Hearing

• Anatomy of the auditory pathway

• Hair cells and transduction of sound waves

• Regional specialization of the cochlea to respond to different frequencies

The Ear

Middle ear

maleus incus

stapes

EardrumOval window

Round window

Middle Ear

Amplification

• The ear has air / water interface– Poor for transmitting vibration

• Amplification1. Large eardrum small oval window

2. Mechanical levers of the middle ear bones

Scala tympaniPerilymph – high Na+, low K+

Scala vestibuli

Scala mediaEndolymph – low Na+, high K+

Basilar membrane

cochlear nerve

Cross section of the Cochlea

Hair bundle

OuterHair Cell

Tectorial membrane

Basilar membrane

Inner Hair CellVibrates in response to sound

Shear force generated

Hinge Points

3 rows of outer hair cells

1 row of inner hair cells

• Inner Hair Cell – The actual sensory receptors

• Outer Hair Cell– Act as mechanical amplifier to sharpen

response of basilar membrane

Hair Cell

Hair Cell

Kinocilium – the tallest one

Stereocilia30-100

Tip Link

K+

Rest

Tip Link

Active Adaptation

K+

K+

Depolarization

Ca++Ca++

Voltage gated Ca channel

Sensory neuron Sensory neuron

Synaptic vesicles

Sequence of Events

• Sound waves transmitted to oval window of cochlea

• Compression of oval window vibrates the basilar membrane

• Shear forces between basilar membrane and tectorial membrane deflect stereocilia of hair cells

• mechanical opening of cation channel via the ‘tip link’

• K+ flows into cilia depolarizing the hair cell

• Opens voltage-gated Ca++ channel

• Leads to fusion of synaptic vesicles

• Activates neurotransmitter receptors on the sensory neuron

At rest, some channels open Deflection away from kinocilium, all

channels close hyperpolarization

K+

Vm

0

excitationInhibition

• How do the hair cells repolarize?

Tight Junctions – separate the extracellular fluids, & create two extracellularenvironments

perily

K+

K+

Ca++Ca++

K+ K+

PerilymphLow K+, High Na+

Repolarization

EndolymphHigh K+, Low Na+

Voltage gated K channel

Tuning of the sensory response

1. Basilar membrane is specialized to respond to certain frequencies along its length

Oval window

Round Window

Basilar Membrane

Frequency response of the basilar membraneUnrolled cochlea

Distance from oval windowMem

bran

e D

ispl

acem

ent

10,000 Hz 1000 Hz 100 Hz

BaseApex

20 Hz

Basilar membrane

• At the base, narrow & stiff high frequency vibration

• At the apex, wide & flexible low frequency vibration

• Therefore, sensory neurons originating from different regions of the cochlea carry frequency information

• Apical end low frequency information

• Basal end high frequency information