WHAT IS PAIN?

Pain results from a series of exchanges among three major 
components of your nervous system:


YOUR PERIPHERAL NERVES

These nerves extend from your spinal cord to your skin, muscles and internal organs. Some peripheral nerve fibers end with receptors that respond to touch, pressure, vibration, cold and warmth. Other types of nerve fibers end with nociceptors (no-sih-SEP-turs) which are receptors that detect actual or potential tissue damage.

Nociceptors are most concentrated in areas prone to injury, such as your fingers and toes. When nociceptors detect a harmful stimulus such as the hard surface that stubbed your toe they relay pain messages in the form of electrical impulses along a peripheral nerve to your spinal cord and brain. Sensations of severe pain are transmitted almost instantaneously.

YOUR SPINAL CORD

The nerve fibers that transmit pain messages such as the throbbing pain from that stubbed toe enter the spinal cord in an area called the dorsal horn. There, they release chemicals (neurotransmitters) that activate other nerve cells in the spinal cord, which process the information and then transmit it up to the brain.

YOUR BRAIN

When news of your stubbed toe travels up the spinal cord, it arrives at the thalamus a sorting and switching station deep inside your brain. The thalamus forwards the message simultaneously to three specialized regions of the brain: the physical sensation region (somatosensory cortex), the emotional feeling region (limbic system) and the thinking region (frontal cortex). Your brain responds to pain by sending messages that moderate the pain in the spinal cord.

The location of your pain can affect how you perceive it. A headache that interferes with work or concentration may be more bothersome and therefore receive a stronger response than arthritic pain in your knee or a cut to your finger.

Excerpt from "How you feel pain" Mayo Clinic