Pain is the means by which the peripheral nervous system (PNS) warns the central nervous system (CNS) of injury or potential injury to the body. The CNS comprises the brain and spinal cord, and the PNS is composed of the nerves that stem from andlead into the CNS. The PNS includes all nerves throughout the body except the brain and spinal cord.
A pain message is transmitted to the CNS by special PNS nerve cells called nociceptors. Nociceptors are distributed throughout the body and respond to different stimulidepending on their location. For example, nociceptors that extend from the skin are stimulated by sensations such as pressure, temperature, and chemical changes.
When a nociceptor is stimulated, neurotransmitters are released within the cell. Neurotransmitters are chemicals found within the nervous system that facilitate nerve cellcommunication. The nociceptor transmits its signal to nerve cells within the spinal cord, which conveys the pain message to the thalamus, a specific region in the brain.
Once the brain has received and processed the pain message and coordinated an appropriate response, pain has served its purpose. The body uses natural pain killers, calledendorphins, that are meant to derail further pain messages from the same source. However, these natural pain killers may not adequately dampen a continuing pain message.Also, depending on how the brain has processed the pain information, certain hormones, such as prostaglandins, may be released. These hormones enhance the pain messageand play a role in immune system responses to injury, such as inflammation. Certain neurotransmitters, especially substance P and calcitonin gene-related peptide, activelyenhance the pain message at the injury site and within the spinal cord.
Pain is generally divided into two categories: acute and chronic. Nociceptive pain, or the pain that is transmitted by nociceptors, is typically called acute pain. This kind of pain isassociated with injury, headaches, disease, and many other conditions. It usually resolves once the condition that caused it is resolved.
Following some disorders, pain does not resolve. Even after healing or a cure has been achieved, the brain continues to perceive pain. In this situation, the pain may beconsidered chronic. The time limit used to define chronic pain typically ranges from three to six months, although some healthcare professionals prefer a more flexible definition,and consider chronic pain as pain that endures beyond a normal healing time. The pain associated with cancer, persistent and degenerative conditions, and neuropathy, or nervedamage, is included in the chronic category. Also, unremitting pain that lacks an identifiable physical cause, such as the majority of cases of low back pain, may be consideredchronic. The underlying biochemistry of chronic pain appears to be different from regular nociceptive pain.
Some researchers have said that uninterrupted and unrelenting pain can induce changes in the spinal cord. In the past, intractable pain has been treated by severing a nerve’sconnection to the CNS. However, the lack of any sensory information being relayed by that nerve can cause pain transmission in the spinal cord to go into overdrive, asevidenced by the phantom limb pain experienced by amputees. Evidence is accumulating that unrelenting pain or the complete lack of nerve signals increases the number of painreceptors in the spinal cord. Nerve cells in the spinal cord may also begin secreting pain-amplifying neurotransmitters independent of actual pain signals from the body. Immunechemicals, primarily cytokines, may play a prominent role in such changes.
Scientists have long recognized a relationship between depression and chronic pain. In 2004, a survey of California adults diagnosed with major depressive disorder revealed thatmore than one-half of them also suffered from chronic pain.