1. Muscular Tissue I
Reading: Chapter 10

2. Are you tired of sand being kicked in your face
Are you tired of sand being kicked in your face? I promise you new muscles in days! Charles Atlas
I just use my muscles as a conversation piece, like someone walking a cheetah down 42nd Street. Arnold Schwarzenegger

3. Three types: Skeletal, Cardiac, and Smooth
Muscle Tissue
Three types: Skeletal, Cardiac, and Smooth
1. Skeletal-striated, voluntary, controlled by nerves
2. Cardiac-striated, only found in the heart, involuntary control
3. Smooth-non-striated, found in walls of hollow internal structures, involuntary control

4. Properties of Muscle Tissue
Functions of Muscle Tissue
Producing body movement
Stabilizing body position
Storing body substances (e.g. food in stomach-outlet closed by
sphincter)
4. Generating heat
Properties of Muscle Tissue
Electrical excitability
Contractility
Extensibility
Elasticity

5. Connective tissue components of Muscle Tissue
Fascia-sheet of fibrous connective tissue supporting and surrounding muscles
Superficial-subcutaneous layer separates muscle from skin composed
of areolar connective tissue and adipose tissue.
Deep-dense irregular connective tissue lines body wall, carries nerves
and blood/lymphatic vessels.
Protective connective tissue-extends from fascia:
Epimysium-outermost layer
Perimysium-surrounds small groups of muscle fibers (10-100) bundling
them into fascicles.
Endomysium-separates individual muscle fibers within a fascicle.
All three layers may extend beyond the muscle fiber to form a tendon
(e.g. Achilles tendon of the gastronemius attaches the muscle to the
calcaneus bone). Some tendons are enclosed within sheaths with visceral
(attached to the tendon surface) and parietal (attached to bone) layers with
a film of synovial fluid separating the layers.

6. Organization of skeletal muscle tissue

7. Organization of a Skeletal Muscle fiber
1. Development
Myoblasts fuse to form myotubes:
Average diameter um, length 10cm
2. Growth
Hypertrophy-enlargement of existing fibers
Hyperplasia-Increase in the number of fibers
3. Anatomy
Sarcolemma-plasma membrane of a muscle cell
Transverse (T) tubules- invaginations in sarcolemma-toward
center of the fiber filled with interstitial fluid.
Sarcoplasm- cytoplasm of muscle fiber that contains glycogen
and myoglobin.
Myofibrils- small fibers found in the sarcoplasm (2um diameter
and the length of the fiber).
Sarcoplasmic reticulum- circles myofibril. Enlarged ends
touch T-tubules forming a triad. Stores Ca2+.

8. Microscopic Organization of Skeletal Muscle
Muscle Atrophy-muscle wasting
Muscle hypertrophy-increase in diameter of muscle fibers
increased production of myofibrils-results in more
forceful contractions.

9. Anatomy of a myofibril
Myofibrils are composed of two types of filaments:
Thick filaments: 16nm diameter, 1-2 um length
Thin filaments: 8nm diameter, 1-2 um length
For each thick filament there are two thin filaments.
Filaments are arranged in compartments called sarcomeres
that are separated by z-discs.
The sarcomere contains a variety of zones.

10. The arrangement of filaments within a Sarcomere
Muscle damage-exercise can result in torn sarcolemmas,
damaged myofibrils, and disrupted z-discs

11. Composition of myofibrils
Myofibrils-composed of:
Contractile proteins
Regulatory proteins
Structural proteins
Contractile proteins: myosin and actin.
Thick filaments- Myosin (motor protein converts ATP into movement) molecules of myosin=1 thick filament.
Thin filaments- are anchored to z-discs and are composed of actin and regulatory proteins. Actin contains a myosin binding site.
Regulatory proteins: tropomyosin and troponin. Tropomyosin blocks myosin binding to actin by blocking the myosin site on actin in relaxed muscle. The tropomyosin is held in place by troponin.
Structural proteins: Titin-anchors thick filament to a z-disc and M
line. Other proteins contribute to alignment, stability, and elasticity.

12. How do muscles contract?
Myosin motors pull thin actin filaments together
shortening the distance between z-discs.

13. The Contraction Cycle

14. What allows Ca2+ to be available for muscle contraction?
The Action Potential
What allows Ca2+ to be available for muscle contraction?

15. The role of Calcium

16. The Contraction Cycle
crossbridge

17. Myosin movement on actin filaments
ATP
Ca2+
Tropomyosin
Actin
troponin

18. The Neuromuscular Junction

19. Neuromusclular Junction (NMJ) transmission

20. Muscle fiber contraction-relaxation

21. NMJ transmission is required for muscle contraction

22. Muscle Metabolism-Fueling Muscle Contraction
ATP is the immediate source of energy for muscle contraction.
Although a muscle fiber contains only enough ATP to power a few twitches, its ATP "pool" is replenished as needed. There are three sources of high-energy phosphate to keep the ATP pool filled:
1. Creatine phosphate
2. Glycolysis of glycogen
3. Cellular respiration in the mitochondria of the fibers.
a. anaerobic-producing ATP without oxygen (e.g. breakdown of glycogen in muscle tissue- glycolysis, produce 4 ATPs by using 2)
b. aerobic-oxygen requiring reactions that produce ATP in mitochondria
Two sources of oxygen for muscle:
Diffusion from blood
Oxygen release from muscle myoglobin

23. ATP Production Creatine phosphate
The phosphate group in creatine phosphate is attached by a "high-energy" bond like that in ATP. Creatine phosphate derives its high-energy phosphate from ATP and can donate it back to ADP to form ATP. Creatine phosphate + ADP <--> creatine + ATP
The pool of creatine phosphate in the fiber is about 10 times larger than that of ATP and thus serves as a modest reservoir of ATP.
Glycolysis
Skeletal muscle fibers contain about 1% glycogen. The muscle fiber can degrade this glycogen by glycolysis. Glycolysis yields two molecules of ATP for each pair of lactic acid molecules produced. Not much, but enough to keep the muscle functioning if it fails to receive sufficient oxygen to meet its ATP needs by respiration. However, this source is limited and eventually the muscle must depend on cellular respiration.
Cellular respiration
Cellular respiration not only is required to meet the ATP needs of a muscle engaged in prolonged activity (thus causing more rapid and deeper breathing), but is also required afterwards to enable the body to resynthesize glycogen from the lactic acid produced earlier (deep breathing continues for a time after exercise is stopped). The body must repay its oxygen debt (convert lactic acid back into glycogen, resynthesize creatine PO4, and replace oxygen removed from myoglobin).

24. Control of Muscle Tension
Motor Unit
A typical skeletal muscle contains thousands of muscle fibers. Although some motor neurons control a few muscle fibers, most control hundreds of them. All the muscle fibers controlled by a single motor neuron constitute a motor unit. The size of a motor unit is an indication of how fine the control of movement can be (e.g. muscles of the eye a motor neuron may control 4-6 muscle fibers. In the leg a single motor neuron may control muscle fibers.
Twitch contraction
A twitch is a single stimulus-contraction-relaxation sequence in a muscle fiber. Twitches vary in duration. Twitches in one eye muscle fiber can be as brief as 7.5 msec, but a twitch in a muscle fiber from the soleus, a small calf muscle, lasts about 100 msec.

25. Motor Unit Recruitment
Increases in the number of active motor units. Helps with smooth movements.
Muscle Tone
In any skeletal muscle, some motor units are always active, even when the entire muscle is not contracting. Their contractions do not produce enough tension to cause movement, but they do tense and firm the muscle. This resting tension in a skeletal muscle is called muscle tone. A muscle with little muscle tone appears limp and flaccid, whereas one with moderate muscle tone is firm and solid. The identity of the stimulated motor units changes constantly, so a constant tension in the attached tendon is maintained, but individual muscle fibers can relax.

26. Muscle Tone
Hypotonia- decreased muscle tone, flaccid
Flaccid paralysis- loss of muscle tone, loss of tendon reflexes, atrophy
Hypertonia- increased muscle tone, spacity or rigidity.
Spasticity- increased muscle tone, increased tendon reflexes (e.g. Babinski).
Rigidity- increased muscle tone where reflexes are not affected (e.g. tetanus)

27. Effect of Stimulation Frequency on Contraction

28. Muscle Contraction
Isotonic Contractions- tension rises and the skeletal muscle's length changes. Lifting an object off a desk, walking, and running involve isotonic contractions
Two types of isotonic contractions:
Concentric- the muscle tension exceeds the resistance and the muscle shortens
Eccentric- the peak tension developed is less than the resistance, and the muscle elongates owing to the contraction of another muscle or the pull of gravity.
Isometric contraction, the muscle as a whole does not change length, and the tension produced never exceeds the resistance.

29. Smooth Muscle
Compared to skeletal muscle, smooth-muscle cells are small. They are spindle-shaped, about 50 to 200 microns long and only 2 to 10 microns in diameter. They have no striations or sarcomeres. Instead, they have bundles of thin and thick filaments (as opposed to well-developed bands) that correspond to myofibrils. In smooth-muscle cells, intermediate filaments are interlaced through the cell much like the threads in a pair of "fish-net" stockings. The intermediate filaments anchor the thin filaments and correspond to the Z-disks of skeletal muscle. Unlike skeletal-muscle cells, smooth-muscle cells have no troponin, tropomyosin or organized sarcoplasmic reticulum.

30. Smooth Muscle Contraction
As in skeletal-muscle cells, contraction in a smooth-muscle cell involves the forming of crossbridges and thin filaments sliding past thick filaments. However, because smooth muscle is not as organized as skeletal muscle, shortening occurs in all directions. During contraction, the smooth-muscle cell's intermediate filaments help to draw the cell up, like closing a drawstring purse.
Calcium ions regulate contraction in smooth muscle, but they do it in a slightly different way than in skeletal muscle:
1. Calcium ions come from outside of the cell.
2. Calcium ions bind to calmodulin in the cytosol and then to an enzyme on myosin, called myosin light chain kinase.
3. The enzyme uses ATP to transfer a phosphate group to myosin.
4. This phosphate transfer activates myosin.
5. Myosin forms crossbridges with actin (as occurs in skeletal muscle).
6. When calcium is pumped out of the cell, the phosphate gets removed from myosin by another enzyme.
7. The myosin becomes inactive, and the muscle relaxes.

31. Cardiac Muscle Tissue
Cardiac or heart muscle resembles skeletal muscle in some ways: it is striated and each cell contains sarcomeres with sliding filaments of actin and myosin. However, cardiac muscle has a number of unique features that reflect its function of pumping blood.
1. The myofibrils of each cell (and cardiac muscle is made of single cells - each with a single nucleus) are branched. The branches interlock with those of adjacent fibers by adherens junctions. These strong junctions enable the heart to contract forcefully without ripping the fibers apart.
2. The action potential that triggers the heartbeat is generated within the heart itself. Motor nerves (of the autonomic nervous system) do run to the heart, but their effect is simply to modulate - increase or decrease - the intrinsic rate and the strength of the heartbeat. Even if the nerves are destroyed (as they are in a transplanted heart), the heart continues to beat.
3. The action potential that drives contraction of the heart passes from fiber to fiber through gap junctions. Significance: All the fibers contract in a synchronous wave that sweeps from the atria down through the ventricles and pumps blood out of the heart. Anything that interferes with this synchronous wave (such as damage to part of the heart muscle from a heart attack) may cause the fibers of the heart to beat at random - called fibrillation. Fibrillation is the ultimate cause of most deaths and its reversal is the function of defibrillators.

32. Cardiac Muscle
The refractory period in heart muscle is longer than the period it takes for the muscle to contract (systole) and relax (diastole). Thus tetanus is not possible (a good thing, too!).
Cardiac muscle has a much richer supply of mitochondria than skeletal muscle. This reflects its greater dependence on cellular respiration for ATP.
Cardiac muscle has little glycogen and gets little benefit from glycolysis when the supply of oxygen is limited. Thus anything that interrupts the flow of oxygenated blood to the heart leads quickly to damage - even death - of the affected part. This is what happens in heart attacks.

33. “Everything should be made as simple as possible,
but not simpler.”
Albert Einstein

34. End of Chapter 10