Contraction of muscles

Contraction of muscle follows an increase of Ca " in the muscle cell as a result of nerve stimulation. This initiates processes which cause the proteins myosin and actin to be drawn together making the cell shorter and thicker. The return of the Ca " to its storage site, the sarcoplasmic reticulum, by an active pump mechanism allows the contracted muscle to relax (27). Calcium ion, also a factor in the release of acetylcholine on stimulation of nerve cells, influences the permeabiUty of cell membranes activates enzymes, such as adenosine triphosphatase (ATPase), Hpase, and some proteolytic enzymes and facihtates intestinal absorption of vitamin B 2 [68-19-9] (28). 

In general, movement is an intrinsic property of living creatures. It occurs at different structural levels, including ion transfer through membranes, separation of replicated chromosomes, beating of cilia and flagella or, the most common, contraction of muscles. These contractions enable... 

CTien contraction of muscle is stimulated (via events involving Ca +, troponin, tropomyosin, and actin, which are described below), actin becomes accessible and the S-1 head of myosin finds it, binds it, and forms the actin-myosin-ADP-P complex indicated. 

The contraction of muscles from all sources occurs by the general mechanism described above. Muscles from different organisms and from different cells and tissues within the same organism may have different molecular mechanisms responsible for the regulation of their contraction and relaxation. In all systems, plays a key regulatory role. There are two general mechanisms of regulation of muscle contraction actin-based and myosin-based. The former operates in skeletal and cardiac muscle, the latter in smooth muscle. 

GTC seizures may be preceded by premonitory symptoms (i.e., an aura). A tonic-clonic seizure that is preceded by an aura is likely a partial seizure that is secondarily generalized. Tonic-clonic seizures begin with a short tonic contraction of muscles followed by a period of rigidity. The patient may lose sphincter control, bite the tongue, or become cyanotic. The episode may be followed by unconsciousness, and frequently the patient goes into a deep sleep. 

Langley JN (1905) On the reaction of cells and of nerve-endings to certain poisons, chiefly as regards the reaction of striated muscle to nicotine and curari. J Physiol 33 374 13 Langley JN (1907) On the contraction of muscle, chiefly in relation to the presence of receptive substances. Part I, J Physiol 36 347-384... 

In addition to its structural role in bone, and in teeth, the calcium ion is a carrier of chemical messages. It influences secretion, contraction of muscle, cell division, growth, transcription, as well as other key physiological processes. 

An increase in the rate of cross-bridge cychng increases force of contraction of muscle, which decreases the cytosolic concentration of ATP and increases that of ADP. This results, via the adenine nucleotide translocase, in a similar change in direction of ATP and ADP concentrations within the mitochondrial matrix (i.e. a decrease in the ATP/ADP concentration ratio). 

Figure 13.9 k soldier wounded at the Battle of Corunna (a battle in the Peninsular War) suffering simultaneous contraction of all muscles after infection with the bacterium, Clostridium tetani. Both agonist and antagonist muscles are active in this condition. The bacterium is found in the earth, especially in places where animal faeces have been present. Bacteria invade the body through a wound, especially in soldiers in battle. The bacterium secretes a toxin that is absorbed into the motor nerves which then become acutely responsive to mild stimuli. It can lead to death unless treated (from Bell 1824). The toxin is now used in cosmetic manipulation to stimulate contraction of muscles in the face to tighten the skin which removes or conceals wrinkles (Botox). 

Tetanus Clostridium tetani tetanus toxin Blocks neurotransmitter release in the brain leading to chronic contraction of muscles (See Figure 13.9)... 

Tetanus immunoglobulin is an example of an antibody preparation used to induee passive immunization against a mierobial toxin. Tetanus (lockjaw) is an infectious disease caused by the bacterium, Clostridium tetani. Bacterial spores can commonly contaminate surface wounds and the resulting bacterial cells produce a toxin as they multiply. The toxin interferes with normal neurological function, particularly at neuromuscular junctions. The result is spasmodic contraction of muscles and, if untreated, mortality rates are high. Treatment with antibiotics and anti-toxin, however, is highly effective if administered promptly. 

Troponin C in muscle is structurally closely related to catmodulin. It has 4 EF structures, of which only two can be occupied by Ca. Troponin C is a component of the contraction apparatus of muscle. Ca binding to troponin C induces a conformational change in the troponin complex that leads to contraction of muscle. 

Ca brings about its regulatory effect at the level of phosphorylase kinase. Phosphorylase kinase is activated by Ca. Ca influx comes into play particularly in contraction of muscle, a process by which Ca is released from storage and activates phosphorylase kinase. 

The function of nerves, which cause contraction of muscles, is attacked by the toxin, and this leads to weakness of muscles causing blurred vision and difficulty in swallowing and breathing, and breathing may stop completely with fatal result if sufficient toxin has been absorbed into the body. 

ACh, cholinomimetics and acetylcholinesterase inhibitors have been shown to inhibit the movement of flatworms and to cause flaccid paralysis (for reviews, see Fbx et a/., 1996 Halton et a/., 1997 Halton and Maule, 2004 Ribeiro et a/., 2005). This is in contrast to the mainly myoexcitatory effects of ACh at the vertebrate neuromuscular junction and in many other invertebrates including nematodes. Although ACh has predominantly inhibitory actions on flatworm muscle it has been shown to have inconsistent effects on the muscle of the monogenean, Diclidophora merlangi (Maule et a/., 1989), to induce the contraction of muscle fibres dispersed from planarians (Blair and Anderson, 1994 Moneypenny eta/., 2001) and, to cause increased muscle activity... 

B. Myoclonic seizures Sudden, brief, "shocklike contractions of muscles in the face and trunk, or in one or more extremities contractions may be single or multiple consciousness may be impaired... 

Quinidine (Figure 6.88) is the principal cinchona alkaloid used therapeutically, and is administered to treat cardiac arrhymias. It inhibits fibrillation, the uncoordinated contraction of muscle fibres in the heart. It is rapidly absorbed by the gastrointestinal tract and overdose can be hazardous, leading to diastolic arrest. 

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