Lecture Notes: Unit II: Principles of Support and Movement
Chapter Ten – Muscle Tissue
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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. Overview of Muscle Tissue
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. Excitability – ability to respond to certain stimuli by producing electrical signals (AP’s 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. Skeletal muscle tissue
A. Each skeletal muscle is a separate organ composed of cells called myofibers
B. Connective tissue “wrappings” associated with skeletal muscle
1. Fascia--connective tissue sheet which surrounds muscles and other organs of the body
a. Superficial fascia (aka: Subcutaneous layer (Sub-Q) or Hypodermis
(1) Separates muscle from skin
(2) Composed of areolar connective tissue and adipose connective 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 organ
d. Perimypsium--surrounds groups of myofibers (fascicles)
e. Endomysium--thin sheet of areolar connective tissue which surrounds individual myofibers
2. Tendon--cord of dense collagenous c.t. which attaches a muscle to the periosteum of a bone
3. Aponeurosis -broad, flat layer connecting muscles across the midline rather than cord-like structure
C. Microscopic anatomy of a skeletal myofiber
1. Myoblasts - small mesodermal cells that fuse together to form the skeletal muscle cell (myofiber)
2. Satellite cells - myoblasts found in mature skeletal muscle that help repair damaged muscle fibers
3. Sarcolemma--cell membrane of the skeletal myofiber
4. Transverse tubules (T tubules)--membranous invaginations which extend from the surface of
the myofiber to the depths of the cell (action potentials from the surface travel down tubules)
5. Sarcoplasm--cytoplasm of a myofiber
6. Myoglobin--oxygen-storing protein which is found only in myofibers
7. Myofibrils--contractile elements of the myofiber, rod-like structures consisting of end-to-end sarcomeres
8. Sarcoplasmic reticulum--membranous network which stores calcium ions (calcium ions are
released to cause muscle contraction)
(a) Terminal cisterns– dilated end sacs of sarcoplasmic reticulum; 2 are associated with each T-tubule
(b) Triad – one T tubule flanked by two terminal cisterns (one on either side)
9. Sarcomere--basic functional units of the myofiber; what myofibrils are arranged in.
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a. Composition of the “thin myofilaments”
(1) Actin—globular, contractile proteins each containing a myosin binding site; arranged in two “strings of beads”
(2) Tropomyosin--regulatory protein
(3) Troponin--regulatory protein
b. Composition of the “thick myofilaments”: polymerized Myosin (filamentous, contractile protein molecules)
c. A band--darker middle portion of the sarcomere (entire length of the thick myofilaments)
d. I bands—2 lighter, less dense area in each sarcomere which only contain thin myofilaments
e. Z disc--attachment of thin myofilaments (the sarcomere is considered to be everything 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. Other (structural) proteins of the sarcomere:
(1) titin--anchorsthe 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.TheSliding Filament Mechanism of muscle contraction
1. An impulse is sent down along a motor neuron
2. Acetylcholine (ACh), a neurotransmitter, is released via exocytosis by the motor neuron axonal end bulbs
3. ACh binds with Ach receptor molecules located imbedded in the sarcolemma (at the motor end plate)
4. A muscle action potential is created.\
5. Acetylcholinesterase (AChE)is an enzyme which breaks down Ach so that stimulation is brief (it ceases
almost as soon as it starts)
6. The action potential passes down along the sarcolemma and down the T tubules
7. The action potential causes the release of calcium ions through the Ca ++ release channels of the terminal cisterns
8. The calcium ions bind to troponin causing the troponin-tropomyosin complex to move away from the myosin binding-sites.
9. Myosin heads which are "powered" or "high energy" due to ATP’s 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 A TP 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 A TP production
a. Creatine phosphate ..
(1) Creatine kinase transfers phosphate groups 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 ACh released by the motor unit (the one motor neuron at the motor end plates with which it synapses)
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
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b. Resynthesize creatine phosphate and A TP
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 - one motor neuron and all the myofibers 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—a record of the events in any muscle contraction. A twitch myogram is a record of a twitch
contraction and includes four events: the stimulus; latent period, contraction period, + relaxation period.
b. Refractory period—time when a myofiber has temporarily lost excitability; skeletal myofibers have a short
refractory period and cardiac myofibers have 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 1arge 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 anyone 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
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IV. Cardiac muscle tissue
A. Cardiac muscle tissue is only found in the heart wall
1. Myofibers 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 specialized autorrhythmic myofibers (modified myofibers that
function more as nerve cells, in that they spontaneously depolarize at a certain rate; that rate may be altered – made more
rapid or less rapid – by input from the nervous system.
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. fFbers 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 stressrelaxation 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