Why Do You Feel Pain?

The experience of pain is a protective mechanism to bring into consciousness the awareness of tissue damage. At the time of the pain experience, however, it is unlikely to feel very protective. Unlike other bodily sensations, the experience of pain is accompanied by motivation and behavioral responses, such as crying of fear. These experiences are an integral part of the pain experience and this become important in its diagnosis and treatment.

Scientists have distinguished among three kinds of pain perception. The first is mechanical nociception (pain perception) that results from mechanical damage to the tissue of the body. The second is thermal damage or the experience of pain due to temperature exposure. The third is referred to as polymodal nociception, a general category referring to pain that triggers chemical reactions from tissue damage.

Nociception in the peripheral nerves first sense injury and, in response, release chemical messengers, which are conducted to the spinal cord., where they are passed directly to the reticular formation and thalamus and into the cerebral cortex. These regions of the brain in turn identify the site of the injury and send messages back down the spinal column m which led to muscle contractions, which can help block the pai and change in other bodily functions, such as breathing.

Two major types of peripheral nerve fibers are involved in nociception. A-delta fibers are small, especially to mechanical or thermal pain, transmitting sharp brief pains rapidly. C-fibers are unmylineted nerve fibers, involved in polymodal pain, that transmit dull or aching pain (myelination increases the speed of transmission, so sudden and intense pain is more rapidly conducted to the cerebral cortex than is the slower, dull aching pain of the C-fibers).

Peripheral nerve fibers enter he spinal column in the dorsal horn,. Sensory aspects of pain are heavily determined by activity in the A-delta fibers, which projects onto areas in the thalamus and the sensory area of the cerebral cortex.

The motivational and affective elements of pain appear to be influenced more strongly by the C-fibers, which project onto different thalamic, hyperthalamus and cortical areas. The experience of pain, then, is determined by the balance of activity in these nerve fibers which reflects the pattern and intensity of stimulation.
Neurotransmitters also affect the transmission of pain. In particular, Substance P glutamate appears to be implicated in pain transmission. Several other regions of the brain are also involved in the modulation of pain.

Reference: http://en.wikipedia.org/wiki/Pain