Describe the mechanism of hearing. Explain impedance matching, the transduction by hair cells and the analysis of pitch.

Range of Hearing

• Intensity measured in decibels; threshold 0 dB, conversation 60 dB, damage above 90–100 dB

Conduction of Sound

Sound waves → external auditory meatus → Tympanic membrane vibrates → Malleus → incus → stapes → Oval window → Perilymph of scala vestibuli → Basilar membrane vibrates → organ OF CORTI → Hair cells → cochlear nerve

Impedance matching — the function of the ossicles

• Sound must pass from air to fluid; without help, 99.9% would be reflected
• Areal ratio — tympanic membrane 55 mm2 to oval window 3.2 mm2 = 17:1
• Lever action of the ossicles = 1.3:1
• Total gain 17 × 1.3 = about 22-fold, or 25–30 dB
• Loss of the ossicles therefore causes a conductive deafness of about 30 dB

Cochlear Fluids and Potentials

• Endolymph is secreted by the stria vascularis
• Endocochlear potential = +80 mV in the scala media
• Hair cell interior is −60 mV, so the total potential across the apical membrane is 140 mV — the largest in the body, which makes the hair cell exquisitely sensitive

Transduction BY Hair Cells

Basilar membrane moves → Shearing between tectorial membrane and hair cells → Stereocilia bend towards the kinocilium → K+ channels open (tip links) → K+ enters from endolymph → depolarisation → Ca2+ entry → transmitter release → Cochlear nerve fires

• Unusually, it is K+ influx that depolarises the hair cell, because endolymph is K+-rich
• Inner hair cells (3500) are the true sensory receptors, carrying 95% of the afferents
• Outer hair cells (20,000) act as a cochlear amplifier, changing length to sharpen tuning; they generate otoacoustic emissions, used in newborn hearing screening

Analysis of Pitch — Place Theory

• Von Békésy travelling wave theory
• Base of the cochlea — narrow and stiff basilar membrane → responds to high frequencies
• Apex — wide and floppy → responds to low frequencies
• Each frequency therefore produces maximum displacement at a particular place — a tonotopic map preserved all the way to the cortex
• Loudness is coded by the number of hair cells stimulated and the firing frequency

Auditory Pathway

Spiral ganglion → Cochlear nuclei (medulla) → Superior olivary nucleus (partly crossed) → Lateral lemniscus → Inferior colliculus → Medial geniculate body → Auditory cortex — areas 41, 42 (superior temporal gyrus)

• The pathway is bilateral from the cochlear nuclei onwards — therefore a unilateral cortical lesion does not cause deafness
• Deafness from a single lesion means the lesion is in the ear or the cochlear nerve

Functions of the Middle Ear and Eustachian Tube

• Eustachian tube connects the middle ear to the nasopharynx
• Function — equalises pressure on either side of the tympanic membrane; drains middle ear secretions
• Normally closed; opens on swallowing, yawning and the Valsalva manoeuvre
• In children it is shorter, wider and more horizontal → why otitis media is commoner in children
• Blockage → negative middle ear pressure → retracted drum, conductive deafness, effusion

Masking and Bone Conduction

• Bone conduction bypasses the external and middle ear and stimulates the cochlea directly — the basis of tuning fork tests
• Masking — the better ear must be masked with noise when testing the worse, or sound crosses the skull and gives a false result
• Interaural attenuation is about 40 dB for air conduction but only 0–10 dB for bone conduction

Localisation of Sound

• Interaural time difference — used for low frequencies below 1500 Hz
• Interaural intensity difference — the head casts a sound shadow; used for high frequencies
• Processed in the superior olivary nucleus, the first site of binaural convergence
• The pinna helps distinguish front from back and above from below

Applied Aspects

• Attenuation reflex — tensor tympani and stapedius contract to loud sound (> 70 dB) → protects the inner ear; latency 40–80 ms, so it cannot protect against an explosion
• Noise-induced hearing loss — begins at 4000 Hz ("acoustic dip"); irreversible outer hair cell damage
• Presbycusis — age-related loss, beginning with high frequencies
• Cochlear implant bypasses the hair cells and stimulates the cochlear nerve directly — useful in sensorineural but not in retrocochlear deafness
• Tinnitus — perception of sound without an external source; usually accompanies sensorineural loss
• Recruitment — abnormally rapid growth of loudness; characteristic of cochlear rather than retrocochlear disease