II.

Physiology of Pain

The signals that warn the body of tissue damage are transmitted through the nervous system. In this system, the basic unit is the nerve cell or neuron. A nerve cell is composed of three parts: a central cell body, a single major branching fiber called an axon, and a series of smaller branching fibers known as dendrites. Each nerve cell meets other nerve cells at certain points on the axons and dendrites, forming a dense network of interconnected nerve fibers that transmit sensory information about touch, pressure, or warmth, as well as pain.

Sensory information is transmitted from the different parts of the body to the brain via the spinal cord, which is a complex set of nerves that extends from the brain down along the back, protected by the bones of the spine. About as wide as a finger, the spinal cord is like a cable packed with many bundles of wires. The bundles are nerve pathways for transmitting information. But the spinal cord is more than just a message transmitter, it is also an extension of the brain. It contains neurons that process incoming sensory information, and generates messages to be sent back down to cells in other parts of the body.

Information being transmitted between and within the brain and spinal cord travels through the nervous system using both chemical and electrical mechanisms. A message-carrying impulse travels from one end of a nerve cell to another by means of an electric signal. When the electric signal reaches the terminal end of a nerve cell, a gap called a synapse prevents the electric signal from crossing to the next cell. The electric signal triggers the cell to release chemicals called neurotransmitters, which float across the synapse to the neighboring nerve cell. These neurotransmitters fit into specialized receptors found on the adjacent nerve cell, much as a key fits into a lock, generating an electric impulse in the neighboring cell. This new impulse travels to the end of the long cell, in turn triggering the release of neurotransmitters to carry the message across the next synapse. Not all neurotransmitters initiate a message in a neighboring nerve cell. Some specialize in preventing neighboring cells from generating an electrical signal, while others function as helpers, fascilitating the message's journey to the brain.

While most of the sensory nerves in the skin and other body tissues have special structures covering their nerve endings, those nerves that signal injury have free nerve endings. These simple nerve endings specialize in detecting noxious stimuli—a catchall term for injury-causing stimuli such as intense heat, extreme pressure, or sharp pricks or cuts. The nerve endings that detect pain are called nociceptors, and the process of transmitting pain signals when harmful stimulation occurs is called nociception. Several million nociceptors are interlaced through the tissues and organs of the body.

An injury triggers pain signals in two types of nociceptors, one with large, insulated axons known as A-delta fibers and one with small, uninsulated axons known as C fibers. The large A-delta fibers conduct signals quickly, and the smaller C fibers transmit information slowly. The difference in the functions of these two fibers becomes obvious to a person who stubs a toe. At first the injured person is aware of a sharp, flashing pain at the point of injury. Generated by the A-delta fibers, this short-lived pain intrudes upon the thoughts and perceptions occurring in the brain. Just as this first pain subsides, a second pain begins that is vague, throbbing, and persistent. This sensation is derived from the C fibers.

Pain information from the A-delta and C fibers travels through the spinal cord to the brain. When it receives the pain message, the spinal cord generates impulses that travel back down to muscles, which lead to a reflexive contraction that pulls the body away from the source of injury. Other reflexes may affect skin temperature, blood flow, sweating, and other changes.

While this reflex action is underway, the pain message continues up the spinal cord to relay centers in the brain. The sensory information is routed to many other parts of the brain, including the cortex, where thinking processes occur.