In between every vertebrae in the spine is a cushion of jelly (the disc) which acts as a shock absorber and cushions each and every step. When the disc bulges backwards it traps the nerve where it leaves the spinal canal. This causes pressure which results in pain but where the pain is felt depends on how much pressure the disc is exerting on the nerve. The body works like a telephone exchange. If there is a problem on the line you can only tell which phone line is out of order not where the fault actually lies. The body is very similar. If there is a problem between the brain and say the ankle the brain can not tell exactly where along the nerve the fault lies.
The brain doesn’t know whether you hit your ankle on a chair, or whether the nerve between the brain and the ankle is being trapped in the back. Each nerve is arranged so that the nerve fibres which have furthest to travel lie in the centre of the nerve and those which are going to branch off sooner lie around the outside. Therefore the more pressure the disc exerts on the nerve the further away the patient feels the pain. For example pain in the hip would result from less pressure on the nerve than pain in the ankle. This is called referred pain. When there is so much pressure on the nerve that its conduction of messages is affected the sensation of pins and needles is felt. If the pressure is enough to stop the nerve working completely the part of the body supplied by that nerve feels numb to the touch. See slipped disc and back pain.