Physiology of muscle performance - exercise therapy.pptx

Physiology of muscle performance - part of exercise therapy

1. Physiology of muscle performance Presented by Dr. Anaz A (MPT – MSS)

2. Contents Structure of skeletal muscle, Chemical & mechanical events during contraction &relaxation, Motor unit, Muscle fiber type, Force gradation. Causes of decreased muscle performance Physiologic adaptation to training: strength & power, endurance.

3. Structure of skeletal muscle Skeletal muscles are composed of muscle tissue (contractile) and connective tissue (no contractile). The muscle tissue has the ability to develop tension in response to chemical, electrical, or mechanical stimuli. The connective tissue , on the other hand, develops tension in response to passive loading.

4. Composition of a muscle fiber Contractile Proteins A skeletal muscle is composed of many thousands of muscle fibers. A single muscle contains many fascicles. The arrangement, number, size, and type of these fibers may vary from muscle to muscle, each fiber is a single muscle cell that is enclosed in a cell membrane called the sarcolemma Like other cells in the body, the muscle fiber is composed of cytoplasm, which in a muscle is called sarcoplasm.

5. The sarcoplasm contains myofibrils which are the contractile structures of a muscle fiber, and nonmyofibrillar structures such as ribosomes, glycogen, and mitochondria, which are required for cell metabolism Myofibril Thick myofilament – Myosin Thin myofilament - actin Interaction causes muscle contraction !!!

6. The thin myofilaments are formed of two chainlike strings of actin molecules wound around each other. Molecules of the globular protein troponin are found in notches between the two actin strings, and the protein tropomyosin is attached to each troponin molecule The troponin and tropomyosin molecules control the binding of actin and myosin myofilaments.

7. Each of the myosin molecules has globular enlargements called head groups (Fig. 3–2B).4 The head groups, which are able to swivel and are the binding sites for attachment to the actin, play a critical role in muscle contraction and relaxation When the entire myofibril is viewed through a microscope, the alternation of thick (myosin) and thin (actin) myofilaments forms a distinctive striped pattern, called striated muscle.

8. Non contractile protein provide a structural scaffold for the muscle fiber, involved in the transmission of force along the fiber and to adjoining fibers (e.g., desmin ). One has a particularly important role in maintaining the position of the thick filament during a muscle contraction and in the development of passive tension. Eg : Titin is a large protein that is attached along the thick filament and spans the gap from the thick filament to the Z discs

9. Chemical & mechanical events during contraction &relaxation

10. Muscle contraction Depolarization of the muscle membrane spreads through the muscle Causes muscle contraction (mechanical event)

11. Important structural details Sarcolemma Conduct AP over the surface of the muscle fibre T tubules Invagination of sarcolemmal membrane Conduct AP deep into the muscle fibre The sarcoplasmic reticulum (SR) specialized form of the endoplasmic reticulum of muscle cells dedicated to calcium ion (Ca 2+ ) handling, necessary for muscle contraction and relaxation.

12. Events AP spreads through t tubule into the muscle tissue Close to the sarcoplasmic reticulum DHP receptor ( dihydropyridine receptor) senses the membrane depolarization activates the ryanodine receptor ( RyR ) that releases Ca2+ from the SR Ca flows to the myoplasm in the vicinity of actin & myosin

13. Actin Composed of 3 different proteins: actin, tropomyosin & troponin Actin has myosin binding sites They are normally covered by tropomyosin Troponin contains Ca++ binding sites Myosin Myosin contains protein chains with bent heads which forms the cross bridges Myosin head contain ATPase. An ATP molecule is attached to it. ATP is broken down during sliding

14. Ca ++ binds to troponin Troponin shifts tropomyosin Myosin binding sites in actin filament uncovered Myosin head binds with actin Cross bridges form Filaments slide with ATP being broken down Muscle shortens New ATP occupies myosin head Myosin head detaches Filaments slide back Cycling continues as long as Ca is available

15. Walk-along theory of contraction Heads of myosin filament is known to forms cross bridges by attachment Then head bends causing actin filament to slide Then head detaches from the actin filament and walked to a new site in actin filament and attaches again This process continue to happen As if myosin head walk-along actin filament

16. Relaxation This occurs when Ca++ is removed from myoplasm by Ca++ pump located in the sarcoplasmic reticulum When Ca++ conc is decreased Troponin returns to original state Tropomyosin covers myosin binding sites Cross-bridge cycling stops

17. Motor unit The motor unit consists of the alpha motor neuron and all of the muscle fibers it innervates. The stimulus that the muscle fiber receives initiating the contractile process is transmitted through an alpha motor neuron The cell body of the neuron is located in the ventral horn of the spinal cord. The nerve cell axon extends from the cell body to the muscle, where it divides into either a few or as many as thousands of smaller branches. Each of the smaller branches terminates in a motor end plate that lies in close approximation to the sarcolemma of a single muscle fiber.

18. All of the muscle fibers on which a branch of the axon terminates are part of one motor unit, along with the cell body and the axon. The contraction of the entire muscle is the result of many motor units firing asynchronously and repeatedly.

19. Some motor units have small cell bodies, and others have large cell bodies. in the small-diameter units, a stimulus will take longer to reach the muscle fibers than it will in a unit with a large-diameter axon. The size of the motor unit is determined by the number of muscle fibers that it contains and the size of the motor nerve axon Muscles that are used to produce large increments of force and large movements usually have a predominance of large motor units, large cell bodies, and large diameter axons. The motor units of the small muscles that control eye motions may contain as few as nine muscle fibers, whereas the gastrocnemius muscles have motor units that contain about 2,000 muscle fibers

20. Recruitment of motor units Size principle of motor unit recruitment. The motor units with the small cell bodies and few motor fibers are recruited first by the nervous system and then, as force is increased, larger motor units are recruited This recruitment strategy is referred to as the size principle of motor unit recruitment Energy conserving . Small muscle – less energy Multitask muscles eg :biceps brachii type of muscle action

21. Muscle fiber type Three primary muscle fiber types will be referred to as type I (slow), type IIA (intermediate), and type IIX (fast) Each skeletal muscle in the body is composed of a combination of each of the three types of fibers, But variations exist among individuals in the percentage of each fiber type. The soleus muscle, contains up to 80% type i fibers . Muscles that have a relatively high proportion of type i fibers in relation to type ii fibers, are able to carry on sustained activity because the type i fibers do not fatigue rapidly (stability or postural muscles)

22. Muscles that have a higher proportion of the type II fibers, such as the hamstring and gastrocnemius muscles, are sometimes designated as mobility or nonpostural muscles. These muscles are involved in producing a large range of motion (ROM) of the bony components. Type II fibers produce slightly more force per fiber than type I fibers and are able to contract at a significantly higher velocity , thus producing a higher power output than type I fibers

23. Force gradation – Active tension Tension may be increased by increasing the frequency of firing of a motor unit or by increasing the number of motor units that are firing. Tension may be increased by recruiting motor units with a larger number of fibers. The greater the number of cross-bridges that are formed, the greater the tension. Muscles that have large physiological cross-sections are capable of producing more tension than are muscles that have small cross-sections. Tension increases as the velocity of active shortening decreases

24. Intrinsic and Extrinsic Factors Involving Active Muscle Tension The velocity of muscle contraction is affected by: recruitment order of the motor units: Units with slow conduction velocities are generally recruited first. type of muscle fibers in the motor units: Units with type II muscle fibers can develop maximum tension more rapidly than units with type I muscle fibers; rate of cross-bridge formation, breaking, and re-formation may vary. the length of the muscle fibers : Long fibers have a higher shortening velocity than do shorter fibers.

25. The magnitude of the active tension is affected by: size of motor units: Larger units produce greater tension. number and size of the muscle fibers in a cross-section of the muscle : The larger the cross-section, the greater the amount of tension that a muscle may produce. number of motor units firing : The greater the number of motor units firing in a muscle, the greater the tension. frequency of firing of motor units: The higher the frequency of firing of motor units, the greater the tension. sarcomere length: The closer the length is to optimal length, the greater the amount of isometric tension that can be generated. type of muscle contraction : eccentric contraction > > isometric contraction >> concentric contraction;