The Structure & Function of the Autonomic Nervous System

The Structure & function of the

Autonomic Nervous System

Introduction:

The organs of our body are controlled by many systems in order to function correctly and efficiently in order to survive within the environment we live in. These include the heart, stomach and intestines and other vital organs and body systems. All of the systems in our body are regulated by a part of the nervous system called the autonomic nervous system (ANS). The ANS is part of the peripheral nervous system and it controls many organs and muscles within the body. Rather bizarrely we are unable to determine or feel its presence in our bodies as it is working involuntary, as a reflexive manner. A common example of this involuntary action is best understood when you think about your circulatory system. We do not notice when blood vessels change size or when our heart beats faster, unless of course the change is shockingly dramatic. But otherwise it's an internal change to your bodies internal environment that you do not have any mental or physical control upon.

However, it is known that, some people can train themselves to control some functions of the ANS such as heart rate or blood pressure. Deep sea diving with out the use of underwater breathing equipment, is an efficient use for this technique as it allows you to reduce your heart rate. This then allows the bodies oxygen consumption rate to be reduced, resulting in an overall increase in the ability to hold your breath to dive deeper depths (up to 60ft has been recorded). But don't be mislead, this is not an ability that anyone could simply undertake on the family holiday. It requires strict self-control and powerful meditational skills.

The Sympathetic and parasympathetic Nervous System:

The ANS is divided into two parts, the Sympathetic Nervous System and the Parasympathetic Nervous System. These have opposite (antagonistic) effects on the organs, which they supply. Generally, the sympathetic system prepares the body for stressful/emergency situations and the Para-sympathetic system reduces stress. The responses of the ANS are co-ordinated by two subconscious brain regions, the medulla oblongata and the hypothalamus.

The sympathetic nervous system plays a major role in our reactions and responses to the environment surrounding us, these are important when making responsive life saving decisions. This particular subsystem is commonly known as a flight of fight response, a quick example of this is your ability to run away in the sight of life endangerment. Take notice of figure.1 in the illustrated diagram that the sympathetic nervous system originates in the spinal cord. Specifically, the cell bodies of the first neuron are located in the thoracic and lumbar spinal cord. Axons from these neurons project to a chain of ganglia located near the spinal cord. In most cases, this neuron makes a synapse with another postganglionic neuron in the ganglion. A few preganglionic neurons go to other ganglia outside of the sympathetic chain and synapse there. The postganglionic neuron then projects to the "target" - either a muscle or a gland.

Two more facts about the sympathetic nervous system are the synapse in the sympathetic ganglion, uses acetylcholine as a neurotransmitter. The synapse of the postganglionic neuron with the target organ uses the neurotransmitter called norepinephrine. There is one exception to this, the sympathetic postganglionic neuron that terminates on the sweat glands uses acetylcholine.

The Parasympathetic nervous system is an antagonist to the sympathetic nervous system. Its primary function is to give you the ability to rest and relax, in times when you are tired or stressed. The parasympathetic nervous system works to save energy and allow your blood pressure to also decrease. Your heartbeat reduces becoming much slower, and digestion can start. Notice again in figure 1 on the left, that the cell bodies of the parasympathetic nervous system are located in the spinal cord (sacral region) and in the medulla. In the medulla, the cranial nerves III, VII, IX and X form the preganglionic parasympathetic fibres. The preganglionic fibre from the medulla or spinal cord projects to ganglia very close to the target organ and makes a synapse. This synapse uses the neurotransmitter called acetylcholine. From this ganglion, the postganglionic neuron projects to the target organ and uses acetylcholine again at its terminal.

Figure. 1: Autonomic nervous system

Para-sympathetic and sympathetic differences:

The sympathetic and parasympathetic nervous system generally have opposing effects on organs they supply, and this enables the body to make rapid and precise adjustments of involuntary activities in order to maintain a steady state. An example is an increase in heart rate due to the release of noradrenalin by sympathetic neurones, this is then compensated for by, the release of acetylcholine by parasympathetic neurones. This prevents heart rate from increasing beyond its working capabilities and allows it to reduce and settle back to its resting state.

The effects of sympathetic and parasympathetic stimulation are tabulated below in figure. 2. Notice that the effects are generally in opposition to each other, in relation to each organ affected by a particular reflex.

Feature Sympathetic Parasympathetic

Origin of Neurones Emerge from cranial,