PAIN: DESTINATION OF NERVE FIBRES
The spinal cord as it enters the skull merges with the medulla, an opened-up version of the cord containing sensory and motor areas. Further forward, the medulla becomes the pons, on top of which the cerebellum resides. Further still is the midbrain, which again closes up to become a cord-like cylinder with sensory areas on top and motor areas below. This brainstem made up of medulla, pons and midbrain receives ascending messages from the cord and a special input from the viscera, and adds inputs from eyes, ears and face, mouth and head. In addition, it distributes orders to eye muscles, face and jaws.
Finally, beyond the brainstem is the forebrain. At its core is a mass of cells, the thalamus and basal ganglia. Below the thalamus is the hypothalamus, which is concerned with all our internal business: feeding, blood flow, temperature, hormones and so on. Wrapped completely around the thalamus is the cortex. It consists of a vast, folded continuous sheet of cells in six layers which receive inputs from everywhere, send impulses everywhere, and for good measure talk to each other.
Every one of the structures mentioned receives powerful input signals generated by tissue damage and coincident with the production of pain. In the rest of this book, we have to unravel this apparent mess. No one area has the monopoly of capturing the one and only input signal associated with pain. One thing is certain: we are not going to find a single pain centre as proposed by Descartes. At the same time, this huge accumulation of nerve cells is not a random network because it contains specialized zones which provide some hope for the discovery of an overall plan which we will pursue. It is obviously time to put our thinking caps on top of our brains in order to propose an acceptable subtle theory which takes the anatomical and physiological facts into account.
One fact we have already stressed is that the brain does not sit passively reading the sensory messages sent to it from the tissues and spinal cord. It sends out descending control systems which shape the received messages. Most of these descending control systems originate from cells in the core of the brain stem, in the medulla, pons and midbrain. These cell groups are in turn affected by inputs from the cord and from the forebrain. We are therefore faced with a feedback in which the input is affected by the brain as well as by events in the periphery. The most famous descending control runs all the way from cortex to cord. It has therefore been assumed to be the motor system by which the brain orders the muscles to contract. It now turns out that even this corticospinal pyramidal motor system also influences the sensory message received by the cortex.
In summary, this chapter has outlined several stages. When tissue is damaged, a sequence of events produces inflammation with pain. The spinal cord is informed of tissue damage by way of sensory nerves. Cells in the spinal cord react immediately to the input but the amount of their output depends on small cells that can enhance or diminish the output message. The setting of the small cells depends on orders descending from the brain. Once bombarded by injury messages, spinal-cord cells exaggerate their sensitivity. Chemical messages from damaged tissue, and especially from cut nerves, further increase the sensitivity of cord cells. The cord cells signal to many parts of the brain that injury exists. Many parts of the brain then feed back onto the cord cells and amplify or reduce their output messages.
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