Reflexes occur to protect the body from tissue damage; they are rapid involuntary actions. For example, the arm will suddenly be withdrawn if it touches a very hot object. A reflex arc describes an arrangement of three neurones which detect the stimulus and relay the information across the spinal cord to a motor neurone which will move an appropriate muscle.
The initial sensation is detected by peripheral sensory receptors and transmitted along the dendrite of a sensory neurone to the cell body. From the cell body it is then transmitted along the axon of the sensory neurone. This fibre then goes into the spinal cord where it synapses with a relay neurone. The relay neurone will carry the impulse across the spinal cord from the back, where the sensory neurones enter, to the front where the motor neurones leave. This relay neurone then synapses with the cell body of a motor neurone which carries the impulse away from the spinal cord to the appropriate muscle which is innervated to contract, withdrawing the hand away from the hot object.
In addition an ascending neurone carries the information up the spinal cord to inform the brain of what has happened. This means that the reflex can occurbefore the brain becomes aware that there is a problem. This saves a lot of time; if the message had to go up to the brain and a further message sent down to the muscle there would be a time delay during which injury could occur. There are many other examples of reflexes including the eyelash and gag reflexes.
Sensory neurones always approach the spinal cord from the back and the motor neurones always leave the spinal cord from the front. The nerve containing the sensory neurones which pass into the spinal cord is termed the posterior root and the one containing motor neurones the anterior root. Posterior always means towards the back and anterior towards the front.