Transcript Slide 1
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 Middle Ear maleus incus stapes Eardrum Oval window Round window Amplification • The ear has air / water interface – Poor for transmitting vibration • Amplification 1. Large eardrum small oval window 2. Mechanical levers of the middle ear bones Cross section of the Cochlea Scala vestibuli Scala media Endolymph – low Na+, high K+ Scala tympani Perilymph – high Na+, low K+ cochlear nerve Basilar membrane Shear force generated Tectorial membrane Hair bundle Outer Hair Cell Basilar membrane Inner Hair Cell Hinge Points Vibrates in response to sound 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 Stereocilia 30-100 Tip Link Rest Active Tip Link K+ Adaptation K+ K+ Depolarization Voltage gated Ca channel Ca++ Ca++ Synaptic vesicles Sensory neuron Sensory neuron 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 K+ At rest, some channels open Deflection away from kinocilium, all channels close hyperpolarization Inhibition excitation Vm 0 • How do the hair cells repolarize? Tight Junctions – separate the extracellular fluids, & create two extracellular environments K+ Repolarization K+ Endolymph High K+, Low Na+ Voltage gated K channel K+ K+ Ca++ Ca++ perily Perilymph Low K+, High Na+ Tuning of the sensory response 1. Basilar membrane is specialized to respond to certain frequencies along its length Frequency response of the basilar membrane Unrolled cochlea Oval window Basilar Membrane Base Membrane Displacement Round Window 10,000 Hz 1000 Hz 100 Hz Distance from oval window 20 Hz Apex 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