Skeletal Muscle Contraction Flashcards
(48 cards)
What is a sarcomere?
functional unit of the muscle fiber
Describe sarcomere structure
Three-dimensional cylinder-like arrangement
bordered by structures called Z-discs/Z-lines
Have thick (myosin) and thin filaments (actin)
Has:
A bands
H zone
I bands
Bisecting the H zone is the M line
What are Z-lines?
Structures to which actin myofilaments are anchored
What are M-lines?
Structures to which myosin filaments are anchored
What do M-lines contain?
a myosin-binding protein myomesin (holds the thick filaments in place)
creatine kinase (catalyzes transfer of phosphate groups from phosphocreatine)
What are A bands?
The dark bands (where there are myosin myofilaments)
What are I bands?
Light bands (portions of the thin filaments that do not overlap the thick filaments)
What is the H zone?
Part of the sarcomere which only has myosin and is part of A band
What is excitation-contraction coupling?
for a skeletal muscle fiber to contract, its membrane must first be “excited” (stimulated to fire an action potential)
The muscle fiber action potential, which sweeps along the sarcolemma as a wave, is “coupled” to the actual contraction through the release of calcium ions (Ca++) from the SR.
List the steps of the excitation-contraction coupling.
1.action potential travels along the axon of a motor neuron.
Individual branches to terminate at the NMJ.
- At the NMJ, the axon terminal neurotransmitter, acetylcholine (ACh) is released.
- ACh molecules diffuse across the synaptic cleft and bind to ACh receptors within the motor end-plate
- A channel in the ACh receptor opens and positively charged ions (Na+) can pass through into the muscle fiber, causing it to depolarize, thus producing a muscle action potential
- As the membrane depolarizes, voltage-gated sodium channels are triggered to open.
- Sodium ions enter the muscle fiber and an action potential rapidly spreads along t-tubules to initiate excitation-contraction coupling.
- triggers the release of calcium ions (Ca++) from its storage in the cell’s SR.
- Ca++ then binds to troponin changing its shape and moving tropomyosin on the F-actin to expose the myosin-binding active sites and allow crossbridges to form.
- The Ca++ then initiates contraction, which is sustained by ATP
- Immediately following depolarization of the membrane, it repolarizes
Meanwhile, the ACh is degraded
What kind of channel does the Ach receptor contain?
nonselective cation channel that opens upon neurotransmitter binding
What is the purpose of T-tubules?
T-tubules ensure that the membrane can get close to the terminal cisterns of SR in the sarcoplasm.
What is a triad?
Arrangement of T tubule with two closely associated small cisterns of sarcoplasmic reticulum on each side
What are the two substances absolutely necessary for muscle contraction
ATP and Calcium ions
What is Ach degraded by?
acetylcholinesterase in the synaptic cleft
What is the all or nothing law?
Individual striated muscle fibers do not show graded contraction—they contract either all the way or not at all.
How do skeletal muscles produce graded contractions?
To vary the force of contraction: the fibers within a muscle fascicle do not all contract at the same time.
With large muscles composed of many motor units, the firing of a single motor axon will generate tension proportional to the number of muscle fibres it innervates
What is a motor unit?
axon and all the muscle fibers in contact with its branches
What is the cross-bridge cycle?
For thin filaments to continue to slide past thick filaments during muscle contraction, myosin heads must pull the actin at the binding sites, detach, re-cock, attach to more binding sites, pull, detach, re-cock, etc.
Describe the steps of cross-bridge cycling?
- The active site on actin is exposed as calcium binds to troponin. Troponin changes shape and moves tropomyosin out of the way to expose binding site on actin.
- Cross-bridge formation
- Pi is released, causing myosin to form a stronger attachment to the actin
- power stroke
- ATP binding causes the myosin head to detach from the actin ATP is converted to ADP and Pi by the intrinsic ATPase activity of myosin.
- The energy released during ATP hydrolysis changes the angle of the myosin head into a cocked position. The myosin head is now in position for further movement.
What is cross-bridge formation?
occurs when the S1 region of myosin attaches to the actin while adenosine diphosphate (ADP) and inorganic phosphate (Pi) are still bound to myosin
Describe the power stroke
myosin head moves toward the M-line, pulling the actin along with it.
This conformational change is brought about by the rotating of the S1 and S2 regions of myosin.
In the absence of ATP, the myosin head will not detach from actin.
What state is the myosin head i when it is cocked?
high-energy configuration.
This energy is expended as the myosin head moves through the power stroke
Describe the state of myosin at the end of the power stroke
at the end of the power stroke, the myosin head is in a low-energy position. After the power stroke, ADP is released; however, the formed cross-bridge is still in place, and actin and myosin are bound together.
