The Auditory and Vestibular System

1
The Auditory and Vestibular System

• Chapter 11

2
The Auditory and Vestibular System

• Auditory System - sense of hearing
• Used to detect sound
• We are also able to interpret nuances of sound.
• Important in communication and survival
• Able to evoke sensations and emotions
• Vestibular system - sense of balance
• Informs nervous system about the relative
  position and movement of the head
• Subconsciously controls muscle to reorient body
  and eye position.

3
The Nature of Sound

• Hearing is a response to vibrating air molecules.
• Sounds are audible variations in air pressure
• Moving objects compress air as they move forward
  and decrease the density of air as they move
  away.
• Waves move at 343 m / sec or 767 mph.

4
The Nature of Sound

• Pitch is determined by the frequency of vibration
• Frequency the number of compressions per
  second.
• One cycle is the distance between the waves of
  compression
• Frequency is expressed in hertz (Hz)
• Hearing range is 20 to 20,000 Hz.
• Most sensitive to frequencies ranging from 1,500
  to 4,000 Hz.
• Decreases with age or exposure to loud sounds.
• There are high and low sounds that our ears
  cannot hear. (Just like light)
• Loudness difference in pressure between
  compressed and rarefied patches of air
• Range is tremendous
• loudest sound without ear damage is a trillion
  times greater than the faintest sound we can hear
• Intensity is expressed in decibels (dB)
• 120 to 140 dB causing pain in most people.
• Real world sounds rarely consist of simple
  periodic sound waves at one frequency or
  intensity.

5
(No Transcript)
6
The Structure of the Auditory System

• Outer Ear
• Auricle or Pinna
• External Auditory Meatus
• Tympanic Membrane
• Middle Ear
• Ear ossicles
• Inner Ear
• Vestibule
• Cochlea

7
(No Transcript)
8
Middle Ear

• Ear Ossicles
• Malleus, Incus, Stapes
• Act as a lever system to amplify sound.
• Eustachian tube
• Equalizes pressure in middle ear
• Attenuation Reflex
• Tensor Tympani and Stapedius Muscle
• Reduces hearing saturation levels
• Protects the inner ear.
• Reduces low-frequency background noise.

9
The Inner Ear

• Vestibule
• Semicircular Canals
• Cochlea
• Chambers
• Scala Vestibuli
• Scala Media
• Scala Tympani
• Membranes
• Basilar membrane
• Reissners (vestibular) membrane
• Tectorial membrane
• Stria vascularis

10
Cochlear Structures

• Oval Window
• Round window
• Helicotrema
• Basilar membrane
• widens toward the Apex of cochlea.
• Perilymph
• Fills Scala Vestiblia and Scala Tympani
• Low K, High Na
• Endolymph
• Fills Scala media
• High K, Low Na
• Produces and endocochlear potential that enhances
  auditory transduction

11
Basilar Membrane

• Structural properties determine how the membrane
  responds to sound.
• Wider at apex than at the base by 5 times.
• Stiffest at base and most flexible at apex.
• Movement of stapes causes endolymph to flow
  causing a traveling wave in the membrane
• The distance the wave travels depends on the
  frequency of the wave.
• Different locations of the basilar membrane are
  maximally deformed at different frequencies
  creating a placed code.

12
Response of the Basilar Membrane to Sound

• High frequency waves dissipate near the narrow,
  stiff base.
• Low frequency waves dissipate near the wide
  flexible apex.
• A place code where maximum amplitude deflection
  occurs is responsible for the neural coding of
  pitch.

13
Organ of Corti

• Hair cells
• Three rows of outer hair cells and one inner.
• Stereocilia are embedded in the reticular lamina
  and tectorial membrane.
• Have no axons
• Interact with bipolar spiral ganglion cells that
  form the cochlear (auditory) nerve.

14
(No Transcript)
15
Transduction by Hair Cells.

• Vibration in the basilar membrane results in the
  bending of stereocilia
• Stereocilia are cross linked by filaments and
  move together as a unit.
• Bending of steriocilia causes changes in the
  polarization of the hair cells.
• Displacement of only 0.3 nm can be detected
  (diameter of a large molecule).
• Loud noises that saturate hair cells move
  stereocilia by only 20 nm.

16

• Bending depolarizes or hyperpolarizes hair cells
  depending on the direction the stereocilia are
  pulled.
• Hair cell receptor potentials closely follow the
  air pressure changes during a low-frequency
  sound.

17
Depolarization of Hair Cells.

• K channels on the stereocilia are linked by
  elastic filaments.
• Displacement of cilia opens or closes K channels
• K entering cell causes depolarization.
• Note K entry generally causes
  hyperpolarization.
• Endolymph has a high concentration of K.
• Depolarization causes voltage gated Ca channels
  to open
• Ca triggers the release of neurotransmitter.

High K Low Na
High Na Low K
18
The Innervation of Hair Cells

• Auditory nerves are bipolar with nuclei in the
  spiral ganglian.
• 95 of spiral ganglian neurons communicate with
  inner hair cells.
• Each inner hair cells feeds about 10 spiral
  ganglian cells
• Most detection of sound occurs on the inner hair
  cell.
• One spiral ganglian cell will connect to multiple
  outer hair cells.

19
Amplification by Outer Hair Cells

• Motor proteins in the outer hair cells can
  shorten hair cells.
• Shortening of hair cells increases the bending of
  the basilar membrane.
• Amplification of basilar membrane vibration
  causes the stereocilia on the inner hair cells
  bend more.
• Furosemide inactivates outer hair cell motor
  proteins thus reducing transduction.

20
Central Auditory Processes

• Spiral ganglion neurons travel through the
  vestibulo-cochlear nerve to the medulla and
  branch to enter both the dorsal and ventral
  cochlear nucleus.
• Neural signals travel through numerous pathways.
• The ventral cochlear nucleus projects to the
  superior olive on both sides of the brain then
  through the lateral lemniscus.
• The dorsal path bipasses the superior olive.
• All paths converge at the Inferior Colliculus
  then go on to the Medial Geniculate Nucleus then
  into the Auditory Cortex.

21
Central Auditory Processes

• Inferior colliculus communicates with the
  superior colliculus to integrate with visual
  input.
• There is an extensive feedback system in the
  auditory system
• Other than the cochlear nuclei, auditory nuclei
  receive input from both ears.

22
Response Properties of Spiral Ganglion Cells

• Spiral ganglion cells are frequency tuned.
• Each cell responds at a characteristic frequency.
• Response properties of nuclei are diverse
• Cochlear nuclei -specialized for varying time
  with frequency
• MGN- Vocalization
• Superior Olive - Sound localization

23
Encoding Sound Intensity and Frequency

• Sounds are diverse and complex
• Our brain must analyze the important ones and
  ignore the noise
• Sound is differentiated based upon intensity,
  frequency and point of origin
• Each of these features is represented differently
  in the auditory pathway.

24
Stimulus Intensity

• Encoded by the firing rates of neurons and by the
  number of active neurons.

25
Stimulus Frequency and Tonotopy
26

• Phase Locking firing at the same phase of a
  sound wave
• Necessary because low frequency are difficult to
  distinguish and displacement of the basilar
  membrane changes with intensity

• Below 4000Hz phase locking is necessary.
• At intermediate ranges both phase locking and
  tonotopy are used
• At high frequencies only tonotopy is use.l

27
Interaural time delay as a cue to the
localization of sound
28

• Continuous tones are difficult to localize.
• Use phase of sound for low frequency
• Use interaural intensity differences and time
  delays created by a sound shadow at high
  frequency

29
(No Transcript)
30
(No Transcript)
31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
(No Transcript)
37
(No Transcript)
38
(No Transcript)
39
(No Transcript)
40
(No Transcript)
41
(No Transcript)
42
(No Transcript)