The Nervous System
The nervous system consists of two parts, the CNS
(Central Nervous System) and a series of other nerves linking the
CNS to other organs.
The CNS is split into another two sections; the brain and the
spinal chord. As the brain is enclosed by the cranium and the
spinal chord by the backbone, the CNS is completely covered in
bone. The nerves coming from the Brain are called Cranial Nerves
and those coming from the spine are called Spinal Nerves.
The main job of nerves is to carry messages from one part ofd
the body to another. These messages take the form of tiny
electrical impulses called nerve impulses. 
A reflex action is an immediate response to a stimulus. An
example is moving a hand away from a hot-plate.
When you pull your handf award from a hot object, the arm muscle
contracts as it receives messages from the sensory endings in the
skin of the hand.
A relfex arc is a route taken by impulses through the nervous system bringing about a reflex action.
There are five parts of a reflex arc:
Receptor - receives stimulus (e.g. from Skin)
Sensory Nerve Fibre - carries impulse to spinal chord
Intermediate Nerve Fibre - carries impulse from the
upper to lower side of the spinal chord
Motor Nerve Fibre - carries the message from the chord
to the muscle
Effector - response (e.g. Biceps)
The three types of nerve fibre are thread-like extensions of
nerve cells (neurons) and are connected to each other by synapses
inside the spinal chord. These synapses only let impulses travel
in one direction. All the nerve fibres must be working properly
for the impulse to "get through".
Polio is an example of the impulses not getting through and this
leads to paralysis.
A few reflexes e.g. Knee Jerk do not involve the brain. Most do however. This is important as it allows you to feel pain, also it allows you to exert voluntary control over the reflex.
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