HEADACHES: THE PATHWAY OF PAIN
July 4th, 2011There are pathways in the nervous system which help us appreciate our environment, whether by sight, touch, or hearing. Each sense organ can be regarded as an extension, or the sensitive area, of a nerve which is linked to the brain. In the case of the eyes, the light-sensitive cells in the back of the eyeball (retina) are linked together in specific patterns which are then transmitted by the optic nerves to a relay station at the base of the brain. In lower animals, much organization of incoming information is carried out here but, in man and other primates, most of the processing occurs in the grey matter of the brain, the cortex. Here, the shape perceived is compared with stored information and recognition occurs when the pattern is matched with memory. A label is supplied by an area of the brain concerned with memory and we have a conscious appreciation of what the object is. A similar process occurs with the recognition of sounds but, in this case, different areas of the brain are involved.Touch (tactile sensation) is different in that the impulses have to travel a greater distance to the brain up the spinal cord (except in the case of sensation on the face). The other varieties of sensation besides touch include temperature, superficial pain, and deep pain. Impulses travel along nerve fibres from sensitive structures in the skin to the spinal cord where they relay with long nerve fibres grouped together. These travels to the brain where two things happen: the first is that the sensation is localized, because only that group of nerve fibres in the brain concerned solely with sensation from the specifically activated area is stimulated; secondly, other areas of the brain go into a state of expectancy, a general alerting reaction. Because of this activation (which can be measured electrically) there is an increased flow of blood to the nerve cells. Complex patterns of skin stimulation can be analyzed in much the same way as patterns of light and sound perceived by eyes and ears.The structures in the skin or other tissues which receive painful stimuli are of a specific type, and are different from those sensitive to touch and temperature. Pain sensation is served by two types of nerve fibre, fast and slow, each of which transmits pain of a different character: the fast fibre transmits discrete, sharp pain whilst the slow fibre produces dull and diffuse pain. Slow-conducting fibres also transmit the sensation of itch; this is the reason why scratching, in other words, blocking the itch sensation by pain, is efficacious.
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