Physiology lecture 25

Hearing and the ear

LOs

• Explain the basic concepts of sound​

• Describe the role of the outer and middle ears in hearing ​

• Describe the organisation of the cochlea​

• Explain how sound frequencies are analysed on the basilar membrane​

• Describe the roles of inner and outer hair cells in sound transduction

Why is hearing important?​

• Early warning system​

• 360 degree coverage​

• Works at a distance​

• No line of sight needed​

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• Information​

• Environment sounds - knowledge of world around us​

• Sophisticated means communication in humans and animals​

• Speech - most highly developed form​

• Music - originally perhaps form of social cohesion

What is sound and what is it defined by?

• Longitudinal pressure wave travelling through air or other

• Defined by​:

• Frequency​

• Pitch (Tone) of sound​

• measured in Hertz (Hz) = cycles per second​

• Amplitude (intensity)​

• Loudness​

• Measured in dB (deciBels)​

• log scale – to better match perception medium​

Sound frequency and intensity (i.e., amplitude)​

t = period (s)​

Freq = 1/ t Hertz (s-1)​

Sound levels in common situations​

Human hearing: range of sound frequency and level​

The ear​

three parts - outer, middle, inner

all have diff functions

The outer ear​

• Pinna​

• not (significantly) mobile in humans​

• In other species, sound gathered by orientation towards source​

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• Externalisation of sounds​

• Listen without pinna, e.g. over headphones​

• Sound perceived inside the head​

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• Localisation of sounds in vertical plane​

• Sounds waves interfere on surface of pinna​

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• Acoustic gain (amplification of sound) due to ear canal resonance (28mm in length)​

• 10 to 15 dB amplification for 1.5 kHz to 7 kHz

The middle ear​

• Middle ear – air filled space​

• Need equal pressure either side of tympanic membrane​

• Air travel, diving, blocked Eustachian tube​

• Consequences of unequal pressure across tympanic membrane, hearing is impaired

Contain three bones

Movement via sound waves affects the liquid inner ear

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Why we need a middle ear​?

• Problem of transferring energy from one medium to another​

• Sound waves travelling from air to water mostly reflected​

• Inner ear is fluid filled​

• Without middle ear, poor transfer​ of energy from air to cochlear fluid​

• Middle ear is impedance transformer​

• Fish don’t have an outer or middle ear!

Function of the middle ear​

• Middle ear overcomes air - fluid mismatch​

• Area of tympanic membrane greater than footplate of the stapes​

• Force on tympanic membrane acts over smaller area of stapes footplate​

• (increases the pressure = force/surface area)​

• Ossicles also act as levers

The inner ear - Inside the cochlea​

In reality, uncoiled cochlea is a cylinder 2mm in diameter and 35mm long.​

Inside the cochlea​

scala - cavities

red = into cochlea

blue - out of cochlea

This stimulates the cochlear nerve allowing you to hear

Cochlea in cross section​

three scala

diff membranes and spaces

Cross section through the organ of Corti​

need to propagate sound

human hair cells

• Human ears have ​

• 1 row of inner hair cells (3500)​

• ~3 rows of outer hair cells (12,000)

More outer than inner

what are at the tips of hair cells?

small processes called stereocilia​

​- Stereocilia bend in response to pressure changes

​- Stereocilia of the tallest row of outer hair cells are embedded in the tectorial membrane

Separation of sound frequencies in the cochlea​

(NB for clarity scala media shown as single line)​

Stapes acts on membrane and the fluid causing movement

High frequency at base, lower at apex

Travelling wave on the basilar membrane

position of maximum displacement depends on frequency​

Frequency analysis by the cochlea​

When sound enters cochlea:

• Causes basilar membrane to vibrate​

• Travelling waves travel up basilar membrane from base to apex​

• Position of maximal displacement depends on frequency of sound​

• Low frequencies produce maximal displacement at apex of cochlea​

• High frequencies produce maximal displacement at the base​

• Displacement of basilar membrane excites hair cells

what does movement of basilar membrane relative to tectorial membrane do?

causes bending of hair stereocillia which can stimulate the cranial nerve

what does Excitation of inner hair cells lead to?

transmitter release and excitation of auditory nerve fibres​

bending of hairs

only when it bends in certain directions is a nerve impulse stimulated

in other bend the hair cell is hyperpolarised and cannot fire

Activation of hair cells​

This applies to both inner and outer hair cells​

• Movement of basilar membrane + interaction with tectorial membrane​

• causes bending of stereocilia of inner and outer hair cells​

• Bending towards tallest stereocilium​

• Increased inflow of potassium ions - depolarisation​

• Bending towards shortest stereocilium​

• Reduced inflow of potassium ions - hyperpolarisation​

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• Inner hair cells only​

• release neurotransmitter​

• activate cochlear nerve fibres associated with the hair cell

Frequency analysis​

Cochlea filters by frequency in two ways:​

1. Each neuron responds best to one frequency​

2. Each location along the cochlea amplifies one frequency

what are outer hair cells?

cellular motors

• Outer hair cells (OHC)​

• change length as stereocilia bend back and forth​

• shorten and lengthen in response to sound stimulation​

• driven by motor protein in OHC membrane – Prestin​

• inject energy and amplify basilar membrane (BM) movement​

• Reason hearing is so sensitive - detect BM movements innm range.

what are OHCs damaged by?

• Damaged by ototoxic drugs​

• some antibiotics, e.g. aminoglycosides​

• other drugs, e.g. cisplatin (cancer drug), furosemide (diuretic),salicylate (aspirin)

Can impact upon hearing

Amplification of basilar membrane motion

by motor proteins in outer hair cells​

Elongation and shortening of outer hair cells increases displacement of basilar membrane

Increased displacement of basilar membrane. Enhances hearing sensitivity and frequency selectivity

deciBels​