Special senses
Introduit| English | Sens |
|---|---|
| optic nerve/ˈɒptɪk nɜːv/ | The bundle of nerve fibres that carries visual signals from the retina toward the brain. |
| optic chiasm | optic chiasm |
| bitemporal hemianopia | bitemporal hemianopia |
| homonymous hemianopia | homonymous hemianopia |
| tonotopic | tonotopic |
Clipping the left wing mirror
- After a stroke, a driver keeps hitting things on the left. His eyes are healthy; his brain has lost the left half of what both eyes see.
- Vision and hearing are wired as maps, so the pattern of what is lost tells you exactly where the damage is.
The visual pathway
- Retina → optic nerve 视神经 → optic chiasm 视交叉, where the fibres from the nasal half of each retina cross → optic tract → lateral geniculate nucleus → optic radiation → visual cortex.
- The nasal retina sees the temporal (outer) field, so the crossing fibres carry the outer half of each eye's view.
Put the visual pathway in order.
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Only nasal fibres cross at the chiasm; everything behind it carries one half of the world.
Each site stamps a defect
- An optic nerve lesion blinds one eye. A chiasm lesion, often a pituitary tumour, cuts the crossing fibres: bitemporal hemianopia 双颞侧偏盲, loss of both outer fields.
- Any lesion behind the chiasm gives homonymous hemianopia 同向偏盲, the same half lost in both eyes, on the side opposite the lesion. A temporal-lobe lesion of the lower radiation fibres (Meyer's loop) takes the opposite upper quarter.
Match each lesion to its field defect.
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The map is preserved from retina to cortex, so each site stamps its own defect.
A pituitary tumour presses up on the optic chiasm. Which defect is typical?
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The crossing nasal fibres carry the temporal fields, and they are the ones squeezed.
The cochlea is a keyboard
- The cochlea is tonotopic 音调定位: its base responds to high frequencies and its apex to low ones.
- Young adults hear about 20–20 000 Hz, so an audiogram's shape shows which part of the cochlea is damaged.
The base of the cochlea responds to low-frequency sounds.
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The base responds to high frequencies; the apex to low ones.
Hearing spans 20 Hz to 20 000 Hz. Each octave doubles the frequency. About how many octaves is that?
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20 000 ÷ 20 = 1000, and log₂ 1000 ≈ 9.97, about 10 octaves.
Rinne and Weber
- Conductive loss (outer or middle ear): bone conduction beats air (Rinne negative), and the Weber tuning fork sounds louder in the affected ear.
- Sensorineural loss (cochlea or nerve): air still beats bone (Rinne positive) though both are reduced, and Weber sounds louder in the better ear.
Rinne is negative on the left and Weber is louder on the left. What kind of hearing loss is it?
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Bone beating air and sound moving to the bad ear both point to a conductive problem.
Balance
- Three semicircular canals at right angles detect head rotation; the otolith organs detect straight-line acceleration and tilt.
- Nystagmus, the rhythmic jerking of the eyes, is the balance reflex made visible.
Worked example: naming the field defect
- (Fictional) The driver has lost the left half of the visual field in both eyes: a LEFT homonymous hemianopia, named after the lost half.
- Both eyes are affected in the same way, so the lesion is behind the chiasm, and it is on the RIGHT, opposite the lost fields.
The left half of the visual field is lost in both eyes. Where is the lesion?
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Same half in both eyes means behind the chiasm, on the side opposite the lost field.
Worked example: deaf after a cold
- (Fictional) After a cold, a woman hears poorly in her right ear. Rinne is negative on the right; Weber sounds louder on the right.
- Bone beats air and the sound moves to the bad ear: conductive loss, most likely fluid in the middle ear.
Three common slips
- Calling a homonymous hemianopia one-eyed. It is the same half in BOTH eyes.
- Using Rinne without Weber. The pair, not either alone, separates the two kinds of hearing loss.
- Forgetting that the outer field falls on the nasal retina. That is why chiasm lesions take both outer fields.
Recap
- Optic nerve: one eye. Chiasm: bitemporal. Behind the chiasm: opposite homonymous hemianopia. Meyer's loop: opposite upper quarter.
- The cochlea's base hears high notes, its apex low, about 20–20 000 Hz.
- Conductive: Rinne negative, Weber to the bad ear. Sensorineural: Rinne positive, Weber to the good ear.