The ANS has certain fundamental differences from the Somatic Nervous System ( SoNS ) which are summarised in the table shown below. 

Characteristic Somatic Nervous System Autonomic Nervous System
Effectors Voluntary (skeletal) muscle Smooth muscle, Cardiac Muscle, Glands
General function Adjustment to external environment. Adjustment within the internal environment (homeostasis)
Number of neurones from CNS to effector 1 2
Ganglia outside the CNS 0 Chain ganglia, collateral ganglia or terminal ganglia
Neurotransmitter Acetylcholine Acetylcholine, adrenaline, noradrenaline
Effect of nerve damage on the effector Paralysis and muscle wastage (atrophy) Effector remains functional but not able to quickly respond to the changing needs of the body.

 

The overall function of the ANS can be summarised as follows.
 

The Autonomic Nervous System in the Human:A Checklist of Basic Facts
1 Involuntary : it's actions are not under conscious control from the CNS
2 The system sends efferent (motor) neurones to  the following : smooth muscle of the inner organs such as the digestive tract and the bladder, blood vessels, glands, ciliary muscle of the eye, cardiac muscle in the heart.
3 The system's main activity is to maintain homeostasis
4 The system prepares the body for dealing with threatening situations by providing what is known as the Fight or Flight Response
5 There are 2 major divisions of the ANS which are:
6 Sympathetic Nervous System (SyNS) sometimes called the Thoracicolumbar division of the ANS.
7 Parasympathetic Nervous System (PaNS) sometime called the Craniosacral division of the ANS.
8 Within the system there are 3 types of ganglia  (Vertebral, Prevertebral, Terminal) and two types of neurones (Preganglionic : from the CNS to the ANS and Postganglionic : from the autonomic ganglia to the internal organs)

Glossary :

Afferent (sensory neurone) carries nerve impulses towards the brain.
CNS is the Central Nervous System comprising the Brain and the Spinal Cord.
Efferent (motor) neurones carry nerve impulses away form the brain.
Ganglia are clusters of nerve cells.

As can be seen from the above the ANS is further divided into two sub-systems the Sympathetic Nervous System (hereafter referred to as the SyNS) and the Parasympathetic Nervous System (referred to as the PaNS).

Anatomically  and Pharmacologically the ANS is of great importance as it is the only part of the nervous system where synapses occur outside the protection of the skull and the vertebrae. However it is important to remember that they all form part of an integrated whole and that both the SyNS and the PaNS are operating continuously along with the rest of the nervous system. They do not switch on and off as and when required but are responding in varying degrees to information provided by the sensory component of the nervous system. However the SyNS dominates during stressful times and the PaNS dominates during times of emotional calm and/or physical rest.

For example the SyNS would dominate if you were late for an important appointment, taking an examination or being interviewed for a job. In contrast the PaNS would dominate after an enjoyable meal with friends or relaxing in front of the TV watching a favourite programme.

The SyNS is associated with mobilising the body during stressful or physically strenuous situations. It's nerves send impulses which will increase the Blood Pressure, speed up rate and force of heart beat, increase blood sugar concentration and re-route blood flow so that skeletal muscle receives the amount of blood required for maximum effort.

Conversely the PaNS works in virtually the opposite manner. It's nerves send impulses that decrease the Blood Pressure, decrease heart rate and stimulate the digestive processes.

Many organs are stimulated by nerves from both the SyNS and the PaNS however in most instances one or the other will be the dominant one.

Last updated on Wednesday, 07 April 1999 10:57 +0100

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