What is a sarcomere?
A sarcomere is the basic contractile unit of a muscle fiber, formed from thick (myosin) proteins and thin (actin) proteins.
What are the primary thick and thin filaments?
The thick filament is myosin , and the thin filament is actin.
How is the A-band defined? What does it contain?
The A-band is the darker, thicker band on a micrograph. It is formed from myosin and the overlapping regions of actin.
How is the I-band defined? What does it contain?
The I-band is the lighter band. It contains only the thin actin protein fibers
What structures define the boundaries of a sarcomere?
The Z-lines (or Z-discs) form the end boundaries of each sarcomere
What structures are in the center of the sarcomere?
The M-line runs through the middle, holding the myosin fibers in place. The H-zone is the region surrounding the M-line that contains only the thick myosin protein fibers
What is the effect of contraction on the sarcomere and its parts?
Contraction causes the sarcomere to compress (shorten). The H-zone and I-band shrink , and the zone of overlap increases , but the individual filament lengths do not change.
Where do the neuron and muscle fiber meet?
At the neuromuscular junction (NMJ)
What are the three components of the neuromuscular junction?
The synaptic terminal of the neuron, the motor end plate of the muscle fiber, and the gap between them, the synaptic cleft.
What is the neurotransmitter released at the NMJ?
Acetylcholine (ACh)
What sequence of events is initiated by ACh binding to the motor end plate?
ACh binds to receptors, opens sodium ion channels , and sodium rushes in, generating a depolarization response that leads to an action potential in the sarcolemma (muscle cell membrane)
What happens to ACh after it initiates a signal?
The enzyme acetylcholinesterase inactivates ACh in the synaptic cleft to stop further receptor activation.
How does the action potential lead to the release of calcium?
The action potential travels along the sarcolemma and down the invaginations called T-tubules. Depolarization of the T-tubules stimulates the release of calcium ions from the sarcoplasmic reticulum (SR)
What structure within the muscle fiber stores calcium?
The sarcoplasmic reticulum.
In a resting muscle cell, what covers the active binding sites on actin?
Troponin molecules, held in place by tropomyosin molecules, cover the actin binding sites.
What is the immediate role of calcium in the contraction process?
Calcium binds with troponin , causing it to shift position and expose the actin active binding sites.
What happens once the actin binding sites are exposed?
Myosin heads, which are already cocked with stored energy from ATP , bind to the exposed actin active sites.
Describe the power stroke
The myosin head pivots toward the M-line, pulling the actin filament along. This movement releases the stored energy, and ADP and Pi are released
How does the myosin head detach from actin after the power stroke?
A new molecule of ATP binds to the myosin head, allowing it to release from the actin.
What role does ATP play after detachment?
The ATP is hydrolyzed to ADP and Pi, which cocks the myosin head into its resting position, ready for another power stroke.
Why does rigor mortis occur?
Ca^2+ is still present to stimulate contraction, but ATP is depleted, so the myosin heads cannot detach and relaxation cannot occur until muscle proteins decompose
Are sarcomeres visible in smooth muscle?
No, sarcomeres are not visible in smooth muscle, although it still contains actin and myosin
In smooth muscle, what protein does calcium interact with instead of troponin?
Calcium interacts with the protein calmodulin.
What is the ultimate action of calmodulin in smooth muscle contraction?
Calmodulin interacts with tropomyosin to regulate the actin and myosin binding sites.
