Skeletal Muscle
Each muscle fiber is a single long cylindrical muscle cell. Several terms are unique to muscle cells.
Sarcolemma – is the muscle cell membrane.
Sarcoplasm – is the muscle cell’s cytoplasm, which contains many mitochondria and nuclei.
Myofibrils – thick and thin filaments in muscle, used for contraction.
Myosin – thick filament
Actin – thin filament
Troponin and Tropomyosin – Protein complex that covers actin-myosin binding site
The organization of myosin and actin produce the striations seen under a microscope.
Sarcomere – From Z line to Z line. The Z line is a partition in a muscle cell.
When looking under a microscope several defined areas or bands can be seen.
I bands (light bands) are made of actin filaments and are anchored to Z lines.
A bands (dark bands) is the region of overlapping thick and thin filaments.
H bands is the center of the band and consist of myosin filaments only
Questions
Which filament is attached to the Z line? ___________________________________________
Which filament is thinner, actin or myosin? ________________________________________
Which filament is pulled across the other? _________________________________________
Which protein covers the actin binding site? ________________________________________
What function does Troponin and Tropomyosin perform? ______________________________
Skeletal muscle fibers have a unique site called the neuromuscular junction. The nerve communicates to the muscle from the presynaptic nerve terminal to the motor end plate. The nerve releases acetylcholine that crosses the synapse and binds to receptors on the motor end plate.
The muscle fiber membrane is modified to form the motor end plate. There are an abundant amount of nuclei and mitochondria in this region. The sarcolemma is tightly folded.
Presynaptic region – Nerve End
The cytoplasm of a motor neuron contains numerous mitochondria and synaptic vesicles store neurotransmitters. When the nerve action potential reaches the end of the nerve it causes the release of neurotransmitters (acetylcholine) that cross the space and bind to the motor endplate of muscle, which activates the sarcolemma for contraction.
A muscle will not contract unless told to do so by the motor nerve.
Questions
What tells the motor nerve to signal the muscle? _____________________________________
What neurotransmitter does the motor axon release? __________________________________
What is a motor neuron? _________________________________________________________
What is a neurotransmitter? ______________________________________________________
What happens when a neurotransmitter binds to the motor end plate? _____________________
What happens when a neurotransmitter does not bind to the motor end plate? ______________
What is the name of the space between the nerve and motor end plate? ___________________
A motor unit -The motor nerve and all the muscle fibers it controls.
A single motor neuron axon will give branches to multiple muscle fibers. So one nerve controls multiple muscle fibers.
Does one motor neuron communicate with one or many nerve fibers? ___________________
Muscle Contraction
A muscle contraction is the shortening of a sarcomere, and the pulling of muscles against its attachment sites. For this process be accomplished myosin pulls actin across it, which shortens the sarcomere. (Z lines move closer together). With numerous sarcomere shortening at once the overall length of the muscle fiber decreases producing a visible contraction.
Muscle contraction occurs when the myosin pulls the actin fiber and across it. The myosin protein fiber head remains in its high-energy state ready to bind onto the first available actin binding site. Myosin is usually prevented from binding onto actin because of a combination of proteins called troponin and tropomyosin. Troponin and tropomyosin cover the actin binding site and prevent myosin from attaching to it without the appropriate signals.
Troponin and tropomyosin prevents binding until calcium rushes into the muscle cell and binds to troponin and tropomyosin complex. Calcium binding causes troponin and tropomyosin to shift shape, which results in the uncovering of the actin binding site. Now myosin immediately grabs onto actin and pulls the fiber. Myosin pulls from its high energy position into its low energy position. The pulling action shortens the distance between Z lines, since actin is attached to the end wall of the fiber (Z line).
ATP then comes over and binds to myosin, causing it to release actin. Myosin then breaks the ATP (into ADP and P) and utilizes its energy to shift back into the high-energy state where it can immediately grab onto the next available actin binding site. This process continues until the actin binding site is recovered by troponin and tropomyosin (because calcium is removed which causes troponin and tropomyosin shift back to its original shape and recover the binding site).
ATP is the energy source for muscle contraction. Binding of ATP molecule on the myosin protein causes myosin to release actin, and myosin then utilizes ATP energy to shift back into its high-energy state. ATP is required for myosin to release actin. Without ATP myosin would maintain its low-energy state attached to the actin fiber, which is what occurs with rigor mortis.
Which protein is the larger protein that pulls the smaller protein? _______________________
What usually prevents myosin from binding to actin? ________________________________
What role does calcium play in contraction? _______________________________________
What happens if calcium is not in the muscle fiber? ________________________________
What happens when calcium enters the fiber? _____________________________________
What causes troponin and tropomyosin to change shape and cover the actin binding site?
__________________________________________________________________________
Calcium is normally stored in the sarcoplasmic reticulum and T tubules. The motor neuron releases the neurotransmitter acetylcholine from the presynaptic terminal which crosses the synaptic cleft and binds onto the motor plate. Binding of acetylcholine causes the motor end plate to send an impulse over the surface of the sarcoplasmic reticulum and into the T tubules. This impulse causes the release of calcium into the sarcoplasm of the muscle fiber. The high concentration of calcium rushes down its concentration gradient quickly into the sarcoplasm where it binds with the troponin and tropomyosin complex.
Calcium is removed from the sarcoplasm and is pulled back into its storage area in the sarcoplasmic reticulum. As the concentration of calcium decreases less of the actin binding sites are available for myosin to grab onto. This results in the stopping of the muscle contraction.
Where is calcium usually stored? _____________________________________________
What stimulates the release of calcium? _______________________________________
Is there a difference between concentrations of calcium inside vs outside the cell? ________
Does calcium require energy to move into the cell? _______________________________
Does calcium require energy to move out of the cell? _____________________________
Name the type of diffusion that calcium moves into the cell by? _____________________
Name the type of diffusion that calcium moves out of the cell by? ___________________
What happens to troponin and tropomyosin when calcium is removed from the muscle fiber? ________
With abundant amounts of calcium rushing in and binding to troponin and tropomyosin numerous actin binding sites are available to myosin. With continued release of calcium more can bind to troponin and tropomyosin causing increased shifting away from actin. The more myosin that can pull actin the greater the contraction. More calcium in a muscle fiber allows for a stronger contraction due to the volume of myosin heads binding and pulling actin. As calcium is removed from the muscle cell there are less actin binding sites available and the contraction force decreases. Eventually all of the calcium is removed and myosin is unable to bind to actin.
Muscle relaxation is caused by the active transport of calcium into the sarcoplasmic reticulum, removing calcium from the sarcoplasm (covering actin).
The more available binding sites available the more binding and pulling is possible = Greater contraction.
Why does the force of contraction depend on the amount of calcium inside a cell? ________________________
Why is the force of contraction dependent on the concentration of calcium in a muscle fiber? ______________
What happens if the cell was unable to pump calcium out of the sarcoplasm? ____________________
What happens if calcium is unable to enter the cell? _________________________________
Acetyl acetylcholinesterase is an enzyme that breaks down acetylcholine in the synaptic cleft.
ATPase is the enzyme involved with ATP in the muscle fiber.
The process of muscle contraction is highly controlled. It depends on the nervous system stimulating the motor end plate of a muscle fiber. You can consciously tell a motor neuron to contract a muscle, which is why this is a voluntary contraction.
Energy for muscle contraction comes from molecules of ATP. ATP is produced by mitochondria. Since muscle contraction requires a high concentration of ATP, muscle fibers have large amounts of mitochondria within the cells. A more active and trained muscle cell has more mitochondria to produce vast amounts of ATP. Likewise, an inactive muscle fiber has few mitochondria. You earn your density of mitochondria.
Creatine phosphate stores excess energy produced by the mitochondria. When sufficient levels of ATP are formed the excess energy is transferred to the intermediate molecule creatine phosphate. When ATP levels begin to drop with activity creatine phosphate transfers the energy to the ADP and phosphate producing ATP. Think of creatine phosphate as an intermediate storage molecule for ATP.
Creatine phosphokinase promotes the synthesis of creatine phosphate.