Excitation of a muscle cell's membrane, known as the sarcolemma, initiates contraction through a well-coordinated series of events that involve the generation of an action potential and the subsequent release of calcium ions (Ca++) from the sarcoplasmic reticulum (SR). Here’s how this process works:
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Action Potential Generation: The process begins when a motor neuron releases the neurotransmitter acetylcholine at the neuromuscular junction. This neurotransmitter binds to receptors on the sarcolemma, leading to depolarization of the membrane. If the depolarization reaches a certain threshold, it triggers an action potential.
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Propagation of Action Potential: The action potential travels along the sarcolemma and then spreads into the muscle cell through invaginations in the membrane called T-tubules (or transverse tubules). This allows the action potential to penetrate deep into the muscle fibers, ensuring that the depolarization reaches all areas of the cell.
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Activation of the Sarcoplasmic Reticulum: The arrival of the action potential at the T-tubules triggers the sarcoplasmic reticulum (SR), a specialized type of smooth endoplasmic reticulum surrounding the myofibrils, to respond. The action potential causes the SR to open Ca++ channels located in its membrane.
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Release of Calcium Ions: When the Ca++ channels in the SR open, Ca++ ions are released into the cytoplasm of the muscle cell. The increase in intracellular calcium concentration is critical for muscle contraction.
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Muscle Contraction: The released Ca++ ions bind to the protein troponin, which causes a conformational change in another protein, tropomyosin, that normally blocks binding sites on the actin filaments. This allows myosin heads to attach to the exposed binding sites on actin, forming cross-bridges. The interaction between actin and myosin leads to the sliding filament mechanism of contraction, resulting in muscle shortening and tension.
In summary, the excitation of a muscle cell's membrane leads to the generation of an action potential that travels along the sarcolemma and through T-tubules, activating the sarcoplasmic reticulum to release Ca++ ions. This increase in Ca++ concentration ultimately triggers the contraction of the muscle cell through the interaction of actin and myosin filaments.