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Hearing
(Mechanoreceptors)
I). Anatomy
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A). Outer ear (external)
1). Pinna or Auricle: shell shaped cartilage projection
2). External auditory canal: tube that extends to eardrum
3). Tympanic membrane: ear drum
B). Middle Ear
1). Tympanic cavity: Separates external & internal ears
2). Ossicles:
Malleus (hammer) Incus (anvil) Stapes (stirrups) 3). Oval Window with secondary tympanic window
4). Auditory tube or Eustachian tube: Continuous with pharynx opens to equalize pressure
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C). Inner Ear
1). Bony or Osseous Labyrinth
: central cavity; houses equilibrium receptors.i). Vestibule
ii). Semicircular Canals:
iii). Cochlea: fluid filled; spiral chamber houses
- Organ of Corti: receptor for hearing.
2). Membranous Labyrinth: series of sacs & ducts within the bony labyrinth.
II). Properties of Sound
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is a pressure disturbance originating from a vibrating object and propagated by molecules of the medium.A). Sound
- The molecules in the air just vibrate against each other.
B). Frequency: tone or pitch. (units: Hertz)
- Number of waves that pass a given point.
- Short wavelength (distance between waves) higher the sound.
C). Amplitude: intensity or loudness (units: Decibels)
- Height of the waves
III). Process of Hearing
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A). Steps
1). Sound waves are propagated through the air.
2). The pinna and the auditory canal focus the waves.
3). Sound wave strikes the tympanic membrane and starts it vibrating.
4). The malleus is secured to the membrane and passes the same total forces through the stapes and into the oval window.
5). The oval window is smaller so the force becomes concentrated. (pressure is related to area)
6). The wave is passed on to the cochlea and the fluid filled chamber vibrates. It also causes the fluid to vibrate the round window.
- Fluids cannot be compressed.
- So when the stapes moves in and out and the membrane moves up and down the round or cochlear window will also move in and out.
7). Hair cells in the Organ of Corti are stimulated by activity in the membrane
- High pitch activates cells near oval window and low pitch activates cells further away.
8). Stimuli are sent to the spiral ganglia and the cochlear nerve.
9). To the auditory reflex center of the midbrain.
10). To the thalamus.
11). To the auditory cortex in the temporal lobe.
12). Each auditory cortex receives input from both ears.
13). Cortical processing distinguishes wavelengths and sounds.
B). Distinguishing sounds
1) Pitch
- Specific cochlear cells are stimulated for different pitch and the cortex has a tonotopic map of the regions of the organ of Corti.
2). Detection of loudness
- Some cochlear cells have a higher threshold.
3). Localization of sound
- Determined by relative timing and relative intensity. (ie. overhead: both ears received sound waves at the same time)
