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Ear and hearing (Middle ear (Transmit sound waves from outer ear to inner…
Ear and hearing
Middle ear
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Auditory tube - Connects nasopharynx to middle ear, equalises air pressure on both sides of ear drum
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Otitis media
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Discharge collects in bulla, head tilts towards affected side
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Horners syndrome
Prolonged middle ear infection can damage facial nerve and sympathetic fibres of cervical ganglion:
- Facial paralysis
- Constricted iris
- 3rd eyelid displaces across eye
- Upper eyelid droops
Outer ear
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Elastic cartilage (pinna) - Gathers and directs sound down ear canal. Communicates emotion. Outer side more hairy than inner.
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Mechanism of hearing
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- Ear drum vibrates
- Ossicles vibrate
- Stirrup pushes oval window in & out
- Perilymph moves
- Pressure waves transmit from scala vestibuli to s. tympani then to endolymph of s. media
- Basilar membrane deflects, hair bundles bend converting vibration to electrochemical signal
Ear wax = Cerumen
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Ceruminous glands = Modified sebaceous glands, secrete wax to protect canal from damage/infection
Otitis externa
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Can be caused by mites, bacterial/fungal infection
Results in: Pain, dischrge, self-trauma
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Inner ear
Functions:
- Hearing: convert middle ear vibrations to electrochemical signals
- Balance: Enable sensing of position of head in space
Bony and membranous labyrinths - Separated by perilymph and endolymph
[refer to pictures from lecture]
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Nerve deafness
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Could be due to:
- Genetics
- Inner ear infections
- Drug toxicity
- Noise trauma
- Age-related degeneration
Conduction deafness
Sound cannot pass into ear due to:
Tumours, perforation of drum, outer and middle ear infections, wax build up, ear mites
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Auditory pathway
- Sensory neurones in cochlear brance of CN VIII terminate in (ipsilateral) medulla oblongata (cochlear nuclei)
- Axons carry auditory signals projecting to nuclei in pons
- Axons ascend to caudal colliculi and then to medial geniculate nucleus in thalamus
- Auditory signals to auditory cortex. Because many auditory axons decussate in medulla while some remain same side, left and right auditory cortex recieve impulses of both sides