Musculoskeletal Systems. Anatomy: Arrangement of Cells Physiology: Contractions

Transcription

1 Musculoskeletal Systems Anatomy: Arrangement of Cells Physiology: Contractions

2 Characteristics of all muscle Contractile: it shortens Excitable: receives & responds to electrical signals Extensible: stretches Elastic: Returns to original length after extending

3 Skeletal Unbranched; striated; multinucleate; looooong Smooth Unbranched; unstriated; uninucleate Cardiac Branched; striated; intercalated discs

4 Arrangements Hydrostatic Endo Exo

5 Arrangement: tubes within tubes Many fascicles make up a muscle Many muscle fibers (cells) make up a fascicle Many myofibrils make up a muscle fiber All are surrounded by collagen and elastin

6 Arrangement: tubes within tubes Myofibrils are made of many sarcomeres stacked end to end. Individual sarcomeres contract to shorten a muscle

7 Sarcomeres Consist of interdigitating thick and thin protein filaments Thick = myosin thin = actin

8 Contraction Occurs when these proteins attach to and pull on each other. This interaction is ANCIENT!

9 Evolutionary history of muscles & muscle proteins Bacterial origin! Actin and myosin in Eukaryotes Tubulin and kinesin in Bacteria Sequences differ greatly, but AA sequence at key binding sites is conserved, and so produces same structure Plantae Archaea Amoebozoans Fungi Choanoflagellate Cnidaria Bilaterians Bacteria Skeletal & smooth muscle divergence Actin/Tubulin autopolymerizer Myosin/kinesin motor protein

10 Skeletal muscle cells have the usual cell structures BUT they have different names Sarcolemma: plasma membrane Sarcoplasm: cytoplasm Sarcoplasmic reticulum: SER; fuse and form terminal cisternae, which house Ca 2+ ions Unique to skeletal muscle cells Transverse tubules: conduct signal (Na 2+ ions) to contract

11 Working unit of skeletal muscle cell How is a signal transferred from neuron to muscle cell?

12 Signal transduction 1. AP presynaptic terminal; causes Ca 2+ channels to open 2. Ca 2+ ions enter & stimulate neurotransmitter release (ACh) from synaptic vessicles into synaptic cleft

13 3. ACh diffuses across synaptic cleft & binds to ACh receptors on Na + channel proteins in sarcolemma of muscle cell Signal transduction

14 Signal transduction 4. Influx of Na + ions produces depolarization of postsynaptic membrane; when threshold is reached, postsynaptic cell (muscle cell) fires an AP

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16 Exposure of attachment sight: Ca 2+ binds to troponin; allows tropomyosin to move, exposing myosin attachment sight Cross-bridge formation: Myosin heads attach to actin subunits.

17 Power Stroke: Stored E in myosin heads used to pull actin filament toward M line. ADP releases from myosin head New ATP phosphorylates myosin head: causes myosin to release actin Cross-bridge release: ATP broken down to ADP + P. Myosin head releases Recovery Stroke: Myosin heads return to resting position. E still stored in myosin head

18 Twitch Contraction Three phases Latent: AP reaches sarcolemma; SR releases Ca 2+ ; 2ms Contraction: Cross-bridge formation; Ca 2+, troponin; 15ms Relaxation: Ca 2 + uptake; tropomyosin covers actin; 25ms

19 Ennervation of whole muscles sin.html Notice the differences between Vertebrate and Arthropod ennervation patterns and motor unit arrangements

20 Recruitment & Summation We know single muscle cells contract when an AP arrives A single AP stimulus produces a single Twitch Twitches produce muscle tension How do twitches coordinate to achieve whole muscle contraction? They build Tension 1. Multiple motor units are stimulated (recruitment) 2. AP s arrive more frequently (summation)

21 What happens when AP frequency increases? relaxation phase Complete relaxation phase Incomplete TETANUS relaxation phase Eliminated

22 What about when multiple neurons are stimulated?

23 Motor units control tension 1 motor unit = all the muscle fibers controlled by a single motor neuron Size of motor units vary Why would it vary? Level of control required Muscles of the eye - precise control; 4-6 fibers per unit Muscles of the leg - gross control; 1-2k fibers per unit

24 Motor Units

25 Motor unit size & recruitment

26 What ultimately controls muscle tension within 1 fiber? Presence of Ca 2+ ions More Ca 2+ ions present = more to potentially bind to troponin Stronger contraction (more tension produced)

27 What fuels muscles? 1. Adenosine triphosphate (ATP) 2. Creatine Phosphate (CP) 3. Glucose 4. Fats

28 Replenishing ATP: Glucose metabolism In cytoplasm In mitochondria Fatty acids

29 Resting muscle Plenty of 0 2 around Fatty acids are burned through aerobic metabolism to make reserves of ATP, CP & glycogen

30 Moderate muscle activity Enough 0 2 around Fatty acids and glucose burned through aerobic metabolism to make ATP as it is used up to power contractions

31 Peak muscle activity Not enough 0 2 around Most (~ 66%) ATP produced via glycolysis. Lactic acid is a byproduct At high concentrations, interferes with actinmyosin binding & other cellular enzymes

32 Muscle fiber types = Continuum Fast twitch Primarily glycolytic Large diameter rapid contraction phase few mitochondria poorly vascularized huge ATP hogs Slow twitch Primarily oxidative Small diameter slow contraction phase tons of mitochondria heavily vascularized Lots of myoglobin (0 2 storage protein)

33 Fast vs. Slow

34 Muscle Adaptations Rapid adaptations Increase glycogen storage Increase ATP and CP storage Increase glucose transport proteins Increase lactate export proteins Increase neuromuscular coordination Long-term adaptations Increased mitochondrial density Increased capillary density Hypertrophy: increase in size of fibers Only caused by repeated exhaustive stimulation