Topic 12 The Auditory and Vestibular Systems Lange

1. Topic 12 The Auditory and Vestibular Systems Lange Biology 463 - Neurobiology

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 Middle Ear Components of the Middle Ear

7. 5 – Stapedius muscle 9 – Tensor Tympani muscle

8. 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 The Middle Ear

9. 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

10. 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

11. Georg von Békésy - Hungarian biophysicist born in Budapest. In 1961, he was awarded the Nobel Prize in Physiology or Medicine for his research on the function of the cochlea in the mammalian hearing .

12. The Inner Ear Travelling wave in the Basilar Membrane

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

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

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

16. Central Auditory Processes Auditory Pathway

17. 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

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

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

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

21. 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

22. 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

23. Figure 15.35: Structure of a macula, p. 594. Macula of saccule Macula of utricle Kinocilium Otolithic membrane Stereocilia Otoliths Hair bundle Hair cells Supporting cells Vestibular nerve fibers

24. Figure 15.36: The effect of gravitational pull on a macula receptor cell in the utricle, p. 595. 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

25. Figure 15.37: Location and sturcture of a crista ampullaris, p. 596. Flow of endolymph Crista ampullaris (a) Fibers of vestibular nerve Cupula (b) Turning motion Ampulla of right ear Ampulla of left ear Cupula Cupula at rest Position of cupula during turn Position of cupula during turn Fluid motion in ducts Horizontal ducts Increased firing Decreased firing (d) (c) Afferent fibers of vestibular nerve

26. The Vestibular System The Semicircular Canal Structure

27. 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:

28. The Vestibular System The Vestibulo-Ocular Reflex (VOR) • also known as the oculocephalic reflex • a reflex eye movement that stabilizes images on the retina during head movement • Stabilization occurs by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field

29. END.