Download Micro Muscle: Muscle signal response and myosin activity

Micro Muscle: Muscle signal response and myosin activity
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I. Signal Transmission
Muscle fibers begin to contract when they receive signals from the nervous system to do
so. Many different aspects of physiology interact to allow this to take place. Recall from the
nervous system is made up of networks of nervous tissue. This nervous tissue is made of cells
called neurons that can interact with other types of tissue. Neurons that control muscle tissue are
called motor neurons. Motor neurons are responsible for receiving electrical impulses from the
central nervous system and transmitting that signal to the muscle fibers.
Each muscle fiber interacts with an axon of a motor neuron. The site where the neuron
meets the muscle fiber is called the synapse, a space where information can pass from one cell to
another without physical contact. The chemical signals that neurons send to other cells are called
neurotransmitters, which are released into the synapse and give signals to the other cells. In
muscle tissue, the site where neurons and muscle fibers meet is called a neuromuscular
junction. At this location, the muscle fiber of the membrane is specialized to form a motor end
plate that has higher concentrations of mitochondria and nuclei are present and the sarcolemma
is extensively folded.
Typically, a muscle fiber will have a single motor end plate. Motor neurons however
contain many branches which allow the axon to interact with multiple different muscle fibers.
Muscle fibers that are controlled by the same neuron create a unit called a motor unit. The small
gap that separated the membrane of the neuron from the membrane of the muscle fiber is called
the synaptic cleft. The distal end of the nerve fiber contains many small vesicles that contain
neurotransmitters that will be released into the synaptic cleft.
Label the following diagram using the new book page 290
Acetylcholine (ACh) is the neurotransmitter that motor neurons use to control skeletal
muscle contraction. It is created in the cytoplasm of the motor neuron and stored in vesicles in
the axon, close to the synapse. When the action potential reaches the end of the axon, some of
the vesicles release ACh into the synapse of the neuromuscular junction. Once in the synapse,
ACh binds quickly to receptors found on the muscle fiber.
When ACh binds to the receptors, it makes the muscle fiber membrane more permeable
to sodium ions. When the ions rush into the muscle fiber, it creates an electrical response called
a muscle impulse. The muscle impulse then travels all throughout the membrane of the muscle
fiber much like a nerve impulse travels across a nerve cell. This impulse reaches the T- tubules,
into the sarcoplasm, and finally it reaches that sarcoplasmic reticulum and cisternae. Once the
signal has been received in the sarcoplasmic reticulum, it releases calcium ions into the
myofibrils where they will be used by the sarcomeres.
After nerve impulses stop two events occur that relax the muscle tissue. The first of
which is release of acetylcholinesterase. Acetylcholinesterase is a protein that degrades ACh so
that it is no longer in the synapse. This prevents the contraction signal from being constantly
active because the signal molecule is being removed. The secondly, the calcium ion pumps
quickly move calcium ions out of the myofibrils and puts them back into the sarcoplasmic
reticulum where it is stored for the next contraction.
1. Neurons that control muscle function are called?
2. What are neurotransmitters? Which neurotransmitters do motor neurons release?
3. What happens when muscle fiber receptors bind acetylcholine?
4. What protein degrades ACh and why is it important to degrade the neurotrassmitter?
II. Myosin/ Actin filament interactions
Recall from the previous lesson that inside the muscle fiber it is made up of mostly
myofibrils. In those myofibrils they are made up of units called sarcomeres which are units of
contraction that contain thick and thin filaments: thin filaments being made of actin; thick
filaments being comprised of myosin bundles. Myosin is a motor protein that wants to bind to
actin filaments. When the muscle is at rest though, the protein tropomyosin prevents the myosin
from binding with actin. Without myosin being able to attach to actin, muscle contraction cannot
occur.
When muscles receive a signal to contract, calcium ions are released from the
sarcoplasmic reticulum into the myofibrils. The calcium ions quickly bind to a protein located in
the thin filament called troponin. Once troponin binds calcium ions, it moves the tropomyosin
away from the actin-myosin binding sites on the thin filament so that myosin can bind to the
actin.
1. What does tropomyosin do?
2. What function does troponin accomplish?
3. Where do the calcium ions come from that troponin bind to? (List the name of a muscle cell
organelle)
III. Myosin activity (walking)
Once calcium bound troponin moves tropomyosin off the active binding site for myosin,
myosin is now free to perform the steps needed to carry out muscle contractions.
1) The first step is that myosin binds to the actin on the thin filament.
2 )When myosin binds to actin, myosin immediately pulls on the filament once, bringing the
myosin closer to the Z disk of the sarcomere. This pulling motion changes the possition of the
myosin so that it needs to be cocked back into place if it is to perform another pulling motion.
3) After the myosin pulls the actin, it exposes an ATP binding site on the myosin protein and
ATP quickly attaches to the myosin. When ATP is bound to myosin it quickly releases from the
actin filament.
4) Once the filament is released, the myosin uses the energy in ATP to return to its cocked
position so it can pull again.
These steps will continually repeat until the myofibril runs out of ATP or calcium ions. That is
why muscle tissues have high numbers of mitochondria so it can supply large amounts of ATP to
the muscle fibers. Remember that this is an action of a single myosin protein from the thick
filament. In real life a thick filament has much more myosin completing these same steps too and
not in sync with other myosin proteins. When the ACh is no longer bound to the ACh receptors,
calcium ions are no longer released into the myofibrils and troponin no longer can inhibit
tropomyosin. Tropomyosin in turn returns to its resting position of blocking myosin from
binding to actin. This causes the muscle fiber then to relax.
1. What stops myosin from binding actin when the muscles are relaxed?
2. How is ACh removed so that the muscles can relax?
3. Using the ordered steps above, label the diagram below with the step number. (Note ADP is
not ATP)
Review Questions: page 293 of the new book 5-9
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