The Muscular System

1. The Muscular System

2. The Muscular System Function of Muscles: • Produce movement • Maintain posture • Stabilize joints • Generate heat Three basic muscle types are found in the body • Skeletal muscle • Cardiac muscle • Smooth muscle

3. Characteristics of Muscles • Muscle cells are elongated, and are called muscle fibers (muscle cell = muscle fiber) • Contraction of muscles is due to the movement of microfilaments • All muscles share some terminology • Prefix myo refers to muscle • Prefix mys refers to muscle • Prefix sarco refers to flesh

4. Skeletal Muscle Characteristics Skeletal Muscle: • Most are attached by tendons to bones • Multinucleate – more than 1 nucleus • Striated – have visible banding • Voluntary – subject to conscious control • Cells are surrounded and bundled by connective tissue

5. Connective Tissue Wrappings of Skeletal Muscle • Endomysium – around single muscle fiber • Perimysium – around a fascicle (bundle) of fibers • Epimysium – covers the entire skeletal muscle • Fascia – on the outside of the epimysium Figure 6.1

6. Skeletal Muscle Attachments • Muscles attach to bones by: • Tendons – cord-like structure • Aponeuroses – sheet-like structure • Sites of muscle attachment • Bones • Cartilages • Connective tissue coverings

7. Smooth Muscle Characteristics Smooth Muscle: • Has no striations • Spindle-shaped cells • Single nucleus • Involuntary – no conscious control • Found mainly in the walls of hollow organs Figure 6.2a

8. Cardiac Muscle Characteristics Cardiac Muscle: • Has striations • Usually has a single nucleus • Joined to another muscle cell at an intercalated disc • Involuntary • Found only in the heart Figure 6.2b

9. Microscopic Anatomy of Skeletal Muscle • Sarcolemma – specialized plasma membrane • Sarcoplasmic reticulum – specialized smooth endoplasmic reticulum • Sarcomere – contractile unit of a muscle fiber Figure 6.3a

10. Microscopic Anatomy of Skeletal Muscle • Myofibril • Bundles of myofilaments – actin & myosin • Myofibrils are aligned to give distinct bands • I band = light band • A band = dark band Figure 6.3b

11. Microscopic Anatomy of Skeletal Muscle • Organization of the sarcomere • Thick filaments = myosin filaments • Made of the protein myosin • Thin filaments = actin filaments • Composed of the protein actin Figure 6.3c

12. Microscopic Anatomy of Skeletal Muscle • Myosin filaments have heads (extensions, or cross bridges) • At rest, there is a bare zone that lacks actin filaments • Sarcoplasmic reticulum (SR) – storescalcium used in contractions - During contraction myosin & actin slide past eachother Figure 6.3d

13. Nerve Stimulus to Muscles • Irritability –receive and respond to stimulus • Contractility – shorten when stimulated • Skeletal muscles must be stimulated by a nerve to contract • Motor unit • One neuron • Muscle cells stimulated by that neuron Figure 6.4a

14. Nerve Stimulus to Muscles • Neuromuscular junctions – association site of nerve and muscle (where they meet) Figure 6.5b

15. Nerve Stimulus to Muscles • Synaptic cleft – gap between nerve and muscle • Nerve and muscle do not touch • Area between nerve and muscle filled with interstitial fluid • Neurotransmitters travel across space Figure 6.5b

16. Transmission of Nerve Impulse to Muscle • Neurotransmitter acetylcholine – chemical released by nerve upon arrival of impulse • Acetylcholine attaches to receptors on the sarcolemma of muscle • Sarcolemma becomes permeable to sodium (Na+) • Sodium rushes into the cell and generates an action potential (stimulus) for contraction • Once started, muscle contraction cannot be stopped

17. The Sliding Filament Theory of Muscle Contraction • Activation by nerve causes myosin heads (crossbridges) to attach to binding sites on actin • Myosin heads then bind to the next site actin • This continued action slides myosin along actin • The result is that the muscle is shortened (contracted) Figure 6.7

18. The Sliding Filament Theory Figure 6.8

19. Contraction of a Skeletal Muscle Skeletal Muscle Contraction: • Muscle fiber contraction is “all or none” • Within a skeletal muscle, not all fibers may be stimulated during the same interval • Different combinations of muscle fiber contractions may give differing responses • Graded responses – different degrees of skeletal muscle shortening

20. Types of Graded Responses • Twitch • Single, brief contraction • Not a normal muscle function Figure 6.9a–b

21. Types of Graded Responses • Tetanus (summing of contractions) • One contraction is immediately followed by another • Contraction is smooth and sustained Figure 6.9a–b

22. Muscle Response to Strong Stimuli • Muscle force depends upon the number of fibers stimulated • More fibers contracting results in greater muscle tension • Muscles can continue to contract unless they run out of energy

23. Energy for Muscle Contraction Energy for Muscle Contraction: • First, muscles used stored ATP for energy • Bonds of ATP are broken to release energy (lasts only few seconds) • After this initial time, cells need to undergo aerobic or anaerobic respiration to produce energy for contraction

24. Energy for Muscle Contraction • Aerobic Respiration • Occurs in the mitochondria • Glucose is broken down to carbon dioxide, water and ATP (energy) • This is a slower reaction that requires continuous oxygen Figure 6.10b

25. Energy for Muscle Contraction • Anaerobic glycolysis • Breaks down glucose without oxygen • Glucose is broken down to pyruvic acid, then lactic acid, to produce some ATP • Not as efficient as aerobic respiration Build up of lactic acid Produces muscle fatigue Figure 6.10c

26. Types of Muscle Contractions Types of Muscle Contractions • Isotonic contractions • Myofilaments are able to slide past each other during contractions • The muscle shortens • Isometric contractions • Tension in the muscles increases • The muscle is unable to shorten Different fibers contract at different times to provide muscle tone

27. Muscles and Body Movements • Movement is attained due to a muscle moving an attached bone • Muscles are attached to at least two points • Origin – attachment to an immoveable bone • Insertion – attachment to a movable bone Figure 6.12

28. Effects of Exercise on Muscle • Results of increased muscle use • Increase in muscle size • Increase in muscle strength • Increase in muscle efficiency • Muscle becomes more fatigue resistant

29. Types of Ordinary Body Movements Types of body movements due to muscles • Flexion: brings bones closer together • Extension: opposite of flex, increases distance between bones • Rotation: movement around longitudinal axis (shaking head no) • Abduction: moving limb away from body • Adduction: opposite, move limb toward body • Circumduction: proximal end is stationary and distal end moves in a circle

30. Body Movements Figure 6.13a–c

31. Body Movements Figure 6.13d

32. Special Movements • Dorsifelxion: toes up, walking on heels • Plantar flexion: toes down, walking on toes • Inversion: turn sole medially (in toward body) • Eversion: turn sole laterally (away from body) • Supination: turn palm up (facing anteriorly) • Pronation: turn palm down (facing posteriorly) • Opposition: ability to move thumb to touch other fingers

33. Types of Muscles • Prime mover (Agonist) – muscle with the major responsibility for a certain movement • Antagonist – muscle that opposes or reverses a prime mover • Synergist – muscle that aids a prime mover in a movement and helps prevent rotation • Fixator – stabilizes the origin of a prime mover

34. Naming of Skeletal Muscles • Direction of muscle fibers: • Example: rectus (straight) • Relative size of the muscle: • Example: maximus (largest) • Location of the muscle: • Example: temporalis (temporal bone) • Number of origins: • Example: triceps (three heads) • Shape of the muscle • Example: deltoid (triangular) • Action of the muscle • Example: flexor and extensor

35. Head and Neck Muscles Figure 6.15

36. Trunk Muscles Figure 6.16

37. Deep Trunk and Arm Muscles Figure 6.17

38. Muscles of the Pelvis, Hip, and Thigh Figure 6.19c

39. Muscles of the Lower Leg Figure 6.20

40. Superficial Muscles: Anterior Figure 6.21

41. Superficial Muscles: Posterior Figure 6.22