1. Chapter 11 Physiology of the Muscular System

2. Introduction Muscular system is responsible for moving the framework of the body In addition to movement, muscle tissue performs various other functions

3. General Functions Movement of the body as a whole or of its parts Heat production Posture

4. Function of Skeletal Muscle Tissue Characteristics of skeletal muscle cells –Excitability (irritability)— ability to be stimulated –Contractility— ability to contract, or shorten, and produce body movement –Extensibility— ability to extend, or stretch, allowing muscles to return to their resting length

5. Overview of the muscle cell Muscle cells are called fibers because of their threadlike shape –Sarcolemma—plasma membrane of muscle fibers –Sarcoplasmic reticulum (SR) Network of tubules and sacs found within muscle fibers Membrane of the sarcoplasmic reticulum continually pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release

6. –Muscle fibers contain many mitochondria and several nuclei –Myofibrils— numerous fine fibers packed close together in sarcoplasm –Sarcomere Segment of myofibril between two successive Z lines Each myofibril consists of many sarcomeres Contractile unit of muscle fibers Overview of the muscle cell

7. Striated muscle Dark stripes called A bands –light H zone runs across midsection of each dark A band Light stripes called I bands –dark Z line extends across center of each light I band Overview of the muscle cell

8. Tranverse tubules (T Tubules) T tubules extend across sarcoplasm at right angles to long axis of muscle fiber Formed by inward folds of sarcolemma Membrane has ion pumps that continually transport Ca++ ions inward from sarcoplasm Allow electrical impulses traveling along sarcolemma to move deeper into cell Overview of the muscle cell

11. Triad Triplet of tubes a T tubule sandwiched between two sacs of SR allows an electrical impulse traveling along a T tubule to stimulate the adjacent SR Overview of the muscle cell An electron micrograph of a section through a triad junction of a frog tonic fibre, showing a central t- tubular element flanked on either side by the sarcoplasmic reticulum.

12. Myofilaments –Each myofibril contains thousands of thick and thin myofilaments Myofilaments

13. Thick Filament Myofilaments Four different proteins make up myofilaments Thin Filament

14. Myofilaments Myosin –Makes up almost all the thick filament –Myosin “heads” are attracted to actin –Myosin “heads” attach to actin and form cross bridges Thick Filament

15. Actin—protein that forms two strands that twist around each other to form the thin filament –Myosin attaches to actin on the active sites –Contains tropomyosin and troponin Tropomyosin—protein that blocks the active sites on actin Troponin—protein that holds tropomyosin in place Myofilaments

16. –Thin filaments attach to both Z lines (Z disks) of a sarcomere and extend partway toward the center –Thick myosin filaments do not attach to the Z lines Myofilaments

17. Mechanism of contraction Excitation and contraction A skeletal muscle fiber remains at rest until stimulated by a motor neuron Neuromuscular junction (NMJ) — motor neurons connect to sarcolemma Neuromuscular junction is a synapse where neurotransmitter transmit signals

18. Acetylcholine (ACh)— neurotransmitter released into synaptic cleft that diffuses across gap, stimulates receptors, and initiates impulse in sarcolemma Nerve impulse travels over sarcolemma and inward along T tubules  triggers release of calcium ions Mechanism of contraction

19. Calcium binds to troponin  tropomyosin to shift and expose active sites on actin ACh  Calcium  troponin  tropomyosin Mechanism of contraction

20. –Excitation and contraction (cont.) Sliding filament model 1.When active sites on actin are exposed, myosin heads bind to them 2.Myosin heads bend, pulling the thin filaments past them 3.Each head releases, binds to next active site, and pulls again 4.Entire myofibril shortens Mechanism of contraction ACh  Calcium  troponin  tropomyosin  myosin  actin (contraction)

23. Relaxation Immediately after Ca ++ ions are released, SR begins actively pumping them back into sacs Ca ++ ions are removed from troponin molecules, shutting down contraction Mechanism of contraction

24. SR Actin Myosin

25. Hydrolysis of ATP yields energy required for muscular contraction Adenosine triphosphate (ATP) binds to myosin head  transfers its energy to myosin head to perform work of pulling thin filament during contraction Energy sources for muscle contraction

27. Catabolism by muscle fibers requires glucose and oxygen Aerobic respiration: occurs when adequate O 2 is available Anaerobic respiration: occurs when low levels of O 2  results in formation of lactic acid (muscle soreness) Glucose and oxygen supplied to muscle fibers by blood capillaries Skeletal muscle contraction produces waste heat that can be used to help maintain body temperature (HOMEOSTASIS) Energy sources for muscle contraction

28. Function of Skeletal Muscle Organs Muscles are composed of bundles of muscle fibers that are held together by fibrous connective tissue Motor unit: motor neuron plus the muscle fibers to which it attaches –Some motor units consist of only a few muscle fibers, whereas others consist of numerous fibers –Generally, the smaller the number of fibers in a motor unit, the more precise the movement available; the larger the number of fibers in a motor unit, the more powerful the contraction available