Chapter 10: Muscular Tissue

1. Chapter 10: Muscular Tissue

2. Objectives How do entire muscles contract? Where does skeletal muscle get its energy? How does aging affect skeletal muscles How does cardiac muscle function? How does smooth muscle function?

3. Putting it all together

4. Length-tension relationship Forcefulness of a muscle contraction depends on the length of a sarcomere before contraction Optimal zone of overlap Zone of overlap is short in stretched muscle which allows for fewer myosin heads to contact thin fibers Zone of overlap is lengthened so that actin filaments crumple at the M line then myosin heads cannot contact the actin. Resting fiber length is held very close to optimum by their attachment via tendons to bone.Zone of overlap is short in stretched muscle which allows for fewer myosin heads to contact thin fibers Zone of overlap is lengthened so that actin filaments crumple at the M line then myosin heads cannot contact the actin. Resting fiber length is held very close to optimum by their attachment via tendons to bone.

5. Control of muscle tension A single nerve impulse elicits a single muscular action potential. The force that a muscle fiber produces varies and�. Depends on the frequency of stimulation. Depends on the amount of stretch before a contraction is elicited. Depends on availability of nutrients: glucose, oxygen, and ATP.

6. Motor units Each muscle fiber has a singular NMJ. A motor unit is the sum of muscle fibers innervated by a somatic motor neuron (~150 muscle fibers). The muscle fibers in a motor unit contract in unison. They are NOT clustered together but are dispersed throughout the muscle. Muscles that require precise movement have many motor units. Muscles controlling the larynx have as few as 2 or 3 muscle fibers in a motor unit. Eye movements have 10 to 20 muscle fibers and muscles involved in power movements have 2-3K muscle fibers in a motor unit. The strength of a motor unit depends on the number of muscle fibers it innervates.Muscles controlling the larynx have as few as 2 or 3 muscle fibers in a motor unit. Eye movements have 10 to 20 muscle fibers and muscles involved in power movements have 2-3K muscle fibers in a motor unit. The strength of a motor unit depends on the number of muscle fibers it innervates.

7. Motor units

8. Twitch contraction A brief contraction of all the muscle fibers in a motor unit. Latent period Contraction period Relaxation period Record of a contraction is called a myogram. Latent period�muscle AP sweeps over the sarcolemma and Ca2+ is released from the SR. Contraction period lasts 10-100ms. Ca2+ binds to troponin, myosin binding sites are exposed and crossbridges are formed. Peak tension develops. Relaxation period�Ca2+ is transported back into the SR, myosin detaches from actin and the muscle relaxes. Refractory period is the time during which the muscle is not excitable and cannot respond. Skeletal muscle has a refractory period of 5ms. Cardiac muscle has one of 300ms.Record of a contraction is called a myogram. Latent period�muscle AP sweeps over the sarcolemma and Ca2+ is released from the SR. Contraction period lasts 10-100ms. Ca2+ binds to troponin, myosin binding sites are exposed and crossbridges are formed. Peak tension develops. Relaxation period�Ca2+ is transported back into the SR, myosin detaches from actin and the muscle relaxes. Refractory period is the time during which the muscle is not excitable and cannot respond. Skeletal muscle has a refractory period of 5ms. Cardiac muscle has one of 300ms.

9. Frequency of stimulation

10. Motor unit recruitment Recruitment is responsible for producing smooth muscle movements. Different motor units are activated at various times. Weak motor units are recruited first and stronger ones later as needed. Recruitment of different motor units also reduces fatigue.

11. Muscle tone Muscle tone is sustained by the alternation of active and inactive motor units in a constantly shifting pattern. Muscle tone keeps muscles firm but does not lead to movement. Muscles that lose muscle tone are called flaccid (hypotonia). Muscles that have increased muscle tone can be spastic or rigid (hypertonia).

12. Isometric and isotonic contractions Isometric�tension varies and muscle length is constant Isotonic contractions--tension remains constant in a muscle but muscle length varies.

13. Isometric contractions Isometric contraction of shoulder and arm muscles.

14. Concentric isotonic contractions Concentric isotonic contractions�tension used to generate movement exceeds the resistance of the object to be moved (muscle shortens).

15. Eccentric isotonic contractions The length of the muscle increases during the contraction ( biceps lengthen)

16. Muscle metabolism How muscle maintains ATP levels for contractions Creatine phosphate�creates enough energy for 15s. Anaerobic cellular respiration�creates enough energy for 30-40s. Aerobic cellular respiration�creates all the ATP for extended periods of exercise. Creatine phosphate is unique to muscle cells. IT stores the extra ATP made when muscles are at rest through the activity of creatine kinase which removes a phosphate group from ATP and leaves it as ADP. When ADP levels rise during contraction K transfers a P onto ADP. Anaerobic cellular metabolism. The process of glycolysis breaks down glucose into pyruvic acid. The process uses 2 ATP to produce 4 ATP for a gain of 2 ATP. Provides 30-40 s of maximal muscle activity. Anaerobic cellular respiration often produces lactic acid which diffuses into blood and is converted back to glucose in the liver. Aerobic Respiration relies on ATP produced in mitochondria. Pyruvic acid enters into the mitochondria and with oxygen produces 36 ATP from one molecule of glucose and 100ATP from one fatty acid molecule. Oxygen comes from the blood and it comes from myoglobin which stores O2Creatine phosphate is unique to muscle cells. IT stores the extra ATP made when muscles are at rest through the activity of creatine kinase which removes a phosphate group from ATP and leaves it as ADP. When ADP levels rise during contraction K transfers a P onto ADP. Anaerobic cellular metabolism. The process of glycolysis breaks down glucose into pyruvic acid. The process uses 2 ATP to produce 4 ATP for a gain of 2 ATP. Provides 30-40 s of maximal muscle activity. Anaerobic cellular respiration often produces lactic acid which diffuses into blood and is converted back to glucose in the liver. Aerobic Respiration relies on ATP produced in mitochondria. Pyruvic acid enters into the mitochondria and with oxygen produces 36 ATP from one molecule of glucose and 100ATP from one fatty acid molecule. Oxygen comes from the blood and it comes from myoglobin which stores O2

17. Muscle Fatigue The inability of a muscle to maintain force of contraction after prolonged activity. Inadequate release of Ca2+ from SR. Depletion of creatine phosphate. Not enough O2 Depletion of glycogen and other nutrients Buildup of lactic acid and ADP Failure of motor neurons to release ACh.

18. O2 consumption after exercise Oxygen debt or recovery oxygen uptake refers to the extra oxygen needed after exercise above resting levels. Used to restore metabolic conditions. Converts lactic acid to glycogen. Resynthesize creatine phosphate. Replace O2 bound by myoglobin. Elevated body temperatures increase metabolic reactions. Faster reactions use more ATP The heart and muscles are still working hard Tissue repair is increasing Faster reactions use more ATP The heart and muscles are still working hard Tissue repair is increasing

19. Types of skeletal muscle fibers Slow oxidative fibers Fast oxidative-glycolitic fibers Fast glycolytic fibers Fibers vary in fiber diameter, myoglobin content, mitochondria, capillaries, and colorFibers vary in fiber diameter, myoglobin content, mitochondria, capillaries, and color

20. Types of muscle fiber Fibre Type Type I fibres SO; Type II A fibres FOG; Type II B fibres FG Contraction time Slow Fast Very Fast Size of motor neuron Small Large Very Large Resistance to fatigue High Intermediate Low Activity Used for Aerobic Long term anaerobic Short term anaerobic Force production Low High Very High Mitochondrial density High High Low Capillary density High Intermediate Low Oxidative capacity High High Low Glycolytic capacity Low High High Major storage fuel TriglyceridesCP, GlycogenCP, GlycogenFibre Type Type I fibres SO; Type II A fibres FOG; Type II B fibres FG Contraction time Slow Fast Very Fast Size of motor neuron Small Large Very Large Resistance to fatigue High Intermediate Low Activity Used for Aerobic Long term anaerobic Short term anaerobic Force production Low High Very High Mitochondrial density High High Low Capillary density High Intermediate Low Oxidative capacity High High Low Glycolytic capacity Low High High Major storage fuel TriglyceridesCP, GlycogenCP, Glycogen

21. Slow oxidative fibers They are slow because the ATPase hydrolyzes the ATP on the myosin head slowly and contraction is slowed, Twitch contraction lasts 200-300ms and they take longer to reach peak tension. Fatigue resistant. Adapted for maintaining posture and endurance activities.

22. Fast oxidative-glycolytic fibers Hydrolyzation of ATP by ATPase is much faster. Twitch contraction lasts less than 100ms. FOG fibers contribute to walking and sprinting.

23. Fast glycolytic fibers Fibers contract strongly and quickly Hydrolyzation of ATP by ATPase is much faster. Twitch contraction lasts less than 100ms. FG fibers are used for short intense anaerobic movements such as weight lifting or throwing a ball. Recruited to provide force. They increase in size because of increase in synthesis of muscle proteins.

24. Exercise and types of skeletal muscle Genes determine the amount of each type of skeletal muscle fibers. Exercise can induce changes in the types of fibers. Endurance exercises can gradually transform some FG fibers into FOG fibers. Muscle fibers can increase in diameter, increase the amount of mitochondria, increase blood supply, and increase in strength.

25. Cardiac Muscle Cardiac muscle is similar to skeletal muscle except for presence of intercalated discs. They are composed of: Thickenings of the sarcolemma that use demosomes to connect cardiac fibers Gap junctions that allow the muscle action potential to travel to other muscle fibers. Contraction time is 10 to 15 times longer than skeletal muscle.

26. Cardiac Muscle

27. Cardiac Muscle Ca2+ arises from SR and interstitial fluid. Ca2+ from interstitial fluid has prolonged flow into cell Autorhythmic muscle fibers provide stimulation leading to contraction (75 pulses per minute). Mitochondria are larger and more numerous Cardiac muscle uses aerobic cellular respiration.

28. Smooth muscle Visceral (single-unit) smooth muscle tissue Found in wall of small arteries and veins. Found in stomach, intestines, uterus, and urinary bladder. Multiunit smooth muscle tissue Found in walls of large arteries, airways of lungs In arrector pili muscles attached to hair follicles Iris and ciliary body that adjusts focus of the lens

29. Visceral smooth muscle Fibers connect through gap junctions Stimulation of a single fiber through neurotransmitter, hormone or autorhythmic signal spreads to neighboring fibers�fibers contract in unison

30. Multiunit smooth muscle Individual fibers have their own motor unit terminal. There are a few gap junctions between neighboring fibers.

31. Microscopic anatomy of smooth muscle Each fiber has a single nucleus Sarcoplasm has both thick and thin filaments but not arranged in sarcomeres. Have intermediate filaments Have caveolae instead of transverse tubules. Caveolae store Ca2+ Have small amount of SR.

32. Microscopic anatomy of smooth muscle Thin and intermediate filaments attach to dense bodies. Dense bodies are attached to the sarcolemma. Smooth muscle fibers twist in a helix as they contract. Relaxation is produced by a rotation in opposite direction of contraction.

33. Smooth muscle contraction

34. Smooth muscle physiology Contractions start slowly, last longer and end slowly. Ca2+ from caveolae and SR flow into cytosol. Calmodulin binds to Ca+ in cytosol. Myosin light chain kinase (MLCK) is activated and uses ATP to attach a P to myosin head. Phosphorylation (attaching P) activates ATPase Myosin head binds to actin and initiates a contraction. Contractions are slow because of the time it takes for Ca2+ to flow to filaments in the center of the fiber. Myosin light chain kinase works slowly Prolonged presence of Ca in the cytosolContractions are slow because of the time it takes for Ca2+ to flow to filaments in the center of the fiber. Myosin light chain kinase works slowly Prolonged presence of Ca in the cytosol

35. Smooth muscle physiology Ca2+ combines with calmodulin The Ca2+ -CaM complex activates MLCK, which in turn phosphorylates the light chain. The phosphorylated myosin filament combines with the actin filament and the muscle contracts.

36. Smooth muscle physiology Smooth muscle maintains long term tone Provides steady pressure on GI contents Provides steady pressure in arterioles Smooth muscle fibers contract or relax in response to: Stretching Hormones Change in pH, O2, CO2 Temperature Ion concentrations

37. Smooth muscle contractions The bolus distends the gut, stretching its walls. Stretching stimulates nerves and the muscle becomes "more depolarized." When a slow wave passes over this area of sensitized smooth muscle, spike potentials form and contraction results. A coordinated contraction moves along the gut because the muscle cells are electrically coupled through gap junctions. The bolus distends the gut, stretching its walls. Stretching stimulates nerves in the wall of the gut to release neurotransmitters into smooth muscle at the site of distension - the membrane potential of that section of muscle becomes "more depolarized." When a slow wave passes over this area of sensitized smooth muscle, spike potentials form and contraction results. The contraction moves around and along the gut in the coordinated manner because the muscle cells are electrically coupled through gap junctions. The bolus distends the gut, stretching its walls. Stretching stimulates nerves in the wall of the gut to release neurotransmitters into smooth muscle at the site of distension - the membrane potential of that section of muscle becomes "more depolarized." When a slow wave passes over this area of sensitized smooth muscle, spike potentials form and contraction results. The contraction moves around and along the gut in the coordinated manner because the muscle cells are electrically coupled through gap junctions.

38. Regeneration of muscle Satellite cells can divide and fuse with muscle cells to facilitate growth and to repair damaged fibers. Cardiac muscle can regenerate under some circumstances. Smooth muscle fibers arise from pericytes. Pericytes are stem cells associated with blood capillaries and small veins. Smooth muscle has the greatest powers of regeneration.Pericytes are stem cells associated with blood capillaries and small veins. Smooth muscle has the greatest powers of regeneration.

39. Aging and muscular tissue Loss of skeletal muscle mass is replaced by fibrous connective and adipose tissue. Muscle decreases in strength, speed and flexibility. Slow oxidative fibers increase Aerobics and strength training can slow or reverse decline in muscular performance