Download BIOL 424 Muscle 1 I. Contractile Structure of Skeletal

BIOL 424
Muscle 1
I. Contractile Structure of Skeletal Muscle
A. Myofilaments
1. actin
α-actin in muscle
ß-actin in microtubules
a. two twisted strings of beads (helix)
b. tropomyosin filaments
along groove between actin filaments
expose and cover myosin attachment sites on G-actin
c. troponin molecules attached to tropomyosin
provides Ca++ binding sites
d. thin myofilaments
2. myosin
a. bundles of tiny 2-headed golf clubs
b. thick myofilaments
- 2 identical heavy chains
- four light chains
c. heads can bind to attachment sites on actin
B. Sarcomeres
1. highly organized actin and myosin filaments
2. functional units of skeletal muscle
3. responsible for contraction
4. Z-disk to Z-disk
- titin attaches ends of myosin bundles to Z-disk
C. Bands of the sarcomere
1. I band
a. actin only
b. spans Z-disk
c. ends at myosin filaments
2. A band
a. dark, central
b. extends length of myosin filaments
c. actin and myosin overlap at both ends
3. H zone
a. center of sarcomere
b. myosin only
4. M line
a. line at center of sarcomere
b. anchor point for myosin filaments
c. contains creatine kinase
5. alternating A and I bands ---> striations
II. Sliding Filament Theory
A. Ca++ released from SR sets contractile apparatus of fiber in motion
B. Sliding of thin myofilaments over thick myofilaments
1. shortening of sarcomere
2. shortening of muscle fibers
3. myofilaments do NOT shorten
4. “cross bridges” must form for sliding to occur
C. Contractile mechanism (vertebrate skeletal muscle)
1. at rest, active sites on actin covered by tropomyosin
2.Ca++ released into sarcoplasm
a. binds to troponin
b. troponin changes shape, “pulling” tropomyosin off active site
c. myosin “heads” free to bind actin
3. Z discs pulled toward each other
sarcomere shortens
muscle fiber shortens
4. Myosin ATPase
a. globular “head” has enzymatic activity
b. catalyzes ATP hydrolysis
c. energy stored in head of myosin
d. upon actin binding, energy used to move heads
5. mechanism
a. myosin head “cocked” by ATP breakdown
b. binds to active site on actin
c. myosin head swings
- ADP and Pi released
d. actin filament moved
e. new ATP binds to ATPase site on myosin
f. head detaches
g. ATP hydrolysis returns head to “cocked/energized” position
6. Ca++ required for “rowing” motion to occur
a. “rowing” will continue as long as Ca++ present
b. rigor mortis
D. Other mechanisms
1. Ca++ binds to myosin light chains
2. smooth muscle
E. Length-Tension relationship
1. optimum resting length
- resting length at which contraction of muscle will bring maximal force
a. dependent on the amount of overlap between actin and myosin
b. force generated by sarcomere is proportional to # of cross bridges
2. overstretching 3. too much shortening of muscle 4. normal resting length is maintained by constant neural “tone”
III. Propagating the Action Potential in Twitch Muscles
A. Action potential generated by depolarization at neuromuscular junction
1. carried by T-tubules
a. invaginations of sarcolemma (lumen continuous with ECF)
b. run in a groove between adjacent terminal cisterns
2. action potential transferred to terminal cisternae
B. Ca++ released from terminal cisternae
1. expanded region of sarcoplasmic reticulum
2. stores Ca++
C. Ca++ facilitates muscle contraction
D. Ca++ remains in sarcoplasm for duration of neural stimulation
1. no neural activity → no action potential → no signal down T-tubule
2. Ca++ actively transported back to SR
3. ATP needed for both contraction and relaxation
E. Excitation-contraction coupling
IV. Muscle Contractions
A. Isotonic contractions
1. results in muscle shortening
2. tension produced by contraction overcomes opposing force
B. Isometric contractions
1. muscles contract but do not shorten (can at least decrease elasticity)
2. failure to overcome opposing force
3. can be maintained voluntarily
V. Motor Units of Twitch Muscles (vertebrate skeletal muscle)
A. Somatic motor neuron + all muscle fibers it innervates
1. each fiber ----> single axon terminal
2. cell body of neuron in spinal cord
B. Activation
1. all fibers innervated by single neuron will be stimulated
2. all-or-none response
3. asynchronous between different motor neurons
C. Innervation ratio
1. more control (over strength of contraction) ---> many small motor units
2. more power (of contractions; less fine control) ---> fewer, larger neurons
3. recruitment
a. smaller motor units may initiate contraction
b. greater strength required ----> larger motor units activated