1. Muscle
Specialized for:
Types:

2. Single contraction characteristics of the three muscle types

3. Skeletal Muscle Striated (sarcomere is functional unit); multinucleate
Voluntary (activated by a motor neurons)
Each fiber is a cell; many fibers to a muscle
Different types of fibers
Each fiber has a nerve connection

5. Typing based on succinate dehydrogenase activity
Type I = low activity
Type IIa = moderate activity
Type IIb = high activity
Typing based on speed of myosin (myosin ATPase activity
Type I = low activity
Type II = high activity

6. Basic Structure of a Skeletal Muscle Fiber
Sarcolemma = cell membrane
Fiber (cell) > myofibril (bundles of actin and myosin) > myofilaments (actin and myosin)

7. Functional unit of a myofibril is the sarcomere
Major filamentous proteins are actin and myosin, z proteins (in the z disk)
Many structural proteins; alpha actinin, myomesin, C protein, titin, nebulin
Cytoskeletal proteins; desmin, vimentin, filamin
sarcomere

9. Myosin (thick) filaments
Each filament contains about 400 individual myosin molecules
Bundled so that half of the molecules have their head facing one direction, the other half, the opposite direction
Myosin heads are in a staggered arrangement along the filament

10. Myosin is composed of 6 polypeptide chains, 2 H or heavy chains, and two L or light chains
Myosin head has ATPase activity
Myosin has a hinge region where the molecule is flexible
The myosin head has a high affinity for g actin
In smooth muscle, light chains regulate myosin action; in cardiac and skeletal muscle, light chains partially determine the speed of the myosin ATPase activity

11. Actin Filaments (thin filaments)
Monomers of globular or g actin combine to form long fibers of f actin. Two f actin molecules twist around one another to form a single thin filament
All actin myofilaments are anchored to the proteins of the z disk
Each g actin molecule has a binding site for the myosin head

12. Actin
Tropomyosin – covers active sites on g actin molecules
Troponin – regulates tropomyosin; three subunits – troponin c, troponin I, and troponin m
Troponin c has a binding site for calcium and is bound to the other two subunits
Troponin I keeps the tropomyosin over the myosin binding sites on G actin (inhibits actin/myosin binding)
Troponin m anchors the three subunits to the tropomyosin molecule

15. Ryanodine receptor

16. Excitation-contraction coupling
Motor neuron releases Ach onto the surface of the skeletal muscle fiber. The fiber’s nicotinic receptors are activated, opening K+ and Na+ channels. The cell membrane is depolarized.
The action potential moves away from the motor end plate in all directions, including down the t-tubule system.
Receptors in the t-tubules called dihydropyridine receptors (DHP) are activated by the change in voltage. They are connected to the ryanodine receptors in the lateral sacks of the sarcoplasmic reticulum (SR).
The ryanodine receptors are opened by the change in conformation of the DHP receptors and Ca2+ is released from the SR.
Ca2+ diffuses across the myofilaments.
The Ca2+ binds to troponin C, causing it to change conformation, pulling on troponin I, which in turn pulls on tropomyosin. With the altered conformation in tropomyosin, the myosin binding sites on g actin molecules are exposed.
Myosin heads can now bind to g actin molecules and cross-bridge cycling begins, shortening the sarcomere by pulling on the actin filaments and drawing the z disks closer together

17. Cross-Bridge Cycling

20. Cross-bridge cycling Why?
Continues as long as nerve depolarizes muscle membrane; also dependent on fuel supply as myosin power stroke is dependent on ATP
Pulls z lines closer together; shortens sarcomere
Myosin heads do not pull in a synchronized manner; random rowing
Why?

21. Relaxation
What must happen for a muscle to relax: _

22. Factors Affecting Strength of Muscle Contraction
Energy supply
Anaerobic glycolysis; aerobic glycolysis; FA metabolism; phosphocreatine metabolism
ATP + creatine phosphocreatine + ADP ATP + creatine
Muscle length

24. Frequency of stimulation
Summation; tetanus
Why does the force of contraction increase? What factors are changing at the cellular level?

25. Frequency of stimulus determines the overall cellular [Ca2+]
Frequency of stimulus determines the overall cellular [Ca2+]. The more Ca2+, the greater the binding to troponin. The greater the binding, the more movement of tropomyosin. The more active sites exposed, the greater the number of myosin heads that can bind. The more myosin heads performing a powerstroke, the greater the strength of contraction.

26. Number of motor units activated (recruitment)
Slow twitch, fatigue resistant activated first; weak stimulus activates them
Fast twitch, fatigue-resistant (intermediate) activated as stimulus intensity increases
Fast twitch, glycolytic activated at stimuli of highest intensity
What will happen if you maximally contract a muscle for an extended period of time? Why?

27. Types of Contractions
Isotonic = same stretch; moves a load; as muscle contracts, it shortens
Concentric = muscle shortens as it moves a load
Eccentric = muscle lengthens as it moves a load
Isometric = same length; load is not moved; muscle contracts but doesn’t shorten

28. Smooth muscle Small, not striated Involuntary
Found in hollow organs; other organs important to homeostasis
Can be activated by stretch, neurons, hormones
Little SR; caveolae
Myosin has heads down entire length
Slow ATPase activity
Regulated by myosin light chains
Dense bodies anchor actin
Actin has no troponin or tropomyosin components