Muscle Contraction Flashcards
What are the three types of muscle tissue?
Skeletal muscle, cardiac muscle, smooth muscle
These muscle tissues differ in structure, function, and control mechanisms.
What is the characteristic shape of skeletal muscle cells?
Long cylindrical and multinucleate
Skeletal muscle cells form from the fusion of separate embryonic cells.
How does cardiac muscle differ from skeletal muscle in terms of structure?
Branched and striated
Cardiac muscle is found in the heart and operates involuntarily.
What is the shape of smooth muscle cells?
Fusiform, long and tapered with one nucleus
Smooth muscle lacks striations and is found in hollow organs.
Where is skeletal muscle located in the body?
Attached to bones or skin
Skeletal muscle is under voluntary control.
Where is cardiac muscle located?
In the walls of the heart
Cardiac muscle is under involuntary control.
Where is smooth muscle found?
In the walls of hollow visceral organs
Smooth muscle also operates involuntarily.
What is the membrane surrounding a muscle cell called?
Sarcolemma
The cytoplasm inside a muscle cell is called sarcoplasm.
What are myofibrils?
Long slender contractile organelles in muscle cells
Myofibrils are composed of myofilaments.
What are myofilaments?
Smallest units contained inside myofibrils
They are made of proteins that contribute to muscle contraction.
What causes the striated appearance of skeletal muscle?
Arrangement of myofilaments in myofibrils
Dark and light stripes correspond to overlapping and non-overlapping myofilaments.
What are the two types of myofilaments?
Actin (thin) and myosin (thick)
Actin forms the light bands, while myosin forms the dark bands in striated muscle.
What is a sarcomere?
Smallest functional unit of a muscle cell
It is defined by the distance between Z discs.
What are the components of a sarcomere?
A band, I band, Z disc, M line, H zone
The A band contains overlapping filaments; the I band contains only actin.
What is the function of titin in muscle cells?
Connects myosin filaments to the Z disc
Titin helps maintain the structure of the sarcomere.
What initiates muscle contraction at the neuromuscular junction?
Release of acetylcholine (ACh)
ACh binds to receptors on the sarcolemma, leading to depolarization.
What is the role of calcium ions in muscle contraction?
Bind to troponin, exposing binding sites on actin
This allows myosin heads to attach to actin.
What is the sliding filament mechanism?
Thin filaments slide past thick filaments during contraction
Neither filament changes length; the sarcomere shortens.
What happens to the H zone and I band during muscle contraction?
H zone disappears and I band shortens
This indicates increased overlap of actin and myosin.
What is the action potential’s role in muscle contraction?
Triggers excitation-contraction coupling
It spreads along the sarcolemma and down T-tubules.
What is the role of tropomyosin in muscle contraction?
Blocks myosin-binding sites on actin in a relaxed state
It is moved away by troponin when calcium binds.
What is the function of the sarcoplasmic reticulum?
Stores and releases calcium ions
It regulates intracellular calcium levels during muscle contraction.
What is the relationship between the T tubules and the sarcoplasmic reticulum?
T tubules run between terminal cisternae of the sarcoplasmic reticulum
They facilitate rapid transmission of action potentials.
What is the significance of the M line in a sarcomere?
Holds myosin filaments in place
It is located in the center of the sarcomere.
What changes shape to remove tropomyosin’s blocking action during muscle contraction?
Calcium ions
Calcium ions bind to troponin, leading to a change in the conformation of tropomyosin.
What are the events of excitation-contraction coupling responsible for?
Preparing the muscle for contraction
What is the first step in excitation-contraction coupling?
Action potential generated at the neuromuscular junction propagates along the sarcolemma and down into the T tubules
What happens in Step 2 of excitation-contraction coupling?
Calcium ions are released from the sarcoplasmic reticulum
What alters the shape of the voltage-sensitive tubule proteins?
Action potential travelling down the T tubule
What protein does calcium bind to in the sarcoplasm?
Troponin
What does calcium binding to troponin do?
Removes the blocking action of tropomyosin
What occurs when the myosin binding site is exposed?
Myosin binds to actin forming a cross bridge
What is the cross bridge cycle?
A series of events where myosin heads pull thin filaments toward the center of the sarcomere
What is Step 1 of the cross bridge cycle?
Energised myosin head attaches to the active actin binding site
What occurs during the power stroke in Step 2 of the cross bridge cycle?
Myosin head pivots and bends, pulling the actin filament toward the M line
What causes cross-bridge detachment in Step 3 of the cross bridge cycle?
ATP attaches to myosin, weakening the link between myosin and actin
What happens during the cocking of the myosin head in Step 4?
Myosin head returns to its pre-stroke or high-energy position
What is necessary for the cross bridge cycle to continue?
ATP and calcium bound to troponin
What happens to calcium levels in the cytosol during muscle relaxation?
Calcium is pumped back into the sarcoplasmic reticulum
What returns to cover the binding sites on actin when calcium levels drop?
Tropomyosin
What are striations in skeletal muscle?
Vertically arranged light and dark bands along a muscle fibre
What do dark bands in skeletal muscle correspond to?
Regions of overlapping thick and thin myofilaments
What is the functional contractile unit of a muscle cell?
Sarcomere
What are myofibrils?
Bundles of specially arranged myofilaments
What changes occur in the banding pattern when a muscle fibre contracts?
The banding pattern changes
List the steps of excitation-contraction coupling.
- Spread of action potential along the sarcolemma to the T-tubules
- Release of calcium from sarcoplasmic reticulum
- Binding of calcium to troponin and exposure of active site
What is necessary for muscle relaxation to occur?
ATP to destabilize cross bridges and pump calcium ions back into the sarcoplasmic reticulum
