Muscle Contraction

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Muscle Contraction
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I. Skeletal Muscle
A. Muscle fiber
1. Sarcolemma
2. Sarcoplasm
3. Myofibrils contractile elements
a. Actin myofilament

• F actin strands

• tropomyosin

• troponin (T, I, C)

b. Myosin myofilament
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4. Sarcomere

• arrangement of myofibrils

a. Z disk attaches actin
b. I band actin myofilament
c. A band both actin and myosin
H zone only myosin
5. T Tubules

• invagination of sarcolemma

6. Sarcoplasmic Reticulum

• high conc. of calcium

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B. Signal transmission
1. Motor neuron
2. Presynaptic terminal
3. Endplate

• region of skeletal fiber where synapse occurs

4. Nicotinic receptor
C. Muscle Contraction
1. Action Potential -gt sarcolemma -gtT
tubules
2. T tubules -gt Sarcoplasmic Retic
3. Voltage gated Ca channels open
4. Ca -gt sarcoplasm
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5. Calcium binds to troponin (C)
6. Tropomyosin is deflected
7. Active sites of actin exposed
8. ATP attaches to myosin head
9. ATP is hydrolyzed (ADP P)
10. Myosin head is phosphorylated
cocks
11. Myosin head binds to actin (cross
bridge)
12. Myosin head dephosphorylates (head
moves) ADP
released (power stroke)
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D. Muscle relaxation

• Calcium pumped into Sarco Retic

E. Phases of muscle movement
1. Lag Phase

• AP in motor neuron to exposure of active sites

2. Contraction Phase

• crossbridge -gt power stroke

3. Relaxation Phase

• calcium pumped into S. R.

4. Mechanical signal

• measured as tension

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F. Stimulus vs contraction

• all or none response

• subthreshold stimulus -gt no

• Threshold -gt AP -gt contraction

• increase Ca increase force

G. Stimulus frequency

• no refractory period

• freq of AP freq of contractions

• tetanus

calcium not pumped back
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II. Cardiac Muscle
A. Contractions like skeletal

• striated (sarcomeres)

B. Intercalated disks
1. attachment
2. Z disks
3. Gap junction
III. Smooth Muscle
A. Structure

• elongated spindle shaped

• no striations (no sarcomeres)

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• actin myosin loose bundles

• intermediate filaments noncontractile

B. Contraction
1. ANS via 2nd messenger
2. Opens Na and Ca channel
3. Depolarization
4. Muscarinic receptor -gtIP3
5. IP3 -gt Sarcoplasmic Retic. (Ca
released)
6. Calcium activates calmodulin
7. Ca/Calmod activates myosin
kinase
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8. Myosin kinase hydrolyzes ATP

• phosphorylate myosin head

9. Myosin binds to actin -gt
contraction
10. Relaxation dephosphorylation
(Myosin Phosphatase)