crossbridges go through the cycle simultaneously but out of phase out of step

Crossbridges go through the cycle simultaneously but

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Excitation-Contraction Coupling: How Muscle Contractions Are Turned On and Of Muscle cells are excitable, meaning that they are capable of generating action potentials if their plasma membranes are depolarized to a sufficient degree When a muscle cell receives input from a motor neuron, the cell depolarizes and fires an action potential that then stimulates contraction The sequence of events that links the action potential to the contraction is referred to as excitation-contraction coupling The Role of the Neuromuscular Junction in Excitation-Contraction Coupling Connection between a motor neuron and a muscle cell, referred to specifically as a neuromuscular junction, is fundamentally similar to an “ordinary” synapse between two neurons in the nervous system Motor neuron (the presynaptic cell) transmits an action potential and secretes the neurotransmitter acetylcholine upon its arrival at the axon terminal A specialized region of the sarcolemma called the motor end plate, which is highly folded and contains a high density of acetylcholine receptors The resulting depolarization (called an end-plate potential) is much larger than an ordinary postsynaptic potential—so large, in fact, that it is always above threshold and triggers an action potential in the muscle cell As the action potential travels through the T tubules, it triggers the release of calcium from the lateral sacs of the sarcoplasmic reticulum The Roles of Calcium, Troponin, and Tropomyosin in Excitation-Contraction Coupling When a muscle cell is relaxed, the concentration of calcium in the cytosol is very low, and little binding occurs between calcium and troponin
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