AUTONOMIC NERVOUS SYSTEM

AUTONOMIC NERVOUS SYSTEM

INTRODUCTION

The word 'autonomic' comes from two Greek words that mean 'self' and 'law’. The autonomic nervous system is that part of the nervous system which controls the activity of the viscera. Moreover the autonomic nervous system is a collection of neurons that carry messages from the central nervous system to the heart, smooth muscles, and glands generally not as a result of conscious action on the part of the brain.

CLASSIFICATION OF ANS

It is traditional to break the autonomic nervous system (ANS) into a central and peripheral component, and then to further subdivide the peripheral ANS into:

1.     sympathetic nervous system (SNS)

2.     parasympathetic nervous system (PNS)

General Characteristics of the ANS:

1.     It is a two-neuron pathway.
2. Sensory signals from viscera and skin send signals to autonomic neurons in brain and spinal cord.
3. A preganglionic neuron cell body is located within the CNS (brain stem or spinal cord).
4. Preganglionic fibers (efferent fibers) synapse with a ganglionic neuron located in the PNS
5. A postganglionic fiber terminates on the effector organ (heart, stomach, etc).

 

Basic ANS anatomy

• It is important in the process of homeostasis
• Helps control the heart rate, blood pressure, digestion, respiration, blood pH and other bodily functions
• These controls are done automatically below the conscious level
• The hypothalamus has important role in co-ordinating autonomic function
• In the ANS there are two nerves between the central nervous system (CNS) and the end organ
• The nerve cell bodies for the second nerve are organized into ganglia
• The ANS effects its function via neural transmission as follows:
• Central nervous system
• Preganglionic nerve
• Ganglion
• Postganglionic nerve
• End organ
• At each junction neurotransmitters are released
• The ANS has two divisions that differ in anatomy and function

 

 

What does the ANS control?

Resisting the temptation to say 'everything', we note the important functions of the ANS:

·         Control of heart rate;

·         Control of exocrine glands;

·         Influence on certain endocrine glands;

·         Altered tone in almost all smooth muscle, wherever it's found.

Autonomic receptors

• There are multiple types of receptors in the autonomic nervous system
• For the sympathetic system the major receptor types are alpha and beta
• These are subdivided into alpha-, alpha-2, beta-1 and beta-2
• The parasympathetic system has nicotinic and muscarinic receptors
• Receptors subtypes are not evenly distributed throughout the body
• Adrenergic receptors:
• Usually stimulated by noradrenaline or adrenaline
• Alpha-1 type is found in the smooth muscle of most arterioles and in sphincter muscles of the GI tract and bladder
• Alpha-2 type is found in presynaptic nerves and parts of the GI tract
• Beta-1 type is the dominant type in the heart
• Beta-2 type is found in the bronchioles of the lung, the wall muscles of the bladder and other locations
• Cholinergic receptors:
• Usually stimulated by acetylcholine
• Nicotinic types are found in autonomic ganglia
• These receptors are different nicotinic receptor found in neuromuscular junctions

More ANS neurotransmitters
Transmitter	Functions
nitric oxide (NO)	parasympathetic - important in erection and in gastric emptying. Activates guanylate cyclase.
vasoactive intestinal polypeptide (VIP)	parasympathetic - co-release with ACh affects salivation; also in sympathetic cholinergic fibres. May be important throughout the gastrointestinal tract.
adenosine triphosphate (ATP)	sympathetic - blood vessels and vas deferens. Co-released with catecholamines.
neuropeptide Y (NPY)	sympathetic - facilitates effect of noradrenaline (co-released). Causes prolonged vasoconstriction.
serotonin (5HT)	important in enteric neurones (peristalsis)
gamma-amino butyric acid (GABA)	enteric.
dopamine	May mediate vasodilatation in the kidney
gonadotropin releasing hormone (GnRH)	co-transmitter with ACh in sympathetic ganglia.
Substance P	sympathetic ganglia, enteric neurones
calcitonin gene related peptide (CGRP)	contributes to neurogenic inflammation

    Principles of ANS function

As is often done when dealing with any fairly complex system, people have tried to extract simplifying principles. Here are a few (after Rang, Dale & Ritter):

1.     Dale's principle is a gross oversimplification. This principle is that a mature neurone releases the same transmitter(s) at all of its synapses. Although generally true, we now know that not only is release of a 'cocktail' of neurotransmitters the rule rather than the exception, but also that the 'mix' may vary depending on stimulation frequency, and so on. (As an aside, neurones may during their lifetime also change the transmitters they release).

2.     Cannon's law of denervation tells us that if a post-ganglionic neurone has it's pre-ganglionic input removed, then it will become super-sensitive to the normal neurotransmitters that mediate that pre-ganglionic input. There is a variety of reasons for this, including up-regulation of receptors for the neurotransmitter(s), post-receptor effects, and impaired removal of neurotransmitters from the synapse.

3.     The modulation of transmission ('neuromodulation') at a synapse may be either at a presynaptic or postsynaptic level. Presynaptic modulation is discussed within the next section, and post-synaptic effects a bit later

 

 

 

Actions of the ANS

• The sympathetic and parasympathetic systems often have opposing actions on the same organ
• Some examples of autonomic functions are as follows

	Sympathetic	Parasympathetic
Eye	Iris dilates	Iris constricts
Heart	Increased heart rate	Decreased heart rate
Bronchioles	Bronchodilatation	Bronchoconstriction
Bladder	Sphincter constricts	Sphincter relaxes
	Detrusor muscle relaxes	Detrusor muscle contracts
Intestine	Secretions decrease	Secretions increase
	Motility decreases	Motility increases
Rectum	Sphincter relaxes	Sphincter constricts
	Muscle wall contracts	Muscle wall relaxes