Download Control of Muscle Fiber Contraction 1. Contraction is under control of

Control of Muscle Fiber Contraction
1. Contraction is under control of the nervous system
2. Communication between nervous system and a skeletal muscle fiber (cell)
occurs at an intercellular connection = Neuromuscular Junction (NMJ)
3. Nerve cell (neuron):
a. Dendrites and nucleus in the cell body
b. Axon
c. Synaptic terminals at the ends.
4. NMJa. Each muscle fiber (cell) is controlled by a nerve cell = motor neuron
b. Axon branches within the perimysium to form synaptic terminals
throughout the perimysium.
c. Synaptic terminals communicate chemically with the muscle fiber (p.
191 of text)
d. Cytoplasm of synaptic terminals contain mitochondria and vesicles
filled with acetylcholine (Ach) = one type of neurotransmitter (a
chemical that is released by a neuron to communicate with other
cells).
e. Ach, changes the permeability of the sarcolemma, which will result in
the triggering of the contraction of the muscle fiber.
f. Synaptic Cleft = the space between the synaptic terminals and the
sarcolemma.
g. Motor End Plate = the part of the sarcolemma that has receptors for
Ach
h. Acetylcholinesterase (AChE) = an enzyme in the synaptic cleft and
the motor end plate which breaks down Ach.
i. Action Potential = Electrical impulse that travels down an axon to the
synaptic terminals
5. Steps leading up to contraction: (see p. 191)
a. Arrival of an action potential at the synaptic terminals.
b. Release of Ach from vesicles in the synaptic terminals into the
synaptic cleft.
c. Ach binds to the receptors on the sarcolemma at the motor end plate.
This changes the permeability of the sarcolemma to sodium ions.
d. Sodium ions rush into the sarcoplasm, producing an action potential in
the sarcolemma
e. The action potential spreads over the entire sarcolemma surface.
i. Travels down all of the T-tubules, to the terminal cisternae that
encircles the sarcomere.
f. The action potential in the cisternae triggers a sudden massive release
of calcium ions into the sarcoplasm. Hi conc. of calcium ions bind to
the troponin causing the troponin to change shape and pull the
tropomyosin off of the active sites on the actin.
g. Cross-bridge interactions occur between the myosin and the actin and
a contraction begins.
h. Simultaneously, Ach is broken down by AChE
6. Steps of the Contraction Cycle
a. Myosin head has an ADP and a phosphate group attached to it. This is
stored energy just waiting to be used.
b. Calcium flooded the sarcoplasm the active site of the actin is exposed.
c. Myosin cross bridge forms and attaches to the exposed active site on
the actin.
d. The attached head pivots toward the center of the sarcomere and ADP
and phosphate are released.
e. Myosin head binds to an ATP floating by and the cross bridge
detaches (the affinity of the head for the active site is so strong that it
requires the binding of ATP to detach it).
f. Myosin head splits the ATP it captured and releases its energy (and
storing it by pivoting the head back or cocking it into its original
position, Ready to go with an ADP and phosphate again. The myosin
head is “cocked” so that when it attaches and pivots it snaps and pulls
the actin in towards the center of the sarcomere like a powerstroke.
The ATP is needed to “cock” the head again so it can pull on the actin
again and make it even shorter.
g. Myosin heads are continuously attaching, pivoting, detaching, and
reattaching.
h. The cycle keeps repeating (with steps c-f) until the calcium ion
concentration is back to normal resting levels. (Active transport
pumps it back into the sarcoplasmic reticulum and the terminal
cisternae.
i. 1 action potential, the contraction will be very brief.
j. Many action potentials, one after another, and calcium conc. stay hi,
will be a sustained contraction.
k. As the calcium is pumped out of sarcoplasm, then the calcium lets go
of the troponin and the tropomyosin once again covers the active site.
l. Muscle relaxation occurs.
7. Rigor Mortis:
a. At death, circulation ceases and muscles do not receive nutrients and
oxygen.
b. Over a few hours, the muscle fibers run out of ATP, and the
sarcoplasmic reticulum cannot remove calcium ion from the
sarcoplasm anymore.
c. Calcium builds up in the sarcoplasm and triggers a sustained
contraction.
d. No ATP means the cross bridges cannot detach from the active sites
and the muscle locks in the contracted position.
e. All muscles are involved = “stiff as a board”.
f. Lasts until the lysosomal enzymes released by autolysis break down
the myofilaments 15-25 hours later.