Auditory System

Lesson 15

The Stimulus

What kind of energy is sound? mechanical movement of air molecules

Waves intensity = amplitude pitch = frequency (wavelength)

Audible range in humans 20 Hz – 20 kHz ~

Anatomy of the Ear Tympanic membrane Middle ear - ossicles

Malleus (hammer) Anvil (incus) Stapes (stirrup)

Inner ear - Oval window Cochlea - fluid filled Round window ~

Cochlea

Structure: Cochlea

2 continuous chambers Scala Vestibuli Scala Tympani filled with perilymph

Scala Media between others filled with endolymph high K+ concentration ~

Scala Media

Scala Tympani

Scala Vestibuli

Uncoiled Cochlea

Scala Media

Scala Vestibuli

Scala Tympani

Round Window

Oval Window

Organ of Corti

Auditory receptor organ in scala media

Tectorial membrane roof of scala media

Basilar membrane floor of scala media

Hair cells ~

Hair Cells - Auditory Receptors Mechanoreceptors

Anchored to basilar membrane Cilia

tiny appendages (“eyelashes”) top of hair cells ~

Basilar Membrane

Tectorial Membrane

Organ of Corti

Receptor Potential Depolarization K+ influx NT release from receptor

Cilia tip-links K+ channels cilia bend open/close channels ~

Hair Cells - Transduction

Hair Cell

Cilia

K+

Tip Link

Hair Cells - Transduction

Force Force

Inhibition Resting rate Excitation

APs in Cochlear Nerve Axon

Hair Cells - Transduction

The Auditory Pathway

Complex pathway focus on most direct hierarchical processing parallel processing

Bilateral output contralateral stronger ipsilateral weaker

Cochlear Nerve Cochlear Nucleus ~

The Auditory Pathway

Left Right

Hair Cell

More NT

Auditory Nerve - VIII

Cochlear NucleiMedulla

Inferior Colliculus

Thalamus: MGN

Superior OlivePons

Auditory Pathway

A1 A1

Pitch Perception

Frequency Theory Rate neurons

vibration of basilar membrane frequency vibration frequency APs text max. frequency = 1000 Hz (1 kHz)

Low frequency sounds only < 100 - 200 Hz ~

Pitch Perception

Place Theory Place neurons

Part of basilar membrane stimulated high frequency near oval window low frequency near end ~

Base Apex

High Medium Low

Loudness Perception

Intensity amplitude of wave

place neurons rate of firing

rate neurons number of neurons firing ~

Sound Localization

Superior Olives Low frequency

Phase differences High frequency

Intensity differences ~

< 3000 Hz

> 3000 Hz