how myofibrils arranged longitudinally in smooth muscle fibers
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Skeletal muscle cells are innervated by motor neurons. A motor unit is defined as the neuron and the fibers it supplies. Some motor neurons innervate one or a few muscle cells whereas other motor neurons can innervate hundreds of muscle cells. Muscles that require fine control have motor neurons that innervate fewer muscle cells; muscles that participate in less controlled movements may have many fibers innervated by each neuron. Motor axons terminate in a neuromuscular junction on the surface of skeletal muscle fibers. The neuromuscular junction is composed of a pre-synaptic nerve terminal and a post-synaptic muscle fiber. Upon depolarization, the pre-synaptic vesicles containing the neurotransmitter acetylcholine fuse with the membrane, releasing acetylcholine into the cleft. Acetylcholine binds to receptors on the post-synaptic membrane and causes depolarization of the muscle fiber, which leads to its contraction. Typically, one action potential in the neuron releases enough neurotransmitter to cause one contraction in the muscle fiber.
In muscle cells, the sarcolemma or plasma membrane extends transversely into the sarcoplasm to surround each myofibril, establishing the T-tubule system. These T-tubules allow for the synchronous contraction of all sarcomeres in the myofibril. The T-tubules are found at the junction of the A- and I- bands and their lumina are continuous with the extracellular space. At such junctions, the T-tubules are in close contact with the sarcoplasmic reticulum, which forms a network surrounding each myofibril. The part of the sarcoplasmic reticulum associated with the T-tubules is termed the terminal cisternae because of its flattened cisternal arrangement. When an excitation signal arrives at the neuromuscular junction, the depolarization of the sarcolemma quickly travels through the T-tubule system and comes in contact with the sarcoplasmic reticulum, causing the release of calcium and resulting in muscle contraction.
In muscle cells, the sarcolemma or plasma membrane extends transversely into the sarcoplasm to surround each myofibril, establishing the T-tubule system. These T-tubules allow for the synchronous contraction of all sarcomeres in the myofibril. The T-tubules are found at the junction of the A- and I- bands and their lumina are continuous with the extracellular space. At such junctions, the T-tubules are in close contact with the sarcoplasmic reticulum, which forms a network surrounding each myofibril. The part of the sarcoplasmic reticulum associated with the T-tubules is termed the terminal cisternae because of its flattened cisternal arrangement. When an excitation signal arrives at the neuromuscular junction, the depolarization of the sarcolemma quickly travels through the T-tubule system and comes in contact with the sarcoplasmic reticulum, causing the release of calcium and resulting in muscle contraction.
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