Excitation and Contraction of Skeletal Muscle
The area of contact between the end of a motor nerve and a skeletal muscle cell is called the motor end plate. Small branches of the motor nerve form contacts (boutons) with the muscle cell in a roughly eliptical area. The excitatory transmitter at the motor end plate is acetylcholine. The space between the boutons and the muscle fibre is called primary synaptic cleft. Numerous infoldings of the sarcolemma in the area of the motor end plate form secondary synaptic clefts. Motor end plates typically concentrate in a narrow zone close to the middle of the belly of a muscle. The excitable sarcolemma of skeletal muscle cells will allow the stimulus to spread, from this zone, over the entire muscle cell.
The spread of excitation over the sarcolemma is mediated by voltage-gated ion channels.
Invaginations of the sarcolemma form the T-tubule system which "leads" the excitation into the muscle fibre, close to the border between the A- and I-bands of the myofibrils. Here, the T-tubules are in close apposition with cisternae formed by the sarcoplasmatic reticulum. This association is called a triad. The narrow gap between the T-tubule and the cisternae of the sarcoplasmatic reticulum is spanned by proteins which mediate the excitation-contraction coupling.
Proteins in the sarcolemma which forms the wall of the T-tubule (dihydropyridine (DHP) receptors)change conformation, i.e. they change their shape, in response to the excitation travelling over the sarcolemma. These proteins are in touch with calcium channels (ryanodine receptors) which are embedded in the membrane of the cisternae of the sarcoplasmatic reticulum. The change in the shape of the proteins belonging to the T-tubule opens the calcium channels of the sarcoplasmatic reticulum. Calcium can now move from stores in the sarcoplasmatic reticulum into the cytoplasm surrounding the myofilaments.
Sites of interaction between actin and myosin are in resting muscle cells "hidden" by tropomyosin. Tropomyosin is kept in place by a complex of proteins collectively called troponin. The binding of calcium to troponin-C induces a conformational change in the troponin-tropomyosin complex which permits the interaction between myosin and actin and, as a consequence of this interaction, contraction.
ATP-dependent calcium pumps in the membrane of the sarcoplasmatic reticulum typically restore the concentration of Ca to resting levels within 30 milliseconds after the activation of the muscle fibre.
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