Your nervous system is divided into two parts: the central and the peripheral nervous system.
The central nervous system (CNS) is made up of the brain and the spinal cord.
The peripheral nervous system is all the other nerve fibres that connect to it.
Together, they take in information about the outside world through special receptor cells. Each cell responds to a specific stimulus.
They then pass the information on as electrical impulses to the billions of nerve cells (neurones) that make up the nervous system.
Receptors are specialised nerve cells, which are adapted to respond to a stimulus. For example, each of our 'senses' has a particular group of receptors, which respond to a stimulus.
Each receptor is specifically adapted to the stimulus to which it is sensitive. For example, the eye (see 'The Eye') is beautifully adapted to receive light stimuli. It might surprise you to learn that your ear is responsible for keeping you in balance! How bizarre!
The following table tells you where the receptors for each stimulus aresituated:
|Description of stimlus:||Location of receptors:|
|The lush smell of tasty food||Nose|
|The sound of your favourite music||Ear|
|The taste of your favourite food||Tongue|
|The feel of the shape of your computer mouse||Skin|
|The image on your computer screen||Eye|
Receptors pass electrical impulses to other neurones at tiny junctions called synapses.
These signals allow the nervous system to co-ordinate a response.
Neurones passing electrical impulses on their own isn't much use. It's the same as people not talking.
Neurones 'talk' by passing a small amount of a chemical messenger between them across the synapse. This neurotransmitter then sets up the electrical impulse in the second neurone, and so it carries on.
Drugs, poisons and other chemicals can effect synapses by interfering with how the neurotransmitter is dealt.
There are billions of these nerve cells inside you. There are more of them than there are people on the Earth! One estimate is 1,000,000,000,000 neurones.
Neurones are beautiful cells which come in a huge variety of shapes and sizes. Their sizes range from about a two hundredth of a millimetre to a tenth of a millimetre.
However, whatever shape they are, neurones share basic features in common.
Like all cells, neurones have a cell body and a nucleus surrounded by a cell membrane.
They have extensions from the cell membrane called dendrons, which have even finer branches called dendrites.
The dendrites overlap between neurones and allow communication between themselves at sites called synapses. Each neurone can make between 200 and 10000 synapses.
Many neurones have one dendrite called the axon, which is longer than the others.
In some neurones, the axon has other cells wrapped around it like sausages threaded onto a string. These cells contain a fatty chemical called myelin. This myelin sheath of cells insulates the axon and speeds up the nerve impulses passing down it.
We can put neurones into three groups on the basis of what they do:
|1||sensory neurones||Carries impulses from receptors to the CNS.|
|2||motor neurones||Carries impulses from the CNS to the effector.|
|3||relay neurones||Co-ordinate the response. Are link neurones in the CNS.|
These different types of neurone fit together to allow a co-ordinated response. One example of this is in a reflex.
A reflex is a very fast, pre-programmed response to a stimulus. They are automatic so that you don't need to think about it beforehand. They act to protect the body.
Reflexes follow this series of steps:
The stimulus is picked up by a receptor, which transmits an impulse to a sensory neurone.
This neurone passes the impulse to the co-ordinator, the central nervous system. The CNS co-ordinates the signal and transmits back a response via a motor neurone. The response is carried out by the effector organ, which is either a gland or a muscle.
Now lets look at a real reflex, like what happens when you put your hand on something hot.
Sort the descriptions of each event into the correct order:
The whole mechanism of the reflex is called a reflex arc, as it isn't a complete loop.
You've probably also noticed that the brain isn't directly involved here. So how is it that we know when we do something like this?
A side branch of the sensory neurone goes up to the brain - so that we shout 'Ow', or something slightly ruder!
This also means that the brain can try and override the reflex.
Click on the 'yes' or 'no' buttons to say whether or not the statement is an example of a reflex. Then, check your answer: