CHAPTER 10: MUSCLE TISSUE

I. Introduction
    A. Motion results from alternating contraction and relaxation of the muscles (the skeletal system provides leverage and a supportive framework for movement
    B.
myology--scientific study of muscles
II. Muscle tissue overview
    A. Types of muscle tissue
        1.
skeletal muscle--attached to bone, striated, voluntary
        2.
cardiac muscle--wall of the heart, striated, involuntary
        3.
smooth muscle--located in viscera, nonstriated, involuntary
    B. Functions of muscle tissue
        1. production of body movements
        2. stabilizing body positions
        3. regulates organ volume
        4. moving substances within the body
        5. generates heat
    C. Properties of muscle tissue
        1.
electrical excitability--ability to respond to certain stimuli by producing electrical signals (action potentials)
        2.
contractility--ability to shorten and thicken (contract), generating force to do work
            a.
isometric contraction--muscle develops tension but does not shorten
            b.
isotonic contraction--tension remains constant while the muscle shortens
        3.
extensibility--ability to be extended (stretched) without damaging the tissue
        4.
elasticity--ability to return to the original shape after contraction or extension
III.
Sleletal muscle tissue
    A. Each skeletal muscle is a separate organ composed of cells called fibers
    B. Connective tissue of skletal muscle
        1.
fascia--connective tissue sheet which surrounds muscles and other organs of the body
            a.
superficial fascia
                (1) separates muscle from skin
                (2) composed of aerolar connective tissue and adipose tissue
                (3) provides a pathway for nerves and blood vessels to enter and exit muscles
            b.
deep fascia
               (1) dense, irregular connective tissue
                (2) holds muscle with similar functions together
                (3) carries nerves and blood and lymphatic vessels
                (4) fills spaces between muscles
            c.
epimysium--encircles the entire muscle
            d.
perimysium--surrounds groups (fascicles) of muscle fibers
            e.
endomysium--thin sheet of aerolar connective tissue which covers individual muscle fibers
        2.
tendon--cord of dense regular collaginous connective tissue which attaches a muscle to the periosteum of a bone
        3.
aponeurosis--broad, flat layer rather than a cord-like structure
    C. Microscopic anatomy of a skeletal muscle fiber
        1.
myoblasts--small mesodermal cells which fuse together to form the skeletal muscle cell (fiber)
        2.
satellite cells--myoblasts which are found in mature skeletal muscle and help repair damaged muscle fibers
        3.
sarcolemma--cell membrane of the skeletal muscle cell
        4.
transverse tubules (T tubules)--membranous invaginations which extend from the surface of the muscle cell to the depths of the cell (action potentials from the surface can travel down the T tubules)
        5.
sarcoplasm--cytoplasm of the muscle cell
        6.
myoglobin--oxygen-storing protein which is only found in muscle cells
        7.
myofibrils--contractile elements of the muscle cell
        8.
sarcoplasmic reticulum--membranous network which stores calcium ions (calcium ions are released to cause muscle contraction)
            a.
terminal cisterna--dilated end sacs of the sarcoplasmic reticulum which is associated with the T tubules
            b.
triad--a T tubule and two terminal cisterna (one on either side)
        9.
sarcomere--basic functional units of the myofibril
            a. 
thin myofilaments
               (1) actin--contains a binding site for myosin; contractile protein
                (2)
tropomyosin--regulatory protein
                (3)
troponin--regulatory protein
            b.
thick myofilaments--composed of myosin (contractile protein)
            c. A band--darker middle portion of the saromere (entire length of the thick myofilaments)
            d.
I band--lighter, less dense area which only contains thin myofilaments
            e.
Z disc--attachment of thin myofilaments (the sarcomere is considered to be that which is between two Z discs)
            f.
H zone--found in the center of the A band; contains thick but no thin myofilaments
            g.
M line--supporting proteins which hold the thick filaments together
            h. structural proteins of the sarcomere
                (1)
titin--anchors the thick myofilament to the Z disc and the M line
                (2)
myomesin--bind to titin and connect adjacent thick filaments to each other
                (3)
nebulin--maintains alignment of the thin filaments
                (4)
dystrophin--links thin filaments to integral membrane proteins of the sarcolemma
    D. The Sliding Filament Mechanism of muscle contraction
        1. An impulse is sent down along a motor neuron
        2.
Acetylcholine, a neurotransmitter, is released via exocytosis at the motor neuron axonal end
        3. Acetylcholine binds with acetylcholine receptors on the sarcolemma
        4. An action potential is created
        5.
Acetylcholinesterase is an enzyme which breaks down acetylcholine so that the stimulation is brief
        6. The action potential passes down along the sarcolemma and down the T tubules
        7. The action potential causes the release of calium ions through the
Ca++ release channels of the terminal cisterna
        8. The calcium ions bind to troponin which causes the troponin-tropomyosin complex to move away from the myosin binding sites
        9. Myosin heads which are "powered" or "high energy" due to ATP hydrolysis bind to the actin sites
        10. The myosin heads undergo a
power stroke where they pull the thin filaments toward each other causing the sarcomere to shorten (muscle contraction has occurred)
        11. When ATP binds to myosin the myosin head break off from the actin
        12. Once the calcium release channels close active transport mechanisms transport calcium ions back into the sarcoplasmic reticulum where the calcium ions bind to a protein called
calsequestrin
    E. Muscle Metabolism
        1. Sources of ATP production
            a.
creatine phosphate
               (1) creatine kinase transfers phosphate from creatine phosphate to ADP to form ATP
                (2) can be used for approximately 15 seconds
           b. anaerobic cellular respiration
               (1) does not require oxygen
                (2) glycolysis yields two net ATP molecules and two pyruvic acid molecules from one glucose molecule
                (3) pyruvic acid molecules are converted into lactic acid which allows this process to continue for 30-40 sec.
           c. aerobic cellular respiration
               (1) oxygen-requiring mitochondrial reactions
                (2) following glycolysis pyruvic acid enters the mitochondria where it is broken down into carbon dioxide, water and heat
                (3) this process can continue as long as sufficient quantities of oxygen and nutrients are available
                (4) oxygen is delivered to muscles in two ways:
                    (a) diffusion into muscle from the blood
                    (b) release from myoglobin
        2.
muscle fatigue--inability of a muscle to contract forcefully after prolonged activity
            a. inadequate release of calcium ions from the sarcoplasmic reticulum
            b. depletion of creatine phosphate
            c. insufficient oxygen
            d. depletion of glycogen
            e. buildup of lactic acid and ADP
            f. insufficient levels of acetylcholine released at the motor unit
        3.
oxygen debt (recovery oxygen uptake) is the added oxygen, over and above the normal, resting oxygen consumption levels
            a. converts lactic acid back into glycogen stores in the liver
            b. resynthesize creatine phosphate and ATP
            c. replaces oxygen removed from myoglobin
            d. increased rate of chemical reactions due to higher temperatures
            e. tissue repair processes are occurring at a faster rate
    F. Control of Muscle Tension
        1.When considering the contraction of a whole muscle, the tension it can generate depends on the number of fibers that are contracting in unison
        2.
motor unit--motor neuron and the muscle fibers it stimulates (from 10 to 2,000 fibers--average 150)
        3.
twitch contraction--a brief contraction of all the muscle fibers in a motor unit in response to a single action potential
            a.
myogram--is a record of a twitch contraction and includes three periods: latent, contraction, relaxation
            b.
refractory period--time when a muscle has temporarily lost excitability with skeletal muscles having a short refractory period and cardiac muscle having  a long refractory period
        4. frequency of stimulation
            a.
wave summation--increased strength of a contraction resulting from the application of a second stimulus before the muscle has completely relaxed after a previous stimulus
            b. A sustained muscle contraction that permits partial relaxation between stimuli is called
incomplete (unfused) tetanus; a sustained contraction that lacks even partial relaxation between stimuli is called complete (fused) tetanus
        5. recruitment is the process of increasing the number of active motor units
        6.
muscle tone is the firmness of the muscle resulting from a sustained partial contraction of portions of relaxed skeletal muscle
        7.
isotonic contraction occurs when a constant load is moved through the range of motions possible at a joint
            a.
concentric contraction
           b. eccentric contraction
        8. isometric contraction occurs when the muscle does not shorten but tension increases
    G. Types of skeletal muscle fibers
        1.characteristics which vary among skeletal muscle fibers
            a. color varies according to the content of
myoglobin (oxygen-storing reddish pigment)
            b. fiber diameter
            c. amount of mitochondria
            d. blood capillaries
            e. sarcoplasmic reticulum
            f. contraction velocity
            g. resistance to fatigue
        2. classification of skeletal muscle fibers
            a.
slow oxidative fibers
                (1) smallest in diameter
                (2) contain large amounts of myoglobin (dark red in color)
                (3) large number of capillaries
                (4) ATPase of the myosin heads hydrolyze ATP at a slow rate
                (5) resistant to fatigue and can sustain contractions for many hours (postural and muscles associated with endurance training)
           b. fast oxidative-glycolytic fibers
                (1) intermediate in diameter
                (2) contain large amounts of myoglogin
                (3) large number of capillaries
                (4) ATPase of the myosin heads hydrolyze ATP at a fast rate
                (5) moderately resistant to fatigue (muscles involved with such activities as walking and sprinting)
           c. fast glycolytic fibers
                (1) largest in diameter
                (2) can generate the most powerful contractions due to having the greatest number of myofibrils
                (3) low myoglobin content (appears white in color)
                (4) relatively few capillaries and few mitochondria
                (5) can hydrolyze ATP at a rapid rate
               (6) fatigue quickly (used for activities such as weight lifting)
        3. Distribution and recruitment of different types of fibers
            a. Most skeletal muscle contain a mixture of all three fiber types, their proportions varying with the usual action of the muscle.  All fibers of any one motor unit, however, are the same.
            b. Although the number of different skeletal muscle fibers does not change, the characteristics of those present can be altered by various types of exercise
IV. Cardiac muscle tissue
    A. cardiac muscle tissue is only found in the heart wall
        1. fibers are arranged similarly to skeletal muscle fibers
        2.
intercalated discs (containing gap junctions and desmosomes) are present to connect adjacent cardiac muscle cells together
    B. cardiac muscle contractions last longer than the skeletal muscle twitch due to the prolonged delivery of calcium ions from the sarcoplasmic reticulum and the extracellular fluid
    C. cardiac muscle fibers contract when stimulated by their own autorhythmic fibers
V. Smooth Muscle
    A. nonstriated
    B. involuntary
    C. two types:
        1.
visceral (single unit) smooth muscle
           a. found in the walls of hollow viscera and small blood vessels
            b. fibers are arranged in a network
        2.
multiunit smooth muscle
           a. found in large blood vessels, large airways, arrector pili muscles, iris of the eye
            b. fibers operate singly rather than as a unit
    D. Microscopic anatomy of smooth muscle
        1. the duration of contraction and relaxation of smooth muscle is longer than in skeletal muscle
        2. in smooth muscle, the regulator protein that binds calcium ions in the cytosol is
calmodulin (in place of the role of troponin in striated muscle); calmodulin activates the enzyme myosin light chain kinase which facilitates myosin-actin binding and allows contraction to occur at a relatively slow rate
        3. the prolonged presence of calcium ions in the cytosol of smooth muscle fibers provides for smooth muscle tone, a state of prolonged partial contraction
        4. smooth muscle fibers can stretch considerably  without developing tension; this phenomenon is termed the
stress-relaxation response
        5. dense bodies
            a. structures which functionally are similar to the Z discs of striated muscle
            b. intermediate fibers radiate out from dense bodies
            c. the sliding filament mechanism generates tension on the intermediate filaments which pull the dense bodies leading to the shortening of the muscle fiber

LEARNING OBJECTIVES FOR CHAPTER 10

Following successful study of this chapter, students should be able to:

1. Define or describe each of the following terms: myology, electrical excitability, contractility, isometric contraction, isotonic contraction, extensibility, elasticity, muscle fibers, fascia, superficial fascia, deep fascia, epimysium, perimysium, fascicles, endomysium, tendon, aponeurosis, neuromuscular junction, sarcolemma, transverse tubules, sarcoplasm, myoglobin, myofibrils, sarcoplasmic reticulum, filaments, sarcomeres, A band, I band, Z disc, H zone, M line, myosin, actin, tropomyosin, troponin, titin, myomesin, nebulin, dystrophin,calcium ion release channels, calcium ion active transport pumps, neurotransmitter, synaptic vesicles, acetylcholine, motor end plate, acetylcholinesterase, muscle fatigue, oxygen debt, recovery oxygen uptake, motor unit, twitch contraction, myogram, latent period, contraction period, relaxation period, refractory period, wave summation, unfused tetanus, fused tetanus, motor unit recruitment, muscle tone, isotonic contraction, isometric contraction, desmosomes, gap junctions, intercalated discs, visceral smooth muscle tissue, multiunit smooth muscle tissue, dense bodies, calmodulin, stress-relaxation response
2. Contrast skeletal, cardiac and smooth muscle (refer to the table found in your textbook)
3. List and describe the functions of muscle tissue
4. Draw a labeled diagram or label a diagram showing the connective tissue layers associated with muscle (epimysium, perimysium, endomysium, fascicles)
5. Draw a labeled diagram or label a diagram showing the component parts of a sarcomere (Z disc, A band, I band, H zone, M line, thin myofilaments, thick myofilaments)
6. Draw a series of diagrams and provide a word explanation for the sliding filament mechanism of muscle contraction.
7. List and describe the various mechanisms by which muscle can generate the energy needed for contraction (creatine phosphate, anaerobic cellular respiration, aerobic cellular respiration)
8. List and describe the three types of skeletal muscle fibers (slow oxidative fibers, fast oxidative-glycolytic fibers, fast glycolytic fibers)