BIOL1019 Chap.3 The Autonomic Nervous System
The Autonomic Nervous System
The autonomic nervous system controls cardiac muscle, smooth muscle, glands and adipose tissue without conscious effort.
This chapter contrasts it with the somatic system, which uses a single motor neuron, and builds the two-neuron chain of each autonomic division: preganglionic neurons always release acetylcholine onto nicotinic receptors, while postganglionic neurons release noradrenaline onto adrenergic receptors in the sympathetic division and acetylcholine onto muscarinic receptors in the parasympathetic division.
It then explains dual innervation, tonic control and autonomic tone, which together give the precise control of organs such as the heart, and applies the receptor rules to drug questions like the use of atropine in bradycardia.
Expect this material in Test 1 and again in the final exam, often joined to other chapters: the pupil and ciliary muscle in the special senses, the adrenal medulla in the endocrine chapters, insulin secretion in the pancreas and sweat glands in thermoregulation all depend on the same transmitter and receptor rules.
What this chapter covers
- 01
Somatic versus autonomic motor pathways
- 02
Thoracolumbar and craniosacral origins
- 03
Preganglionic and postganglionic transmitters
- 04
Nicotinic, muscarinic and adrenergic receptors
- 05
Adrenal medulla as a modified ganglion
- 06
Dual innervation and tonic control
- 07
Autonomic tone and drug effects
- 08
The hypothalamus as autonomic integrator
Worked example · free
Pupils and lens in a dark cinema
- +1In the dark the pupils dilate, a sympathetic effect on the pupillary muscle.
- +1Sympathetic postganglionic neurons release noradrenaline onto adrenergic receptors.
- +1For the close ticket the ciliary muscle contracts and the lens rounds, a parasympathetic effect.
- +1Parasympathetic postganglionic neurons release acetylcholine onto muscarinic receptors, M3 at the ciliary muscle.
Key terms
- Preganglionic neuron
- The first autonomic neuron, running from the CNS to a ganglion and releasing acetylcholine onto nicotinic receptors.
- Muscarinic receptor
- A cholinergic G protein-coupled receptor that mediates every parasympathetic response on effector cells.
- Autonomic tone
- The normal background level of activity in an autonomic pathway, present even at rest.
- Nicotinic receptor
- An acetylcholine-gated ion channel found at autonomic ganglia and at the skeletal muscle end plate.
The Autonomic Nervous System FAQ
Why is the adrenal medulla described as part of the sympathetic system?
The adrenal medulla behaves as a modified sympathetic ganglion. Sympathetic preganglionic fibres stimulate it directly, and instead of passing the signal to a postganglionic neuron, it releases noradrenaline and adrenaline into the blood, so the fight or flight response is spread through the body as a hormonal signal as well as a neural one.
What is tonic control and how does it differ from dual innervation?
In tonic control only one division supplies the organ, usually the sympathetic division, as in most blood vessel smooth muscle. A background signal is always present, so increasing it constricts the vessel and reducing it lets the vessel dilate. Dual innervation instead uses two divisions with opposing effects on the same organ, such as the heart.
Why is atropine given for a very slow heart rate?
Atropine blocks muscarinic receptors. In the heart, parasympathetic acetylcholine acting on muscarinic receptors slows the rate, so blocking those receptors removes the parasympathetic brake. Sympathetic input through adrenergic receptors is then unopposed, and heart rate rises, which is why the lectorial describes intravenous atropine for acute bradycardia.
Which organs receive opposite instructions from the two divisions?
The course's dual innervation table lists the heart, where sympathetic activity raises rate and force and parasympathetic activity lowers them; the gut, where motility falls and rises; the bladder, which relaxes or contracts; the pupil, which dilates or constricts; and the bronchioles, which dilate or constrict. Insulin secretion is inhibited sympathetically and stimulated parasympathetically.
Exam move
Draw three rows, somatic, sympathetic and parasympathetic, and label every synapse with its transmitter and receptor until you can do it in under a minute. Learn the dual innervation table organ by organ, saying the direction aloud for each division. Then practise drug questions by asking three things in order: which receptor the drug acts on, whether it mimics or blocks, and what the organ does next.
Finish with one integrated answer: describe a sudden fright from the sympathetic chain and adrenal medulla through to recovery under parasympathetic control, naming the transmitter at every synapse.
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