.At this stage ATP molecules get attached with the myosin heads and some ADP & Pi are formed which (= ADP & Pi) remain within the same sites of the my

attached with the active sites within the actin .At this stage ATP molecules get attached with the myosin heads and some ADP & Pi are formed which (= ADP & Pi) remain within the same sites of the myosm head.. The energy released also remain stored up within the myosm head. 2. Next, the myosm head becomes attached with the active site of the actin ->The stored energy of the head mentioned above is released this released energy provides the energy which is needed for twist of the myosm head (which ultimately produces the "power stroke) mentioned earlier. Fig.9.1.4 Description in the text Fig.9.1.3 Descripton in the text 3. The ADP & Pi now move awayfrom the myosm head. 4. Fresh ATP molecules get attached with the ATP sites of the myosm head, now lying unoccupied. What exactly happens nexl is not very clear. One view is the intact molecule of ATP causes deattachment of the myosm head and actm ATP now splits into ADP & Pi and the released energy is stored ->the myosm head reattaches itself into a new site the whole cycle repeats. Another view is, ATP molecule splits before the deattachment. Anyway, for the deattachment ATP molecules are necessary. In short, the two roles of ATP are in, (i) the rotational movement of the myosm head within the groove (active site) of actm, and (li) deattachment of myosm head from the actm. Applied physiolog.y Rigor mortis Usually, some hours after death the muscles are stiffened and the phenomenon is called 'rigor mortis'. In the medical urisprudence, this phenomenon is important, as this helps fixation of the hour of death after a murder and thus helps to track the criminal. Cause of rigor mortis can now be explained. After death, fresh supply of ATP becomes impossible. Therefore, once the local store of ATP molecules are exhausted, the deattachment of myosm from actm cannot take place resulting in permanent state of contraction (= stiffening) of the muscles. If before the death, the person or the animal was amidst violent activity, rigor mortis sets in very quickly. Thus rigor mortis may occur almost immediately after the death in the soldiers killed in the battle field or in animals killed during an exciting and prolonged chase. This is due to the fact that severe antemortem activity in such cases, causes exhaustion of ATP rapidly. Excitation contraction coupling Normally when a muscle is stimulated satisfactonly (^stimulated adequately), it becomes excited and develops an action potential (AP) This is excitation and is basical-, ly an electrical phenomenon. When a muscle is excited, it contracts (either isometrically or isotonically) What is the link or connection (coupling, agent) between these two different types of phenomenon, excitation (electrical) and contraction (mechanical) ? The answer is as follows' when an AP develops in a muscle fiber, it travels into the interior of the muscle fiber via the T tubules (and causes the development of T tabule potential). As it travels down the T tubules, it comes to the zones of cisterns where the T tubules lie very close of the L system. L systems are rich in Ca++ As the AP reaches the zones called cisterns (triad), the L systems are excited and they release Ca++ into the fibril and the cross bridges develop the muscle contracts. Subsequently, the Ca++ go back again into the L system and relaxation occurs. In short, the Ca++ ions are the linking or coupling material between the excitation and contraction. N. B. The student at this stage may compare the microscopic anatomy and mechanism of contraction of skeletal muscle with those of the cardiac muscles and find that they are very closely similar. Fast and slow muscles Muscles, in subhuman mammals can be divided into two types, (i) slow and (li) fast muscles. Anatomically speaking, fast muscles are, in most cases, pale looking, whereas the slow muscles are more red. This is so, because, the slow muscles are richer in myoglobms Also slow muscles have greater capillary network Slow muscles are therefore also called red muscles, whereas the fast muscles are called pale muscles. These two types of muscles differ in between them in their functions and response to chemical agents (drugs! hormones). Only some of the important differences will be highlighted; 1. The fast muscles have a greater speed of contraction, and are capable, of more intense activity (although the duration of activity is short) and fatigue develops earlier than in the slow muscles. 2. The fast muscle is more sensitive to tubocurarme but less sensitive to decamethonium. These are muscle relaxants, which are used by anesthesists In human beings, most muscles contain a variable mixture of fast and slow muscle libers. The postural muscles are largely slow muscles. Extrinsic muscles of eye (which cause movement of the eye balls) are one of the fastest muscles of our body (Imagine, the speed EUROMUSCUUtf? JUNCTION Introduction 2. Functional anatomy 3. Chemical transmission. 4. Applied physiology Introduction. The unctional region between the motor nerve fiber and the corresponding skeletal muscle fiber is called 'neuromusculariunction' Both the supplying nerve and the muscle supplied have contributions in making a neuromuscular unction. A typical neuromuscular lunction is seen only in the skeletal muscle. Smooth muscles or cardiac muscles do not have such typical structure. There is some confusion regarding the term motor end plate' According to many authors, neuromuscular junction = motor end plate. However, some authors use this term (motor end plate) to mean only that part of the neuro muscular junction which is contributed by the muscle, that is, the muscle membrane underlying the bulbous expansion of the nerve. In this book, the term motor end plate will be used interchangeably with the term neuromusculariunction (le, motor end plate = neuromuscular junction) Functional anatomy A motor nerve (= the nerve which transmits impulses to the muscles. When this nerve & stimulated, the muscle contracts), is an axon whose soma (or nerve cell body) is situated either in the anterior horn cell of the spinal cord, or in a cranial nerve nucleus of the brain. A single such axon usually supplies several muscle libers (recall, a muscle fiber = a muscle cell) When the axon approaches the muscle, it splits into several terminal1 branches, each such terminal branch (twig) supplying one and only one muscle fiber. Fig 9.1.5. Neuromuscular junction. The fine terminal branch of the axon (twig) approaches the muscle fiber, close to the muscle fiber, it loses its myelm sheath (fig 9.1.5), and the terminal part of the axis cylinder (of the terminal branch) develops a bulb like expansion. The cell membrane (= plasma