7 Auditory system

= mechanical radiant energy transmitted by longitudinal pressure waves in the air

= objective cause of hearing

Number of times per second at which waves repeat (unit: Hertz)

= size of the pressure changes (unit: Pascal or Decibel)

20-20 000 Hertz

detection

• 20 - 20 000 Hz

discrimination

• differ 2 similar sound stimuli e.g. 1000 and 1002 Hz

• phase locking in brainstem

localization

• intraaural time difference (ITD)

  • time difference between the 2 ears

• intraaural intensity difference (IID)

  • head as sound shadow

• Pinna

cochlear amplifier

outer hair cells = motor = amplify the motion 100 times

—> can produce sound itself “echo”

frequency, intensity, sound source, complexity

—> subjective experiences: pitch, loudness, location, timbre

• frequency = pitch

• intensity = loudness

• sound source = sound location

• complexity (simultaneous combinations of different frequencies at varying intensities) = timbre

intensity we can hear differs about 1 trillion

decibel scale is logarithmic and compresses the numbers to 0-120 db

• pitch

  • assignment of tones

  • frequencies Hz: high, low

• loudness

  • intensity (db): loud, silent

• location

  • intraaural time/level difference (ITD,ILD), Pinna: source

• timbre

  • same tone (both high, loud), different instrument

= sounds that are actively generated by the ear itself

• movement of outer hair cells drives the cochlear fluids and membranes

• —> ossicles—> tympanic membrane —> vibrates, producing actual sound waves in the outside air

= characteristic of a healthy ear, usually central auditory neurons suppress the perception of this internal noise

with fluid —> ossicles amplify the waves (factor 20 in comparison with tympani)

= uncoiled cochlea

ossicles = malleus, incus, stapes

cochlea = fluid filled, Corti organ (4000 hair cells)

scala media fluid filled, +80mV

Stria vascularis pumps K+ into Scala media

left: high frequencies

right: low frequencies

because basilar membrane is 5 times wider at the apex

= Corti organ

outer ear

• pinna

  • collects sound waves

• auditory canal

• tympanic membrane

  • vibration begins

middle ear

• ossicles (malleus, incus, stapes)

  • amplification

inner ear

• fluid displacement

  • stapes —> oval window —> perilymph (fluid) in scala vestibuli —> round window bulges outward

• basilar membrane

  • traveling wave along the membrane base —> apex

    • low frequencies travel longer

• organ of corti

  • stereocilia of hair cells bend against the tectorial membrane

  • mechonsensitive K+ channels open —> K+ influx in hair cell

  • depolarization triggers AP on auditory nerves (Glutamate release)

mechanosensitve ion channels open, when stereociles are moving

—> K+ influx

—> depolarization

—> Ca2+ influx through voltage gated channels

—> Glutamate release

—> spiral ganglion neurite

—> axonal conduction to brain

scala vestibuli = perilymph

scala media = endolymph

scala tympani = perilymph

Perilymphe = 0 mV, K+ low

Endolymph = +85 mV, K+ high

inner hair cell = -70 mV, K+ low

potential difference= +155 mV

—> K+ flows into the hair cells because of the massive electrochemical gradient

—> spiral ganglion

—> ventral/dorsal cochlear nucleus (Medulla)

—> bilateral to superior olive (Medulla)

—> lateral lemniscus —> inferior colliculus (Midbrain)

—> MGN=Medial Geniculate Nucleus (Thalamus)

—> auditory cortex A1

pitch perception = coding of sound frequency

• fiber identity (labeled line)

• phase locking = temporal code

loudness perception = coding of intensity

• firing rates

• nr. of active neurons

primary auditory cortex = 41

secondary auditory cortex = 42

outer haircells = cochlear amplifier

inner haircells = acoustic, convert mechanical energy into electrochemical

= mechanical support

• stabilize the components of Corti organ

• make vibration transmission to hair cells possible