Pain is one of the most common human experiences. Yet pain has never been fully accepted as a medical problem. One reason may be because pain is a subjective and highly individualized experience. You can measure pain even though you can't touch it, feel it (unless it's your own), image it or prove its existence. Even a pinprick creates differing sensations of pain for different people.

Nevertheless, chronic pain affects more than 70 million Americans.

Pain is the body's way of sending a warning to the brain that something is wrong. Aches are felt when pain messages, carried by chemicals called neurotransmitters, travel from the nerves, along the spinal cord to the brain. In the brain, pain messages are meshed with thoughts, emotions and expectations that shape our interpretation and response to pain.

Both emotions and drugs can change the perception of pain because both affect neurotransmitter levels. Both emotions and chemicals also alter the amount of endorphins, the body's natural pain relievers, which block the relay of pain messages to the brain. Depending on your mood and mental state, pain messages can be slowed, strengthened or stopped entirely. For example, fear, anger and worry can mask or heighten pain, while calming, positive thoughts can ease it.

There are two types of pain. "Nociceptive" pain is pain caused by tissue damage in the skin, muscles, bone or internal organs. Joint pain caused by arthritis, for instance, is an example of nociceptive pain. "Neuropathic" pain involves the nerve cells that transmit pain messages to the brain because either the nerves themselves are damaged or because they are not functioning properly and are sending out persistent messages.

You may hear nociceptive and neuropathic pain referred to as "acute" and "chronic" pain. Pain specialists prefer the terms nociceptive and neuropathic because they better define the parameters of the pain experience. The more scientific terms enable clinicians to zero in on where and how to begin diagnosis and treatment.

These newer classifications are important because they are based on current understanding of pain pathophysiology of the nervous system. The terms, "acute pain" and "chronic pain" are not accurate because there is no time relationship between when pain changes from being acute to being chronic. An arbitrary time frame—typically three months—was identified as the point at which the pain experience changed from acute to chronic.

But time has no relationship to changes in the nervous system. For example, phantom pain, which is an excellent example of neuropathic pain, can occur within 24 hours of an amputation and be permanent. The newer classification helps us better understand complex pain problems.

Understanding Nociceptive and Neuropathic Pain

Nociceptive pain basically represents pain associated with a pain receptor. This kind of pain is a signal to the body that it's being damaged in some way that needs immediate attention. Trauma, infection or illness can cause nociceptive pain. Toothaches, sprains, backaches or a broken bone are other common causes. Although unpleasant, most injuries resulting in nociceptive pain are short-lived and are easily treated with rest or medications.

Neuropathic pain refers to pain that is not associated with specific pain receptors and probably represents damage to or sensitization of the nervous system (this is when pain becomes the disease process itself, rather than representing a "warning" of underlying pathology). It is constant, often lasting for months after an injury or trauma and can be disabling.

Neuropathic pain can cause fatigue, concentration problems and appetite changes and lead to suppression of the immune system, depression, anxiety and even suicide. Conditions that cause such pain include osteoarthritis and fibromyalgia, and are more common in women than in men. This form of pain is also associated with progressive illnesses such as arthritis and cancer. Cancer pain is, more often than not, nociceptive, due to a tumor invading bones or organs or certain cancer treatments, such as surgery or radiation therapy.

While undertreated nociceptive pain can lead to neuropathic pain, not all neuropathic pain needs to have been preceded by nociceptive pain, and there is no specific timeframe for when it can occur. Perhaps the worst aspect of either persistent nociceptive or neuropathic pain is not knowing how long it will last or what can relieve it, which makes coping with it difficult.