You must know what's going on around you, your nervous system makes this possible. Your nervous system carries electrical signals (impulses) that travel fast. This means that you can respond to changes very quickly.
Any changes (stimuli) are picked up by cells called receptors. Receptor cells have a nucleus, cytoplasm and cell membrane. They are found clustered together in sense organs, such as your eyes.
Once your sensory receptor detects a stimulus, the information (sent as an electrical impulse) passes along special cells called neurons, which are found in their thousands, in nerves. The impulse travels along here until it reaches the Central Nervous system (CNS) (brain and spinal cord). The cells that take the impulse to the CNS are called sensory neurons.
Your brain gets a lot of information from the sensory receptors. It coordinates the information and sends impulses along special cells called motor neurons. The carry impulses to your effector organs, which respond.
Effector organs are muscles or glands, muscles respong by contracting and glands respond by secreting chemical substances.
Your nervous system lets you take in information from your surroundings and then react, but some responses are so fast that you don't even have time to think.
If you touch something hot or sharp, you pull your hand away before you feel the pain and if something comes near your face you blink. These are known as reflexes. It would be very tricky if you had to think about these things, also forgetting to breathe would be quite bad.
Reflexes involve: sensory neurons, motor neurons and relay neurons in the CNS (which connect the two).
An electrical impulse passes along the sensory neuron to the CNS . It then passes along a relay neuron (in the spinal cord) and straight back along a motor neuron. From there the impulse arrives at the effector organ. This is called a reflex arc. The key thing is that it bypases the brain, meaning the time is as short as possible and that you don't have to think about it.
Your nerves aren't joined directly. There are junctions between called synapses. The electrical impulses have to cross thses synapses, and cannot leap the gap. When an impulse arrives at the junction, chemicals are released which cross the synapse and arrive at the receptor sites of the next neuron. This starts up an electrical impulse in the next neuron. There are synapses in the reflex arc between the sensory, relay and motor neuron.
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