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Chapter 9

Chapter 9. Muscles and Muscle Tissue. J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D. Some Muscle Terminology. Myology: the scientific study of muscle muscle fibers = muscle cells myo, mys & sarco: word roots referring to muscle. Three Types of Muscle.

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Chapter 9

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  1. Chapter 9 Muscles and Muscle Tissue J.F. Thompson, Ph.D. & J.R. Schiller, Ph.D. & G. Pitts, Ph.D.

  2. Some Muscle Terminology Myology: the scientific study of muscle muscle fibers = muscle cells myo, mys & sarco: word roots referring to muscle

  3. Three Types of Muscle Skeletal, cardiac, and smooth muscle differ in: • Microscopic anatomy • Location • Regulation by the endocrine system and the nervous system

  4. Characteristics of Skeletal Muscle • Attached primarily to bones • Voluntary (conscious) control (usually) • Contracts quickly, tires easily (fatigable) • Allows for wide range of forces to be generated

  5. Skeletal Muscle Cells • Long, cylindrical cells • Striated (banded) • Multinucleate

  6. Characteristics of Cardiac Muscle • Forms most of heart wall (myocardium) • Involuntary (unconscious) • Autorhythmicity (contracts without external stimuli) • Fast contraction, non-fatigable • Beats at constant rhythm that can be modified by neural and hormonal signals

  7. Cardiac Muscle Cells • Branched cells • Uninucleate (may occasionally be binucleate) • Striated • Intercalated discs

  8. Characteristics of Smooth Muscle • Found in the walls of hollow internal structures (digestive, respiratory, reproductive tracts, blood vessels) • Arrector pili, pupil of the eye, etc. • Involuntary (unconscious) • Long, slow contractions, non-fatigable

  9. Smooth Muscle Cells • Nonstriated = smooth • Uninucleate

  10. Functions of Muscle Tissue • Motion: external (walking, running, talking, looking) and internal (heartbeat, blood pressure, digestion, elimination) body part movements • Posture: maintain body posture • Stabilization: stabilize joints – muscles have tone even at rest • Thermogenesis: generating heat by normal contractions and by shivering

  11. Functional Characteristics • Excitability (irritability) • the ability to receive and respond to a stimulus (chemical signal molecules) • Contractility • ability of muscle tissue to shorten • Extensibility • the ability to be stretched without damage • most muscles are arranged in functionally opposing pairs – as one contracts, the other relaxes, which permits the relaxing muscle to be stretched back • Elasticity • the ability to return to its original shape • Conductivity (impulse transmission) • the ability to conduct excitation over length of muscle

  12. Connective Tissue Wrappings of Skeletal Muscle Tissue • Superficial Fascia: "hypodermis" • Deep Fascia: lines body walls & extremities; binds muscle together, separating them into functional groups • Epimysium: wraps an entire muscle • Perimysium: subdivides each muscle into fascicles, bundles of 10-100 muscle fibers • Endomysium: wraps individual muscle fibers

  13. Nerve and Blood Supply • Each muscle fiber is supplied by a branch of a motor nerve • Each muscle is supplied by its own arteries and veins • Blood vessels branch profusely to provide each muscle fiber with a direct blood supply

  14. Attachments (to bone) • Origin: the part of a muscle attached to the stationary bone (relative to a particular motion) • Insertion: the part of a muscle attached to the bone that moves (relative to a particular motion) • Attachments are extensions of connective tissue sheaths beyond a muscle, attaching it to other structures • Direct attachment: epimysium fused to periosteum

  15. Attachment Structure • Indirect attachment: connective tissue wrappings gathered into a tendon or aponeurosis which attaches to an origin or insertion on bone • Tendon: cord (of dense regular connective tissue) • Aponeurosis: sheet (of dense regular connective tissue)

  16. Microscopic Anatomy of A Skeletal Muscle Fiber • Muscle fibers (cells): long, cylindrical, and multinucleate (individual muscle cells fuse during embryonic development) • Sarcolemma: the cell membrane of a muscle fiber • Sarcoplasm: the cytoplasm of a muscle fiber, rich in oxygen-storing myoglobin protein

  17. Myofibrils of A Skeletal Muscle Fiber • Myofibrils: bundles of contractile protein filaments (myofilaments) arranged in parallel, fill most of the cytoplasm of each muscle fiber; 100’s to 1000’s per cell • Sarcomeres:the repeating unit of contraction in each myofibril

  18. Organelles of A Skeletal Muscle Fiber • Mitochondria: provide the ATP required for contraction • Sarcoplasmic reticulum (smooth ER): stores Ca2+ ions which serve as second messengers for contraction

  19. Striations/Sarcomeres • Z discs (lines): the boundary between sarcomeres; proteins anchor the thin filaments; bisects each I band • A (anisotropic) band: overlap of thick (myosin) filaments & thin filaments • I (isotropic) band: thin (actin) filaments only • H zone: thick filaments only • M line: proteins anchor the adjacent thick filaments

  20. Myofilaments • Thin filaments: actin (plus some tropomyosin & troponin) • Thick filaments: myosin • Elastic filaments: titin (connectin) attaches myosin to the Z discs (very high mol. wt.)

  21. Sarcomeres • Components of the muscle fiber with myofilaments arranged into contractile units • The functional unit of striated muscle contraction • Produce the visible banding pattern (striations) • The myofilaments between two successive z discs

  22. Summary of Muscle Structure

  23. Myosin Protein • Rod-like tail with two heads • Each head contains ATPase and an actin-binding site; point to the Z line • Tails point to the M line • Splitting ATP releases energy which causes the head to “ratchet” and pull on actinfibers

  24. Thick (Myosin) Myofilaments • Each thick filament contains many myosin units woven together

  25. Thin (Actin) Myofilaments Two G actin strands are arranged into helical strands • Each G actin has a binding site for myosin • Two tropomyosin filaments spiral around the actin strands • Troponin regulatory proteins (“switch molecules”) may bind to actin and tropomyosin & have Ca2+binding sites

  26. Muscle FiberTriads • Triads: 2 terminal cisternae + 1 T tubule • Sarcoplasmic reticulum (SER): modified smooth ER, stores Ca2+ ions • Terminal cisternae: large flattened sacs of the SER • Transverse (T) tubules: inward folding of the sarcolemma

  27. Regulation of Contraction & The Neuromuscular Junction The Neuromuscular Junction: • Where motor neurons communicate with the muscle fibers • Composed of an axon terminal & motor end plate • Axon terminal: end of the motor neuron’s branches (axon) • Motor end plate: the specialized region of the muscle cell plasma membrane adjacent to the axon terminal

  28. The Neuromuscular Junction: • Synapse: point of communication is a small gap • Synaptic cleft: the space between axon terminal & motor end plate • Synaptic vesicles: membrane-enclosed sacs in the axon terminals containing the neurotransmitter

  29. The Neuromuscular Junction: • Neurotransmitter: the chemical that travels across the synapse, i.e., acetylcholine, ACh) • Acetylcholine (ACh) receptors: integral membrane proteins which bind ACh

  30. axonal terminal motor end plate Generation of an Action Potential(Excitation) • Binding of neurotransmitter (ACh) causes the ligand-gated Na+ channels to open • Opening of the Na+ channels depolarizes the sarcolemma (cell membrane)

  31. Generation of an Action Potential • Initial depolarization causes adjacent voltage-gated Na+ channels to open; Na+ ions flow in, beginning an action potential • Action potential: a large transient depolarization of the membrane potential • transmitted over the entire sarcolemma (and down the T tubules)

  32. Generation of an Action Potential • Repolarization: the return to polarization due to the closing voltage-gated Na+ channels and the opening of voltage gated K+ channels • Refractory period: the time during membrane repolarization when the muscle fiber cannot respond to a new stimulus (a few milliseconds) • All-or-none response: once an action potential is initiated it results in a complete contraction of the muscle cell

  33. Excitation-Contraction Coupling • The action potential (excitation) travels over the sarcolemma, including T-tubules • DHP receptors serve as voltage sensors on the T-tubules and cause ryanodine receptors on the SR to open and release Ca2+ ions • And now, for the interactions between calcium and the sarcomere…

  34. The Sliding Filament Model of Muscle Contraction • Thin and thick filaments slide past each other to shorten each sarcomere and, thus, each myofibril • The cumulative effect is to shorten the muscle

  35. http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#SKELETALhttp://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#SKELETAL • This simulation of the sliding filament model can also be viewed on line at the web site below along with additional information on muscle tissue

  36. Calcium (Ca2+) The “on-off switch”: allows myosin to bind to actin off on

  37. Calcium Movements Inside Muscle Fibers Action potential causes release of Ca2+ ions (from the cisternae of the SR) Ca2+ combines with troponin, causing a change in the position of tropomyosin, allowing actin to bind to myosin and be pulled (“slide”) Ca2+ pumps on the SR remove calcium ions from the sarcoplasm when the stimulus ends

  38. The Power Stroke & ATP • Cross bridge attachment. myosin binds to actin • The working stroke. myosin changes shape (pulls actin toward it); releases ADP + Pi • Cross bridge detachment. myosin binds to new ATP; releases actin

  39. The Power Stroke & ATP 4. "Cocking" of the myosin head. ATP hydrolyzed (split) to ADP + Pi; provides potential energy for the next stroke

  40. The “Ratchet Effect” Power Stroke Attach Repeat steps 1-4: The “ratchet action” repeats the process, shortening the sarcomeres and myofibrils, until Ca2+ ions are removed from the sarcoplasm or the ATP supply is exhausted Repeat Release

  41. RATCHET EFFECT ANIMATION http://www.sci.sdsu.edu/movies/actin_myosin_gif.html

  42. Excitation-Contraction Coupling • The action potential (excitation) travels over the sarcolemma, including T-tubules • DHP receptors serve as voltage sensors on the T-tubules and cause ryanodine receptors on the SR to open and release Ca2+ ions • Ca2+ binds to troponin, causing tropomyosin to move out of its blocking position • Myosin forms cross bridges to actin, the power stroke occurs, filaments slide, muscle shortens • Calsequestrin and calmodulin help regulate Ca2+ levels inside muscle cells

  43. Destruction of Acetylcholine • Acetylcholinesterase: an enzyme that rapidly breaks down acetylcholine is located in the neuromuscular junction • Prevents continuous excitation (generation of more action potentials) • Many drugs and diseases interfere with events in the neuromuscular junction • Myasthenia gravis: loss of function at ACh receptors (autoimmune disease?) • Curare (poison arrow toxin): binds irreversibly to and blocks the ACh receptors

  44. MUSCLE CONTRACTION • One power stroke shortens a muscle about 1% • Normal muscle contraction shortens a muscle by about 35% • Cross bridge (ratchet effect) cycle repeats • continue repeating power strokes, continue pulling • increasing overlap of fibers; Z lines come together • About half the myosin molecules are attached at any time • Cross bridges are maintained until Ca2+ levels decrease • Ca2+ released in response to action potential delivered by motor neuron • Ca2+ATPase pumps Ca2+ ions back into the SR

  45. RIGOR MORTIS IN DEATH • Ca2+ ions leak from SR causing binding of actin and myosin and some contraction of the muscles • Lasts ~24 hours, then enzymatic tissue disintegration eliminates it in another 12 hours

  46. Skeletal Muscle Motor Units • The Motor Unit = Motor Neuron + Muscle Fibers to which it connects (Synapses)

  47. Skeletal Muscle Motor Units • The size of Motor Units varies: • Small - two muscle fibers/unit (larynx, eyes) • Large – hundreds to thousands/unit (biceps, gastrocnemius, lower back muscles) • The individual muscle cells/fibers of each unit are spread throughout the muscle for smooth efficient operation of the muscle as a whole

  48. The Myogram • Myogram: a recording of muscle contraction • Stimulus: nerve impulse or electrical charge • Twitch: a single contraction of all the muscle fibers in a motor unit (one nerve signal)    

  49. Myogram • Latent period: delay between stimulus and response • Contraction phase: tension or shortening occurs • Relaxation phase: relaxation or lengthening

  50. Muscle Twitches All or none rule: All the muscle fibers of a motor unit contract all the way when stimulated

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