How does pain work?

However unwelcome pain may be, it's clear that it has a function: survival. It's a reflex that makes us withdraw a finger from a flame, faster than thought, an irresistible impulse to massage a stubbed toe, or the urge to rest an injured limb. Pain — particularly chronic pain — is never a pleasant experience, but it can protect us. What's not so clear is how it does its work.
At bottom, pain (all sensation for that matter) results from the operations of the nervous system. More complex than the most advanced electronic circuitry, it's the most baffling of all the body's systems. We've known something of its workings since the time of the ancient Greeks, who described and analyzed the sets of peripheral nerves, sensory and motor, that run to and from the spinal cord and connect with the brain.
In the centuries that followed, incremental advances were made in identifying and mapping the trail of nerves, but it was the French philosopher and scientist René Descartes, in his 1664 book, 'Description of the Human Body,' who first suggested that pain travels specific pathways from extremities in the skin to the brain. With certain modifications, and within certain limits, that basic understanding has been with us down to the present day.
But as Jeremy S.H. Taylor and R.M. Gaze of the University of Edinburgh pointed out in the 1987 edition of 'The Oxford Companion to the Mind,' there's more to this than meets the eye. Our understanding of neural structure (that is, the nerves and the nervous system) is far greater than our understanding of neural function. That's because, except for the nervous system's most obvious roles, we don't really know what the function of the nervous system actually is.
We tend to think of the function of the nervous system and brain as 'information-processing,' but that doesn't really explain anything. We understand mechanical information-processing machines because we built them in a particular way and for a particular purpose. We don't have that understanding of the nervous system because we didn't construct it, and, so far at least, we haven't been able even to define some of its operations.
Part of what makes the nervous system so astonishingly complex is the fact that it's virtually everywhere in the human body, from the tips of our toes to the scalp on our heads. Its endpoints lie everywhere just beneath the surface of the skin, and connect with the muscles and other tissues of all the organs of our bodies. There's almost no place in our bodies that isn't touched — and controlled — by it.
The nervous system consists of three interconnected parts: afferent nerve fibres and their receptors, efferent fibres with their muscles and glands, and the central nervous system — the spinal cord and brain. Like all living tissue, the entire network is made up of cell matter, primarily nerve cells, or neurons. (We're actually born with the cells of the central nervous system, which, unlike all other cells in the body, are irreplaceable.)
In most other ways, nerve cells are like other cells of the body, except that at one end they have a number of root-like projections called dendrites, and at the other end each nerve cell has a long, whiplike tail called an axon. Grouped in bundles like the fibres of a rope, nerve cells may be the merest part of a centimetre long or run a metre or more from the tip of the toe to the base of the spine. It's their fantastic interconnectedness — a large neuron, for example, may be in contact with the dendrites of as many as 200,000 cells — that gives us such a rich sensory apparatus.
The afferent and efferent fibres, which comprise the peripheral nervous system, are like one-way transmission lines. The afferent nerves conduct messages to the spinal cord and brain; the efferent nerves conduct signals away, to the muscles and glands. The kind of messages efferent fibres carry to their end organs are generally command signals. If the destination is a muscle, it may be a command to contract; if it's an organ — the stomach, say — the signal may be an order to release digestive enzymes.
The afferent fibres' messages, on the other hand, convey sensory information — sensations such as heat, cold, touch and, unfortunately, pain, from bumps, bruises and other failings of the flesh. Be they via afferent or efferent fibres, messages are sent in a sort of Morse CodeÑseries of sequential dots grouped in volleys of electrochemical nerve impulses, racing along at anywhere from two to 120 metres per second. As fast as that sounds, pain is hardly instantaneous, as everyone knows: Stub your toe, and you know you will feel pain before you actually do. In chronic pain, the signal from the nerve endings seems instantaneous, but only because it never really stops.
This kind of bare-bones description makes neural structure sound simple, and it makes the Cartesian model of pain (remember Descartes?) seem perfectly sound: You stub your toe, and the mass of bruised tissue releases chemicals (such as substance P, prostaglandins or bradykinin) to sensitize the adjacent nerve endings. Once stimulated, these send forth a volley of electrochemical nerve impulses to the spinal cord via afferent fibres. In turn, the spinal cord passes on the signals to the brain, which deciphers them as 'pain.' It then transmits secondary instructions, via the efferent pathways, to various muscle groups and glands, which initiate a series of motor activities: You lift your foot, reach down and massage the toe, while the vocal chords produce a resounding 'Ow!', in addition to the usual blood-borne 'healing cascade.' What could be more straightforward?