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The Nervous System
Written by Tim Sheppard MBBS BSc. Created 15/6/10; last updated 15/6/10

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What is the nervous system?

The nervous system is an organ system which includes all of the nerves in the body. There are lots of ways to split it up, but the clearest and most obvious is between the central nervous system (or CNS) and the peripheral nervous system (PNS).

Every good business has an effective method of communication. In the building trade, for example, t's no good having one person in one place unable to communicate with another person somewhere else, because the job won't get done properly. You need some way to get messages from one end to another.

In the body, this needs to happen incredibly quickly. For instance, if you are trying to catch a ball, the eye needs to tell the brain where the ball is, and the brain needs to tell the hand where to move in order to catch the ball.

Nerves are very cleverly designed cells to communicate from one end of the body to the either - like phone lines, but even quicker. Messages travel at extraordinary speeds in order to get to where they need to be - and some nerves are able to transmit messages even faster than others!

The main organs of the nervous system are the brain and the spinal cord, but the system includes nerves that travel to every organ in the body. Nerve fibres make their ways into every nook and cranny, to make sure that signals can get to where they need to.


What is the central nervous system?

The central nervous system is the brain and spinal cord. It is the bit of the nervous system protected by bone (the skull and vertebrae) and wrapped in the special coverings called the meninges. Importantly, it is where the whole nervous system is controlled. It is the most valuable part of the nervous system, because repairs to the central nervous system are much, much more difficult - often impossible.

In many ways the central nervous system is the control room of the body. It is from here that messages are processed, decisions made, and actions commanded. If you find out that your house is on fire, it is in the brain that your legs are told to move. If you taste a delicious meal, it is in your brain that you realise just how tasty it is. Even reflexes happen in the central nervous system - if you touch a hot plate, the signal needs to get as far as the spinal cord before you decide to pull back your hand.

Because the central nervous system is so important, it has a special security barrier that keeps things out called the blood-brain barrier. Although oxygen and important nutrients can still get through, many things are kept out, to make sure it is kept safe. This is an important difference between the CNS and the peripheral nervous system.


What is the peripheral nervous system?

The peripheral nervous system does all the down-to-earth work. It's the peripheral nerves which stretch into every part of the body to do the two main things which nerves do: sense things, and bring about a change. Nerves come in two major types - afferent and efferent nerves, which mean they are either sending messages back to the central nervous system, or they're getting messages from the central nervous system to take back out to the periphery.

Peripheral nerves often take similar routes to blood vessels, joined together with arteries and veins in neurovascular bundles. This means they are able to use the same safe routes to get to a particular location. Unfortunately there isn't always a safe route, and sometimes nerves are prone to damage. A nerve running around the elbow called the ulnar nerve is especially prone, and it's that nerve which you knock when you 'hit your funny bone'.


What is the somatic nervous system?

The somatic nervous system is the bit of the peripheral nervous system that you are aware of - the 'conscious' nervous system. When you feel pain, it's because your somatic nervous system has been activated. When you move your hand, it's because your somatic nervous system has moved it.


What is the autonomic nervous system?

The autonomic nervous system is the "automatic" bit of your peripheral nervous system. It's the bit which does things without you being consciously aware - like changing the rate that your heart beats, and making your pupils change size in order to focus. I can't make my heart beat go down or make my pupils smaller just by concentrating on them - it's my autonomic nervous system that handles that.

The autonomic nervous system is basically split into two major parts - the sympathetic nervous system and the parasympathetic nervous system.


What is the sympathetic nervous system?

The sympathetic nervous system is the part of the autonomic nervous system which makes you ready for a battle. It's known as the 'Fight-flight' or 'Fright-fight-flight' system - so it brings about those things which would happen if you were frightened, if you needed to run away, or if you were going to get into a fight. It doesn't throw the punches for you, but it gets your heart beating and stimulates adrenaline release so that you've got the energy and power to 'punch beyond your weight' - and hopefully survive the battle!


What is the parasympathetic nervous system?

The parasympathetic nervous system is the part of the autonomic nervous system which handles what goes on when you're relaxed. Someone once told me that imagine relaxing after a big meal in front of the fire in a dark room - that's when your autonomic nervous system kicks.

If the sympathetic nervous system is the 'Fright-flight-fight' nervous system, then the parasympathetic nervous system is the 'Rest-n-digest' system. It manages the functions that aren't quite as urgent - still important, but it doesn't matter if they get held up for a few minutes. It's the kind of job you'd give to the new kid on the block; you still need to trust them, but it doesn't matter too much if it takes them a while to do it. The main functions are processing food, and getting rid of waste (urine and poo!).


What is an afferent nerve?

An afferent nerve is one which takes a signal towards the central nervous system. When we're talking about sensory nerves, we're talking about afferent nerves - they are basically the same thing. A receptor or sensory organ realises something is going on, and transmits a signal through an afferent nerve through to the spinal cord and brain. One way to remember afferent nerves is that they arrive in the CNS.


What is an efferent nerve?

An efferent nerve is one which takes a signal away from the central nervous system. Motor nerves are efferent nerves, but you also get nerves going to places like the salivary glands to cause them to produce saliva, and these are still efferent nerves. I remember that efferent nerves want to have an effect. Another way of remembering them is that efferent nerves exit the CNS.


Further Reading