Muscles

1. Muscles 12

2. The Three Types of Muscle Cylindrical shaped, multinuclei, straited, voluntary, fibers of different speeds Branched, uni-/binuclei, involuntary, striated, rhythmic contractions Spindled shaped, one nucleus, involuntary, non-straited, internal organs Figure 12-1a

3. Muscles: Summary

4. Skeletal Muscle • Usually attached to bones by tendons- sometimes attached directly to bone (pectoralis major) • Origin: closest to the trunk- usually does not move a joint when contracts. • Insertion: more distal- moves joint when contracts • Flexor: brings bones together- decreases angle at joint • Extensor: bones move away- increases angle at joint • Antagonistic muscle groups: flexor-extensor pairs- antagonistic muscles are usually in opposite sides.

5. Anatomy Summary: Skeletal Muscle Figure 12-3a (1 of 2)

6. Anatomy Review: Muscle Fiber Structure

7. Ultrastructure of Muscle Figure 12-3b

8. Anatomy Summary: Skeletal Muscle Figure 12-3a (2 of 2)

9. Ultrastructure of Muscle Myosin are motor proteins. 250 myosins join to form the thick filaments. The thin filament is made up of a string of actin with tropomyosin and tropnin attached. Titin and nebulin anchor and stabilize. Figure 12-3e

10. Ultrastructure of Muscle Sarcomere A band (c) Z disk Z disk Myofibril M line I band H zone (d) Titin Z disk Z disk M line M line Thin filaments Thick filaments Titin (f) Nebulin Troponin (e) Myosin heads Hinge region Myosin tail Tropomyosin G-actin molecule Actin chain Myosin molecule Actin and myosin form crossbridges Figure 12-3c–f

11. Summary of Muscle Contraction Muscle tension: force created by muscle Load: weight that opposes contraction Contraction: creation of tension in muscle Relaxation: release of tension Figure 12-7

12. Neuromuscular Junction: Overview • Terminal boutons- insulate the site of the neuromuscular juction and secrete supportive growth factors • Synaptic cleft- space between the axon terminal and the sarcolemma • Acetylcholine- neurotransmitter released involves calcium and binds to nicotinic receptors • Motor end plate- folds on the sarcolemma of the muscle • On muscle cell surface • Nicotinic receptors

13. Anatomy of the Neuromuscular Junction Figure 11-12 (1 of 3)

14. Anatomy of the Neuromuscular Junction Figure 11-12 (2 of 3)

15. Anatomy of the Neuromuscular Junction Figure 11-12 (3 of 3)

16. Mechanism of Signal Conduction • Axon terminal (of presynaptic cell) • Action potential signals acetylcholine release • Motor end plate – series of folds in the plasma membrane of the postsynaptic cell • Two acetylcholine bind • Opens cation channel • Na+ influx – K+ efflux • Membrane depolarized • Stimulates fiber contraction as a result in increased intracellular calcium concentration

17. Events at the Neuromuscular Junction Figure 11-13a

18. T-tubules and the Sarcoplasmic Reticulum Figure 12-4

19. Excitation-Contraction Coupling Somatic motor neuron releases ACh at neuro- muscular junction. 1 (a) 1 Axon terminal of somatic motor neuron ACh Muscle fiber Motor end plate Sarcoplasmic reticulum T-tubule Ca2+ DHP receptor Tropomyosin Z disk Troponin Actin M line Myosin head Myosin thick filament Figure 12-11a, step 1

20. Excitation-Contraction Coupling Somatic motor neuron releases ACh at neuro- muscular junction. Net entry of Na+ through ACh receptor-channel initiates a muscle action potential. 1 2 (a) 1 Axon terminal of somatic motor neuron ACh Muscle fiber potential Action K+ Action potential 2 Na+ Motor end plate Sarcoplasmic reticulum T-tubule Ca2+ DHP receptor Tropomyosin Z disk Troponin Actin M line Myosin head Myosin thick filament Figure 12-11a, steps 1–2

21. Excitation-Contraction Coupling Action potential in t-tubule alters conformation of DHP receptor. DHP receptor opens Ca2+ release channels in sarcoplasmic reticulum and Ca2+ enters cytoplasm. 3 4 5 Ca2+ binds to troponin, allowing strong actin- myosin binding. (b) 4 3 Ca2+ Ca2+ released 5 7 6 Myosin thick filament M line Distance actin moves Actin filament slides toward center of sarcomere. Myosin heads execute power stroke. 6 7 PLAY Animation: Muscular System: The Neuromuscular Junction Figure 12-11b

22. Changes in Sarcomere Length during Contraction PLAY Animation: Muscular System: Sliding Filament Theory Figure 12-8

23. Regulatory Role of Tropomyosin and Troponin In the relaxed state the myosin head is at 90o but it is unbound to actin because the binding sites on actin are blocked. Figure 12-10a

24. Regulatory Role of Tropomyosin and Troponin** (b) Initiation of contraction Ca2+ levels increase in cytosol. 1 4 Power stroke Ca2+ binds to troponin. 2 3 Tropomyosin shifts, exposing binding site on G-actin Pi ADP Troponin-Ca2+ complex pulls tropomyosin away from G-actin binding site. 3 TN Myosin binds to actin and completes power stroke. 4 5 2 G-actin moves Actin filament moves. 5 1 Cytosolic Ca2+ Figure 12-10b

25. The Molecular Basis of Contraction Tight binding in the rigor state. The crossbridge is at a 45° angle relative to the filaments. ATP binds to its binding site on the myosin. Myosin then dissociates from actin. 1 2 Myosin filament 45 ° ATP binding site Myosin binding sites ATP 3 2 4 3 2 1 4 1 G-actin molecule Figure 12-9, steps 1–2

26. The Molecular Basis of Contraction The ATPase activity of myosin hydrolyzes the ATP. ADP and Pi remain bound to myosin. The myosin head swings over and binds weakly to a new actin molecule. The cross- bridge is now at 90º relative to the filaments. 3 4 ADP 90° Pi Pi 3 2 4 1 3 2 4 1 Figure 12-9, steps 3–4

27. The Molecular Basis of Contraction Release of Pi initiates the power stroke. The myosin head rotates on its hinge, pushing the actin filament past it. At the end of the power stroke, the myosin head releases ADP and resumes the tightly bound rigor state. 5 6 ADP Pi 3 2 3 2 4 1 4 1 5 5 Actin filament moves toward M line. Figure 12-9, steps 5–6

28. The Molecular Basis of Contraction Myosin filament Tight binding in the rigor state. The crossbridge is at a 45° angle relative to the filaments. ATP binds to its binding site on the myosin. Myosin then dissociates from actin. 45° 1 2 ATP binding site 3 2 4 1 G-actin molecule ADP ATP 3 2 4 1 3 2 1 4 5 At the end of the power stroke, the myosin head releases ADP and resumes the tightly bound rigor state. 6 The ATPase activity of myosin hydrolyzes the ATP. ADP and Pi remain bound to myosin. 3 ADP Contraction- relaxation Pi Pi Sliding filament 3 2 4 1 3 2 4 1 5 Actin filament moves toward M line. 90° Pi 3 2 4 1 Release of Pi initiates the power stroke. The myosin head rotates on its hinge, pushing the actin filament past it. 5 The myosin head swings over and binds weakly to a new actin molecule. The crossbridge is now at 90º relative to the filaments. 4 Myosin binding sites Figure 12-9

29. Muscle Fatigue: Multiple Causes • Extended submaximal exercise • Depletion of glycogen stores • Short-duration maximal exertion • Increased levels of inorganic phosphate • May slow Pi release from myosin • Decrease calcium release • Potassium is another factor in fatigue

30. Length-Tension Relationships in Contracting Muscle The strength of the contraction is related to the length before the muscle contracts. Very short fibers do not produce much tension because there is a lot of overlap not allowing for much sliding and not many new crossbridges. At optimum lenght there is an optimum number of cross-bridges to there is optimum tension. At a longer length there is less overlap and less ability to produce optimal force Figure 12-16

31. Electrical and Mechanical Events in Muscle Contraction A twitch is a single contraction-relaxation cycle Figure 12-12

32. Summation of Contractions Stimuli is too far apart and allows the muscle to relax and lose tension If action potentials come in at a closer time they recruit more fibers and the additive effect results in increased muscle tension Figure 12-17a

33. Summation of Contractions The more stimulus the more fibers recruited until there is a maximum tension but is there is alot of time between the stimulus the muscle relaxes resulting in an unfused tetanus Figure 12-17c

34. Summation of Contractions Complete tetanus results when action potentials arrive close enough to not allow the muscle to relax. Maximum tension can only be sustained for a limited time because fatigue Figure 12-17d

35. Motor Units: Fine motor movements have more innervations

36. Mechanics of Body Movement • Isotonic contractions create force and move load- creates force and moves a load. • Concentric action is a shortening action- contraction that flexes the joint while working against a load • Eccentric action is a lengthening action- contraction that extends the joint while resisting a load • Isometric contractions create force without moving a load- the muscle produces tension and contracts but does not move the joint.

37. Isotonic and Isometric Contractions Figure 12-19

38. Muscle Contraction Duration of muscle contraction of the three types of muscle- in smooth muscle contraction and relaxation happen slower and can be sustained for a longer time. Figure 12-24

39. Smooth Muscle: Properties • Uses less energy-can maintain maximum tension while using only a small percentage of the total maximum cross bridge • Maintain force for long periods- allows organs to be tonically contracted and maintain tension for a long time (sphincter muscles) • Low oxygen consumption- allows for to maintain tension for a long time without fatiguing (bladder).

40. Smooth Muscle • Smooth muscle is not studied as much as skeletal muscle because • It has more variety-impossible to come up with a single muscle function model- special types for vascular, gastrointestinal, urinary, respiratory, reproductive, and ocular • Anatomy makes functional studies difficult-fibers within cells and muscle layers within organs run indifferent directions. • It is controlled by hormones, paracrines, and neurotransmitters • It has variable electrical properties- contraction is not triggered only action potential • Multiple pathways influence contraction and relaxation-acts as an integrating center to interpret mutiple excitatory and inhibitory signals that may arrive at the same time

41. IV. Smooth Muscle- A tissue formed by uninucleated spindle shaped cells found in six areas of the body: blood vessel walls, respiratory tract, digestive tubes, urinary organs, reproductive organs, and the eye. Smooth Muscle Locations

42. Smooth Muscle layer orientations

43. Cellular details of smooth muscle

44. Muscle Disorders • Muscle cramp: sustained painful contraction – hyperexcitability of the motor unit, countered with stretching • Overuse – excessive use that causes tearing in the muscle structures (fibers, sheaths, tendon connection) • Disuse- loss of muscle activity causes muscle atrophy because of loss of blood flow, can recover is disuse is less than a year • Acquired disorders – infectious diseases and toxin poisoning that lead to muscle weakness or paralysis • Inherited disorders - • Duchenne’s muscular dystrophy – muscle degenrates from pelvis up, happens most often in women, people live to be 20-30, die of respiratory failure • Dystrophin –links actin to proteins in cell membrane • McArdle’s disease – limited exercise tolerance • Glycogen to glucose-6-phosphate – enzyme missing thus muscles do not have the energy source available.