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| Departments
» Medical Sciences » A&P » Aural Pathways by Peter Stevenson |
| Anatomy | Physiology | Notes |
| Outer (External) Ear | ||
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Collects sound waves and funnels into inner parts of ear. | Hairs and earwax (Cerumen) prevent entry of and expel foreign bodies. |
| Middle Ear | ||
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Sound waves strike the tympanic membrane causing it to vibrate. | The tensor tympani muscle limits movement and increases tension on the ear drum to prevent damage to the inner ear from continuous loud noises. |
Auditory
Ossicles
|
Vibrations
are transferred from the tympanic membrane to an inner membrane
called the oval window through the tiny bones called the auditory
ossicles.
The Malleus is attached to the tympanic membrane and the Stapes is connected to a membrane covering an opening called the oval window. |
The oval
window is much smaller than the ear drum. It vibrates 20 times more
vigorously than the tympanic membrane.
The stapedius muscle is the smallest skeletal muscle. It has a role in protecting the inner ear from damage by dampening vibrations from loud noises. |
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The round window resides at the termination of the scala tympani of the cochlea directly below the oval window. It is covered with a membrane called the secondary tympanic membrane. | The round window sends pressure waves back into the middle ear from the inner. Depending on the phasing of the pressure waves this could either interfere with or enhance the primary sound wave. |
| Inner Ear | ||
Cochlea
|
The vibrations of the oval window are transferred into the perilymph contained in the scala vesibuli and scala tympani of the cochlea | The inner
ear is also called the labyrinth as it contains a complicated series
of canals. It is involved in maintaining balance.
The cochlea is part of the inner ear containing hearing receptors. It resembles a snails shell and makes three turns around a central bony core. |
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In between
the two vestibules is the cochlear duct filled with endolymph. The
cochlear duct is partitioned from the scala vestibuli by the
vestibular membrane and the scala tympani by the basilar membrane.
The pressure waves in the perilymph induce pressure waves in the endolymph. |
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The spiral
organ rests on the basilar membrane and it is the organ of hearing.
It contains hair cells which are the receptors for auditory
sensations. The hair cells have projecting microvilli which are
covered by the tectorial membrane.
When the endolymph moves the bassilar membrane moves causing the hair cells to move against the tectorial membrane. This produces action potentials within the hair cells. |
The
frequency and strength of the vibrations impact the movement of the
microvilli of the hair cells. This impacts the frequency and
strength of the action potentials.
The mechanical bending of the microvilli opens ion channels into the hair cells. This permits the influx of mainly potassium and calcium ions. This depolarisation causes calcium channels at the base of the cell to open allowing the influx of more calcium. This causes the release of a neurotransmitter firing the nerves that synapse with the hair cells. Bending the microvilli in the other direction closes the ion channels causing a repolarisation of the cell and a reduction in the release of the neurotransmitter. |
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The hair cells synapse with first-order sensory and motor neurons from the cochlear branch of the vestibularcochlear nerve. | |
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Impulses are conducted to the cochlear nuclei in the medulla oblongata. | |
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Most nerve fibres cross to the opposite side, extend through the midbrain and terminate in the Thalamus. | |
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Signals travel from the Thalamus to the primary auditary ara in the termporal lobe of the cerebral cortex. |