The parasympathetic nervous system (PNS) is one of the two main divisions of the autonomic nervous system (ANS). It acts in conjunction with the sympathetic nervous system (SNS) to maintain homeostasis within the body.
The collective action of the parasympathetic nervous system can be described as “rest and digest”. It dominates during periods of quiet activity and helps to return the body to a state of relaxation.
In this article, we shall look at the anatomy of the parasympathetic nervous system – its components, actions, and clinical correlations.
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Anatomical Structure
The parasympathetic nervous system consists of the following components:
- Pre-ganglionic fibres – originate within the brainstem and sacral division of the spinal cord. They end by synapsing with post-ganglionic fibres at peripheral ganglia.
- Peripheral ganglia – a group of neuronal cell bodies where the pre-ganglionic and post-ganglionic fibres synapse.
- Post–ganglionic fibres – begin at the peripheral ganglia and carry parasympathetic fibres to the effector organs.
We shall now look at each component of the parasympathetic nervous system in more detail.
Fig 1
The parasympathetic nervous system is comprised of pre-ganglionic neurones, ganglia, and post-ganglionic neurones.
Pre-Ganglionic Neurones
The pre-ganglionic neurones originate in the brainstem and sacral segments (S2-S4) of the spinal cord. They are myelinated, long and release acetylcholine.
There are two routes by which they exit the central nervous system:
- Cranial nerves III, VII, IX, X
- S2-S4 spinal nerves
(For this reason, the parasympathetic nervous system is also referred to as the craniosacral division of the autonomic nervous system).
When the pre-ganglionic neurones reach the peripheral ganglia, they synapse with post-ganglionic neurones.
Peripheral Ganglia
The peripheral ganglia refer to groups of neuronal cell bodies where the pre-ganglionic and post-ganglionic fibres synapse. They are typically located close to the structure that they innervate.
Post-Ganglionic Neurones
The post-ganglionic neurones originate in the peripheral ganglia and carry signals to the target organs.
They are typically short, unmyelinated and release acetylcholine.
| Pre-Ganglionic Fibre | Post-Ganglionic Fibre | |
| Length | Long | Short |
| Myelination | Yes | No |
| Neurotransmitter released | Acetylcholine | Acetylcholine |
Table 1: Neurones of the parasympathetic nervous system.
Head & Neck
The parasympathetic innervation of the head and neck originates in the brainstem and leaves via cranial nerves III, VII, IX and X.
The fibres synapse at four discrete ganglia in the head and neck:
- Otic
- Ciliary
- Pterygopalatine
- Submandibular
Parasympathetic fibres leave the ganglia and reach the effector organs via short post-ganglionic neurones.
To explore the parasympathetic supply to the head and neck in more detail, see here.
Fig 2
The pterygopalatine ganglion of the head and neck. Pre-ganglionic fibres travel with the greater petrosal nerve (a branch of the facial nerve).
Trunk & Limbs
The parasympathetic innervation to the trunk and limbs is from the vagus nerve and sacral spinal nerves.
Vagus Nerve (CN X)
The paired vagus nerves are a major contributor of parasympathetic supply, providing approximately ~75% of the total outflow. They supply the thoracic viscera and most of the gastrointestinal tract.
The vagus nerve originates from the medulla oblongata of the brainstem. It carries pre-ganglionic fibres to ganglia located within the nerve plexuses that are in close proximity to the target organs. rom the ganglia, short post-ganglionic neurones carry parasympathetic innervation to the target organs.
Sacral Spinal Nerves
The sacral spinal nerves supply parasympathetic innervation to the distal colon, rectum, bladder and reproductive organs.
The pre-ganglionic fibres arise from the anterior rami of the sacral spinal nerves from S2-S4. These form the pelvic splanchnic nerves, which carry parasympathetic fibres to the ganglia (located in the hypogastric plexus or in the walls of the target organ).
From the ganglia, short post-ganglionic neurones carry parasympathetic innervation to the target organs.
Fig 3
The organisation of the parasympathetic outflow.
Parasympathetic Actions
The parasympathetic nervous system acts on a range of systems within the human body:
- Eyes – constriction of pupils
- Lacrimal and salivary glands – secretion
- Cardiovascular – decreases heart rate
- Endocrine – increases insulin secretion from pancreas
- Respiratory – reduces airway diameter
- Digestive – increased activity, dilatation of sphincters in the gastrointestinal tract, secretion from salivary and digestive glands, glycogen synthesis from the liver, exocrine secretion from the pancreas
- Urinary – contraction of detrusor muscle in the bladder, relaxation of the internal urethral sphincter
- Reproductive – penile erection in males; vaginal lubrication, reproductive organ secretion, clitoral erection and uterine relaxation in females
Clinical Relevance
Truncal Vagotomy
The vagus nerve provides parasympathetic innervation to the stomach, where it stimulates acid secretion and smooth muscle contraction.
Prior to the introduction of proton pump inhibitors, a truncal vagotomy was used in the management of peptic ulcer disease.
In this procedure, the vagal supply to the stomach would be severed – reducing the parasympathetic supply. This would lead to a reduction in acid production. A common complication of this procedure was gastroparesis, due to a reduction in smooth muscle contraction.