To understand the mechanism of headaches, it is necessary to have some understanding of the concerned human anatomy and physiology. The skin perceives sensations of pain, touch and temperature. Pain perceived as being in the head region is referred to as ‘headache’. Sensations which arise from the skin, muscles or joints are termed general somatic sensations. Sensory receptors, when adequately stimulated, generate impulses which are conveyed to the central nervous system through the afferent sensory fibers, i.e. those which convey impulses to rather than from the central nervous system. Receptors present in the skin are called exteroceptors and those present in the muscles and joints are called proprioceptors. Exteroceptors convey sensations of warmth, cold, pain and touch, while proprioceptors convey body position, sense of the body in space, vibration, force, duration and range of movement of the joints, and deep pressure sensations. Sensations which arise from the viscera, i.e. the lungs, heart, intestine, etc., are called interceptive sensations, most of which do not reach our consciousness. Finally, there are special senses like vision, hearing, smell and taste.
Skin receptors (exteroceptors) are of several types as recognized both in terms of macro-structure (morphology) and micro-structure of tissues (histology). Morphologically, they vary from free nerve terminal to highly branched and encapsulated structures. These complicated structures in the skin, these end-organs of sensation, were given different names by the scientists who first described them. They are connected to the brain by a network of nerve fibers which transmit sensory messages or impulses to the brain for interpretation and recognition of the sensations. These nerve fibers do not form a continuous system but are interrupted at a few places. There are many good reasons why nature should have evolved these interruptions but it is difficult to explain them adequately to a non-physiologist. The site of each interruption is called a synapse. Here a message brought in by one fiber is conducted to another cell through a chemical transmitter called a neurotransmitter. Remember, there is no anatomical continuity at the synapses.
Nerve fibers, the conducting channels, like electric wires, vary largely in size, thickness and the capacity of their insulation. One can understand why a thick electric wire with heavier insulation and better conductivity is used for air-conditioners or other heavy-load equipment, in comparison to that used for bulbs, fans, and other light-load equipment. Here we are concerned with pain sensations which are conducted by the so-called A-delta fibers and C-fibers. A-delta fibers (1.0 to 5.0 um in diameter) are covered by a very thin insulating membrane, a myelin sheath. These fibers conduct at the rate of 10 to 20 meters/second. C-fibers (0.4 to 1.1 um in diameter) have no myelin sheath and conduct at the rate of 1 to 2 meters/second. The characteristics of these two fiber systems and their termination is the spinal cord is quite complex and, fortunately, such details are not necessary for a general understanding of our subject.
