Skeletal Muscle Physiology

1. Skeletal Muscle Physiology

2. Muscular System Functions • Body movement (Locomotion) • Maintenance of posture • Respiration • Diaphragm and intercostal contractions • Communication (Verbal and Facial) • Constriction of organs and vessels • Peristalsis of intestinal tract • Vasoconstriction of b.v. and other structures (pupils) • Heart beat • Production of body heat (Thermogenesis)

3. Properties of Muscle • Excitability: capacity of muscle to respond to a stimulus • Contractility: ability of a muscle to shorten and generate pulling force • Extensibility: muscle can be stretched back to its original length • Elasticity: ability of muscle to recoil to original resting length after stretched

4. Types of Muscle • Skeletal • Attached to bones • Makes up 40% of body weight • Responsible for locomotion, facial expressions, posture, respiratory movements, other types of body movement • Voluntary in action; controlled by somatic motor neurons • Smooth • In the walls of hollow organs, blood vessels, eye, glands, uterus, skin • Some functions: propel urine, mix food in digestive tract, dilating/constricting pupils, regulating blood flow, • In some locations, autorhythmic • Controlled involuntarily by endocrine and autonomic nervous systems • Cardiac • Heart: major source of movement of blood • Autorhythmic • Controlled involuntarily by endocrine and autonomic nervous systems

5. Connective Tissue Sheaths • Connective Tissue of a Muscle • Epimysium. Dense regular c.t. surrounding entire muscle • Separates muscle from surrounding tissues and organs • Perimysium. Collagen and elastic fibers surrounding a group of muscle fibers called a fascicle • Endomysium. Loose connective tissue that surrounds individual muscle fibers • Collagen fibers of all 3 layers come together at each end of muscle to form a tendon or aponeurosis.

6. Nerve and Blood Vessel Supply • Motor neurons • stimulate muscle fibers to contract • Capillary beds surround muscle fibers • Muscles require large amts of energy • Extensive vascular network delivers necessary oxygen and nutrients and carries away metabolic waste produced by muscle fibers

7. Muscle Tissue Types

8. Skeletal Muscle • Long cylindrical cells • Many nuclei per cell • Striated • Voluntary • Rapid contractions

9. Basic Features of a Skeletal Muscle • Muscle attachments • Most skeletal muscles run from one bone to another • One bone will move – other bone remains fixed • Origin – less movable attach- ment • Insertion – more movable attach- ment

10. Basic Features of a Skeletal Muscle • Muscle attachments (continued) • Muscles attach to origins and insertions by connective tissue • Fleshy attachments – connective tissue fibers are short • Indirect attachments – connective tissue forms a tendon or aponeurosis • Bone markings present where tendons meet bones • Tubercles, trochanters, and crests

11. Skeletal Muscle Structure • Composed of muscle cells (fibers), connective tissue, blood vessels, nerves • Fibers are long, cylindrical, and multinucleated • Tend to be smaller diameter in small muscles and larger in large muscles. 1 mm- 4 cm in length • Develop from myoblasts; numbers remain constant • Striated appearance • Nuclei are peripherally located

12. Muscle Attachments

13. Antagonistic Muscles

14. Microanatomy of Skeletal Muscle

15. Parts of a Muscle

16. Motor Unit: The Nerve-Muscle Functional Unit • A motor unit is a motor neuron and all the muscle fibers it supplies • The number of muscle fibers per motor unit can vary from a few (4-6) to hundreds (1200-1500) • Muscles that control fine movements (fingers, eyes) have small motor units • Large weight-bearing muscles (thighs, hips) have large motor units

17. Motor Unit: The Nerve-Muscle Functional Unit • Muscle fibers from a motor unit are spread throughout the muscle • Not confined to one fascicle (bundle of skeletal muscle fibers surrounded by perimysium) • Therefore, contraction of a single motor unit causes weak contraction of the entire muscle • Stronger and stronger contractions of a muscle require more and more motor units being stimulated (recruited)

18. Motor UnitAll the muscle cells controlled by one nerve cell

19. Power Output: The Most Physiologically Relevant Marker of Performance Power = work / time = force x distance / time = force x velocity Peak power obtained at intermediate loads and intermediate velocities. Figure from Berne and Levy, Physiology Mosby—Year Book, Inc., 1993.

20. Most likely to cause muscle injury Three Potential Actions During Muscle Contraction: Biceps muscle shortens during contraction • shortening Isometric-muscle does not change its length • isometric • lengthening Biceps muscle lengthens during contraction

21. Spinal cord To increase force: • Recruit more M.U.s • Increase freq. (force –frequency) Recall The Motor Unit: motor neuron and the muscle fibers it innervates • The smallest amount of muscle that can be activated voluntarily. • Gradation of force in skeletal muscle is coordinated largely by the nervous system. • Recruitment of motor units is the most important means of controlling muscle tension. • Since all fibers in the motor unit contract simultaneously, pressures for gene expression (e.g. frequency of stimulation, load) are identical in all fibers of a motor unit.

22. Increased use: strength training Early gains in strength appear to be predominantly due to neural factors…optimizing recruitment patterns. Long term gains almost solely the result of hypertrophy i.e. increased size.

23. Performance Declines with Aging --despite maintenance of physical activity 100 80 60 Performance (% of peak) 40 Shotput/Discus Marathon 20 Basketball (rebounds/game) 0 20 30 40 50 60 10 Age (years) D.H. Moore (1975) Nature 253:264-265. NBA Register, 1992-1993 Edition

24. Number of motor units declines during aging AGE-ASSOCIATED ATROPHY DUE TO BOTH… Individual fiber atrophy (which may be at least partially preventable and reversible through exercise). Loss of fibers (which as yet appears irreversible). Campbell et al., (1973) J Neurol Neurosurg Psych 36:74-182.

25. Motor unit remodeling with aging Central nervous system Muscle Motor neuron loss AGING • Fewer motor units • More fibers/motor unit

26. Mean Motor Unit Forces: • FF motor units get smaller in old age and decrease in number • S motor units get bigger with no change in number • Decreased rate of force generation and POWER!! 225 200 Adult 175 Old 150 125 Maximum Isometric Force (mN) 100 75 50 25 0 FF FI FR S Kadhiresan et al., (1996) J Physiol 493:543-552. Motor Unit Classification

27. Muscle injury may play a role in the development of atrophy with aging. • Muscles in old animals are more susceptible to contraction- induced injury than those in young or adult animals. • Muscles in old animals show delayed and impaired recovery following contraction-induced injury. • Following severe injury, muscles in old animals display prolonged, possibly irreversible, structural and functional deficits.

28. Disorders of Muscle Tissue • Muscle tissues experience few disorders • Heart muscle is the exception • Skeletal muscle – remarkably resistant to infection • Smooth muscle – problems stem from external irritants

29. Disorders of Muscle Tissue • Muscular dystrophy – a group of inherited muscle destroying disease • Affected muscles enlarge with fat and connective tissue • Muscles degenerate • Types of muscular dystrophy • Duchenne muscular dystrophy • Myotonic dystrophy

30. Disorders of Muscle Tissue • Myofascial pain syndrome – pain is caused by tightened bands of muscle fibers • Fibromyalgia – a mysterious chronic-pain syndrome • Affects mostly women • Symptoms – fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

31. Aerobic Respiration • Needs oxygen for respiration • Glucose + Oxygen  Carbon Dioxide + Water + Energy • Energy=ATP

32. ATP

35. Anaerobic Respiration • Without oxygen for respiration • Glucose  Lactic Acid + Energy • Incomplete breakdown of glucose • 5% of energy released by aerobic respiration • Lactic Acid-produces an oxygen debt because oxygen is needed to oxidize lactic acid (liver) • REST

38. Muscle Fatigue • Lack of oxygen causes ATP deficit • Lactic acid builds up from anaerobic respiration

39. Muscle Fatigue

40. Muscle Atrophy • Weakening and shrinking of a muscle • May be caused • Immobilization • Loss of neural stimulation

41. Muscle Hypertrophy • Enlargement of a muscle • More capillaries • More mitochondria • Caused by • Strenuous exercise • Steroid hormones

42. Steroid Hormones • Stimulate muscle growth and hypertrophy

43. Muscle Tonus • Tightness of a muscle • Some fibers always contracted

44. Tetany • Sustained contraction of a muscle • Result of a rapid succession of nerve impulses

45. Tetanus

46. Refractory Period • Brief period of time in which muscle cells will not respond to a stimulus

47. Refractory

48. Refractory Periods Skeletal Muscle Cardiac Muscle

49. Isometric Contraction • Produces no movement • Used in • Standing • Sitting • Posture

50. Isotonic Contraction • Produces movement • Used in • Walking • Moving any part of the body