1. Chapter 11 The Auditory and Vestibular Systems

2. Introduction
Sensory Systems
Sense of hearing, audition
Detect sound
Perceive and interpret nuances
Sense of balance, vestibular system
Head and body location
Head and body movements

3. The Nature of Sound
Sound
Audible variations in air pressure
Sound frequency: Number of cycles per second expressed in units called hertz (Hz)
Cycle: Distance between successive compressed patches

4. The Nature of Sound
Sound
Range: 20 Hz to 20,000 Hz
Pitch: High pitch = high frequency; low frequency = low pitch
Intensity: High intensity louder than low intensity

5. The Structure of the Auditory System

6. The Structure of the Auditory System
Auditory pathway stages
Sound waves
Tympanic membrane
Ossicles
Oval window
Cochlear fluid
Sensory neuron response

7. The Middle Ear
Components of the Middle Ear

8. 5 – Stapedius muscle
9 – Tensor Tympani muscle

9. The Middle Ear
Sound Force Amplification by the Ossicles
Pressure: Force by surface area
Greater pressure at oval window than tympanic membrane, moves fluids
The Attenuation Reflex
Response where onset of loud sound causes tensor tympani and stapedius muscle contraction
Function: Adapt ear to loud sounds, understand speech better

10. The Inner Ear
Anatomy of the Cochlea
Perilymph: Fluid in scala vestibuli and scala tympani
Endolymph: Fluid in scala media
Endocochlear potential: Endolymph electric potential 80 mV more positive than perilymph

11. The Inner Ear
Physiology of the Cochlea
Pressure at oval window, pushes perilymph into scala vestibuli, round window membrane bulges out
The Response of Basilar Membrane to Sound
Structural properties: Wider at apex, stiffness decreases from base to apex
Research: Georg von Békésy
Endolymph movement bends basilar membrane near base, wave moves towards apex

12. Georg von Békésy

13. The Inner Ear
Travelling wave in the Basilar Membrane

14. The Inner Ear
The Organ of Corti and Associated Structures

15. The Inner Ear
Transduction by Hair Cells
Research: A.J. Hudspeth.
Sound: Basilar membrane upward, reticular lamina up and stereocilia bends outward

16. External ear
Middle ear
Internal ear
Air
External
acoustic
meatus
Malleus, incus,
stapes
(ossicles)
Tympanic
membrane
Oval
window
Fluids in cochlear canals
Pinna
Upper and middle
Lower
Pressure
Time
Spiral organ
(of Corti)
stimulated
One
vibration
Amplitude
Amplification
in middle ear

17. Central Auditory Processes
Auditory Pathway

18. Central Auditory Processes
Response Properties of Neurons in Auditory Pathway
Characteristic frequency: Frequency at which neuron is most responsive - from cochlea to cortex
Response Properties more complex and diverse beyond the brain stem
Binaural neurons are present in the superior olive

19. Mechanisms of Sound Localization
Techniques for Sound Localization
Horizontal: Left-right, Vertical: Up-down
Localization of Sound in Horizontal Plane
Interaural time delay: Time taken for sound to reach from ear to ear
Interaural intensity difference: Sound at high frequency from one side of ear
Duplex theory of sound localization:
Interaural time delay: Hz
Interaural intensity difference: Hz

20. Mechanisms of Sound Localization
Interaural time delay and interaural intensity difference

21. Mechanisms of Sound Localization
The Sensitivity of Binaural Neurons to Sound Location

22. Mechanisms of Sound Localization
Delay Lines and Neuronal Sensitivity to Interaural Delay
Sound from left side, activity in left cochlear nucleus, sent to superior olive
Sound reaches right ear, activity in right cochlear nucleus, first impulse far
Impulses reach olivary neuron at the same time summation action potential

23. Mechanisms of Sound Localization
Localization of Sound in Vertical Plane
Vertical sound localization based on reflections from the pinna

24. Auditory Cortex
Primary Auditory Cortex
Axons leaving MGN project to auditory cortex via internal capsule in an array
Structure of A1 and secondary auditory areas: Similar to corresponding visual cortex areas

25. The Vestibular System Importance of Vestibular System
Balance, equilibrium, posture, head, body, eye movement
Vestibular Labyrinth
Otolith organs - gravity and tilt
Semicircular canals - head rotation
Use hair cells, like auditory system, to detect changes

26. The Vestibular System
The Otolith Organs: Detect changes in head angle, linear acceleration
Macular hair cells responding to tilt

27. Otolithic
membrane
Kinocilium
Ster
eocilia
Depolarization
Hyperpolarization
Receptor
potential
(Hairs bent towar
kinocilium) d
(Hairs bent away
from kinocilium)
Nerve
impulses
generated in
vestibular fiber
Increased
impulse frequency
Decreased
impulse frequency
Excitation
Inhibition

28. The Vestibular System
The Semicircular Canal
Structure

29. The Vestibular System Push-Pull Activation of Semicircular Canals
Three semicircular canals on one side
Helps sense all possible head-rotation angles
Each paired with another on opposite side of head
Push-pull arrangement of vestibular axons:

30. The Vestibular System
The Vestibulo-Ocular Reflex (VOR)
Function: Line of sight fixed on visual target
Mechanism: Senses rotations of head, commands compensatory movement of eyes in opposite direction
Connections from semicircular canals, to vestibular nucleus, to cranial nerve nuclei  excite extraocular muscles

31. The Vestibular System
The Vestibulo-Ocular Reflex (VOR)

32. Vestibular
receptors
Visual
receptors
Somatic
receptors
Input
Vestibular
nuclear
complex
Reticular
nuclei
Cerebellum
Central nervous
system processing
Oculomotor control
(cranial nerve nuclei
III, IV, VI)
(eye movements)
Spinal motor control
(cranial nerve nuclei XI
and vestibulospinal tracts)
(neck movements)
Output