Muscle contraction strength

Describe the factors that influence the strength of skeletal muscle contraction including multiple motor unit summation, asynchronous motor unit summation, frequency of nerve stimulation, length-tension relationship, and diameter of the muscle fiber... 

Number of muscle fibers contracting. As the number of contracting muscle fibers increases, the strength of skeletal muscle contraction increases. Two major factors determine the number of muscle fibers activated at any given... 

Many factors influence the contractile activity of smooth muscle. The strength of contraction of multiunit smooth muscle may be enhanced by stimulation of a greater number of cells, or contractile units. This mechanism is directly comparable to motor-unit recruitment employed by skeletal muscle. As the number of contracting muscle cells increases, so does the strength of contraction. However, this mechanism is of no value in single-unit smooth muscle. Due to the presence of gap junctions, all of the muscle cells in the tissue are activated at once. 

Cardiotonic agents are sometimes called positive inotropic drags, i.e. substances that enhance the strength of muscle contractions, and in this case those that enhance the strength of myocardium contraction. Cardiotonic drugs are intended for treating cardiac insufficiency. 

Muscle contraction responses to different patterns of nerve stimulation used in monitoring skeletal muscle relaxation. The alterations produced by a nondepolarizing blocker and depolarizing and desensitizing blockade by succinylcholine are shown. In the train of four (TOF) pattern, four stimuli are applied at 2 Hz. The TOF ratio (TOF-R) is calculated from the strength of the fourth contraction divided by that of the first. In the double burst pattern, three stimuli are applied at 50 Hz, followed by a 700 ms rest period and then repeated. In the posttetanic potentiation pattern, several seconds of 50 Hz stimulation are applied, followed by several seconds of rest and then by single stimuli at a slow rate (eg, 0.5 Hz). The number of detectable posttetanic twitches is the posttetanic count (PTC)., first posttetanic contraction. 

A dramatic example of how transporters can alter the efficacy and toxicity of a commonly prescribed drug is provided by the following scenario. Three 70-kg males with identical symptoms see a physician and are diagnosed with mild cardiac insufficiency. For all a standard treatment with digoxin, a cardiac glycoside that increases the strength of heart muscle contractions, is recommended and all follow the... 

Research into the control of glycolytic rate in muscle has revealed that enzyme activity may also be controlled by reversible formation of enzyme-F-actin complexes. F-actin is a polymer of actin molecules and makes up one of the two muscle filaments that participate in muscle contraction. It can be shown that enzymes, such as PK, readily bind to F-actin filaments under conditions of low ionic strength in vitro (Chan et al., 1986). Extrapolation of the conditions in the test tube to conditions found in cells suggests that a significant proportion of PK may be bound in vivo (Brooks and Storey, 1991 a). In the case of PK, binding decreases the enzyme activity by increasing the Km value for PEP. F-actin, therefore, acts like ATP and alanine in allosterically inhibiting the enzyme. 

The major structural property of a coiled coil superstructure of a-helices is its great mechanical strength. This property is applied very efficiently in both the fibrous proteins of skin and those of muscle. As you can imagine, these proteins must be very strong to carry out their functions of mechanical support and muscle contraction. 

Muscle strength implies the force or torque production capacity of muscles. However, to measure strength, the term must be operationally defined. One definition modified from Clarkson [2000] states that muscular strength is the maximal amount of torque or force that a muscle or muscle groups can voluntarily exert in one maximal effort, when type of muscle contraction, limb velocity, and joint angle(s) are specified. 

Solutions of F-actin and myosin at high ionic strength = 0.6) in vitro form a complex called actomyosin. The formation of the complex is reflected by an increase in viscosity and occurs in a deflnite molar ratio 1 molecule of myosin per 2 molecules of G-actin, the basic unit of the double-helical F-actin strand. It appears that a spike-like structure is formed, which consists of myosin molecules embedded in a backbone made of the F-actin double helix. Addition of ATP to actomyosin causes a sudden drop in viscosity due to dissociation of the complex. When this addition of ATP is followed by addition of Ca +, the myosin ATPase is activated, ATP is hydrolyzed and the actomyosin complex again restored after the ATP concentration decreases. Upon spinning of an actomyosin solution into water, flbers are obtained which, analogous to muscle flbers, contract in the presence of ATP. Glycerol extraction of muscle fibers removes all the soluble components and abolishes the semipermeability of the membrane. Such a model muscle system shows all the reactions of in vivo muscle contraction after the readdition of ATP and Ca +. This and similar model studies demonstrate that the muscle contraction mechanism is understood in principle, although some molecular details are still not clarified. 

Proteins can also seh-assemble into rather large aggregates. Collagen (Atlas P4), the most abundant protein in mammals and responsible for imparting mechanical strength to tissues and organs, consists of three long hehces wound around each other. The protein actin forms thin, rodlike filaments that, when associated with several copies of the protein myosin, play an important role in the mechanism of muscle contraction. The microtubules that participate in the... 

When evaluating muscle function or using muscle contractive force therapeutically, the clinician must be cognizant of the differences in strength and action between the superficial, powerfiil muscles and the deep, weaker muscles. 

The initial phase of whiplash injury is characterized by muscle contraction and limitation of head and neck motion. Soft tissues feel warm and boggy. These tissues require ice for the first 18 hours after the injury, to stop the microhemorrhages, and rest for 24 to 48 hours to allow healing to begin. Moist heat may be used at home after that time. A soft collar may be used to prevent unwanted motion of the neck for the first 24 to 48 hours. It should be removed after that time so that the cervical muscles do not lose strength. Areas adjacent to those most severely injured should be treated with osteopathic manipulation, particularly the cranium and the sacrum. Nonsteroidal anti-inflammatory medication may be given, if appropriate, to ease the discomfort. 

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