Skeletal Muscle: Structure and Function. Objectives. Draw and label the microstructure of skeletal muscle. Define satellite cells. How do these cells differ from the nuclei located within skeletal muscle fibers? List the chain of events that occur during muscular contraction. - PowerPoint PPT Presentation
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Scott K. Powers • Edward T. HowleyScott K. Powers • Edward T. Howley
Theory and Application to Fitness and PerformanceTheory and Application to Fitness and PerformanceSEVENTH EDITION
Chapter
Presentation prepared by:
Brian B. Parr, Ph.D.University of South Carolina Aiken
In SummaryIn Summary The human body contains over 400 voluntary
skeletal muscles, which constitute 40% to 50% of the total body weight. Skeletal muscle performs three major functions: (1) force production for locomotion and breathing, (2) force production for postural support, and (3) heat production during cold stress.
Individual muscle fibers are composed of hundreds of threadlike protein filaments called myofibrils. Myofibrils contain two major types of contractile protein: (1) actin (part of the thin filaments) and (2) myosin (major component of the thick filaments).
In SummaryIn Summary The region of cytoplasm surrounding an individual
nucleus is termed the myonuclear domain. The importance of the myonuclear domain is that a single nucleus is responsible for the gene expression for its surrounding cytoplasm.
Motor neurons extend outward from the spinal cord and innervate individual muscle fibers. The site where the motor neuron and muscle cell meet is called the neuromuscular junction. Acetylcholine is the neurotransmitter that stimulates the muscle fiber to depolarize, which is the signal to start the contractile process.
Step-by-Step Summary of Excitation-Step-by-Step Summary of Excitation-Contraction CouplingContraction Coupling
• Contraction1. At rest, myosin cross-bridges in weak binding state.2. Ca+2 binds to troponin, causes shift in tropomyosin to
uncover active sites, and cross-bridge forms strong binding state.
3. Pi released from myosin, cross-bridge movement occurs.
4. ADP released from myosin.5. ATP attaches to myosin, breaking the cross-bridge
and forming weak binding state. Then ATP binds to myosin, broken down to ADP+Pi, which energizes myosin. Continues as long as Ca+2 and ATP are present.
In SummaryIn Summary The process of muscular contraction can be best explained by
the sliding filament model, which proposes that muscle shortening occurs due to movement of the actin filament over the myosin filament.
The steps in muscular contraction are:§ The nerve impulse travels down the transverse tubules and
reaches the sarcoplasmic reticulum, and Ca+2 is released.§ Ca+2 binds to the protein troponin.§ Ca+2 binding to troponin causes a position change in
tropomyosin away from the “active sites” on the actin molecule and permits a strong binding state between actin and myosin.
§ Muscular contraction occurs by multiple cycles of cross-bridge activity. Shortening will continue as long as energy is available and Ca+2 is free to bind to troponin.
When neural activity ceases at the neuromuscular junction, Ca+2 is removed from the sarcoplasmic reticulum by the Ca+2 pump. This results in tropomyosin moving to cover the active site on actin, and the muscle relaxes.
Human skeletal muscle fiber types can be divided into three general classes of fibers based on their biochemical and contractile properties properties. Two categories of fast fibers exist, type IIx and type IIa. One type of slow slow fiber exists, type I fibers.
The biochemical and contractile properties characteristic of all muscle fiber types are summarized in table 8.1.
In SummaryIn Summary Although classifying skeletal muscle fibers
into three general groups is a convenient system to study the properties of muscle fibers, it is important to appreciate that human skeletal muscle fibers exhibit a wide range of contractile and biochemical properties. That is, the biochemical and contractile properties of type IIx, type IIa, and type I fibers represent a continuum instead of three neat packages.
Successful power athletes (e.g., sprinters) generally possess a large percentage of fast muscle fibers and, therefore, a low percentage of slow, type I fibers.
In contrast to power athletes, endurance athletes (e.g., marathoners) typically possess a high percentage of slow muscle fibers and a low percentage of fast fibers.
Age-Related Changes in Skeletal Age-Related Changes in Skeletal MuscleMuscle
• Aging is associated with a loss of muscle mass– 10% muscle mass lost between age 25–50 years– Additional 40% lost between age 50–80 years– Also a loss of fast fibers and gain in slow fibers– Also due to reduced physical activity
• Regular exercise training can improve strength and endurance– Cannot completely eliminate the age-related loss in
muscle mass
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In SummaryIn Summary Both endurance and resistance exercise training
have been shown to promote a fast-to-slow shift in skeletal muscle fiber types. However, this exercise-induced shift in fiber type is typically small and does not result in a complete transformation of all fast fibers (type II) into slow fibers (type I).
Prolonged periods of muscle disuse (bed rest, limb immobilization, etc.) result in muscle atrophy. This inactivity-induced atrophy results in a loss of muscle protein due to a reduction in protein synthesis and an increase in the rate of muscle protein breakdown.
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
Aging is associated with a loss of muscle mass. This age-related loss of muscle mass is low from age 25 to 50 years but increases rapidly after 50 years of age.
Regular exercise training can improve skeletal muscle strength and endurance in the elderly but cannot completely eliminate the age-related loss of muscle mass.
Alterations in Skeletal Muscle Due to Exercise, Inactivity, and Aging
In SummaryIn Summary The amount of force generated during muscular
contraction is dependent on the following factors: (1) types and number of motor units recruited, (2) the initial muscle length, and (3) the nature of the motor units’ neural stimulation.
The addition of muscle twitches is termed summation. When the frequency of neural stimulation to a motor unit is increased, individual contractions are fused together in a sustained contraction called tetanus.
The peak force generated by muscle decreases as the speed of movement increases. However, in general, the amount of power generated by a muscle group increases as a function of movement velocity.