skeletal muscles Flashcards
What is the structure of a skeletal muscle?
Skeletal muscle is made up of long muscle fibers (cells) containing myofibrils.
Myofibrils contain repeating units called sarcomeres.
Sarcomeres consist of thick myosin filaments and thin actin filaments.
The muscle fibers are surrounded by a plasma membrane called the sarcolemma and have multiple nuclei.
T-tubules allow the action potential to spread throughout the muscle fiber.
What are the main components of a sarcomere?
The sarcomere is the basic contractile unit of muscle fibers and consists of:
A-band: The dark band that contains both myosin and actin filaments.
I-band: The light band that contains only actin filaments.
H-zone: The region within the A-band where there is no overlap between myosin and actin.
Z-line: The boundary between adjacent sarcomeres.
M-line: The center of the sarcomere, where myosin filaments are anchored.
Describe the sliding filament theory of muscle contraction.
Muscle contraction occurs when actin filaments slide over myosin filaments, shortening the sarcomere.
Myosin heads bind to actin filaments to form cross-bridges.
The myosin head tilts, pulling the actin filament towards the center of the sarcomere (power stroke).
ATP is required for:
Myosin head detachment from actin.
Re-cocking the myosin head.
This process is repeated, leading to the shortening of the sarcomere and muscle contraction.
How do calcium ions regulate muscle contraction?
When an action potential reaches the muscle, it triggers the release of calcium ions from the sarcoplasmic reticulum.
Calcium ions bind to troponin on the actin filament, causing a conformational change.
This change moves tropomyosin, exposing binding sites on actin for the myosin heads to attach, allowing contraction to occur.
When calcium ions are removed, tropomyosin covers the binding sites, and the muscle relaxes.
What role does ATP play in muscle contraction?
ATP is required for:
Myosin head detachment from actin after the power stroke.
Re-cocking the myosin head to its high-energy position.
Pumping calcium ions back into the sarcoplasmic reticulum to stop contraction.
Describe the process of excitation-contraction coupling in skeletal muscles.
An action potential travels along the motor neuron to the neuromuscular junction.
Acetylcholine is released, binding to receptors on the sarcolemma and generating an action potential in the muscle fiber.
The action potential travels down the T-tubules, reaching the sarcoplasmic reticulum.
The sarcoplasmic reticulum releases calcium ions, which bind to troponin, allowing myosin to bind to actin.
This initiates the sliding filament mechanism, causing muscle contraction.
What occurs at the neuromuscular junction?
The neuromuscular junction is the synapse between a motor neuron and a muscle fiber.
When an action potential reaches the motor end plate, acetylcholine is released from the neuron.
Acetylcholine binds to receptors on the sarcolemma, generating an action potential in the muscle fiber.
The action potential then triggers contraction via excitation-contraction coupling.
exam question
Describe the process of muscle contraction, including the role of calcium ions, ATP, and the structure of the sarcomere.
Action potential from a motor neuron triggers the release of calcium ions from the sarcoplasmic reticulum.
Calcium ions bind to troponin, causing tropomyosin to move and expose binding sites on actin for myosin heads.
The myosin heads bind to actin, forming cross-bridges. The power stroke occurs as the myosin heads pivot and pull the actin filament toward the center of the sarcomere, shortening the muscle.
ATP binds to the myosin heads, causing them to detach from actin and re-cock for the next cycle.
When the action potential stops, calcium ions are pumped back into the sarcoplasmic reticulum, and the muscle relaxes.
Exam Question:
Explain the sliding filament theory of muscle contraction.
The myosin head binds to the actin filament to form a cross-bridge.
The myosin head tilts, pulling the actin filament towards the center of the sarcomere (power stroke).
ATP binds to the myosin head, causing it to detach from actin.
The myosin head then re-cocks, and the process repeats.
The sliding of actin over myosin shortens the sarcomere, leading to muscle contraction.
