Download Muscle Contraction PWP File

1.Functions of the muscular system:
-movement,
-posture & body position
-body temperature
-protection.
2. The structure of skeletal muscle:
a. Muscle cells are called muscle fibers.
they are long, tubular cells.
b. Each fiber is surrounded by connective
tissue.
c. The fibers are striated and voluntary.
d. A fascicle is a group of muscle fibers.
Each fascicle is surrounded by
connective tissue. The fascicles in a
given area comprise the muscle and are
surrounded by connective tissue. This
connective tissue = fascia.
Fascicle
Tendons (connect muscle to bone) are
formed from fascia that continues past
the muscle.
3. The structure of the muscle fiber:
a. Sarcolemma – cell membrane around
each muscle fiber (or myofiber)
b. Myofilaments – actin and myosin that
make up the myofibril. Actin (thin) and
myosin (thick) are made of protein.
The actin and myosin overlap
slightly to give the muscle fiber the
striped (striated) appearance
-They are organized in repeating
units called sarcomeres.
c. T Tubules (transverse tubules) – extend
from the cell membrane into the
interior of the muscle fiber.
d. Sarcoplasmic reticulum – store calcium
ions which are necessary for contraction.
myosin
actin
4. The Neuromuscular Junction
a. Skeletal muscles are controlled by motor
neurons. Where they connect is called
a synapse or a neuromuscular junction.
b. Presynaptic terminal – located at the end
of the neuron’s axon.
c. Postsynaptic terminal – synaptic area of
the muscle fiber membrane.
d. Synaptic cleft – the space between the
pre and postsynaptic terminals.
e. Neurotransmitter–chemical (acetylcholine) located in the synaptic bulb.
http://www.blackwellscience.com/matthews/nmj.html
5. So how exactly does a muscle fiber contract?
a. A nerve impulse (action potential) travels
down the axon to the presynaptic terminal.
Ca++ influx causes vesicles (filled with Ach)
to fuse with the membrane of the presynaptic bulb.
b. The neurotransmitter (Ach) is released from the
presynaptic terminal (bulb) and travels across
the cleft.
c. The neurotransmitter binds with receptors
on the postsynaptic terminal (motor end
plate or post synaptic terminal) of the muscle
fiber.
d. The binding of the neurotransmitter and
the receptors triggers a nerve impulse
to travel across the muscle fiber
membrane (sarcolemma).
Synaptic terminal
of motor neuron
Synaptic cleft
PLASMA
MEMBRANE
T TUBULE
SR
ACh
Ca2+
CYTOSOL
Ca2+
e. This nerve impulse (action potential)
travels along the cell
membrane (sarcolemma) and
down through the T tubules to the
interior of the cell.
f. Calcium is released from the sarcoplasmic
reticulum in response to the nerve
impulse.
g. The calcium acts on one of the two
regulatory proteins:
-troponin= attached to actin. When it
binds with calcium it causes a
conformational change to tropomyosin.
-tropomyosin = filament that covers
the myosin binding site on the actin.
Tropomyosin
Ca2+-binding sites
Actin
Troponin complex
Myosin-binding sites blocked.
Ca2+
Myosinbinding site
Myosin-binding sites exposed.
h. The change in the shape of the tropomysin
causes it to move and allows
a cross-bridge to form between the actin
and the myosin..
i. So, how does a cross-bridge form? ATP bound
to myosin is hydrolized = now high energy
(ADP + Pi complex).
and rapidly binds to myosin head.
Then the Pi is released causing a conformational change in the myosin. When the
myosin changes shape, it pulls on the actin it
is attached to. This is called a power stroke.
j. Lastly, the ADP is released. A new ATP binds
to the myosin head allowing it to release from
actin. This ATP is hydrolized and the whole
cycle can begin again.
ATPase Cycle:
Website #1 http://www.nismat.org/physcor/muscle.html
Animation #1 http://www.blackwellscience.com/matthews/myosin.html
Animation #2 http://www.sci.sdsu.edu/movies/actin_myosin.html
6. A contraction ends when there is no
more stimulus (no AP). Ca++ pumps
pump out the calcium. This causes the
tropomyosin to move over the myosin
binding site breaking the cross bridges.
Synaptic terminal
of motor neuron
Synaptic cleft
PLASMA
MEMBRANE
T TUBULE
SR
ACh
Ca2+
CYTOSOL
Ca2+
7. If the stimulus is large enough, the muscle
fiber contracts fully. All or nothing principal
8. Motor Unit – Motor neuron and all of the
muscle fibers controlled by it. Most control
hundreds of muscle fibers. All or nothing
contraction.
Motor Unit
9. Whole muscles respond in a graded fashion.
Stronger stimulus = stronger contraction.
When contractions of greater strength are
needed, larger and larger motor units are
activated = recruitment.
10. Types of Muscle Fibers
a. Slow Twitch – good for endurance, have
many capillaries and many
mitochondria. Distance runners
b. Fast Twitch – higher glucogen content and
faster ATP production. Sprint runners.
“Color”
Contraction
Metabolism
Blood Supply
Fiber Diameter
Myosin ATPase
Mitochondria
Myoglobin
Performance
Fast Twitch, Glycolytic
(white muscles)
White
Fast
Glycolytic / anaerobic
Low
Large
High
Few
Low
Rapid, powerful contractions (jumping)
Slow Twitch,Oxidative
(red muscles)
Red
Slow
Oxidative / Aerobic
High
Small
Low
Many
High
Prolonged, continued
muscle activity
(marathon races)