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The autonomic nervous system matters because psychiatric drugs commonly cause side effects by tipping its two opposing branches out of balance. It controls involuntary body functions, such as heart rate, digestion, and pupil size, through two opposing branches: the sympathetic (fight-or-flight) and parasympathetic (rest-and-digest) systems. For example, imagine a patient started on an antipsychotic who develops a dry mouth, blurred vision, and constipation: these all come from blocking one branch's main neurotransmitterA chemical messenger that nerve cells release to signal to the next cell.Read more →. In practice, recognising which branch a drug affects predicts its exact side-effect pattern.

The sympathetic nervous system releases noradrenaline (a neurotransmitterA chemical messenger that nerve cells release to signal to the next cell.Read more →, also called norepinephrine) and prepares the body for action, raising heart rate, dilating pupils, and diverting blood away from digestion. The parasympathetic nervous system releases acetylcholine (a different neurotransmitter) and does the opposite, slowing heart rate, promoting digestion, and constricting pupils. AnticholinergicBlocking the action of acetylcholine, a nerve chemical, causing effects like a dry mouth and blurred vision.Read more → drugs, including many antipsychotics and tricyclic antidepressants, block acetylcholine's parasympathetic effects, producing dry mouth, blurred vision, constipation, and urinary retention. Alpha-blocking drugs block a subtype of noradrenaline receptor, causing postural hypotension (a drop in blood pressure on standing), since normal sympathetic vessel tone is lost. Picture an older patient on an anticholinergic and alpha-blocking antipsychotic who feels dizzy on standing and is chronically constipated: both effects trace back to blocking these two separate autonomic branches. In practice, checking a drug's autonomic side-effect profile before prescribing helps anticipate these predictable problems in vulnerable patients.

• Which structure functions as a modified sympathetic ganglion, releasing hormone directly into the blood?
• A patient starting an antipsychotic feels faint on standing. Which receptor action explains this?
• Which measure is used as an index of parasympathetic tone?

• Sympathetic system, releases noradrenaline for fight-or-flight, unlike the parasympathetic system's acetylcholine-driven rest-and-digest function.
• Anticholinergic side effects, dry mouth, blurred vision, and constipation, rather than the postural hypotension caused by alpha-blockade.
• Alpha-blocking drugs, cause postural hypotension via lost vessel tone, whereas anticholinergicBlocking the action of acetylcholine, a nerve chemical, causing effects like a dry mouth and blurred vision.Read more → drugs act through a different receptor entirely.
• Parasympathetic activation, slows heart rate and aids digestion, unlike sympathetic activation which raises heart rate and diverts blood from digestion.

• The anterior pituitary is controlled by hypothalamic releasing hormones rather than by sympathetic nerves.
• The parathyroids respond to serum calcium rather than to sympathetic innervation.
• The adrenal cortex secretes steroids under pituitary control.
• The thyroid is regulated by pituitary thyroid-stimulating hormone.
• H1 blockade causes sedation and weight gain.

• Localise the clue to the relevant anatomical level, cell type, pathway or molecular process.
• Map that structure or process to its normal function.
• Predict the deficit or clinical sign produced when it is disrupted.
• Reject options that belong to a neighbouring structure, pathway or level of explanation.