Muscular System: Histology and Physiology

1. Muscular System:Histology and Physiology Chapter 9

6. Muscular System Functions • Body movement • Maintenance of posture • Respiration • Production of body heat • Communication • Constriction of organs and vessels • Heart beat

7. Criteria for Naming Muscles • Shape: romboideus, trapezius, biceps • Location: pectoralis (chest) intercostal (ribs) • Attachment: zygomaticus, sternocleidomastoid • Size: maximus, minimus, brevis, longis • Orientation of fibers: rectus (straight), oblique (slanting) • Relative position (lateral, medial, internal, external) • Function: adductor, flexor, extensor, pronator

9. Properties of Muscle • Contractility • Ability of a muscle to shorten with force • Excitability • Capacity of muscle to respond to a stimulus • Extensibility • Muscle can be stretched to its normal resting length and beyond to a limited degree • Elasticity • Ability of muscle to recoil to original resting length after stretched

10. Skeletal Muscle Cardiac Muscle Smooth Muscle

12. Skeletal Muscle Structure • Muscle fibers or cells • Develop from myoblasts • Numbers remain constant • Hypertrophy – increase in the size of each fiber. • Connective tissue • Nerve and blood vessels

14. Connective Tissue, Nerve, Blood Vessels • Connective tissue • External lamina • Endomysium • Perimysium • Fasciculus • Epimysium • Fascia • Binds adjacent muscles or overlying skin. • Nerve and blood vessels • Abundant

16. Parts of a Muscle

17. Structure of Actin and Myosin

18. Components of Sarcomeres

21. Sliding Filament Model • Actin myofilaments sliding over myosin to shorten sarcomeres • Actin and myosin do not change length • Shortening sarcomeres responsible for skeletal muscle contraction • During relaxation, sarcomeres lengthen

22. Sarcomere Shortening

25. Physiology of Skeletal Muscle • Nervous system • Controls muscle contractions through action potentials • Resting membrane potentials • Membrane voltage difference across membranes (polarized) • Inside cell more negative and more K+ • Outside cell more positive and more Na+ • Must exist for action potential to occur

26. Ion Channels • Types • Ligand-gated • Example: neurotransmitters • Voltage-gated • Open and close in response to small voltage changes across plasma membrane

27. Action Potentials • Phases • Depolarization • Inside plasma membrane becomes less negative • Repolarization • Return of resting membrane potential • All-or-none principle • Like camera flash system • Propagate • Spread from one location to another • Frequency • Number of action potential produced per unit of time

32. Action Potential Propagation

33. Neuromuscular Junction • Synapse or NMJ • Presynaptic terminal • Synaptic cleft • Postsynaptic membrane or motor end-plate • Synaptic vesicles • Acetylcholine: Neurotransmitter • Acetylcholinesterase: A degrading enzyme in synaptic cleft

34. Function of Neuromuscular Junction

35. Excitation-Contraction Coupling • Mechanism by which an action potential causes muscle fiber contraction • Involves • Sarcolemma • Transverse or T tubules • Terminal cisternae • Sarcoplasmic reticulum • Ca2+ • Troponin

36. Action Potentials and Muscle Contraction

37. Cross-Bridge Movement

41. Muscle Twitch • Muscle contraction in response to a stimulus that causes action potential in one or more muscle fibers • Phases • Lag or latent • Contraction • Relaxation

42. Stimulus Strength and Muscle Contraction • All-or-none law for muscle fibers • A motor unit contracts with a consistent force in response to each action potential • Sub-threshold stimulus • Threshold stimulus • Stronger than threshold • Motor units • Single motor neuron and all muscle fibers that it innervates • Graded for whole muscles • Strength of contractions range from weak to strong depending on stimulus strength

43. Multiple Motor Unit Summation • A whole muscle contracts with a small or large force depending on number of motor units stimulated to contract • Muscle performing delicate and precise movements have motor units with smaller numbers of fibers

44. Multiple-Wave Summation • As frequency of action potentials increase, frequency of contraction increases • Incomplete tetanus • Muscle fibers partially relax between contraction • Complete tetanus • No relaxation between contractions • Multiple-wave summation • Muscle tension increases as contraction frequencies increase • Due to increased calcium concentration around myofibrils and more complete stretching of muscle elastic elements

45. Treppe • Increase in the force of contraction during the first few contractions of a rested muscle. • Occurs in muscle rested for prolonged period • Each subsequent contraction is stronger than previous until all equal after few stimuli • Due to Ca++ ion levels around myofibrils and increased temperature of muscle • Enzymes for muscle contraction respond more effectively at higher temperature.

46. Types of Muscle Contractions • Isometric: No change in length but tension increases • Postural muscles of body • Isotonic: Change in length but tension constant • Concentric: Overcomes opposing resistance and muscle shortens • Eccentric: Tension maintained but muscle lengthens • Muscle tone: Constant tension by muscles for long periods of time

47. Muscle Length and Tension

48. Fatigue • Decreased capacity to work and reduced efficiency of performance • Usually follows a period of activity • Types • Psychological (in CNS) • Depends on emotional state of individual • Perception that muscle is too tired ( • Home court advantage • Muscular • Results from ATP depletion in muscle • Synaptic • Occurs in NMJ due to lack of acetylcholine

49. Energy Sources • ATP provides immediate energy for muscle contractions from 3 sources • Creatine phosphate • During resting conditions stores energy to synthesize ATP • Exhausted quickly (10-15 sec.) • Anaerobic respiration • Occurs in absence of oxygen and results in breakdown of glucose to yield ATP and lactic acid • Aerobic respiration • Requires oxygen and breaks down glucose to produce ATP, carbon dioxide and water • More efficient than anaerobic • Oxygen Debt • After anaerobic respiration, aerobic respiration is higher than normal to replace creatine phosphate and convert lactic acid to glucose.

50. Slow and Fast Fibers • Slow-twitch or high-oxidative • Contract more slowly, smaller in diameter, well developed blood supply, more mitochondria and high myoglobin content, more fatigue-resistant than fast-twitch • Fast-twitch or low-oxidative • Respond rapidly to nervous stimulation, less blood supply, fewer and smaller mitochondria, lower myoglobin content than slow-twitch, fatigue easily. • Two types: • Fast twitch fatigable fibers • Fast twitch fatigue resistant (highly trained muscle) • Distribution of fast-twitch and slow twitch • Most muscles have both but varies for each muscle