How is the Autonomic Nervous System Organised?
The autonomic nervous system (ANS) controls involuntary functions like heart rate, digestion, and blood pressure. It has two antagonistic divisions: sympathetic ('fight-or-flight') and parasympathetic ('rest-and-digest'), plus the enteric nervous system.
The ANS has three divisions: sympathetic (thoracolumbar T1–L2), parasympathetic (craniosacral S2–S4 + CN III, VII, IX, X), and enteric (gut). They use acetylcholine (ACh) or noradrenaline (NA) and often have opposite physiological effects on the same organ.
- •Origin: T1–L2 spinal cord
- •Neurotransmitter: Noradrenaline (mostly)
- •Effect on heart: increases rate & force
- •Effect on pupils: dilates
- •Effect on digestion: inhibits
- •Effect on blood vessels: constricts
- •Response: 'Fight or flight'
- •Fibre length: long parasympathetic, short sympathetic (pre-ganglionic)
- •Origin: brainstem (CN III, VII, IX, X) + S2–S4
- •Neurotransmitter: Acetylcholine (all)
- •Effect on heart: decreases rate & force
- •Effect on pupils: constricts
- •Effect on digestion: enhances (increases salivation, peristalsis)
- •Effect on blood vessels: dilates
- •Response: 'Rest and digest'
- •Fibre length: short parasympathetic, long sympathetic (post-ganglionic)
Step-by-step worked examples
Why does your heart rate increase during a sprint, and how does it return to normal after?
Sprint activates sympathetic nervous system (T1–L2): - Noradrenaline on sinoatrial (SA) node increases heart rate - Increased cardiac output supports muscle activity After sprint, parasympathetic (vagus, CN X) dominates: - Acetylcholine on SA node decreases heart rate - Calm signal via vagal tone restores baseline
Why does your mouth dry during anxiety, but salivate when eating food?
Anxiety activates sympathetic system: - Noradrenaline inhibits salivary glands - Mouth dries (sympathetic prioritises other organs) Eating food activates parasympathetic: - Acetylcholine (from CN VII, CN IX) enhances salivary glands - Saliva secretion aids digestion
A patient takes a beta-blocker (sympathetic antagonist). What physiological changes occur?
Beta-blockers block noradrenaline at cardiac receptors: - Heart rate decreases - Cardiac force decreases - Blood pressure drops - Useful in hypertension and angina, but can cause fatigue
Flashcards
Quick quiz
Q1.Which spinal levels carry sympathetic fibres?
Q2.A patient's pupil does not dilate in dim light. Which system is impaired?
Q3.The vagus nerve (CN X) carries parasympathetic fibres that slow the heart via which neurotransmitter?
Q4.A patient presents with a dilated, unresponsive pupil and cannot focus (accommodation). Which system is blocked?
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Common mistakes
Thinking sympathetic and parasympathetic are always active or always inactive. — Correct: Both systems are always active to varying degrees (basal tone). A stimulus can shift the balance toward one or the other.
Confusing all autonomic transmitters as either ACh or NA. — Correct: Parasympathetic always uses ACh. Sympathetic uses NA (and epinephrine from adrenal medulla), but some sympathetic preganglionic fibres use ACh on the adrenal medulla.
Assuming parasympathetic ganglia are always in the spinal cord. — Correct: Parasympathetic ganglia are near or within target organs (terminal ganglia). Cranial and sacral parasympathetic fibres are very long.
Thinking sympathetic always speeds things up. — Correct: Sympathetic speeds heart rate but can decrease GI motility and urinary bladder contraction — context and receptor subtype matter.
FAQ
Why is the autonomic nervous system called 'autonomic'?
Because it operates 'autonomously' (automatically) without conscious control, regulating vital functions like heart rate, blood pressure, digestion, and respiration continuously.
What is the enteric nervous system, and is it part of the ANS?
The enteric nervous system is the 'brain of the gut' — ~100 billion neurons in the GI tract wall. It operates semi-independently but receives sympathetic and parasympathetic input for coordination.
Why do some organs have opposing sympathetic and parasympathetic effects, but others don't?
Organs with opposing effects (heart, eye, GI) allow rapid autonomic balance. Some organs (e.g., adrenal medulla) have only sympathetic control because the parasympathetic system doesn't reach them.
What is 'orthostatic hypotension' and why does it happen when you stand up too quickly?
Blood pools in the legs due to gravity. Normally, sympathetic baroreceptor reflex increases heart rate and constricts vessels to maintain BP. Impaired sympathetic response (dehydration, age, drugs) → dizziness on standing.