The document describes the anatomy and physiology of the human ear and central auditory system. It begins by outlining the main components of the hearing mechanism, including the outer, middle and inner ear, as well as the central auditory nervous system. It then provides details on the structures and functions of each part, such as how sound travels through the ear canal, tympanic membrane, ossicles, cochlea and auditory nerve to be processed in the brain. Key structures along the central auditory pathways like the cochlear nuclei, superior olivary complex, lateral lemniscus, inferior colliculus and medial geniculate body are also summarized.

1. 1
ANATOMY AND PHYSIOLOGY OF
THE EAR

2. 2
Main Components of the
Hearing Mechanism:
 Outer Ear
 Middle Ear
 Inner Ear
 Central Auditory Nervous System

3. 3
External Auditory Canal:
 Approx. 1 inch in
length
 “S” shaped
 Lined with cerumen
glands
 Outer 1/3 surrounded
by cartilage
 Inner 2/3’s surrounded
by mastoid bone

4. 4
Mastoid Process
 Bony ridge behind the
auricle
 Provides support to
the external ear and
posterior wall of the
middle ear cavity

5. 5
Tympanic Membrane:
 Thin membrane
 Forms boundary
between outer and
middle ear
 Vibrates in response to
sound
 Changes acoustical
energy into
mechanical energy

6. 6
The Ossicles:
 A: Malleus
 B: Incus
 C: Stapes
– Smallest bones in the body
– Acts as a lever system
– Footplate of stapes enters oval
window of the cochlea
 Stapedius Muscle
– Connects stapes to wall of middle
ear
– Contracts in response to loud
sounds (called the Acoustic Reflex)

7. 7
Eustachian Tube
(AKA: “The Equalizer”)
 Lined with mucous
membrane
 Connects middle ear to
nasopharynx
 “Equalizes” air
pressure

8. 8
Structures of the Inner Ear
 Cochlea
– Snail shaped organ with a
series of fluid-filled tunnels
– Converts mechanical energy
to electrical energy

9. 9
Organ Of Corti:
 The end organ of
hearing
 Contains stereocilia
and hair cells.

10. 10
Hair Cells:
 Frequency specific
– High pitches= base of
cochlea
– Low pitches= apex of
cochlea

11. 11
Vestibular System
 Consists of three semi-
circular canals
 Shares fluid with the
cochlea
 Controls balance

12. 12
Central Auditory System
 VIIIth Cranial nerve or “Auditory Nerve”
– Carries signals from cochlea to brain
 Auditory Cortex
– Temporal lobe of the brain where sound is
perceived and analyzed

13. 13
How Sound Travels Through
The Ear...
1. Acoustic energy, in the form of sound waves, is channeled into the ear canal by
the pinna
2. Sound waves hit the tympanic membrane and cause it to vibrate, like a drum,
changing it into mechanical energy
3. The malleus, which is attached to the tympanic membrane, starts the ossicles into
motion
4. The stapes moves in and out of the oval window of the cochlea creating a fluid
motion
5. The fluid movement causes membranes in the Organ of Corti to shear against
the hair cells
6. This creates an electrical signal which is sent up the Auditory Nerve to the brain
The brain interprets it as sound!

14. The central auditory pathways
Landmarks:
Auditory nerve
Cochlear nucleus
Superior olivary
complex
Lateral lemniscus
Inferior colliculus
Medial geniculate
body

15. The auditory nerve
VIII cranial nerve
Bipolar neurons
Cell bodies from these neurons lie
right outside the cochlea and form
the spiral ganglion
One end innervates the individual
inner and outer HCs of the cochlea
and the other end synapses with
the neurons of the cochlear
nucleus

16. The auditory nerve
Auditory Portion--30,000 fibers
Vestibular Portion--20,000 fibers
Evidence of tonotopic organization
Spiral Ganglion
Cell bodies of the first order neurons
Bipolar
Cerebello-Pontine angle--where the cerebellum, medulla
oblongota and pons meet

17. The auditory nerve
Evidence of
tonotopic
organization

18. The auditory nerve
The individual fibers pass from the
modiolus of the cochlea through the
internal auditory meatus, which exits at
the base of the brain
The IAM also carries fibers from the
utricle, saccule, and semicircular
canals that form the vestibular portion
of the VIII nerve
The vestibular and auditory portions of
the VIII N. separate at the
cerebellopontine angle
The branch of the facial nerve that
courses through the middle ear also
exits here

19. The cochlear nucleus
Two major parts
Dorsal and ventral
Tonotopic organization
“must-synapse” station--second
order fibers
Preserves, but does not enhance,
information received from the
auditory nerve

20. Superior Olivary Complex
 Most of the fibers from the cochlear nuclei cross and
project to the contralateral SOC
 Plays a role in the acoustic reflex
 Analyzes intensity and time-of-arrival differences
between the two ears to help with localization tasks

21. The superior olivery complex
Most (about 80%) of the fibers from
the cochlear nuclei cross and
project to the contralateral SOC
via the trapezoid body
Evidence of tonotopic organization
Plays a major role in the acoustic
reflex
Analyzes intensity and time-of-
arrival differences between the
two ears to help with
localization/lateralization tasks

22. The lateral lemniscus
Highway of
axons that
arise from the
SOC and
terminate in
the midbrain

23. The lateral lemniscus
Tonotopic organization is evident
Nuclei within the lateral lemniscus
have a large proportion of cells
that are sensitive to interaural
time differences, binaural
input, and interaural intensity
differences

24. The inferior colliculus
“must-synapse” station
at the level of the
midbrain
Highly tonotopic
First evidence of
neurons that are
sensitive to sound
duration
Active in binaural
processing

25. Auditory Cortex
Areas of auditory reception are in the temproal lobes on
both sides of the cerebral cortex in an area called the
superior temporal gyrus or Heschl’s gyrus

26. The medial geniculate body
Located in the auditory
thalamus
Last subcortical relay in the
pathway
Evidence of tonotopic
organization
Very active in localization and
lateralization

27. The medial geniculate body
Pathways of neurons
projecting from the medial
geniculate to cortical areas

28. The human brainstem
Structures:
Cochlear nuclei (1)
Lateral lemnisci (2)
Inferior colliculi (3)
Superior colliculi (4)
Medial geniculates (6)
Auditory thalmi (7)

29. The auditory cortex
Areas of auditory reception are in
the temproal lobes on both
sides of the cerebral cortex
The Sylvian or lateral fissure is
the focal point
Primary and secondary auditory
areas are above and below
this point

30. Central Auditory Pathways