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Muscular System Part B

Muscular System Part B. Muscular mechanics. The minimal or smallest amount of stimulation that causes the muscle to contract is called the threshold stimulus . When a muscle cell receives a threshold stimulus, it contracts to its full extent – an all-or-none response.

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Muscular System Part B

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  1. Muscular SystemPart B

  2. Muscular mechanics

  3. The minimal or smallest amount of stimulation that causes the muscle to contract is called the threshold stimulus. • When a muscle cell receives a threshold stimulus, it contracts to its full extent – an all-or-none response.

  4. Give a series of identical stimuli - series of twitch contractions with complete relaxation in between contractions • Strength of contractions increases slightly each time – staircase effect or treppe

  5. Time

  6. Latent period – Ca++ is released, filament movement takes up slack – 2 milliseconds • Contraction period – 10 – 100 milliseconds • Relaxation period - 10 – 100 milliseconds • Refractory period – time after a contraction until the muscle is able to respond to a second stimulus. • Skeletal muscle – 5 msec • Cardiac muscle – 300 msec (0.3 sec)

  7. When stimuli do not allow muscle to relax completely between contractions, the muscle contraction becomes sustained. • If stimulation is great enough, get a sustained contraction called a tetanic contraction or tetanus.

  8. Sustained contraction Tetanic contraction

  9. Muscle fiber length

  10. Whole muscle myogram • A brief, single stimulus results in a twitch contraction. • A twitch is a brief contraction of all the muscle fibers in a motor unit.

  11. When a motor neuron fires, all of its muscle fibers contract fully. • Some motor units are more easily stimulated than others. • If only some of the motor units in a muscle contract, the entire muscle contracts partially. • The process of adding more motor units for a greater muscle contraction is called recruitment or multiple motor unit summation.

  12. To prevent fatigue, there is asynchronous recruitment of motor units. • Recruitment varies with the type of muscle fibers. • Muscles maintain a firmness at rest called muscle tone .

  13. Types of muscle contraction • Isotonic contraction (iso = same, tonus = tension) results in movement at a joint • Because shortening of the muscle occurs it is called a concentric contraction. • When the muscle lengthens it is called an eccentric contraction.

  14. Isometric contraction (iso= same, metric = measure) the force of contraction changes, but the muscle length remains the same.

  15. Cardiac Muscle – similar to skeletal muscle in: • Striations – caused by organization of myofilaments • Contains troponin and tropomyosin – site of activation of cross-bridge activity by Ca++ • Clear length-tension relationship • Numerous mitochondria and myoglobin (for aerobic respiration) • T tubules and moderately well developed sarcoplasmic reticulum (T tubule at Z line)

  16. Cardiac Muscle – differs from skeletal muscle in: • Shorter, larger diameter than skeletal muscle • Branch, forming 3-D networks • Usually only one nucleus • Autorythmicity –influenced by nervous system and hormones • Sarcoplasm is more abundant with more mitochondria • Only one t-tubule per sarcomere

  17. Well developed S.R., but less than skeletal muscle; cisternae store less Ca++. • During contraction a lot of Ca++ enters cell from the extracellular fluid in the t-tubule and extracellular fluid around the cell, so extracellular calcium partially controls the strength and length of contraction. • Intercalated discs – desmosomes; gap junctions • Two networks – atria and ventricles – cells contract together linked by gap junctions - functional syncytium

  18. Cardiac muscle physiology • Contraction starts at the pacemaker or sinoatrial node. • Autorhythmicity • Contraction due in large part to influx of Ca++ from ECF • Resting potential of -90 mV • Opening of voltage-gated Na+ channels reverses polarity to +30 mV

  19. Membrane potential rapidly reverses due to influx of Na+ • Plateau phase lasts several hundred milliseconds due to slow influx of Ca++ (and slowing of exit of K+) • Repolarization is due to rapid out flow of K+ ions. • Remains contracted 10-15 times longer • Long refractory period • Allows for filling of heart chambers • Prevents tetanic contractions

  20. In skeletal muscle the amount of Ca2+ released is sufficient to bind all of the troponin molecules • In cardiac muscle only a portion of troponin has bound Ca2+; allows for changes in contractility • In cardiac muscle SR does not release enough Ca2+ to activate muscle contraction. • Ca2+ entering cell during plateau phase triggers release of calcium from SR (calcium-induced calcium release)

  21. DHP channels: RyR is 1:1 • Release of “calcium sparks” sum to trigger release • Increased cytosolic calcium

  22. Removal of Ca2+ from cytosol • Ca2+ATPase in SR runs continuously and is further activated by high cytoplasmic calcium levels (extracellular Ca++ that entered cell can be stored for next contraction) • Also Ca2+ATPase located in sarcolemma • Na+/Ca++ exchange proteins (3:1 ; secondary active transport)

  23. Effects of extracellular K+ on heart • Changes in K+ in ECF alter the concentration gradient across sarcolemma • Leads to ectopic foci and cardiac arrhythmias • Decrease in action potential leads to weak contractions and dilation of heart • At extremes, heart can stop

  24. Effects of extracellular Ca++ on heart • Rise in ECF Ca++ increases strength of contraction by prolonging plateau phase • Tends to contract spastically • Drugs can influence Ca++ movement across sarcolemma (calcium channel blockers, digitalis e.g.)

  25. Inotropy • Positive inotropes increase contractility of heart • Sympathetic nervous system stimulation • Catecholamine hormones (epinephrine) • Digitalis • Increased heart rate • Negative inotropes decrease contractility • Decreased heart rate • Coronary artery disease • Certain drugs (calcium channel blockers)

  26. Starling’s Law • Within certain physiological limits, an increase in the stretching of the ventricles causes an increase in the force of contraction of the heart. • This allows for instantaneous regulation of contraction for increases in blood entering heart

  27. Smooth muscle • Nonstriated and involuntary • Cells smaller than skeletal muscle cells • Spindle-shaped • Single nucleus • NO T tubules • Different arrangement of myofilaments • Thin, thick and intermediate filaments

  28. Smooth muscle • Thick and thin filaments not arranged in sarcomeres • Thick filaments are longer than in skeletal muscle • Thin filaments lack troponin • 10-15 thin filaments/ thick (skeletal 2:1) • Intermediate fibers act as cytoskeleton • Typically less SR than in skeletal muscle • Intermediate filaments attach to dense bodies ( act like Z discs)

  29. Intermediate fibers connect dense bodies • Thick- and thin-filament contractile units oriented slightly diagonally in a diamond-shaped lattice pattern • Contraction causes the lattice to decrease in length and expand from side to side.

  30. During contraction, the sliding thick and thin filaments generate tension that is transmitted to the intermediate filaments, which pull on the dense bodies in the sarcoplasm and those attached to the sarcolemma. • Isolated smooth muscle cells contract by twisting into a helical shape, but this is prevented in intact tissues due to their attachment to other cells.

  31. Gap Junctions • Often connect smooth muscle cells • May be temporary, and may be under hormonal control • The electrical joining of smooth muscle cells is the basis for classifying smooth muscle into two types: Visceral (single-unit) smooth muscle • Many cells acting together Multiunit smooth muscle • Cells contract in small groups

  32. Single-unit smooth muscle Multiunit Smooth Muscle

  33. Visceral (single-unit) smooth muscle • More common type • Wrap-around sheets • Fibers form networks that contract together • Connected by gap junctions • Some cells also have autorhythmicity • Largely responsible for peristalsis

  34. Multiunit Smooth Muscle • Individual fibers within motor units; few gap junctions • In walls of large arteries, large airways, arrector pili, iris muscles and ciliary body in eye. • Contracts only after stimulation by motor neuron or hormones

  35. Physiology of Smooth Muscle • Contractions start slower and last longer • Can shorten and extend to greater extent • Resting potential is much lower and can vary over time due to automatic cyclical changes in the rate at which Na+ is pumped across the membrane. • “Slow wave”

  36. Sodium is not the major carrier of current during an action potential, instead it is Ca2+ which enters through voltage-gated channels • Also have receptor-activated or chemically activated Ca2+ channels • Repolarization due to outflow of K+ though voltage-gated channels and some channels sensitive to intracellular Ca2+ levels

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