Pain is unpleasant sensation of the body. The receptors of pain are called nociceptors. Nociceptors are present all over the body especially skin (subcutaneously). Any physical, thermal and mechanical pressure stimulates the nociceptors.
The impulses are carried by two types of neuron; Primary or 1st order neuron and Secondary or 2nd order neuron.

1st order neuron:

Primary afferent fibers transmit impulses from the sensory receptors to dorsal horn of spinal cord.

2nd order neuron:

2nd order afferent fibers carry sensory impulses from the dorsal horn of the spinal cord to the brain.
  • A α and A β are large diameter (myelinated) nerve fibers.
  • A Δ and C fibers are the small diameter (unmyelinated) nerve fibers.

The impulses from spinal cord to the brain are carried by Dorsal spinothalamic tract, spinoreticular tract and spinocephalic tract.

Pain Gate Control Theory:

This theory was suggested by Pat wall and Melazak in 1965.
Afferent input is mainly through posterior root of spinal cord and all afferent information must pass through synapses in the Substantia Gelatinosa and Nucleus Propius of the posterior horn.


It was suggested that pain to pass through this gate, there must be unopposed passage for nociceptive information arriving at the synapse. However if the gate is concurrently receiving the impulses produce by the stimulation of large diameter fiber (thermoceptors, mechanoceptors). Then this traffic predominates with the resultant presynaptic inhibition of the small diameter nociceptive information. Many physiotherapeutic agents causing stimulation of the endings connected to the large diameter nerve fibers. The use of Manipulation, TENS, Interferential therapy, Heat, Massage, Ice, Vibration and Movements can produce reduction of pain by closing the pain gate.