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.

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

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 • Reissner’s (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.

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