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The sarcoplasmic reticulum (SR) is a specialized smooth endoplasmic reticulum in muscle fibers. Its main function is to store calcium ions (Ca²⁺) and release them into the sarcoplasm when a muscle fiber is stimulated. Calcium is essential to activate the interaction between actin and myosin during contraction.
When calcium binds to troponin, troponin changes shape. This causes tropomyosin to move away from the active sites on actin, allowing myosin heads to bind to actin and start contraction. Tropomyosin does not block actin after calcium bind, it blocks actin only when the muscle is relaxed.
During muscle contraction, thin filaments slide over thick filaments. The I band (thin filaments only) becomes shorter as overlap increases. The A band stays the same length, because thick filaments do not change length.
Acetylcholine is released from synaptic vesicles in the synaptic knob of a motor neuron at the neuromuscular junction. It diffuses across the synaptic cleft and binds to receptors on the muscle fiber to initiate contraction.
A tendon is a strong connective tissue made of collagen fibers that attaches muscle to bone, allowing muscle contraction to move the skeleton.
When a nerve impulse reaches the synaptic knob, voltage-gated calcium channels open. Calcium enters the neuron and triggers exocytosis of acetylcholine from synaptic vesicles.
Titin is a large, spring-like protein that connects thick filaments to the Z disc. It prevents overstretching, stabilizes the sarcomere, and helps the muscle return to its original length after contraction.
Cardiac muscle has striations, is involuntary, and can contract without nervous stimulation due to its autorhythmic pacemaker cells.
There are only three muscle types: skeletal, smooth, and cardiac. “Liquid” is not a muscle type.
Skeletal muscle fibers are very long cells and can reach up to 30 cm.
Calcium ions bind to troponin and initiate the sliding filament mechanism, making them essential for contraction.
The Z line (Z disc) anchors thin filaments (actin) and elastic filaments (titin), defining the boundaries of a sarcomere.
Smooth muscle does not have striations, unlike skeletal and cardiac muscle. It also contracts slowly and involuntarily.
Epimysium is the outer connective tissue sheath that covers the whole muscle.
- endomysium – connective tissue around muscle cells
- perimysium – connective tissue around muscle fascicles
- epimysium – connective tissue surrounding entire muscle
A sarcomere is the basic unit of contraction, extending from one Z disc to the next.
Acetylcholinesterase (AChE) breaks down acetylcholine in the synaptic cleft, stopping continuous stimulation and allowing muscle relaxation.
Skeletal muscle is under conscious control and attached to bones for body movement.
ATP provides energy for cross-bridge cycling, myosin head movement, and muscle relaxation.
Action potentials move along the sarcolemma and down T-tubules, stimulating the sarcoplasmic reticulum to release calcium.
Synaptic vesicles in the synaptic knob store and release acetylcholine into the synaptic cleft during nerve stimulation.
Perimysium is a thicker connective tissue sheath that groups individual muscle fibers into bundles called fascicles
After death, calcium leaks into the muscle, activating cross-bridges. Relaxation requires ATP to detach myosin from actin, but ATP is no longer produced, so muscles remain stiff until protein decay
SR is a specialized smooth ER that stores calcium ions and releases them during stimulation to trigger contraction
The action potential travels down T-tubules, causing SR to release calcium into the sarcoplasm, which initiates contraction
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