The Auditory and Vestibular System

Chapter 11

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.

The Nature of SoundHearing 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.

Pitch is determined by the frequency of vibrationFrequency = 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.

The Nature of Sound

The Structure of the Auditory System

Outer Ear Auricle or Pinna External Auditory

Meatus Tympanic Membrane

Middle Ear Ear ossicles

Inner Ear Vestibule Cochlea

Middle EarEar 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.

The Inner EarVestibuleSemicircular CanalsCochlea Chambers

Scala Vestibuli Scala Media Scala Tympani

Membranes Basilar membrane Reissner’s (vestibular)

membrane Tectorial membrane Stria vascularis

Cochlear StructuresOval WindowRound windowHelicotremaBasilar 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

Basilar MembraneStructural 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.

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.

Organ of CortiHair 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.

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.

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.

Depolarization of Hair Cells.

K+ channels on the stereocilia are linked by elastic filaments.Displacement of cilia opens or closes K+ channelsK+ 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+

The Innervation of Hair CellsAuditory 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.

Amplification by Outer Hair CellsMotor 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.

Central Auditory ProcessesSpiral 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.

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.

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

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.

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

Stimulus Frequency and Tonotopy

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

Interaural time delay as a cue to the localization of sound

•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