How do we hear

action potentials

electrical signal that occurs when neurons are excited

• at rest inside neuron is slightly negative compared to outside

• incoming neuron signals causes influx of Na ions = depolarisation = more positive

• repolaristion = K flows out and back to negative

• depolar to repolar causes fluctuation in voltage that travels down axon to pass on signal

how are sounds produced and perceived

sound produced when object

sound production

sound when an object vibrates = surrounding air molecules compress/rarefy = pressure changes create sound waves that travel through air at 1100 km/h

hearing

when ear detects changes in air pressure caused by sound waves

range of human hearing

30 – 20,000 Hertz (Hz).

frequency and sound

number of vibration cycles per second., in hertz

3 perceptual dimensions of sound

each corressponds with a particular physical dimension

1. loudness

2. pitch

3. timbre

loudness

determined by degree to which air molecules are pushed together and pulled apart

• physical property - amplitude/intensity

• more vigorous vibrations of object cause larger amplitude sound waves = more intense sounds

pitch

determined by frequency of sound waves produced by vibrrating object

• more sound waves per second = higher pitched sound

timbre

sound quality - determined by the complexity of sound waves

• more little peaks and troughs in waveform = more complex sound

sinusoidal wave

sine wave - simplest type of sound wave produced by single frequency at constant amplitude creating smooth wave

• pure tone with no harmonics

human ear structure

3 parts

1. outer

2. middle

3. inner

outer ear

outer fleshy pinna, auditory canal and tympanic membrane (eardrum)

• tympanic membrane vibrates with soundwaves that enter canal = signal transmitted to middle ear

middle ear

consists of 3 tiny bones called ossicles - malleus (hammer) is connected to tympanic membrane and transmits vibrations via the incus (anvil) to stapes (stirrup). stapes is connected to the cochlea (snail) of inner ear

inner ear

cochlea contains receptors for analysing sound. has 2 small membranes that form windows on its fluid-filled interior

• stapes is connected to oval window

• when sound waves cause stapes to move in and out it pushes oval window = fluid movememnt in ear

• fluid movement stimlates sound receptors in cochlea

• cochlea is closed strucutre = round window acts as flexible membrane allows fluid to move by bulging in and out = relieves pressure

basilar membrane

sheets of tissue containing auditory receptors in cochlea and runs from base to apex - different regions for different sounds

• base (near oval window) - high-frequency sounds

• apex (tip) - low-frequency sounds

• performs frequency discrimination/pitch discrimination

organ of corti

within cochlea - composed of 3 parts all filled with fluid

• basilar membrane (base)

• hair cells - inner and outer receptors (middle) - topped with stereocilia

• tectorial membrane - roof structure

how organ of corti receives sound

sound waves cause basilar relative to tectorial = bends stereocilia = opens ion channels producing receptor potentials = sound waves converted into neural signals

• outer hair - bent by direct contact with tectorial

• inner hair - bent mainly by fluid movement from basilar motion

stereocilia and action potential

1. cilia are connected together in bundle by tip links atatched to ion channels

2. when bending tension in tip links increase = channels open

3. K+ and Ca+ ions enter from surrounding fluid

4. depolarisation

5. neurotransmitters released

6. triggers action potential in spiral ganglion cells

auditory nerve/8th cranium nerve

bundle of spiral ganglion cells that form synapses with neurons in medulla which relay signals to higher brain regions for further processing

• 95% of axons from auditory nerve connect with inner hair cells = crucial for hearing

• 5% connected to outer hair = not directly involved in hearing but modify mechanical properties of basilar and enhance response of inner ear

what does damage to inner hair cells do

significant hearing loss bc primarily responsible for converting sound waves into neural signals

place coding

brain determines frequency/pitch of sound based on where along basilar membrane greatest vibration occurs

primary auditory cortex

area in the brain responsible for processing auditory information, located in the sperior temporal gyrus in temporal lobe

• identifies and interpretes sounds

• majority it buried in lateral fissure = not visible from laateral view

• orangised as tonotopic map - lower frequencies in anterior region and higher more posteriorly.

binaural processing

brain processes info from both ears

• signals from each ear are sent to both cerebral hemispheres = comparison of sound

• comparison = location of sound

cochlear implant

when hair cells are damaged - often congenital

• mircophone - external behind pinna picks up sound

• signal processor - external converts sound into electrical signals

• transmitter - scalp sends signals across skin to implant

• electrode array - surgically inserted into cochlea along basilar

• different electrodes mimic place coding

cochlear implant challenges

• many electrodes needed to reproduce sound well

• fast sound processors needed to determine how much of each frequency is in sound

• optimised for speech - not good for music or natural sounds

how is sound processed

1. sound waves enter pinna and thru cannal = eardrum vibrates

2. ossicles amplify vibrations - stapes moves oval window

3. fluid waves form in cochlea = basilar membrane vibrates

4. hair cells bend = ion channels open = depolarisation = release neurotransmitters

5. spiral ganglion neurons fire action potentials = signal travels to medulla via auditory nerve

6. sent to auditory cortex = binaural processing