1. Muscle Contraction

2. 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

5. 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

8. 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 -> sarcolemma ->T tubules
2. T tubules -> Sarcoplasmic Retic
3. Voltage gated Ca++ channels open
4. Ca++ -> sarcoplasm

12. 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)

14. 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 -> power stroke
3. Relaxation Phase
calcium pumped into S. R.
4. Mechanical signal
measured as tension

16. F. Stimulus vs contraction
all or none response
subthreshold stimulus -> no
Threshold -> AP -> contraction
increase Ca++ = increase force
G. Stimulus frequency
no refractory period
freq of AP = freq of contractions
tetanus
calcium not pumped back

18. 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)

20. 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 ->IP3
5. IP3 -> Sarcoplasmic Retic. (Ca++ released)
6. Calcium activates calmodulin
7. Ca++/Calmod activates myosin kinase

22. 8. Myosin kinase hydrolyzes ATP
phosphorylate myosin head
9. Myosin binds to actin -> contraction
10. Relaxation – dephosphorylation (Myosin Phosphatase)