Muscle Contraction

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Sarcomere Noun

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[sar-koh-meer]

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Sarcomere

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The basic contractile unit of a muscle fiber, defined as the segment between two neighboring Z-discs in a myofibril.

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Example: This diagram shows a sarcomere, the basic unit of a muscle, which is made of thick and thin protein filaments that cause muscle contraction.

Front

Sarcolemma Noun

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[sar-koh-lem-uh]

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Sarcolemma

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The specialized cell membrane that encloses a striated muscle fiber, which receives and conducts stimuli for muscle contraction.

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Example: This diagram shows a muscle fiber, where the sarcolemma is the outer membrane that encloses the internal parts like myofibrils and sarcoplasm.

Front

Sarcoplasm Noun

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[sar-koh-plaz-um]

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Sarcoplasm

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The specialized cytoplasm of a muscle fiber, containing myofibrils, mitochondria, and other organelles necessary for muscle function.

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Example: This diagram shows a muscle fiber is filled with a substance called sarcoplasm, which is the jelly-like cytoplasm of the muscle cell.

Front

Myofibril Noun

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[my-oh-fy-bril]

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Myofibril

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A long, cylindrical organelle found within muscle cells that is composed of actin and myosin filaments and performs contraction.

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Example: This diagram shows a muscle fiber is a bundle of smaller rods called myofibrils, which are the parts that actually contract.

Front

Sarcoplasmic Reticulum Noun

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[sar-koh-plaz-mik reh-tik-yuh-lum]

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Sarcoplasmic Reticulum

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A specialized network of membranes in muscle cells that stores and releases calcium ions to trigger muscle contraction.

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Example: This diagram shows the sarcoplasmic reticulum, a network of tubes that wraps around muscle fibers, storing and releasing the calcium needed for muscle contraction.

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Actin Noun

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[ak-tin]

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Actin

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A protein that forms the thin filaments in muscle fibers, providing the binding sites for myosin heads during contraction.

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Example: This diagram shows an actin filament, a key protein structure in muscle cells, composed of smaller units and having distinct 'plus' and 'minus' ends.

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Myosin Noun

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[my-uh-sin]

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Myosin

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A motor protein that forms the thick filaments in muscle fibers, featuring heads that bind to actin to generate force.

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Example: This diagram shows a myosin molecule, with its distinct 'head' and 'tail' regions, which is a key protein for muscle contraction.

Front

Troponin Noun

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[troh-poh-nin]

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Troponin

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A regulatory protein complex that binds to calcium ions, causing a conformational change that exposes actin-binding sites.

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Example: This diagram shows the troponin protein complex on an actin filament, which acts like a switch to control muscle contraction by interacting with tropomyosin.

Front

Tropomyosin Noun

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[troh-poh-my-uh-sin]

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Tropomyosin

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A regulatory protein that covers the myosin-binding sites on actin filaments, preventing contraction in a relaxed muscle.

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Example: This image shows how the protein tropomyosin covers myosin binding sites on an actin filament, and how it moves to expose them when calcium arrives.

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Neuromuscular Junction Noun

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[noor-oh-mus-kyuh-ler junk-shun]

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Neuromuscular Junction

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The specialized synapse where a motor neuron's axon terminal connects with a muscle fiber to transmit a nerve impulse.

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Example: A motor neuron connects to a skeletal muscle fiber at a special point called the neuromuscular junction (NMJ), where signals for muscle contraction are passed.