2 Neuroscience2.12 Neurobiology of specific treatments
2.12.6 Peripheral and autonomic systems
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Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Routes between brain and body
The peripheral nervous system is made up of the sensory and motor nerves outside the brain and spinal cord. These nerves carry somatic information and visceral (internal organ) information.
Autonomic efferents are the outgoing fibres. They control smooth muscle, cardiac muscle and glands through the sympathetic, parasympathetic and enteric pathways.
Visceral afferents are the incoming fibres. They report the body's internal state to the brainstem, hypothalamus and cortex, which handle interoception (sensing the body's internal state) and regulation.
Pathway | Transmitter and receptor | Example effect |
|---|---|---|
Autonomic preganglionic neurones | Acetylcholine at nicotinic receptors | Activates the postganglionic neurone |
Most sympathetic postganglionic neurones | Noradrenaline at adrenergic receptors | Changes heart and blood vessel activity |
Parasympathetic postganglionic neurones | Acetylcholine at muscarinic receptors | Slows the heart and supports digestion |
Sympathetic fibres to sweat glands | Acetylcholine at muscarinic receptors | Causes sweating |
Somatic motor neurones | Acetylcholine at muscle nicotinic receptors | Makes skeletal muscle contract |
The adrenal medulla receives cholinergic preganglionic input and releases catecholamines into the bloodstream.
The vagus nerve carries both visceral sensory and parasympathetic motor fibres. It is not simply a one-way calming pathway.
Psychiatric relevance
Threat responses: central and peripheral systems act together to cause palpitations, sweating, tremor and gut changes.
Bodily sensations can shape how a person reads threat and where they focus attention. This sets up a feedback loop between autonomic arousal and anxiety.
Drug side effects: blocking muscarinic receptors contributes to dry mouth, constipation and urinary retention. Blocking alpha-1 receptors can cause postural hypotension.
Heart-rate variability reflects several physiological influences. Do not treat it as a specific diagnostic test for a psychiatric disorder.
Peripheral disease or medication effects can mimic anxiety. When you assess, look at the pattern, triggers, physical findings and treatment history.