Stimulus-contraction-coupling

Figure 35. Structure of myofilaments and stimulus contraction coupling...

Calcium plays a vital role ia excitation—contraction coupling, and failure to maintain iatraceUular calcium homeostasis results ia ceU death. The avaUabUity of the calcium antagonists also provides a powerful tool for basic studies of excitation—contraction coupling, stimulus—excretion coupling, and other specific physiological functions. 

Ionized calcium is an important regulator of a variety of cellular processes, including muscle contraction, stimulus-secretion coupling, the blood clotting cascade, enzyme activity, and membrane excitability. It is also an intracellular messenger of hormone action. 

Long stimulus trains will also induce tetanic fade, observed as a decrease in force during a single contraction. Fade is measured as the ratio of final force to initial peak force. Diseased muscle/nerve may show a lower ratio. Accompanying measurement of EMG can reveal if fade is due to failure of the muscle or the nerve. In purely muscle defects (muscle dystrophy) force will drop without any change in EMG whereas reduced force accompanied by reduced EMG suggests a defect in transmission (NMJ) (e.g., (3)) or excitation-contraction coupling. 

Nearly all cells in the body exhibit a difference in electrical voltage between their interior and exterior, the membrane potential. Some cells, including the conducting and contracting cells of the heart, are excitable an appropriate stimulus alters the properties of the cell membrane, ions flow across it and elicit an action potential. This spreads to adjacent cells, i.e. it is conducted as an electrical impulse and, when it reaches a muscle cell, causes it to contract this is excitation-contraction coupling. 

Figure 7.4 The withdrawal reflex coupled with the crossed-extensor reflex. A painful stimulus will elicit the withdrawal reflex, which causes flexor muscles to contract and move the affected body part away from the stimulus. At the same time, the crossed-extensor reflex causes extensor muscles in the opposite limb to contract. The straightening of the opposite limb provides support for the body.

White JG, Rao GHR, Genard JM Tlatdet stimulus activation-contraction-secretion coupling a frequently fractured chain of events . In Mann T lor eds. The regulation of coagulation. New York, NY Elsevier Inc, 1980... 

Histamine released from mast cells plays an important physiological role in immediate hypersensitivity and allergic responses. In addition, histamine functions as a neurotransmitter in the CNS and it is a potent stimulus for gastric acid secretion. These actions depend on the interaction of histamine with two types of receptors, Hi and H2. Hi and H2 receptors are coupled via G proteins to phospholipase C and adenylyl cyclase, respectively. The principal H3 receptor response is stimulation of gastric acid secretion, whereas other actions of histamine (e.g., smooth muscle contraction, vasodilation, increased capillary permeability, pain, and itching) are prunarily mediated by Hi receptors. 

Another important role that calcium ions play is as a second messenger . They can couple a chemical stimulus to a secretion, as in releasing neurotransmitters after receipt of a nervous impulse, whether at synapses or at a neuromuscular junction (Miledi and Slater, 1966), or in the release of adrenaline from the adrenal medulla at the instance of acetylcholine (Douglas, 1968). Likewise calcium is needed to couple the release of a neurotransmitter to a muscular contraction. Calcium seems also to initiate proliferation in many kinds of cells, such as muscle cells, lymphocytes, and fibroblasts. It also plays a role in phagocytosis. Those ionophores (Section 14. i) that transport calcium into cells can initiate many of these effects. Some think that, at the molecular level, calcium functions by giving ATPase access to its substrate (ATP), thus providing the energy for these various actions. 

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