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Muscular System
I. Muscle types
A. All muscle cells are elongated and are called fibers.
1. Muscle cells contract because of the presence of two
types of muscle filaments - myosin and actin.
2. The prefixes “myo” and “sarco” refer to muscles.
B. Skeletal muscle is the type of muscle found attached to the body’s
skeleton.
1. the muscle fibers are cigar shaped and contain multiple nuclei.
2. is also called striated muscle because its fibers appear striped
3. attaches to the skeleton to produce movement
4. is under voluntary control
5. muscle fibers can produce strong contractions because of
several layers of connective tissue sheathes that provide strength.
6. contractions are powerful and rapid; the muscle tires quickly and must
rest
7. 3 key words to remember - skeletal, striated, voluntary
C. Smooth muscle is the type of muscle found in the internal organs and
blood vessels.
1. the fibers have no striations
2. the fibers are under involuntary control
3. found in the walls of the hollow organs
4. fibers are large, spindle shaped, and have a single nucleus
5. fibers are arranged in sheets or layers
6. often there are two sheets - one running circularly and one
running longitudinally
7. as the layers contract and relax they change the size of the
organ
8. contractions are slow and sustained
9. 3 key words to remember - visceral, nonstriated, involutary
D. Cardiac muscle is a type of muscle found in the heart.
1. muscle fibers are striated
2. fibers are under involuntary control
3. fibers are arranged in spiral shaped bundles
4. fibers are branching cells that are joined by intercalated disks
5. 3 key words - cardiac, striated, involuntary
II. Muscles have four roles - produce movement, maintain posture,
stabilize joints, and generating heat
A. Producing movement
1. skeletal muscles are responsible for all voluntary
movement
2. smooth muscles are responsible for the movement of
internal organs
3. cardiac muscle produces our heart beat
B. Maintaining posture
1. counteracts that force of gravity
2. allows us to maintain a sitting or standing position
3. muscles contract almost continuously making tiny
adjustments
C. Skeletal muscles and tendons are important for stabilizing
some joints such as the shoulder or the knee.
D. As a by-product of muscle contraction heat is produced.
1. skeletal muscle is the most important source of body
heat
2. vital for maintaining our constant body temperature
C. Most muscles span across joints attach to bones in at least two places.
1. The bone that moves when a muscle contracts is called the
muscle's insertion.
2. The bone that does not move, or is less moveable, is called the
muscle's origin.
3. The motion produced by the contraction is called the muscle's
action.
V. Microscopic anatomy of skeletal muscle
A. The muscle cell is called a fiber.
1. the cell membrane is called the sarcolemma
2. the fiber contains many nuclei that are pushed against
the sarcolemma
3. the fiber is filled with myofibrils
B. The myofibrils are composed of contractile units called
sarcomeres
1. each sarcomere is composed of two types of protein
filaments known as
myofilaments - thick and thin filaments
3. the protein myosin composes the thick filaments
4. the protein actin composes the thin filaments
B. continued
5. striations represent alternating dark and light bands
dark bands = myosin and actin
light bands = actin
dark bands are called A bands
light bands are called I bands
6. The I bands are interrupted by the Z disk.
connects sarcomeres
A sarcomere extends from Z disk to the next Z disk
anchors the actin filaments of a sarcomere
7. A bands interrupted by the M line
M line represents a protein that anchors the myosin
filaments
8. Ultrastructure of myofilaments
a. the thick filament is made of myosin molecules
b. each myosin molecule has a rod-like tail that ends in
two heads
c. the heads act as cross-bridges that allow temporary
connections to form between myosin and actin during
contraction
d. the thin filaments are made of actin, tropomyosin, and
troponin
e. actin molecules contain active sites where the myosin
cross-bridges attach
f. tropomyosin - stiffens and thin filament
g. troponin - binds to and blocks the active sites on actin
C. When muscles contraction occurs the actin filaments slide
over the myosin filaments and cause the sarcomere to shorten called the sliding filament theory of muscle contraction
1. actin molecules are pulled toward the center of the
sarcomere
2. the distance between Z lines becomes less as each
sarcomere shortens
3. The combined effect of shortened sarcomeres is the
shorten the muscle fiber
4. Contraction results from the attachments of the crossbridges on the myosin molecules to the active sites on the
actin molecules. Each cross-bridge attaches and detaches
several times during one contraction.
5. Contraction requires the presence of calcium ions
Troponin blocks the active sites on actin
Calcium binds with and alters the shape of troponin
the altered shape of troponin opens up the active sites on actin
6. When the active sites on actin are exposed the following four events
occur
a. a cross- bridge attaches to an actin active site
b. the myosin head pivots into a bent shape that drags the actin
toward the center of the sarcomere
c. the cross-bridge de-ttaches
d. the cross-bridge re-forms its straighter, "cocked" position
VI. Contraction of a skeletal muscle
A. Skeletal muscles must be stimulated by nerve impulses to
contract.
1. The nerve input is carried by a motor neuron.
2. One motor neuron may stimulate several or hundreds
of muscle fibers.
3. A motor unit is defined as a motor neuron and the
muscle fibers it stimulates.
4. The neuron branches into a number of axonal terminals
as it nears the fiber. Each terminal forms a junction with
a different muscle fiber. These junctions are called
neuromuscular junctions.
5. The neuron and the fiber never actually touch. There is a
space between the two known as the synaptic cleft.
6. When a nerve impulse reaches the neuromuscular junction, a
chemical, called a neurotransmitter is released. The
neurotransmitter for all skeletal muscles is acetylcholine.
7. The nerve impulse causes the muscle fiber to contract.
B. Contraction of a muscle as a whole
1. Contraction of a muscle fiber is an “all or none”
action.
2. An entire muscle can contract in varying degrees. This
is called a graded response.
3. A graded response can be produced by: 1) varying the
speed with which the muscle is stimulated; and by 2)
changing the number of muscle fibers stimulated.
4. In most muscle activity, nerve impulses are delivered
to the muscle fibers quickly and the combined effect of
successive fiber contractions produces a smooth muscle
contraction.
5. The force with which a muscle contracts is largely
determined by the number of fibers that are stimulated.
C. Energy for muscle contraction
1. The energy needed for muscle contraction comes from
ATP that is produced by aerobic respiration.
2. When a muscle is over worked there is insufficient
oxygen for aerobic respiration to occur and the fiber
switches to anaerobic respiration.
3. Anaerobic respiration produces lactic acid as a byproduct. Lactic acid is the cause of muscle soreness and
stiffness that may occur after intense activity.
D. Muscle fatigue
1. Muscle fatigue occurs if we exercise our muscles continuously
for a long time.
2. Muscle fatigue occurs when a muscle is unable to contract
even though it is being stimulated.
3. This occurs when a muscle is being contracted continuously
without a period of rest. The contractions get shorter and shorter
until they stop completely.
4. Muscle fatigue is believed to be due to the body’s inability to
supply oxygen to the muscles fast enough. Without oxygen the
energy produced by the muscles decreases until there is no
energy for contraction. This is called the “oxygen debt”.
E. Types of muscle contractions
1. Twitch contractions are quick, jerky responses to a stimulus.
They play a minor role in normal muscle activity.
2. Tetanic contractions are longer, more sustained contractions
produced by a series of stimuli reaching the muscle fibers in rapid
succession
tetany refers to a smooth, sustained muscle contraction and NOT
to the disease known as tetanus caused by C. tetani
3. Isotonic contractions occur when the muscle shortens and
movement occurs.
4. Isometric contractions occur when the muscle trys to shorten
but it is met with resistance. The muscle does not shorten but the
tension in the muscle increases.
5. Muscle tone is a state of continuous, partial muscle
contractions that keeps muscles firm and healthy.
F. The effect of exercise on muscle contraction.
1. Aerobic exercises produce stronger, more flexible muscles
that are more resistant to fatigue. This occurs because the blood
supply to the muscle increases as does the number of
mitochondria in each fiber. There is little increase in the size of
the muscles.
2. Resistance exercises, or isometric exercises, train the muscles
against immovable objects. Consistent isometric exercises cause
an increase in the size of muscle cells, not an increase in muscle
cells. Increase fiber size causes an increase in the size of
muscles. Increase in size, not number of cells is called
hypertrophy.
3. Cross training - alternating between aerobic and resistance
exercises provides an effective program for general health.
4. Prolonged inactivity may cause muscle fibers to shrink. This
process is called atrophy and the condition is referred to as
“disuse atrophy”. (Is this principle also at work in "our brains" as
well?)
VII. Muscle contractions and body movements
A. . Muscles usually work together to produce smooth,
coordinated movement
1. Smooth movement is usually the combination of
muscles that produce the movement and muscles that
oppose the movement.
2. The muscle has the major responsibility for a
movement is called the “prime mover” or the "agonist".
3. A muscle that opposes a movement is called an
antagonist.
4. A muscle that helps the prime mover is called a synergist.
5. A fixator is a muscle that assists the prime mover by
stabilizing the origin of the prime mover.
B. Skeletal muscle mechanics
1. The operation of most skeletal muscles involves
leverage and lever systems.
1st class lever
2nd class lever
3rd class lever