Parasympathetic nervous system

The parasympathetic nervous system is one of the two main branches of the autonomic nervous system. It helps manage automatic processes that do not usually require conscious control, including digestion, heart rate, pupil size, and bladder function.

It is often introduced as the “rest and digest” branch because its activity supports recovery and routine maintenance. This does not mean it simply switches on whenever a person is resting. The parasympathetic and sympathetic branches work together, adjusting body functions as circumstances change.

Anatomy of the parasympathetic nervous system

The parasympathetic division is sometimes called the craniosacral division. Its preganglionic neurons begin in selected areas of the brainstem and in the lower part of the spinal cord. Their fibres travel through several cranial nerves and pelvic nerves before reaching ganglia close to, or within, their target organs.

The oculomotor nerve carries signals involved in pupil constriction and near focusing. The facial and glossopharyngeal nerves help regulate selected glands. The vagus nerve carries a large share of parasympathetic signals to organs in the chest and abdomen. Pelvic splanchnic nerves serve parts of the lower bowel, bladder, and reproductive organs.

Each pathway includes a preganglionic neuron and a postganglionic neuron. Parasympathetic ganglia are generally near the target tissue, so the preganglionic fibres tend to be longer and the postganglionic fibres shorter.

Receptors of the parasympathetic nervous system

Acetylcholine is the main neurotransmitter used at parasympathetic synapses. It acts on two broad receptor families: muscarinic and nicotinic receptors.

Muscarinic receptors

Muscarinic receptors are G-protein-coupled receptors found on many target tissues. Their effects depend on the receptor subtype and the cells receiving the signal. They can slow the heart, stimulate glandular secretion, increase digestive activity, constrict the pupil, and support bladder emptying.

Nicotinic receptors

Nicotinic receptors are ion channels. They relay the signal rapidly at autonomic ganglia, where one neuron communicates with the next. Nicotinic receptors are also found at the neuromuscular junction and in the adrenal medulla, although those locations are not themselves parasympathetic target effects.

How does the parasympathetic system work?

When a parasympathetic pathway is activated, acetylcholine is released from nerve endings. It binds to receptors and changes the activity of the target cell. In the heart, for example, this can slow the rate at which the sinoatrial node generates impulses and reduce conduction through the atrioventricular node.

In the digestive tract, parasympathetic signals generally support movement and secretion. In the eye, they constrict the pupil and help the lens focus on nearby objects. The precise response depends on the organ, receptor, and wider signals arriving at the same time.

Effects of the parasympathetic nervous system

The system has different effects in different tissues. The following overview uses broad teaching points rather than treating the two autonomic branches as simple opposites.

Digestive system

Parasympathetic activity generally supports digestive movement and secretion. It helps the gut process food and move its contents along the digestive tract. Stress-related autonomic changes can alter these processes, but digestion is also influenced by hormones, local gut circuits, diet, and health.

Immune function

Nervous, hormonal, and immune systems communicate in complex ways. Parasympathetic signalling can influence inflammatory activity, but it is inaccurate to describe it as a single switch that simply improves or suppresses immunity.

Sleep and wakefulness

Parasympathetic activity is associated with many quiet, restorative states, while sleep is controlled by several interacting brain systems and body rhythms. A balanced sleep routine cannot be explained by parasympathetic activity alone.

Reproductive responses

Autonomic signals contribute to some reproductive responses, including blood-flow changes. These responses involve multiple nerve pathways, blood vessels, hormones, and psychological factors.

Blood vessels

Most routine regulation of blood-vessel diameter is controlled by sympathetic pathways and local mechanisms. Parasympathetic effects on blood vessels are more limited and vary by tissue. It is therefore too broad to say that this system universally dilates vessels or directly normalises blood pressure.

Eyes

Parasympathetic fibres carried by the oculomotor nerve constrict the pupil through the iris sphincter muscle. They also contract the ciliary muscle to help the lens focus on near objects and stimulate tear production through pathways associated with the facial nerve.

Heart

Parasympathetic signals can slow the heart rate and reduce conduction through the atrioventricular node. The vagus nerve is especially important in this regulation. Heart rhythm is also shaped by sympathetic signals, circulating hormones, intrinsic pacemaker activity, and the heart’s local conduction system.

Airways

Parasympathetic signalling can narrow the airways and increase glandular secretion. This is one reason medicines that block muscarinic receptors may be used in selected respiratory conditions. Anyone with breathing symptoms should seek advice from a qualified clinician.

Underactivity and overactivity

Autonomic symptoms can arise from many causes, and they cannot reliably be used to diagnose a problem with one branch of the nervous system. Changes in parasympathetic function may be associated with symptoms such as altered heart rate, dry mouth, changes in sweating, bowel disturbance, urinary symptoms, or changes in pupil responses.

Persistent, severe, or unexplained symptoms should be assessed by a healthcare professional. Treatment depends on the underlying cause rather than on trying to increase or decrease parasympathetic activity independently.

Medicines affecting parasympathetic signalling

Parasympathomimetic medicines act in ways that increase cholinergic effects, while antimuscarinic or other anticholinergic medicines reduce selected effects of acetylcholine. These medicines have specific clinical uses and possible side effects. They should be used only as directed by an appropriately qualified professional.

Summary

• The parasympathetic nervous system is a branch of the autonomic nervous system.
• It supports digestion, glandular activity, pupil constriction, bladder function, and selected heart-rate responses.
• Its pathways have cranial and sacral origins and often use long preganglionic fibres followed by short postganglionic fibres.
• Acetylcholine communicates with nicotinic receptors at autonomic ganglia and muscarinic receptors on many target tissues.
• Parasympathetic activity works alongside sympathetic activity and local organ controls; the two branches are not simple on-off opposites.
• Medicines that alter cholinergic signalling require careful, condition-specific use.

For related reading, explore the functions of the autonomic nervous system, the sympathetic nervous system, the vagus nerve, and cranial nerves.