Nerves - these little wires run the whole body. The biggest nerve in your body is the brain. That's why it's encased in a hard head and surrounded by hard bone with joints. A nerve impulse runs from brain to somehwere, and back. The fastest impulse travels at over 200 miles an hour. Nerves relay messages of pain,heat,cold,pressure as well as making your organs work, digest, eliminate etc. If there is a problem with the nerve or nervous system, many symptoms may be related to that, the most common being pain and muscle weakness. And not only in the low back.