L12 - Physiology and regulation of skeletal muscle contraction Flashcards
Compare between skeletal, cardiac and smooth muscle: Myofibril structure.
Skeletal: Striated
Cardiac: Striated
Smooth:Non-striated (lack of sacromere)
Compare between skeletal, cardiac and smooth muscle: Shape
Skeletal: Long, cylindrical
Cardiac: Small, branched
Smooth: Spindle- shape
Compare between skeletal, cardiac and smooth muscle: Nucleus
Skeletal: Multi-nucleated, peripheral
Cardiac: Single, central
Smooth: Single, central
Compare between skeletal, cardiac and smooth muscle: T- tubules
Skeletal: Long
Cardiac: Short
Smooth: None
Describe the structure of thin filament myofibrils?
Attaches at Z line
F-actin strand with G-actin molecules / subunits (active site)
Troponin-tropomyosin complexes lie parallel to F-actin strands: tropomyosin covers active site on G actin
Describe the structure of thick filament myofibrils?
- Myosin (with head, tail and hinge)
- Titin which spansthedistancefromoneZdisktotheneighboringMline
What are the giant accessory proteins in myofilments?
Titin and Nebulin
Titin = provide elasticity and stabilize myosin
Nebulin = align actin
List the changes to a sacromere during contraction.
sarcomeres shorten simultaneously during contraction
A band stays the same width
I band gets smaller, H bands narrows
Z lines move closer together
How does zero Zone of overlap between myofilaments dictate the amount of force produced?
Too little overlap = contraction cannot occur
- thinandthickfilaments cannotinteractatall
What is the tension generated when the zone of overlap between myofilaments increases beyond the optimal point?
1) At short resting lengths, thin filaments extend across the centre of sacromere (M line)
» Interfere with normal orientation of myofilaments
» reduce tension production
2) When thick filaments contact the Z line, the sacromere cannot shorten
» Myosin heads cannot pivot
» No tension produced
Under what zone of overlap between myofilaments is maximum force generated?
zone of overlap is large but thin filaments do not extend across sarcomere’s center
> > all myosin heads can bind to actin
> > produce maximum tension
What prevents overstretching of sacromeres?
Extreme stretching of muscle fiber is normally prevented by:
Titin filaments (tie thick filaments to Z lines)
Surrounding connective tissues
Describe the role of Ca2+ in excitation-contraction coupling. Start with the NMJ.
1) Somatic motor neuron releases Ach, activate AchR and influx of Na+, initiate muscle AP
2) Action potential propagates down T-tubules
3) Change conformation of dihydropyridine (DHP) receptor (= voltage-gated L-type calcium channel
4) DHP receptor opens ryanodine (RyR) receptor (= calcium-releasing channel) in sarcoplasmic reticulum»_space; Ca2+ enters cytoplasm
5) Ca2+ binds to troponin»_space; allows actin-myosin binding (cross-bridge formation)»_space; Myosin head power stroke …etc
List the events that occur during cross-bridge cycles. Start with Ca2+ entering cytoplasm of sacromere
At rest, tropomyosin covers active binding sites on G-actin
1) Ca2+ increase in cytosol, binds to troponin
2) Troponin-Ca2+ complex rotates, pulls tropomyosin away to expose actin’s myosin-binding site
3) Myosin binds to actin (form cross-bridge)»_space; completes power stroke
5) Actin filament moves
Explain the role of ATP in the cross-bridge cycles in sacromeres?
1) Binding of ATP onto myosin head detaches cross-bridge
2) Hydrolysis of ATP into ADP + Pi causes myosin reactivation»_space; recock into higher energy position
3) Removal of inorganic phosphate (Pi)»_space; cross-bridge strengthening
4) Removal of ADP»_space; Power stroke
What is catabolized for ATP in resting muscle? What is this ATP used for?
Fatty acid from aerobic respiration in mitochondria
Used for building energy reserves of ATP in Creatine phosphate or Glycogen
What is catabolized for ATP in moderate activity muscle? What is this ATP used for?
Glucose (from glycogen) and fatty acids (from mitochondria) are catabolized
Needs adequate supply of oxygen.
Used for power contraction
What is catabolized for ATP in peak activity muscle? What is this ATP used for?
Mostly glucose from glycogen
Also from Creatine phosphate and aerobic mitochondrial activity
Produces lactate as by-product (muscle fatigue)
Which energy source can sustain the longest Duration of isometric tetanic contraction?
Glycogen through Glycolysis (anaerobic) and Aerobic metabolism
What causes Delayed Onset Muscle Soreness?
Eccentric contraction produces more soreness than concentric or isometric contraction
> > Small tears in sacrolemma permit enzymes and chemicals to stimulate nearby nociceptors
> > Tears in connective tiisue and tendon of skeletal muscle
Explain the latent period between muscle AP stimulus and the development of maximum tension?
Time needed for:
Conduction of action potential; and
Subsequent release of Ca2+ by sarcoplasmic reticulum
What is the difference between isotonic and isometric contraction?
Isotonic = Same tension
Isometric = Same length of muscle
What are the different types of contraction resulting from increasing frequency of stimuli?
Single twitch
Summation
Summation leading to Unfused tetanus
Summation leading to complete tetanus
What are the two types of isotonic contractions?
Concentric and Eccentric
