Autonomic Nervous System Introduction

1. What is the Autonomic Nervous System?
2. Organisation of the Autonomic Nervous System
3. Actions of the Autonomic Nervous System
   
   • Sympathetic Nervous System Actions
   • Parasympathetic Nervous System Actions
4. Role of Adrenaline in the Sympathetic Nervous System
5. Drugs Affecting the Autonomic Nervous System
6. SLUDGE Syndrome
7. Quiz

• The autonomic nervous system is responsible for the body’s involuntary functions.
• The autonomic nervous system has three major divisions: sympathetic, parasympathetic, and enteric.
• An autonomic neural pathway is a chain of two neurons: a pre-ganglionic and a post-ganglionic neurone.
• The sympathetic and parasympathetic nervous systems use different neurotransmitters, which act on their respective receptors.
• Actions of the sympathetic nervous system are associated with ‘fight or flight’.
• Actions of the parasympathetic nervous system are associated with ‘rest and digest’.
• We can use drugs which target the autonomic nervous system for therapeutic benefit.
• Overstimulation of the parasympathetic nervous system can lead to SLUDGE syndrome.

What is the Autonomic Nervous System?

The autonomic nervous system (ANS) is the division of the body’s nervous system which controls all involuntary functions - e.g. heart rate, blood pressure, digestion, pupil diameter. It is separate from the somatic motor system, which deals with voluntary actions (i.e. skeletal muscle contraction).

The autonomic nervous system primarily controls involuntary motor output to organs, but it also includes visceral afferent fibres that carry sensory information from the periphery to the central nervous system. These inputs are integrated largely within the brainstem and hypothalamus.

The autonomic nervous system has two major divisions that are usually considered in this context:

1. Sympathetic Nervous System (SNS) - Responsible for the ‘fight or flight’ response, triggered by stressful situations.
2. Parasympathetic Nervous System (PNS) - Regulates basal activities, ‘rest and digest’.

Organisation of the Autonomic Nervous System

The division of the ANS into the sympathetic and parasympathetic systems is purely anatomical:

• SNS - Arises from the thoracic and lumbar divisions of the spinal cord (thoraco-lumbar outflow).
• PNS - Arises from the brainstem and sacral spinal cord (cranio-sacral outflow).

Remember the nervous system is bilaterally symmetrical, so when considering one side it is important to appreciate that the same is happening on the other side.

Diagram - The basic anatomy of the autonomic nervous system

SimpleMed original by Dr. Joshua Bray

Put simply, the autonomic nervous system is a chain of two neurones: a myelinated pre-ganglionic and an unmyelinated post-ganglionic neurone. A ganglion is a collection of neurone cell bodies and is where the pre-ganglionic neurone synapses onto the post-ganglionic neurone.

In the sympathetic nervous system:

• The pre-ganglionic neurone is relatively short and normally synapses within a specialised chain of sympathetic ganglia adjacent to the vertebral column - This is known as the sympathetic chain (or paravertebral chain).
• The post-ganglionic neurone is relatively long and exits the sympathetic chain to travel to the target tissue.

In the parasympathetic nervous system:

• The pre-ganglionic neurone is relatively long and travels along cranial nerves (e.g. vagus nerve), or as the pelvic splanchnic nerves if arising from the sacral cord - They synapse close to or even within the wall of the target tissue.
• The post-ganglionic neurone is consequently relatively short.

Actions of the Autonomic Nervous System

As covered in previous articles, neurones communicate with other cells at synapses, where they release a neurotransmitter which activates receptors on the post-synaptic membrane.

All pre-ganglionic neurones (both sympathetic and parasympathetic) use acetylcholine (ACh) as their neurotransmitter. The acetylcholine acts on nicotinic acetylcholine receptors on the membrane of the post-ganglionic neurone.

For post-ganglionic neurones, the use of neurotransmitter differs between sympathetic and parasympathetic divisions:

• Sympathetic post-ganglionics release noradrenaline, which acts on adrenoceptors on the membrane of the target cell.
• Parasympathetic post-ganglionics release ACh, which acts on muscarinic acetylcholine receptors on the membrane of the target cell.

Adrenoceptors and muscarinic receptors are examples of G-protein coupled receptors (GPCRs). For more information, check out our article on G-Protein Coupled Receptors.

Diagram - The communication between pre- and post-ganglionic neurones of the autonomic nervous system

SimpleMed original by Dr. Joshua Bray

Stimulation of the sympathetic nervous system can result in:

Table - The physiological actions of the sympathetic nervous system

SimpleMed original by Dr. Joshua Bray

Stimulation of the parasympathetic nervous system can result in:

Table - The physiological actions of the parasympathetic nervous system

SimpleMed original by Dr. Joshua Bray

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