Voluntary muscles

Soluble Compounds. The mechanism of barium toxicity is related to its ability to substitute for calcium in muscle contraction. Toxicity results from stimulation of smooth muscles of the gastrointestinal tract, the cardiac muscle, and the voluntary muscles, resulting in paralysis (47). Skeletal, arterial, intestinal, and bronchial muscle all seem to be affected by barium. 

Brucine closely resembles strychnine in action, but is much less poisonous and it also has a more marked curare-like action on the nerve-endings in voluntary muscle. 

Skeletal muscle relaxant. A drug that decreases the tone of voluntary muscles. 

The act of vomiting is a complex process accompanied by several events apart from activation of the motor nerves and various voluntary muscles involved in the increase of intragastric pressure and evacuation of the stomach contents. Vomiting is preceded by a deep inspiration, closure of the glottis and raising of the soft palate to prevent vomitus entering the trachea and nasopharynx, respectively. There is also increased heart rate, pallor, salivation, sweating and lacrimation. 

Voluntary muscle contraction is initiated in the brain-eliciting action potentials which are transmitted via motor nerves to the neuromuscular junction where acetylcholine is released causing a depolarization of the muscle cell membrane. An action potential is formed which is spread over the surface membrane and into the transverse (T) tubular system. The action potential in the T-tubular system triggers Ca " release from the sarcoplasmic reticulum (SR) into the myoplasm where Ca " binds to troponin C and activates actin. This results in crossbridge formation between actin and myosin and muscle contraction. 

This group of conditions, in which myotonia (the failure of voluntary muscle to relax following contraction) may be a feature, can now be classified according to the primary molecular defect responsible for the relevant condition. The clinical features of the different conditions within this group can show some significant differences, even among the diseases now known to be due to mutations within the same genes (Table 1). 

Argov, Z., Mastaglia, F.L. (1988). Drug-induced neuromuscular disorders in man. In Disorders of Voluntary Muscle (Walton, J.N., ed.), pp. 981-1014, Churchill-Livingstone, Edinburgh. 

An overall view of voluntary muscle at several levels of organization is presented in Figure 49-1. 

Figure 49-1. The structure of voluntary muscle. The sarcomere is the region between the...

The development of high-magnification microscopy made it possible to create images of biological materials at the molecular level. Many of these images show structures that have liquid crystalline aspects. Shown here are aligned mosaic virus molecules and protein molecules in voluntary muscles. In addition, all cell walls are picket fences of rod-shaped molecules in regular yet fluid arra. ... 

Both the G- and V-agents have the same physiological action on humans. They are potent inhibitors of the enzyme acetylcholinesterase (AChE), which is required for the function of many nerves and muscles in nearly every multicellular animal. Normally, AChE prevents the accumulation of acetylcholine after its release in the nervous system. Acetylcholine plays a vital role in stimulating voluntary muscles and nerve endings of the autonomic nervous system and many structures within the CNS. Thus, nerve agents that are cholinesterase inhibitors permit acetylcholine to accumulate at those sites, mimicking the effects of a massive release of acetylcholine. The major effects will be on skeletal muscles, parasympathetic end organs, and the CNS. 

Muscle-relaxant A drug that relaxes skeletal (voluntary) muscle either directly by blocking nicotinic cholinergic receptors or indirectly through the activation of inhibitory spinal cord interneurons. 

Signs and Symptoms The first symptoms are the protrusion of the nictitating membrane, and the involvement of facial and jaw muscles leading to lockjaw. Usually develops first in the limbs followed by the muscles of the trunk except in horses, which present from the head down. Causes spasmodic contraction of the voluntary muscles and increased sensitivity to stimuli. Consciousness is not affected. 

Epilepsy implies a periodic recurrence of seizures with or without convulsions. A seizure results from an excessive discharge of cortical neurons and is characterized by changes in electrical activity as measured by the electroencephalogram (EEG). A convulsion implies violent, involuntary contraction(s) of the voluntary muscles. 

Myelopathy. Damage to the spinal cord results in weakness or paralysis of voluntary muscles. 

Voluntary muscles contain a variety of fibre types which are specialized for particular tasks. Most muscles contain a mixture of fibre types although one type may predominate. All human skeletal muscles are composed of several different muscle fibre types. Up to seven different fibre types have been identified histochemically based on the pH stability of myofibrillar adenosine triphosphatase and on the myosin heavy chain profile. Innumerable fibre type transients exist due to continuing adaptation processes. However, three main... 

In conditions of muscle spasticity, there need to be drugs capable of relieving painful muscle spasms that do not take away the ability of voluntary muscle contraction and that do not hamper brain function. 

Musculoskeletal/Cardiac effects When serum sodium or calcium concentration is reduced, moderate elevation of serum potassium may cause toxic effects on the heart and skeletal muscle. Weakness and later paralysis of voluntary muscles, with consequent respiratory distress and dysphagia, are generally late signs, sometimes significantly preceding dangerous or fatal cardiac toxicity. 

Benzodiazepines have the capacity to depress polysynaptic reflexes and have been shown to decrease decerebrate rigidity in cats and spasticity in patients with cerebral palsy. What is not clear is whether they can, in humans, relax voluntary muscles in doses that do not cause considerable central nervous system depression. Nevertheless, benzodiazepines, such as diazepam, are often prescribed for patients who have muscle spasms and pain as a result of injury. In these circumstances, the sedative and anxiolytic properties of the drug also may promote relaxation and relieve tension associated with the condition. 

Myositis (inflammation of voluntary muscle) with or without increased CK, and muscle weakness, occur rarely. These conditions may progress to frank rhabdomy-olysis and renal impairment. 

Ataxia Lack of coordination of voluntary muscles unsteady balance. 

Some sedative-hypnotics, particularly members of the carbamate (eg, meprobamate) and benzodiazepine groups, exert inhibitory effects on polysynaptic reflexes and internuncial transmission and at high doses may also depress transmission at the skeletal neuromuscular junction. Somewhat selective actions of this type that lead to muscle relaxation can be readily demonstrated in animals and have led to claims of usefulness for relaxing contracted voluntary muscle in muscle spasm (see Clinical Pharmacology). Muscle relaxation is not a characteristic action of zolpidem, zaleplon, and eszopiclone. 

By destroying the protein, the toxin prevents the release of the neurotransmitter acetylcholine from small packets at the ends of nerves by exocytosis. These nerves, attached to voluntary muscles, need acetylcholine to allow the flow of signals (impulses) between the nerve and the muscle. By preventing the release of acetylcholine, botulinum toxin blocks muscle contraction, causing paralysis and relaxation. The therapeutic action relies on relaxation of muscles, generally in the face. It is therefore used to treat blepharospasm (uncontrolled contractions) and stroke-induced permanent facial muscle contractions. 

The swimming of bacteria, the flowing motion of the ameba, the rapid contraction of voluntary muscles, and the slower movements of organelles and cytoplasm within cells all depend upon transduction of chemical energy into mechanical work. 

The hierarchy of muscle organization. A voluntary muscle such as the bicep is a composite of many fibers connected to tendons at both ends. Each muscle fiber is composed of several myofibrils that are surrounded by an electrically excitable membrane (sarcolemma). 

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