Anatomy: The Physiology of Pain

The nervous system is one of the most important systems in the body. It consists largely of cells (nerves) that have evolved to be signal channels carrying information from the body to the brain or spine. The autonomic nervous system carries simple urgent information only as far as the spine where "action nerves" send messages to the appropriate muscles; these are the reflex actions that cause someone to withdraw his hand from a hot object, for example. Nonemergency information, such as the texture of an object, is sent to the brain for more leisurely processing.

Significance

• The sensory nervous system protects the body by avoiding dangerous or harmful situations or environments. If a person does something harmful, pain tells him to stop before the damage is too great. If something is too hot or cold, the person avoids it, to prevent further damage to the body.

Function

• Nerve cells have evolved to carry information in the form of electrical impulses. The sensory end of the nerve reacts to stimuli (heat, cold, pressure, compression, etc.) by firing an electrical impulse that travels to other nerves. A chain of nerves carries the information to the central nervous system (the spine or the brain).

Types

• There are two types of sensory nerves: those that send messages to the spine only and those that send messages to the brain (possibly going through the spine). Nerves that communicate with the spine only are the reflex nerves; these cause a person to pull his fingers away from hot objects. This action does not require the brain, and it needs to be done immediately. The nerves that send information to the brain carry information needed for more deliberate action.

Features

• Sensory nerves are some of the longest cells in the body. The sensory nerve that runs from the big toe to the base of the spine is the longest cell in the body. Even inside the spine, sensory nerves tend to be longer than other nerves. The reason for this is speed: it takes time to transfer information between cells. The fewer cells in the chain, the faster the information travels. Pain is important and time-sensitive.

Misconceptions

• The fundamental misconception about pain is that it is a bad thing. Pain prevents us from injuring ourselves in many different ways. Pain sensors in the arms and legs tell when a limb is extended too far or has rotated too far, thereby preventing injury from overextension. Pain in the legs and chest indicate that a person has run too far, to prevent him or her from dropping dead from overexertion.