Chapter 11 The Auditory and

Vestibular Systems

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

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

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

The Structure of the Auditory System

The Structure of the Auditory System

Auditory pathway stages – Sound waves– Tympanic membrane– Ossicles– Oval window– Cochlear fluid– Sensory neuron response

Components of the Middle Ear

The Middle Ear

5 – Stapedius muscle

9 – Tensor Tympani muscle

• 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

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

• 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

The Inner Ear

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 .

Travelling wave in the Basilar Membrane

The Inner Ear

The Organ of Corti and Associated Structures

The Inner Ear

Transduction by Hair Cells– Research: A.J.

Hudspeth.– Sound: Basilar

membrane upward, reticular lamina up and stereocilia bends outward

The Inner Ear

Fluids in cochlear canals

Upper and middle

Internal earExternal ear

PinnaExternalacousticmeatus

Air

Tympanicmembrane

Malleus, incus,stapes

(ossicles)

Ovalwindow Lower

Middle ear

Onevibration

TimeSpiral organ

(of Corti)stimulated

Amplificationin middle ear

Amplitude

Pre

ssu

re

Central Auditory Processes

Auditory Pathway

• 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: 20-2000 Hz• Interaural intensity difference: 2000-20000 Hz

Mechanisms of Sound Localization

Interaural time delay and interaural intensity difference

Mechanisms of Sound LocalizationMechanisms of Sound Localization

The Sensitivity of Binaural Neurons to Sound Location

Mechanisms of Sound LocalizationMechanisms of Sound Localization

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

Mechanisms of Sound Localization

Localization of Sound in Vertical Plane– Vertical sound localization based on reflections from the

pinna

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

Auditory Cortex

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

Human Anatomy and Physiology, 7eby Elaine Marieb & Katja Hoehn

Copyright © 2007 Pearson Education, Inc.,publishing as Benjamin Cummings.

Figure 15.35: Structure of a macula, p. 594.

Macula ofutricle

Macula ofsaccule

Otoliths

Hair bundle

KinociliumStereocilia Otolithic

membrane

Vestibularnerve fibers

Hair cells

Supportingcells

Human Anatomy and Physiology, 7eby Elaine Marieb & Katja Hoehn

Copyright © 2007 Pearson Education, Inc.,publishing as Benjamin Cummings.

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

Otolithicmembrane

Kinocilium

Ster eocilia

Receptorpotential

Nerveimpulsesgenerated investibular fiber

Depolarization

(Hairs bent towarkinocilium)

dHyperpolarization

(Hairs bent awayfrom kinocilium)

Increasedimpulse frequency

Excitation

Decreasedimpulse frequency

Inhibition

Human Anatomy and Physiology, 7eby Elaine Marieb & Katja Hoehn

Copyright © 2007 Pearson Education, Inc.,publishing as Benjamin Cummings.

Figure 15.37: Location and sturcture of a crista ampullaris, p. 596.

(a)

(c) (d)

(b)

CupulaCupula at rest

Position of cupuladuring turn

Turning motion

Fluid motion inducts

Afferent fibers of vestibular nerve

Increased firing Decreased firing

Position of cupuladuring turn

Ampulla of left ear

Ampulla ofright ear

Horizontal ducts

Flow ofendolymph

Cupula

Cristaampullaris

Fibers ofvestibular nerve

The Vestibular System

The Semicircular CanalStructure

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:

The Vestibular System

The Vestibulo-Ocular Reflex (VOR)

The Vestibular System