1. Chapters 9,10 Auditory and Vestibular Systems
Chris Rorden
University of South Carolina
Norman J. Arnold School of Public Health
Department of Communication Sciences and Disorders

2. Audition
The ear converts sound energy into patterns of neural firing: transduction
Outer ear: collect and amplify sound, aid in localization
Middle ear: impedance matching
Cochlea: frequency and intensity analysis
Auditory pathway: complex signal processing

3. Ear structures Peripheral Central Outer ear Middle ear Inner ear
Auditory nerve
Central
Brainstem
Midbrain
Cerebral

4. Anatomy and function
Pinna: the projecting part of the ear lying outside of the head (also called auricle, or just ear lobe)
Reflection of sound in pinna provides spectral cues about elevation of a sound source

5. Outer Ear – Auditory Canal
External auditory meatus
Provides communication between middle and inner ears by conducting sound to the ear drum
S-shaped 2.5cm long 7mm wide
Lining of the lateral 1/3rd of canal has cilia and glands
Cerumen (ear wax): protects ear canal from drying out and prevents intrusion of insects

6. Outer Ear - Ear drum Tympanic membrane Separates outer and middle ear
Compliant
Thin, three-layered sheet
Epithelium of EAM: outer layer
Middle layer: fibrous (strong) tissue
Inner layer of middle ear: mucous membrane

7. Outer Ear - Ear drum Slightly concave to EAM, cone-shaped
Most depressed and thinnest point is called the umbo
End of the attachment of malleus
‘Cone of light’ from umbo to periphery reflects light when viewed with otoscope
Slightly oval, taller than wide
Otoscope: if you pull the pinna up and back the tympanic membrane is visible

8. Middle Ear – Tympanic Membrane

9. Role of outer ear
To augment the sound shadow
Ear canal protects delicate parts of middle and inner ear from impact.
To heighten our sensitivity to sounds
Ear canal boosts sounds 15 to 16 dB between 1.5 and 8 kHz (in the area of speech)
This is due to resonance of ear canal
Just like vocal tract this tube amplifies and dampens certain frequencies based on its length and composition

10. Localization and shadowing
Intensity differences: louder if nearer, less shaded
Inter-aural timing differences
Frequencies influenced by location relative to pinna.

11. Middle Ear – Eustachian Tube
Establishes communication between middle ear and nasopharynx
~ 35 to 38 mm long, typically closed
Biological functions:
To permit middle ear pressure to equalize with external air pressure
On the air plane, change in atmospheric pressure but not pressure in middle ear
Yawning or swallowing opens pharyngeal orifice of tube to equalize pressures
To permit drainage of normal and diseased middle ear secretions into the nasopharynx

12. Middle Ear - Ossicles 3 of the smallest bones
Malleus (hammer)
Incus (anvil)
Stapes (stirrup)
Ossicular chain: Transmits acoustic energy from tympanic membrane to inner ear
Acts as lever: large weak motion of TM causes small forceful movement of stapes.
Takes force from gas (air) and matches impedance to liquid (inner ear).
Muscles allow movement to be attenuated: Prevents the inner ear from being overwhelmed by excessively strong vibrations

13. Middle Ear – Ossicles

14. Middle Ear – Ossicles - Malleus
Malleus (hammer) 9 mm long
Manubrium (handle): attaches to tympanic membrane; pulls the drum medially
Caput (head): jointed (quite inflexibly) to Incus

15. Middle Ear – Ossicles - Incus
The ossicles give the eardrum mechanical advantage via lever action and a reduction in the area of force distribution
Pressure = Force/Area; so less area = more pressure
the resulting vibrations would be much smaller if the sound waves were transmitted directly from the outer ear to the oval window.
The movements of the ossicles is controlled muscles attached to them (the tensor tympani and the stapedius).These muscles can dampen the vibration of the ossicles, in order to protect the inner ear from excessively loud noise and that they give better frequency resolution at higher frequencies by reducing the transmission of low frequencies

16. Middle Ear – Ossicles - Stapes
Head (caput) jointed to incus
Anterior and posterior crura (legs)
Footplate: joins oval window of inner ear (opening in temporal bone) via annular ligament

17. Cochlea and neighbors

18. Inner Ear - Cochlear

19. Osseous cochlea Oval window Round window
Connects scala vestibuli and middle ear
Round window
Connects scala tympani and middle ear

20. Cochleus from 5mo fetus:
Cochlear Structures
Cochleus from 5mo fetus:
Oval window (blue arrow)
Round window (yellow arrow)

21. Tonotopic
Base
High Freq
Apex
Low Freq.

22. Travelling wave
Always starts at the base of the cochlea and moves toward the apex
Its amplitude changes as it traverses the length of the cochlea
The position along the basilar membrane at which its amplitude is highest depends on the frequency of the stimulus

23. High frequencies have peak influence near base and stapes
Traveling wave
High frequencies have peak influence near base and stapes
Low frequencies travel further, have peak near apex
A short movie:
Green line shows 'envelope' of travelling wave: at this frequency most oscillation occurs 28mm from stapes.

24. Cochlear structure
Cross-section shows the coiling of the cochlear duct The red arrow is from the oval window, the blue arrow points to the round window.
Scala media – filled w Endolymph
scala vestibuli filled w Perilymph
scala tympani filled w Perilymph
spiral ganglion
nerve fibres

25. Inner Ear - Labyrinth Endolymph K+ ~100 mV Reissner’s Membrane
Perilymph
Na+ ~20mV
Reissner’s Membrane
Basilar Membrane

26. Inner Ear – Organ of Corti
Both types of hair cells protrude into endolymph of scala media,

27. Inner Ear – OHC & IHC Inner Hair Cells Outer Hair Cells Non-motile
Vibrates when triggered – acts as preamplifier.
Hair cells are mechanically gated ion channels: deflection of hairs depolarizes the cell, resulting in a receptor potential – causing calcium ions to enter, which in turn stimulates the release of neuroreceptors.

28. Neural connections
Inner hair cells: many nerve fibers for each cell (many-to-one innervation) 3500
Outer hair cells: each nerve fiber connected to many hair cells (one- to-many innervation)

29. Function of the cochlea
First stage of auditory processing
1. Spectral analysis
Extracts frequency and amplitude information from sound waves
2. Temporal analysis
Basic temporal characteristics of sounds

30. The ear codes frequency in two ways:
Position of neural responses along basilar membrane changes with frequency - tonotopic organization or the place coding
Timing of neural responses follows the time waveform of sound – phase-locking

31. Place coding
Place coding: Auditory frequency coded by location of stimulation.
Base
High Freq
Apex
Low Freq.

32. The rate of neural firing matches the sound's frequency.
Phase locking
The rate of neural firing matches the sound's frequency.
Problem: some auditory frequencies much faster than neurons can fire
Each neuron can only fire around 200 times per sec.
Solution: volley principle: large numbers of neurons that are phased locked can code high frequencies.

33. Afferent and efferent innervation
Afferent: signals from sense organ to brain
Auditory signals
Efferent: signals from brain to sense organ
Inhibits auditory signals
Both cochlear and ossicles
Improves signal-to-noise ratio by suppressing noise

34. Primary auditory cortex
Medial geniculate
Body (thalamus)
Inferior colliculus
(in midbrain)
Auditory radiation
Cochlear and Superior olivary
Complex in the Medulla

35. Central Auditory Mechanism
Auditory input projects to the cortex bilaterally, with stronger contralateral connections.
The superior olive and the inferior colliculus send efferent fibers back to attenuate motion of the middle ear bones (dampen loud sounds)

36. Cochlear Nucleus
Evidence of signal processing (monaural)
Superior Olivary Complex (SOC)
Binaural processing
Localization of sound source
Low frequency sounds: arrival time compared
High frequency sounds: intensity level compared

37. Inferior Colliculus (IC)
Auditory Pathway
Lateral Lemniscus
Fiber tract within CNS
From SOC to IC
Inferior Colliculus (IC)
Bilateral innervation
Frequency, intensity and temporal processing
Medial Geniculate Body (MGB)
Tonotopic mapping
Complex responses to contralateral signals

38. Cerebral cortex
Signal comes primarily from contralateral ear via ipsilateral MGB
Heschl’s gyrus
Tonotopic mapping in columns
Each column has one characteristic frequency
Neurons in column responsive to different stimulus parameters, like frequency and intensity

39. Anatomy and function
Many sound features are encoded before the signal reaches the cortex
- Cochlear nucleus segregates sound information
- Signals from each ear converge on the superior olivary complex - important for sound localization
- Inferior colliculus is sensitive to location, absolute intensity, rates of intensity change, frequency - important for pattern categorization
- Descending cortical influences modify the input from the medial geniculate nucleus - important as an adaptive ‘filter’
cortex
medial geniculate
body
inferior colliculus
cochlear nucleus
complex
cochlea
superior olivary complex

40. Clinical Notes Conductive Sensorineural Central
Cerumen in canal, Otitis Media of Middle Ear (ear infection)
Sensorineural
Meniere’s disease (abnormality in the fluids of the inner ear = vertigo), Presbycusis (age related hearing loss)
Central
Pathology in cortex
Bilateral auditory cortex lesions result in:
Profound loss of auditory discriminative skills
Impaired speech perception
Hearing loss