R. Melzac and P. Wall's gate control theory presented in 1965, is based on the existence of different types of nerve fibers in charge of "sensation" transmission, from the human body fringe area to the central nervous system.In this way we have:
- Small diameter myelinic fibers type A (delta) and unmyelinated type C. It is the job of these to "carry" the "pain" signal from the algesic fringe zone to the control system. The spinal marrow transmits the signal to the cortical nervous centers where it is recognized and modulated.The conduction speed of these fibers is relatively low (less than 2 meters/sec).
- Large diameter myelinic fibers type A (beta) - their job is to conduct "touch" sensations. These have a much higher conduction speed (around 70 meters/sec).By stimulating the large diameter fibers, without involving the others, the inhibition of small diameter fibers is obtained (through the spinal marrow) and a kind of "brain" pain path block takes place. In practice:
- C fibers are unreceptive impulses, which last less than 200 usec.
- A fibers (delta) are unreceptive to stimulus lasting less than 10 usec.
- A fibers (beta) remain activated even with only 2 usec. pulses.
From this one deduces that stimuli above 300 usec stimulate all three fiber types and therefore, the most suitable impulses for the previously mentioned theory, must last for a maximum of 50 usec.
The Gate Control Theory has been the object of criticism and of successive reconfirmation on the behalf of various authors. We quote "even if the mechanism through which we carry out gate control remains unknown, even if it is dubious that we are dealing with a pre- or post - synaptic inhibition, even if the gelatin substance's role is unknown, the existence of gate control absolutely cannot be questioned, even though its functional role and mechanism details must still be defined. (Wall - 1978)