stimulated. Again, this stimulation can be made only when the nerves terminating on that par! of the brain is stimulated

stimulated. Again, this stimulation can be made only when the nerves terminating on that par! of the brain is stimulated .These nerve fibers are stimulated, normally, in turn, only when the corresponding receptors (eg. Merkel's disc) are stimulated. Ultimately therefore, it means, that it is the receptor, which determines which sensation is going to be perceived. As the receptors are, normally, stimulated by its own specific stimulator (e. g. touch for Merkel's disc) and as the receptor is situated in a specific area of the body, sensory coding is possible. However, it must not be forgotten that ultimately what is required is development of a generator potenti:, that is, excitation in the receptor concerned, which then will be translated into the specific sensation by other components (eg, post central gyrus = somasthetic area 1 , SI) of the sensory system. Put in a little crude way, it means, by 'hook or crook', the concerned receptors or the nerves are to be excited. Take some specific examples: The vibrating tuning fork when placed over the tibia produces a sense of vibration, but the same'vibrating tuning fork when placed over the mastoid process of temporal bone produces a sense of hearing .Or, (whether by means of an electrical or mechanical stimulus) if a sensory nerve fiber is stimulated, it will produce the corresponding sensory sensation in the corresponding anatomical area. In every such example, ultimately an action potential (AP) develops in the nerve the AP travels up and ultimately reaches the specific area of cerebral cortex the specific sensation. (n) Intensity discrimination is possible because of the operation of Weber-Fechner's law. Phantom limb Assume some one's left superior extremity has been amputated at the level of the middle of the arm. If now the exposed central stump of the nerve fibers be irritated, the fibers supposed to be coming from the fingers (which no longer exist now) may be irritated, and the subject feels as if the sensation is coming from the fingers (which no longer exist), thus he develops pain/pruritus etc, in the 'phantom fingers' (phantom = ghost). SUMMARY & HIGHLIGHTS Receptors are structures that interpose between the peripheral most par! of the sense organ (where the sense stimulus is supposed to be applied) and beginning of the sensory nerve. However, with some senses (eg. pain) there is no special receptor, the bare nerve terminal acts as a receptor. When a receptor is stimulated, it develops a local current, called generator potential, if the general potential is sufficiently strong, an action potential develops in the emerging nerve and only now the nerve will conduct the sensory impulse. Stronger the stimulus greater is the perception of the sense and can mathematically be described by Weber-Fechner's law (the interrelationship is on log scale). Of the properties of receptors, adaptation is particularly important: most types of receptors cease to respond when the stimulus becomes protracted: some varieties, notably, pain, gravity receptors, Hermg-Breuer receptor's are exception (and obviously, these exceptions, teleologically, are desirable)> (n) Intensity discrimination is possible because of the operation of Weber-Fechner's law> Phantom limb Assume some one's left superior extremity has been amputated at the level of the middle of the arm. If now the exposed central stump of the nerve fibers be irritated, the fibers supposed to be coming from the fingers (which no longer exist now) may be irritated, and the subject feels as if the sensation is coming from the fingers (which no longer exist): thus he develops pain/pruritus etc, in the 'phantom fingers' (phantom = ghost). SUMMARY & HIGHLIGHTS Receptors are structures that interpose between the peripheral most par! of the sense organ (where the sense stimulus is supposed to be applied) and beginning of the sensory nerve. However, with some senses (eg. pain) there is no special receptor, the bare nerve terminal acts as a receptor. When a receptor is stimulated, it develops a local current, called generator potential: if the general potential is sufficiently strong, an action potential develops in the emerging nerve and only now the nerve will conduct the sensory impulse. Stronger the stimulus greater is the perception of the sense and can mathematically be described by Weber-Fechner's law (the interrelationship is on log scale). Of the properties of receptors, adaptation is particularly important: most types of receptors cease to respond when the stimulus becomes protracted: some varieties, notably, pain, gravity receptors, Hermg-Breuer receptor's are exception (and obviously, these exceptions, teleologically, are desirable). THE PATHOPHYSIOLOGY OF PAIN 1. Introduction 2. Receptors and stimulus for pain 3. Types (classification) of pain 4. Characteristics (properties) of pain 5. Reactions of pain 6. Inter relationship between threshold and reaction of pain. 7. The neurotransmitter and path of pain 8. Visceral pain: (i) characteristic features, (n) referred pain 9. The pain controlling mechanism 10. Applied physiology: (i) analgesics, (n) some (clinically) well known pains, (in) acupuncture. Introduction Pain is produced when a nociceptive (injurious) stimulus is recieved (the nociceptive stimulus either causes actual damage or is a potentially damaging agent of the tissues) The injurious agent may be (i) mechanical (cuts, blows etc), (n) chemicals (acids etc), (in) thermal (burn), or (iv) disease. The term tenderness' means, pain elicited by pressing the part, (eg, abdominal tenderness). Pain and tenderness, therefore, although related, are not identical terms. Importance of pain. Teleology Pain is unpleasant sensation no doubt, but on the whole it is usually beneficial to the man (or animal). Pain makes us conscious of the presence of the injurious agent and