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I.
Types of muscle
A.
B.
SKELETAL muscles attach to the
skeleton. They function in
movement and maintenance of
posture.
1.
They can be long (up to 30 cm).
2.
They are voluntary and contract and relax rapidly
(stimulated by ACETYLCHOLINE).
3.
Cells are multinucleated and tissue is
STRIATED (striped).
SMOOTH muscle is found in the walls of hollow organs
and functions to move organ walls.
1.
PERISTALSIS is the term for rhythmic
contractions and relaxations that occur
in some hollow organs.
2. They are involuntary and contract and
relax slowly. They are rhythmic
(stimulated by acetylcholine,
NOREPINEPHRINE, and hormones).
3. Cells have a single nucleus and tissue
lacks striations.
C. CARDIAC muscle is found in the heart wall where it
functions in pumping the heart.
1. They are involuntary and contract and relax as a unit.
They are rhythmic.
2. Cells have a single nucleus and tissue is striated.
II. Structure of a skeletal muscle
A.
FASCIA is the connective tissue that surrounds each
individual muscle fiber making up a muscle.
1. TENDONS are made up of fascia and connect muscle
to bone.
B.
The EPIMYSIUM surrounds the muscle.
C.
The PERIMYSIUM breaks tissue into small sections.
1. The sections of muscles fibers are called
FASCICLES.
D.
The ENDOMYSIUM
encloses each fascicle.
III. Skeletal muscle fibers
A.
They are made up of single fused
cells.
B.
Each fiber has a SARCOLEMA
(cell membrane), SARCOPLASM
(cytoplasm), SARCOPLASMIC
RETICULLUM (ER), many
mitochondria, and MYOFIBRILS.
1.
Myofibrils are made up of the
proteins ACTIN and MYOSIN
that aid in contraction and
cause striations in tissue.
a. I BANDS (light/made of actin) attach to Z
LINES.
b. A BANDS are made of overlapping actin and myosin
filaments.
c. The H ZONE of the A band is made of only myosin
filaments.
IV. When actin and myosin slide past one another in the
myofibrils, muscle fibers shorten and pull on there
attachments. This is muscle contraction.
A.
Contraction is caused by stimulation from a MOTOR
NEURON. The AXON of the neuron and the muscle
fiber join at the NEUROMUSCULAR JUNCTION.
1. The gap between the axon and muscle fiber is called
the SYNAPTIC CLEFT.
2. NEUROTRANSMITTERS (chemicals that cause
contraction) are released at the distal end of the axon
and bind to receptors on the muscle fibers, thus
stimulating them with a MUSCLE IMPULSE.
a. Muscle impulses are transmitted in all directions
along the cell.
b. Acetylcholine is the neurotransmitter that motor
neurons use to control skeletal muscle.
B. Excitation and the SLIDING FILAMENT THEORY
1. TROPONIN and TROPOMYOSIN are proteins in
actin that prevent muscle contraction in the absence
of calcium ions.
2. When a muscle fiber is relaxed, troponin and
tropomyosin prevent myosin from linking up with
actin.
3. In response to a muscle impulse, the sarcoplasmic
reticulum releases stored calcium ions into the muscle
fiber. They bind with troponin, changing its shape
and shifting the tropomyosin to expose myosin
binding sites.
4. Actin and myosin can then form CROSS-BRIDGES.
5. According to the sliding
filament theory, actin
and myosin filaments
slide past one another to
shorten the sarcomeres
and contract the muscle.
IV. Muscle fibers relax when ACETYLCHOLINESTERASE
decomposes acetylcholine and calcium gets pumped back
into the sarcoplasmic reticulum.
V.
Energy sources for contraction
A.
CREATINE PHOSPHATE can regenerate ATP from
ADP (10 sec.).
B.
Cellular respiration
makes ATP (and gives
off heat).
1.
Blood supplies
oxygen for the
citric acid cycle.
2.
MYOGLOBIN found
in muscles stores
oxygen.
C. When oxygen runs out during strenuous activity,
glycolysis is used to make ATP.
1. Less ATP is released
2. Toxic PYRUVATE molecules are converted into lactic
acid.
3. LACTIC ACID builds up in the muscles (before
diffusing into blood) and causes muscle soreness.
VI. Muscular responses
A. The minimal stimulus strength
required to cause contraction
is called the THRESHOLD
STIMULUS.
B. A TWITCH is the response of a
single muscle fiber to the ACh
released by the neuron.
1. Each twitch has both a period
of contraction and relaxation.
2. The LATENT PERIOD
describes the time
between two muscle
twitches.
3. The REFRACTORY
PERIOD is the amount
of time when a muscle
fiber is unresponsive.
C. Even when muscles appear to be
at rest, there are some sustained
contractions going on. This is
called MUSCLE TONE.
D. Types of contractions
1. In ISOTONIC contractions, force is equal and
muscles change in length.
2. In ISOMETRIC contractions, force changes and
muscles remain the same length.
E. FAST and SLOW muscle twitches
1. Slow twitch fibers always have
oxygen and are resistant to fatigue.
2. Fast twitch fibers can be glycolytic (easily fatigue) or
fatigue resistant.
VII.Skeletal muscle actions
A.
The ORIGIN is the immovable end and the
INSERTION is the moveable end of the muscle.
During contraction, the insertion pulls toward the
origin.
B.
The PRIME MOVER (AGONIST) is the main muscle
responsible for the body movement.
C.
SYNERGISTS contract and help a prime mover.
D.
ANTAGONSITS resist movement of the prime mover
and cause movement in the opposite direction.