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Short Answer and Critical Thinking Essay

15. Name and describe the four functional abilities of muscle that are the basis for muscle response. Contractibility- is the ability to shorten forcibly when adequately stimulated. This ability sets muscle apart from other tissue types. Extensibility- is the ability to extend or stretch. Muscle cells shorten when contracting, but they can stretch even beyond their resting length, when relaxed. Elasticity- is the ability of a muscle cell to recoil and resume its resting length after stretching. 16. Distinguish between (a) direct and indirect muscle attachments and (b) tendon and an aponeurosis. (a.) In direct or fleshy attachments, the epimysium of the muscle is fused to the periosteum of a bone or perichondrium of a cartilage. In indirect attachments, the muscle’s connective tissue wrappings extend beyond the muscle either as a ropelike tendon or as a sheet like aponeurosis.

The tendon or aponeurosis anchors the muscle to the connective tissue covering of a skeletal element (bone or cartilage) or to the fascia of other muscles. (b.) a tendon is a cord of dense fibrous tissue attaching muscle to the bone, and aponeurosis is fibrous or membranous sheet connecting a muscle and the part it moves. 17. (a) Describe the structure of a sarcomere and indicate the relationship of the sarcomere to myofilaments. (b) Explain the sliding filament model of contraction using appropriately labeled diagrams of a relaxed and contracted sarcomere. (a.) The region of a myofibril between two successive Z discs is a sarcomere. It averages 2 micrometers long and is the smallest contractile unit of a muscle fiber. It contains an A band flanked by half an I band at each end. Within each myofibril, the sarcomeres align end to end like boxcars in a train. (b) In a relaxed muscle fiber, the thin and thick filaments overlap only at the ends of the A band.

The sliding filament model of contraction states that during contraction the thin filaments slide pas the thick ones so that the actin and myosin filaments overlap to a greater degree. When the nervous system stimulates muscle fibers, the myosin heads on the thick filaments latch onto myosin-binding sites on the actin in the thin filaments an the sliding begins. These cross bridge attachments form and break several times during a contraction, acting like tiny ratchets to generate tension and propel the think filaments toward the center of the sarcomere. As this event occurs simultaneously in the sarcomeres throughout the cell, the muscle cell shortens. As the thin filaments slide centrally the Z discs to which they attach are pulled toward the M line.

Overall as a muscle cell shortens, the I bands shorten, the distance between successive Z discs shortens, and the H zones disappear and the contiguous A bands move closer together but their length does not change. 19. Explain how a slight (but smooth) contraction differs from a vigorous contraction of the same muscle. Use the concepts of multiple unit summation. A motor unit consists of one motor neuron and all the muscle fibers it innervates or supplies. When a motor neuron fires all the muscle it innervates contract. The number of muscle fibers per motor unit may be as several hundred or as few as four. The more motor units that are recruited, the greater the muscle force. Muscles that exert fine control have small motor units. By contrast, large weight bearing muscles whose movements are less precise have large motor units.

The muscle fibers in a single motor unit are not clustered together but are spread throughout the muscle. As a result, stimulation of a single motor unit causes a wear contraction of the entire muscle. 22. Describe the three distinct types of skeletal muscle fibers. The three types of muscle fibers are (1) fast glycolytic fibers (2) slow oxidative fibers and (3) fast oxidative fibers.

Most muscles contain a mixture of fiber types. 24. Describe some cause(s) of muscle fatigue and define this term clearly. Muscle fatigue is a state of physiological inability to contract even though the muscle still may be receiving stimuli. Although many factors appear to contribute to fatigue, its specific causes are not fully understood. Most experimental evidence indicates that fatigue is due to a problem in excitation contraction compiling or in rare cases, problems at neuromuscular junction. Several ionic imbalances contribute to muscle fatigue and also intense exercise of short duration.

Critical Thinking

1. Jim Fitch decided that his physique left much to be desired, so he joined a local health club and began to “pump iron” three times weekly. After 3 months he noticed that his arm and chest muscles were substantially large. Explain the structural and functional basis of these changes. Muscle hypertrophy— results mainly from high-intensity resistance exercise such as weight lifting, which pits muscles against high resistance or immovable forces. Here, strength not stamina is important. The additional muscle bulk largely reflects the increased size of individual muscle fibers rather than an increased number of muscle fibers. Collectively these changes promote significant increases in muscle strength and size.

4. Michael is answering a series of questions dealing with skeletal muscle cell excitation and contraction. In response to “ What protein changes shape when Ca2+ binds to it?” he writes “tropomysin.” What should he have responded and what result of that calcium ion binding? During excitation-contraction coupling, the tubules of SR release ca+2m but as mentioned it also moves into the cell from the extracellular space via membrane channels. In all striated muscle types, calcium activates myosin by interacting with regulatory molecule called calmodulin, a cytoplasmic calcium binding protein.


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