If the input message comes in only from the large A beta fibres as a result of touch, the cell fires briefly and then is turned off by the action of the small white cell. If, however, the input volley comes from the tissue damage-detection fibres, A delta and C, the cell fires more vigorously and the small black cells also come into action and exaggerate the output. During all this time, the brain is sending down control messages to amplify, diminish or ignore the signal, reflecting the arrival and reception of a sensory message.
Let us examine the action of immediate gate control in real situations. If you hit your thumb with a hammer, it hurts. But then you do many strange things. You wave it about, grip it, rub it and run cold water on it. You are stimulating large, low-threshold A beta fibres, which in turn are stimulating the small white cells which diminish the firing of the big white cells. This is the basis of rubbing, scratching and massage for pain. When William Sweet, chief of neurosurgery at Harvard, and I first realized that this was the effect of the large fibres, we took advantage of the coincidental fact that such fibres can be stimulated very easily by electricity, and we invented transcutaneous nerve stimulation (TENS), to be described later.
If you are about to give blood and you watch carefully as the rather large needle approaches your skin and penetrates the vein with a sudden rush of dark blood into the syringe, you feel a sharp, stinging, tearing pain which can be quite alarming. On the other hand, if you concentrate on some handsome nurse who is wandering around the room, or even better if you close your eyes and snuggle down in the chair, fantasizing that you are on a beach warmed by the sun and listening to the waves, you hardly feel anything beyond the small prick. This is because the brain has sent messages down to the cord that it has more interesting matters to which it has decided to attend. We know that this involves the spinal cord because the effect is not just a matter of sensation, as the local muscle reflexes are correspondingly increased or diminished. We know from the example of the paraplegic soldier that these reflexes are the business of the spinal cord, even though the brain can influence them. Similarly, if the nurse squeezes or slaps the skin before the needle enters, you are distracted and may not feel the needle.