4 Transverse T tubule system consists of tubules formed by finger like

4 transverse t tubule system consists of tubules

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4. Transverse (T) tubule system consists of tubules formed by finger-like invaginations of the sarcolemma deep into the muscle cell cytoplasm. The interior of T tubules are continuous with the extracellular space. T tubules are arranged transversely to the myofibrils and deliver waves of depolarization deeply into myocytes. a. Triads , which are characteristic of skeletal muscle only, are defined as a centrally- located T tubule between two dilated portions of adjacent SR called terminal cisternae . There are two triads per sarcomere, and in mammalian skeletal muscle they are located at the junctions between the A and I bands. b. Excitation-contraction coupling , where depolarization of T tubules is coupled to myocyte contraction, occurs at the level of the triads. 5. Sequence of events. The mechanism of muscle contraction is summarized as follows. a. Innervation. Skeletal muscle cells are innervated by motor neurons whose cell bodies are located in the central nervous system. i. Motor unit. The axon of the motor neuron branches in the muscle to contact few to many muscle cells forming a motor unit . Motor units with fewer muscle cells are characterized by finer motor control. ii. Motor end plate. The terminus of each branch forms a specialization at the muscle surface called a motor end plate . iii. Neuromuscular junction. An action potential from the motor neuron arrives at the motor end plate and activates a specialized synapse structure called the neuromuscular junction . Release of the neurotransmitter acetylcholine from this junction binds to specific receptors on the surface of the adjacent muscle membrane, which causes depolarization of the sarcolemma. b. Muscle cell response. Electrical impulses (depolarizations) travel along the sarcolemma and deep into the cell via the T tubules, which are extensional invaginations of the sarcolemma. i. T - t u b u l e c u r r e n t s (depolarizations) stimulate adjacent profiles of sarcoplasmic reticulum at triads ( excitation- contraction coupling ) to release calcium ions (Ca ). ++ ii. Exposure of myosin binding sites. Released Ca binds to the TnC unit of ++ troponin and induces movement of tropomyosin so that the myosin binding sites on actin are now exposed. iii. Contraction. As a result of tropomyosin displacement, the myosin heads bind to actin filaments, which initiates the motor movement and sarcomere shortening. Muscle relaxation 1. Calcium uptake. The sarcoplasmic reticulum possesses an ATP-driven calcium pumping mechanism, which accumulates calcium within its membrane system and thus reduces the cytoplasmic concentration of calcium around the myofilaments. 2. Actin-myosin inhibition. At low calcium concentrations, TnC is unoccupied and tropomyosin repositions itself so that it sterically blocks the myosin-binding site on each actin molecule.
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  • Summer '05
  • Brown
  • muscle cells

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