BIG PAIN [ MESSENGERS ( THE NERVOUS SYSTEM ) ]
It turns out, the brain has automatic defenses cued up for a quick response to more serious pain. The perception of pain warns the brain of actual or potential tissue damage. The brain’s recognition of pain sets in motion actions to reduce or remove it, and thus the threat.
Most pain receptors consist of the bare ends of sensory nerves embedded throughout all body tissues, except the brain, whose cells cannot experience sensation. These noclceptors react to any ”noxious” stimulation, anything that damages the body’s cells.
Damage makes the cells release chemicals that activate neurotransmitter receptors (substance P is the transmitter for pain) and send pain signals via the peripheral nervous system to the central nervous system, where it may take a while to be felt. Pain doesn’t reach the brain instantly because of the distance the signal must travel; in a tall man, injury to the toe may take rwo seconds to register in the brain.
In the skin, muscles, and joints, cell damage is likely to cause relatively brief and sharp pains. That’s because nerve cells in the spinal cord release natural pain suppressants known as enkephalins, which inhibit the discharge of more pain-exciting neurotransmitters and keep the sensation short. As a result, sharp pains usually fade into dull aches.
Deeper cell damage is more likely to create burns and aches that last longer. The difference lies in the kinds of nerve fibers that transmit the pain signals, and how quickly that information travels.
HIPPOCRATES, the founder of modern medicine, knew that chewing willow bark alleviated pain. Thousands of years later, scientists discovered why: The bark contains salicylic acid. When cells are damaged, they release an enzyme called cyclooxygenase-2. That chemical in turn produces prostaglan-din, which signals to the brain that part of the body is in pain. Prostaglandin also causes the injured flesh to swell and become inflamed. Salicylic acid binds to cyclooxygenase-2, blocking the creation of prostaglandin. Less prostaglandin means fewer pain signals reaching the brain, and less inflammation of the cells around the injury.
Damage to the internal organs, or viscera, usually results in dull aches, burning sensations, and gnawing pain. As the pathways for the visceral and somatic nerves of organs and body converge in the spinal cord, the brain sometimes gets confused and assigns visceral pains to other parts of the body that are not actually injured. A heart attack, for example, may seem to cause shooting arm pams.
