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Communication by sound: The Ear
Overview of The Ear
Key words:
Pinna=External Ear
External auditory meatus=Ear Canal
Tympanum/Tympanic Membrane=Ear Drum
Cochlear Nerve=Auditory Nerve
Introduction
Outer ear comprises pinna, ear canal and eardrum. Pinna important for sound localisation. Ear canal has broad resonance that amplifies frequencies around 3 kHz. Frequency range of human hearing (young adult) is 20Hz-20kHz.
Middle Ear
Key words:
Hammer=Malleus
Anvil=Incus
Stirrup=Stapes
Ossicles=Tiny bones
Methodology
Impedence Matching:
-necessary to optimise sound transmission from air to fluid
-area ratio of eardrum to oval window is 35:1
-malleus is longer than incus (lever ratio=1:1.15)
-effective increase in sound transmission=38dB
Protective Reflex:
-stapedius muscle contracts and stiffens chain of ossicles when ear is exposed to very intense sounds
Detail 1
Detail 1
Detail 2
Inner ear// Cochlea
Key Words:
Vestibular Canal=Scala Vestibuli
Tympanic Canal=Scala Tympani
Scala Media=Cochlear Duct
All chambers are filled with lymph (a fluid)
Cochlear Partition:
...a membranous structure that divides the cochlea along its length. Hollow interior of partition is called the 'cochlear duct'.
Human Cochlear:
-has 2.75 turns from base to apex
-is about 3.5cm long if uncoiled
responds best to low frequencies at apical end; widest (0.5mm), heaviest, and least stiff
Basilar Membrane (BM):
...part of BM next to oval window is called the basal/stapedial end.
...part of BM furthest from oval window is called apical/helicotremal end.
responds best to high frequencies at basal end; narrowest (0.04mm), lightest and most stiff
Key players
Organ of Corti
2 types of hair cells:
...1 row of inner ear cells; sensory cells that respond to movement of BM.
...3-5 rows of outer hair cells; affect how BM moves in response to sound, sharpens ability to discriminate different frequencies of sound.
Timeline
Response of Inner Hair Cells
Inner Hair Cells:
...capped with stereocilia (hair bundle). Cells synapse onto auditory neurons.
...displacement of hair bundle changes membrane potential of hair cell.
...when displacement stretches, tip links between adjacent stereocilia, hair-cell membrane depolarises.
Depolarisation:
...causes neurotransmitter release which excites postsynaptic auditory neuron .
...auditory nerve leaves inner ear consisting of the axons of these auditory neurons.
Results
Auditory Nerve
->Spiral Ganglion (SG) contains cell bodies of auditoy neurons.
->Axons of these auditory neurons are bundled together in the modiolus (hollow core of the cochlea)
->Auditory Nerve (AN) contains around 28,000 axons.
->Anatomical evidence suggests each axon has synaptic connections with inner hair cells only at one place along the BM.
->This indicates that responses of each auditory neuron will be frequency selective; 'a place code'
Analysis
Neural representations of speech sounds
Subtopic 1
excitation is a measure of neural activity.
->log frequency scale corresponds with distance along BM (low freq'=apical, high freq'=basal)
Neural representations of speech sounds
->Plot of excitation level against distance along BM ("place") is called an Excitation Pattern (EP)
Subtopic 2
Neural representations of speech sounds
EP is the "auditory spectrum" of a sound; e.g. how its frequency content is represented in the ear's neural response
Subtopic 3
Summary & Conclusions
Outer ear important for ability to localise sounds. Vibrations of eardrums are transmitted through middle ear to inner ear
Conclusion
The outer ear is important for our ability to localize sounds. Vibrations of the eardrum are transmitted through the middle ear to the inner ear.
The middle ear ensures efficient transmission of sound vibrations from the air to the fluid-filled chambers of the cochlea in the inner ear.
The cochlea acts as a frequency analyser.
Information about the frequency spectrum of speech sounds (and other broadband sounds) is conveyed by the distribution of neural activity by place along the basilar membrane.
Summary & Conclusions
Middle ear ensures efficient transmission of sounds vibrations from the air to the fluid filled chambers of cochlea in the inner ear.
Cochlear acts as a frequency analyser.
The outer ear is important for our ability to localize sounds. Vibrations of the eardrum are transmitted through the middle ear to the inner ear.
The middle ear ensures efficient transmission of sound vibrations from the air to the fluid-filled chambers of the cochlea in the inner ear.
The cochlea acts as a frequency analyser.
Information about the frequency spectrum of speech sounds (and other broadband sounds) is conveyed by the distribution of neural activity by place along the basilar membrane.
What’s next
Thank you
Summary & Conclusions
Information about the frequency spectrum of speech sounds (and other broadband sounds) is portrayed by the distribution of neural activity by place along BM.