Somatosensory and pain pathways
Introduced| English | Meaning |
|---|---|
| dorsal column | dorsal column |
| spinothalamic tract | spinothalamic tract |
| Brown-Séquard syndrome | Brown-Séquard syndrome |
| gate control | gate control |
| referred pain | referred pain |
Vibration lost on one side, pain on the other
- After a knife wound to one side of the spinal cord, one leg cannot feel a tuning fork while the other cannot feel a pin.
- The two main sensory pathways cross the midline at different levels, and that split is the key to finding a lesion.
The dorsal column pathway
- The dorsal column 后索 pathway carries fine touch, vibration and joint-position sense.
- Its fibres enter the cord and rise on the SAME side, then cross in the medulla on their way to the thalamus and cortex.
Put the route of vibration sense from the left foot in order.
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The dorsal column stays on the same side until the medulla.
The spinothalamic tract
- The spinothalamic tract 脊髓丘脑束 carries pain and temperature. Its fibres cross within 1–2 segments of entering the cord, then rise on the OPPOSITE side.
- So pain from the left leg travels up the right side of the cord.
Match each sensation to its pathway.
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Fine touch, vibration and position go one way; pain and temperature the other.
The split cord
- Damage to one half of the cord gives Brown-Séquard syndrome 脊髓半切综合征: below the lesion, vibration and position sense are lost on the same side, pain and temperature on the opposite side.
- The pain loss starts 1–2 segments below the lesion, because those fibres cross a little above where they enter.
The right half of the cord is cut at T8. Where is pain sensation lost?
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Pain fibres have already crossed, so the right tract carries pain from the left side of the body.
A half-cord lesion is at T6, and pain loss starts 2 segments lower. At which thoracic segment does it start? Give the number.
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T6 + 2 = T8, because pain fibres rise 1–2 segments before crossing.
The face is different
- Sensation from the face travels in the trigeminal nerve to the brainstem, not in the spinal cord.
- So a spinal cord lesion never causes numbness of the face.
Sensation from the face travels up the spinal cord.
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It travels in the trigeminal nerve straight to the brainstem.
Gate control
- Large touch fibres (Aβ) excite inhibitory interneurons in the dorsal horn that damp down pain signals from small C fibres: gate control 闸门控制.
- That is why rubbing a knocked elbow helps.
Why does rubbing a knocked elbow reduce the pain?
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This is gate control in the spinal cord.
Referred pain
- In referred pain 牵涉痛, signals from an organ and from an area of skin enter the same spinal segments, and the brain blames the skin.
- Irritation under the diaphragm (C3–C5) is felt at the shoulder tip; heart pain (T1–T5) is felt in the chest and left arm.
Why is heart pain often felt in the left arm?
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Converging inputs make the brain blame the familiar skin area.
Worked example: a left T10 hemisection
- (Fictional) A stab wound cuts the left half of the cord at T10. Below T10 on the left: lost vibration and position sense, plus upper motor neuron weakness.
- On the right: lost pain and temperature from about T11–T12 downwards. Each sign follows one tract and its crossing.
Worked example: pain at the shoulder tip
- (Fictional) After a fall from a bicycle, a cyclist has upper abdominal pain and pain at the left shoulder tip. Blood from a torn spleen is irritating the diaphragm.
- The diaphragm's sensory fibres enter at C3–C5, the same segments as the skin over the shoulder, so the brain reports the shoulder.
Three common slips
- Tracing pain on the same side. Pain fibres cross in the cord, right after entry.
- Expecting a spinal level on the face. The face uses the trigeminal nerve.
- Putting the pain level at the lesion. It sits 1–2 segments lower.
Recap
- Dorsal column: fine touch, vibration, position; same side up the cord, crosses in the medulla.
- Spinothalamic: pain and temperature; crosses within 1–2 segments, opposite side up the cord.
- A half-cord lesion splits sensation; touch can close the pain gate; shared segments refer organ pain to skin.