Feeling pain

Usually pain stimuli coming from the of the skin, joints, fascia, etc. via small nerve fibers go through the dorsal horn of the spinal cord up the to the brain (via the spinothalamic tract), where it is perceived as pain. In the spinal cord there is a connection of different neurons. The so called projection neurons are responsible for the transfer of a pain stimuli from the spinal cord to the brain. But if a painful stimuli really finds its way to the brain is determined by the interplay of the connections between small nerve fibers, large nerve fibers and inhibitory interneurons. This theory was set up by Ron Melzack and Patrick Wall in 1962. They had the idea that pain is not the direct result of the activation of pain receptor neurons (nociceptor), but rather its perception is modulated by the interaction of different neurons. They proposed that a gating mechanism exists within the dorsal horn of the spinal cord: The Gate Control Theory

rough outline

• When there is no input, the gate is closed; the inhibitory neuron prevents the projection neuron from sending signals to the brain.
• When there is more or only large-fiber stimulation (type II, IIIa) a normal somatosensory input happens. Both the inhibitory neuron and the projection neuron are stimulated. The gate remains closed, as the inhibitor neuron prevents the projection neuron from sending signals to the brain.
• Is there more or only small-fiber stimulation (e.g. type IIIb) nociception (pain reception) happens: The inhibitory neuron is inactivated and the projection neuron sends signals to the brain informing it of pain - the gate is open.

The 'neurological gate' either blocks pain signals (coming through large fibers) or allows them to continue on to the brain (coming from small fibers). Hence the organism has a proper self-inhibiting mechanism to avoid pain, which is individually more or less active. Peripheral tracts, as well as descending tracts are able to inhibit pain. Phantom or chronic pain is often explained by this theory. This mechanism is also the basis for a number of pain medication, as opiod analgetica (e.g. morphine). And it also shows in what extent our brain is able to influence the pain perception. Inhibiting pain usually happens in an high emotional arousal (traumata, car accidants, etc.). Serotonine and endorphins (endogenic morphine) are agents in our body which have a high influence on projection neuron and therefore on the gate itself. Cool, ae?!

Be aware that the Gate Control Theory stays a theory. Mechanisms in the human body, especially those of  the brain, are not fully understood (yet). Hence there also exist variating views about pain and its generation.