(10pt each) Apr 27, 2009 Name:____________________
1. Sketch a sarcomere and identify significant structures and constituent proteins.
The thin filaments of the I band are composed primarily of actin, in combination with the regulatory proteins troponin and tropomyosin and the structural protein nebulin.
The thick filaments of the A band are composed primarily of myosin, a hexamer of two heavy chains and four light chains, in combination with the structural/ruler protein titin.
2. Describe the cross bridge cycle.
Actin and myosin begin bound in the rigor state (upper left)
Binding ATP results in release of actin
The free myosin hydrolyzes ATP and undergoes a conformational change that allows myosin to re-associate with actin
Re-binding of S-1 to actin results in a conformational change that releases the hydrolized Pi.
Release of Pi allows the "power stroke" conformational change, opening the ATP binding site and releasing the ADP

3. Describe the major events of excitation-contraction coupling.
Acetylcholine is released from the nerve terminal
ACh diffuses across the synaptic cleft
ACh binds to its receptor on the muscle fiber, allowing Na
+
influx
Sodium influx causes local depolarization that opens voltage gated sodium channels
Action potential is propagated via voltage gated sodium channels
The depolarization is carried deep within the muscle by the T-tubules
Na
+
/K
+
antiporter restores resting potential
Voltage gated dihydropyrmidine receptors (DHPR) allow Ca++ influx from the extracellular space
DHPRs trigger ryanodine receptor to mediate release of Ca++ from sarcoplasmic reticulum
Ca
2+
binds troponin C causing conformational change in TnI/tropomyosin and allowing myosin binding
Myosin binds actin, generating force
SR calcium pump (SERCA) restores Ca
2+
homeostasis
4. Sketch the force-length relationship and relate its prominent features to the sliding filament theory.
Steep ascending limb: 1.3<L<1.6µm thick filaments interfere with Z-disk
Shallow ascending limb: 1.6<L<2.5µm opposing thin filament matricies interfere
Descending limb:
L>4.2µm
2.6<L<4.2µm decreasing overlap reduces force generation
lack of overlap prevents force generation

5. Sketch the force-velocity relationship, including both shortening and lengthening, and relate its shape to the crossbridge model.
Force decreases with increasing shortening velocity because the myosin binding sites on actin are moving relative to the myosin head. Because finite time is required for the crossbridge to attach, this means that the crossbridge length at which myosin bind to actin will be less than that required to stretch the crossbridge maximally, and reduces the transfer of chemical energy to elastic energy. In lengthening, the motion of the filaments adds additional stretch to the attached crossbridges, increasing the force in each one.
6. Describe one of the major muscle sensory organs and the reflex pathway associated with it.
Muscle spindles are specialized muscle fibers wrapped by a sensory axon. Nuclear bag and chain types can be differentiated by the organization of their nuclei. Firing of the spindle afferent depends on either stretch velocity (primary or Ia afferent) or stretch amplitude (secondary).
Spindles are responsible for the stretch reflex and reciprocal inhibition. The stretch reflex increases the activation of a muscle in response to an external stretch, producing a length-feedback servo. Reciprocal inhibition inhibits antagonist muscles through the 1a inhibitory interneuron, which reinforces length feedback.

(5 pt each)
7. Identify and explain a pathway connecting the physiological stresses associated with endurance training to the conversion of "fast" muscle to "slow."
We most associate the fast-to-slow transition with the NFAT and MEF2 transcription factors and their activation by calcineurin. Calcineurin is a calcium dependent phosphatase, and endurance training results in a persistent increase in intracellular calcium.
Endurance training
Î
[Ca i
2+
]
Î
Cn
Î
NFAT/MEF2
Î
Slow oxidative gene program
There are other possibilities, including oxidative stress, PGC1, and AMPK, but the calcium-calcineurin story was most emphasized in class.
8. Explain the pathway linking inactivation of the growth promoting protein Akt (PKB) to increased proteolysis.
Akt is a protein kinase, and among its targets is the FOXO transcription factor family. FOXOs promote transcription of the ubiquitin ligases MuRF and atrogin/MAFBx, but their transcriptional activity is repressed by phosphorylation. Therefore, inactivation of akt reduces FOXO phosphorylation, promoting transcription of MuRF & atrogin. These ubiquitin ligases target sarcomeric proteins and myogenic regulatory factors for degradation by the proteasome, and thus increase proteolysis. akt -| FOXO Î MuRF Î proteasome activity
9. Muscle growth depends on interaction between systemic and local factors. Explain how. IGF-1/insulin and mechanical loads cooperate in the control of protein synthesis
IGF-1 acts through the IGF-1 receptor to activate PI-3 kinase. Mechanical loads stress integrin adhesions, which activates the focal adhesion kinase (FAK), and
FAK activates PI-3 kinase. The two mechanisms converge at PI-3K, with subsequent activation of akt and mTOR, leading to phosphorylation of 4EBP, p70S6k, and the eEF2k, all of which increase protein translation rate

10. The inflammatory response to physical activity has both positive and negative consequences for muscle. Give an example of each.
Positive: removes damaged tissue, releases growth factors that stimulate cell growth, increases blood flow and nutrient delivery, antisepsis
Negative: produces damaging reactive oxygen species, releases cytokines that stimulate apoptosis, promotes connective tissue hypertrophy, produces pain
11. Why are the neuromuscular coupling proteins common venom toxin targets? Give an example, including toxin, its molecular target, and its physiological effect.
Immobilization is an effective means of prey capture. Neuromuscular coupling is a high-gain process, with a lot of amplification at each ion channel and receptor, so the whole process is highly susceptible to disruption.
Bungarotoxin (snake) – acetycholine receptor – flaccid paralysis
Latrotoxin (spider) – SNARE complex – blocks acetycholine release
Tetrodotoxin (fish) – voltage gated sodium channel/AP propogation
Saxitoxin (plankton/mollusc) – Na
V
/AP
Conotoxin (snail) – multiple ion channels – blocks depolarization & Ca2+ influx
Agitoxin (scorpion) – voltage gated potassium channel – prevents repolarization
Taicatoxin (snake) – voltage gated calcium channel – blocks Ca
2+
influx
12. Compare and contrast primary myoblasts with satellite cells
Both 1
°
myoblasts and satellite cells originate in the fetal somites and are committed to the myogenic lineage, which may be indicated by expression of the pax3/pax7 transcription factors. Both cell types migrate from the somites to the distal musculature, but the 1
°
do so long before the satellite cells. 1
°
myoblast migration, differentiation, and fusion is independent of innervation. These cells seem to establish the muscle structure, and are sometimes called "founder" myoblasts. They may be resistant to neurogenic phenotype modification.
Satellite cells migrate, late in development or even postnatal, to the sites of existing muscle, where they may not differentiate and fuse but rather withdraw from the cell cycle into a quiescent stage. A population of these cells persist throughout life, maintained under the basal lamina as reserve cells to facilitate repair and growth of muscle.

(10 pt)
13. A 12 week strength training program results in significant increases in muscle size and in myostatin at both the mRNA and protein levels (Willoughby, 2004). Briefly describe the phenotype of myostatin-deficient animals (eg: Belgian Blue cattle, mstn
-/mouse) and justify the correlation between mystatin and muscle size during
Willoughby's strength training program.
Myostatin deficient animals and humans have abnormally large muscles, because myostatin is one of the major inhibitors of myoblast proliferation.
There are two general themes by which the increase in this negative regulator of muscle size can be reconciled with the observed increase in muscle size. First is to claim that the increased myostatin is ineffective in some way, potentially by a greater increase in sequestering proteins, potentially by a down-regulation of its receptor or some required cofactor. Second is to claim that correlation is not causation, and the hypertrophied, myostatin-enhanced state represents a new equilibrium in a homeostatic feedback loop. ie: that the training increases muscle size via increased protein synthesis, and a feedback loop causes simultaneous upregulation of myostatin to prevent unlimited hypertrophy.