the hair produces
receptor potentials that lead to the
generation of nerve impulses in cochlear
nerve fibres and these nervous impulses
travel down the auditory nerve to the
temporal lobe of the cerebrum where it’s
interpreted

25. Basilar
membrane
with the organ
of Corti
Tectorial
membrane

26. SUMMARY OF THE PHYSIOLOGY OF
HEARING
• Sound waves enter the external auditory
meatus
• Tympanic membrane vibrates
• Auditory ossicles vibrate
• Oval window vibrates
• Perilymph in scala vestibuli & scala
tympani moves
• Basilar membrane moves
• Hairs rub against the tectorial membrane
• Nerve impulse is sent along the
vestibulocochlear nerve to the brain

28. PHYSIOLOGY OF EQUILIBRIUM
➔ There are two types of equilibrium i.e static and
dynamic equilibrium
➔ Static equilibrium which is the maintenance of
body position mainly the head relative to the force
of gravity
➔ Dynamic equilibrium which is the maintenance of
the body position mainly the head in response to
sudden movements such as rotation, acceleration
and deceleration

29. EQUILIBRIUM CONT’D
➔ The receptor organs for equilibrium are;
➔ The vestibular apparatus which includes otolithic
organs i.e the saccule and utricle
➔ semicircular ducts

30. DIAGRAMATIC VIEW

31. EQUILIBRIUM CONT’D
• The utricle and saccule contain a small
thickened region called macula and
these maculae are receptors for static
equilibrium and contribute to some
aspects of dynamic equilibrium
• For static equilibrium, they provide
sensory information on the position of
the head in space and are essential for
maintaining appropriate posture and
balance.

32. EQUILIBRIUM CONT’D
• In dynamic equilibrium i.e. when one is
an elevator that is speeding up or
slowing down, the maculae detect
sensations of linear acceleration and
deceleration

33. EQUILIBRIUM CONT’D
• The maculae are perpendicular to one
another and consist of hair(receptor)
cells and and in between the hair cells
are supporting cells
• The supporting cells secrete the thick
gelatinous glycoprotein called the
otolithic membrane that rests on hair
cells and layer of calcium carbonate
crystals called otoliths extend over the
surface of the otolithic membrane

34. EQUILIBRIUM CONT’D
• If the head is tilted forward, the
otolithic membrane along with the
otoliths is pulled by gravity and slides
down over the hair cells in the direction
of the tilt and stimulates the hair cells.
• Similarly if sitted in a car that
suddenly jerks forward, the otolithic
membrane due to its inertia slides
backward and stimulates the hair cells

36. EQUILIBRIUM CONT’D
• As the otoliths move, they pull on the
gelatinous layer which pulls on the
stereocilia and makes them bend
• Movement of the stereocilia initiate
depolarising receptor potentials in the
vestibular branch of the
vestibulocochlear (viii) nerve

37. DYNAMIC EQUILIBRIUM
• It is maintained by 3 semicircular ducts
together with saccule and utricle. The
ducts lie at right angles to one another
in 3 planes i.e the two vertical ones are
the anterior and posterior semicircular
ducts and the horizontal is the lateral
semicircular duct.
• In the ampulla, the dilated portion of
each duct there are crista which
contain a group of hair cells and
supporting cells covered by gelatinous
material called cupula.

38. ILLUSTRATION OF MACULA

39. DYNAMIC EQUILIBRIUM
• When the head moves, the endolymph
in the semicircular ducts flows over the
hairs and bends them and this
movement of the hair stimulates
sensory neuron and the nerve impulses
which pass along the vestibular branch
of vestibulocochlear nerve

40. EQUILIBRIUM PATHWAYS
• Most of the vestibular branch fibres of the
vestibulocochlear nerve enter the brainstem and
terminate in the vestibular nuclear complex in
the pons, the remaining fibres enter the
cerebellum through the inferior cerebellar
peduncle.
• Fibres from all the vestibular nuclei extend to
nuclei of cranial nerves that control eye
movements i.e. oculomotor(iii), trochlear(iv) and
abducens(vi) and to the accessory(xi) nerve
nucleus that helps control head and neck
movements

41. Receptors are located at the
bases of the semicircular
canals.
DYNAMIC EQUILIBRIUM
the semicircular canals go off in
different directions.
Nerve fibers from the vestibule
and from the semicircular canals
form the vestibular nerve, which
joins the cochlear nerve to form
the vestibulocochlear nerve, the
eighth cranial nerve.

42. EQUILIBRIUM PATHWAY CONT’D
• Fibres from the lateral vestibular
nucleus form the vestibulospinal tract
which conveys impulses to skeletal
muscles that muscle tone in response to
head movements

43. EQUILIBRIUM PATHWAY CONT’D
• The cerebellum continuously receives
sensory information from the utricle
and saccule and using this information,
the cerebellum monitors and makes
corrective adjustments in the motor
activities that originate in the cerebral
cortex.

44. EQUILIBRIUM PATHWAY CONT’D
• Essentially the cerebellum sends
continuous nerve impulses to the motor
areas of the cerebrum in response to
input from the utricle, saccule and
semicircular ducts.

45. DIAGRAM

46. REFERENCES
• Gerrald J. Tortora, Bryan D. Principles
of Anatomy and Physiology 12th
edition.
• Guyton and Hall, Textbook of Medical
physiology 12th
edition.