Skeletal Muscle Tissue Human Anatomy Sonya Schuh-Huerta, Ph.D.

1. Skeletal Muscle Tissue Human Anatomy Sonya Schuh-Huerta, Ph.D.

2. Muscle • Muscle = Latin word for “little mouse” • Muscle is the primary tissue in the: • Heart (cardiac muscle) • Walls of hollow organs (smooth muscle) • Skeletal muscle • All of your voluntary muscles

3. Skeletal Muscle: Basic Function • Voluntary movement • Locomotion • Manipulation of environment • Facial expression • Maintenance of posture • Joint stabilization • Heat generation • Muscle contractions produce heat  • helps maintain normal body temperature Coach Laura

4. Muscle Tissue: Unique Features • Contractility • Excitability • Extensibility • Elasticity

5. Skeletal Muscle Tissue • Skeletal muscle tissue: • Packaged into skeletal muscles • 640 muscles in human body • Makes up ~40% of body weight • 15% more dense than fat • Cells are termed muscle fibers & are striated! Coach Sean

6. Skeletal Muscle • Each muscle is an organ! • Consists mostly of muscle tissue • Skeletal muscle also contains: • Connective tissue • Blood vessels • Nerves

7. Basic Features of a Skeletal Muscle • Connective tissue & fascicles • Sheaths of connective tissue bind a skeletal muscle & its fibers together • Epimysium = dense regular connective tissue surrounding entire muscle • Perimysium = surrounds each fascicle (group of muscle fibers) • Endomysium = a fine sheath of connective tissue wrapping each muscle cell

8. Basic Features of a Skeletal Muscle • Connective tissue sheaths are continuous with tendons • When muscle fibers contract, pull is exerted on all layers of connective tissue & tendon • Sheaths provide elasticity & carry blood vessels & nerves

9. Skeletal Muscle: Levels of Organization Epimysium Bone Tendon Muscle fiber in middle of a fascicle Fascicle (wrapped by perimysium) Muscle fiber (1 cell) Endomysium (between individual muscle fibers) Muscle

10. Basic Features of a Skeletal Muscle • Nerves & blood vessels: • Each skeletal muscle supplied by branches of • 1 nerve • 1 artery • 1 or more veins

11. Capillary Network Surrounding Skeletal Muscle Fibers

12. Basic Features of a Skeletal Muscle • Nerves & blood vessels: • Nerves & vessels branch repeatedly • Smallest nerve branches serve: • Individual muscle fibers!!! • Neuromuscular junction signals the muscle to contract (also called motor endplate)

13. 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 attachment (directly attached to the bone) • Insertion = more movable attachment (attached to bone by a tendon)

14. Muscle contracting Origin by direct attachment Brachialis Tendon Insertion by indirect attachment Muscle Attachments

15. Muscle Attachments • Muscles attach to origins & insertions by CT • Fleshy attachments (direct)  CT fibers are short • Indirect attachments CT forms a tendon or aponeurosis • Bone markings present where tendons meet bones • Tubercles, trochanters, &crests…

16. Muscle At the Microscopic Level

17. Microscopic Structure of the Muscle FiberReview • Huge, long cylindrical cells  10–100 mm in diameter  many centimeters long • Multinucleated  cells formed by fusion of many embryonic cells • Striations visible Striations Nuclei Muscle Fiber (cell) Photomicrograph: Skeletal muscle at 300x

18. Thin (actin) filament Thick (myosin) filament I band A band I band M line Sarcomere Microscopic Structure of the Muscle Fiber Sarcolemma Myofibril Myofilament Nucleus => “Bag of pretzels”

19. Mechanism of Contraction • 2 major types of contraction • Concentric contraction muscle shortens to do work • Most common type of contraction • Eccentric contraction muscle generates force as it lengthens • Muscle acts as a “brake” to resist gravity • “Down” portion of a pushup is an example

20. The Sliding Filament Mechanism of Contraction • Explains concentric contraction • Myosin heads attach to thin filaments at both ends of a sarcomere • Then pull thin filaments toward the center of sarcomere • Thin & thick filamentsdo NOT shorten, the sarcomere shortens! • Initiated by release of Ca2+ from the SR! • Powered by ATP!

21. Ca2+ ATP The Sliding Filament Mechanism of Contraction Thin (actin) filament Movement Thick (myosin) filament Myosin head

22. The Sliding Filament Mechanism of Contraction

23. Sliding Filament Mechanism • Contraction changes the striation pattern • Fully relaxed  thin filaments partially overlap thick filaments • Contraction  thin filaments completely overlap thick filaments, & Z discs move closer together • Sarcomere shortens • I bands shorten, H zone disappears • A band remains same length

24. The Molecular Components • Ca2+ & ATP • myosin binds • ATP  ADP & Pi • powerstroke • new ATP binds • release & recovery stroke • myosin binds again

25. Sliding Filament Mechanism in Action! How your muscles contract… animation by www.encognitive.com

26. Functional Anatomy of Skeletal Muscle • Muscle extension • Muscle is stretched by a movement opposite of that which contracts it • Muscle fiber length & force of contraction • Greatest force produced when a fiber starts out slightly stretched • Myosin heads can pull along the entire length of the thin filaments  do more work & generate greater contraction!

27. The Role of Titin • Titin a spring-like molecule (protein) in sarcomeres (= strong like spider’s silk) • Resists overstretching • Holds thick filaments in place • Unfolds when muscle is stretched

28. Innervation of Skeletal Muscle • Motor neurons innervate skeletal muscle tissue • Neuromuscular junction the point where nerve ending & muscle fiber meet • Axon terminals ends of axons • Store neurotransmitters (Acetylcholine) • Synaptic cleft space between axon terminal & sarcolemma

29. The Neuromuscular Junction Myelinated axon of motor neuron Nerve impulse Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber 1 Nerve impulse stimulates the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft.

30. The Neuromuscular Junction 2 Ach binds to its receptor & causes Na+ to flow into the cell & the membrane to depolarize. This triggers massive Ca2+ to be released from the SR! Axon terminal of motor neuron Neuron Synaptic cleft Synaptic vesicle containing ACh t tubule Sarcoplasmic Reticulum Muscle fiber Ca2+

31. Motor Units Axon terminals at neuromuscular junctions Spinal cord Branching axon to motor unit Motor unit 1 Motor unit 2 Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers (b) Branching axon terminals form neuromuscular junctions, one per muscle fiber (photomicrograph 110). (a) Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle.

32. Quiz – What is this?

33. Types of Skeletal Muscle Fibers • Skeletal muscle fibers are categorized according to 2 characteristics • How they manufacture energy (ATP) • How quickly they contract! • Oxidative fibers produce ATP aerobically • Glycolytic fibers produce ATP anaerobically by glycolysis

34. Types of Skeletal Muscle Fibers • Skeletal muscle fibers are divided into 3 classes: • Slow oxidative fibers • Red slow oxidative fibers • Fast glycolytic fibers • White fast glycolytic fibers • Fast oxidative fibers • Intermediate fibers

35. Types of Skeletal Muscle Fibers • Slow oxidative fibers (slow twitch) • Red color due to abundant myoglobin • Obtain energy from aerobic metabolic reactions • Contain a large number of mitochondria • Richly supplied with capillaries • Contract slowly & resistant to fatigue • Fibers are small in diameter • => O2 store Dark meat?

36. Types of Skeletal Muscle Fibers • Fast glycolytic fibers (fast twitch) • Contain little myoglobin & few mitochondria • About twice the diameter of slow-oxidative fibers • Contain more myofilaments & generate more power!!! • Depend on anaerobic pathways (glycolysis) • Contract rapidly&tire quickly White meat?

37. Types of Skeletal Muscle Fibers • Fast oxidative fibers • Have an intermediate diameter • Contract quickly like fast glycolytic fibers • Are oxygen-dependent (aerobic pathways) • Have high myoglobin content & rich supply of capillaries • Somewhat fatigue-resistant • More powerful than slow oxidative fibers

38. Physiology of Skeletal Muscles • Predominately powered by oxidation of fats & carbs • Fast twitch muscles  operate with little O2 (anaerobic) to break down glucose & produce ATP (energy) for quick powerful bursts of contraction, but tire quickly… –By-product is lactic acid (“Feel the burn!”) –Lactic acid build-up (lactic acid threshold or blood lactate accumulation)

41. Disorders of Muscle Tissue • Muscle tissues experience few disorders • Heart muscle is exception (many problems) • Skeletal muscle • Remarkably resistant to infection! • Smooth muscle • Problems stem from external irritants (things we breathe in & eat)

42. Disorders of Muscle Tissue • Muscular dystrophy • A group of inherited muscle destroying diseases • Affected muscles enlarge with fat & connective tissue • Muscle fibers & muscles degenerate – can lead to paralysis & death • Types of muscular dystrophy • Duchenne muscular dystrophy • Myotonic dystrophy • Others….

43. 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, & headache

44. Muscle Tissue Throughout Life • Muscle tissue develops from myoblasts • Myoblasts fuse to form skeletal muscle fibers • Skeletal muscles contract by the 7th week of development Myotube (immature multinucleate muscle fiber) Embryonic mesoderm cells Myoblasts Satellite cell Mature skeletal muscle fiber Embryonic mesoderm cells undergo cell division (to increase number) and enlarge. Several myoblasts fuse together to form a myotube. 2 1 Myotube matures into skeletal muscle fiber. 3

45. Muscle Tissue Throughout Life • Satellite cells • Surround skeletal muscle fibers • Resemble undifferentiated myoblasts • Fuse into existing muscle fibers to help them grow • Cardiac muscle • Pumps blood in embryo end of week 3

46. What happens with age? • With increased age: • Amount of connective tissue increases in muscles • Number of muscle cells decreases • Loss of muscle mass with age: • Decrease in muscular strength is 50% by age 80 (sarcopenia = “muscle wasting”)

47. What happens with age? • Can your body produce new muscle cells? • NO. -Or very few. • Not many functional stem cells in muscle tissue

48. The Body’s Capacity for Regeneration: Adult Stem Cells • Good to excellent: • Epithelial tissue, bone, dense irregular connective tissue, blood, fat • Moderate: • Smooth muscle, dense regular connective tissue • Weak: • Skeletal muscle, cartilage • None or almost none: • Cardiac muscle, nervous tissue Human stem cells, S. Huerta

49. What can you do about this? • Exercise is Key!!! -For maintaining healthy muscles, bones, & joints well into old age! USE IT OR LOSE IT…

50. Effects of Exercise on the Muscles • So how do you gain more (or maintain) muscle? Especially, when you’re fighting an uphill battle with age? • Strength training, lifting weights, & core exercises (3X week+) • Healthy balanced diet (with protein!) • Proper recovery time & rest • This prevents loss of muscle cells, & increases the strength & size of the individual cells