Autonomic nervous system
Introduced| English | Meaning |
|---|---|
| sympathetic nervous system/ˌsɪmpəˈθetɪk ˈnɜːvəs ˈsɪstəm/ | sympathetic nervous system |
| parasympathetic nervous system/ˌpærəˌsɪmpəˈθetɪk ˈnɜːvəs ˈsɪstəm/ | parasympathetic nervous system |
| Horner syndrome | Horner syndrome |
Fright and a full stomach
- A sudden fright: the heart races, the mouth goes dry and the pupils widen. After a big meal: the heart slows and the gut gets busy.
- Two opposing systems run the organs without you thinking, and one receptor table predicts what each drug or lesion will do.
The sympathetic outflow
- The sympathetic nervous system 交感神经系统 leaves the cord from T1 to L2. Short preganglionic fibres reach the chain of ganglia beside the spine; long postganglionic fibres reach the organs.
- The adrenal medulla is a modified ganglion that releases hormone into the blood: about 80% adrenaline and 20% noradrenaline.
The adrenal medulla releases 50 µg of hormone, 80% of it adrenaline. How many µg of noradrenaline is that?
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20% of 50 µg = 10 µg.
The parasympathetic outflow
- The parasympathetic nervous system 副交感神经系统 leaves through cranial nerves III, VII, IX and X and the sacral segments S2–S4.
- Its long preganglionic fibres reach ganglia in or near the organ wall; short postganglionic fibres finish the job.
Which is NOT a parasympathetic outflow?
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T1–L2 is the sympathetic outflow.
Put the parasympathetic path to the heart in order.
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The parasympathetic ganglia sit close to the organ, so the postganglionic fibre is short.
The transmitter code
- Every preganglionic fibre releases ACh onto nicotinic receptors. Parasympathetic postganglionic fibres release ACh onto muscarinic receptors.
- Sympathetic postganglionic fibres release noradrenaline, except those to sweat glands, which release ACh.
Sympathetic fibres to sweat glands release noradrenaline.
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They are the classic exception and release ACh.
One receptor table
- α1: blood vessels constrict, pupil dilates. β1: heart rate and force rise. β2: airways and muscle blood vessels dilate.
- M2: heart rate falls. M3: glands secrete, gut moves, pupil constricts, airways narrow.
Match each receptor to its main effect.
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The receptor expressed decides the organ's response.
What does a β1 blocker do to the heart?
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β1 receptors drive the heart; blocking them slows it.
The eye shows both systems
- Sympathetic damage gives Horner syndrome 霍纳综合征 on one side: a drooping lid, a small pupil and a dry face.
- A fixed dilated pupil with a drooping lid and an eye turned down and out means the third nerve, which carries the parasympathetic fibres to the pupil.
Worked example: a β2 agonist
- An inhaled selective β2 agonist, as used in asthma, opens the airways and dilates blood vessels in muscle.
- Read the rest off the table: β2 receptors in skeletal muscle give a fine tremor, and the heart speeds a little (a reflex to the dilated vessels, plus some cardiac β2).
Worked example: the drooping lid
- (Fictional) A smoker has a drooping left eyelid, a small left pupil and a dry left forehead. This is Horner syndrome: the sympathetic chain is damaged somewhere between the spinal cord and the eye.
- Its long route passes the top of the lung, so a tumour at the lung apex is one cause to look for. The parasympathetic supply is intact.
A drooping lid, a small pupil and a dry face on one side. What is damaged?
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This is Horner syndrome; a third-nerve lesion gives a large pupil instead.
Three common slips
- "Sympathetic speeds everything up." It slows the gut and constricts skin and gut vessels; the receptor decides.
- Forgetting the sweat glands. They are sympathetic but use ACh.
- Adding a postganglionic neuron to the adrenal medulla. The medulla is itself the ganglion.
Recap
- Sympathetic: T1–L2, chain ganglia, adrenal medulla 80% adrenaline. Parasympathetic: III, VII, IX, X and S2–S4, ganglia in the organ wall.
- Preganglionic: ACh on nicotinic, always. Postganglionic: noradrenaline (sympathetic), ACh on muscarinic (parasympathetic).
- α1 constricts, β1 drives the heart, β2 dilates, M2 slows the heart, M3 secretes. Horner = sympathetic lesion.