This refers to the chain of events that intervene between membrane depolarisation and contractile activation. The sequence of events is as follows:
• The detection of transverse tubular membrane depolarisation by a voltage sensor.
• The voltage sensing event is coupled to the intra-cellular release of stored calcium from the terminal cisternae of the transverse tubular system of the sarcoplasmic reticulum. The T-tubules contain voltage-gated Ca2 + channels, sometimes called dihydropyridine receptors. The major channel protein is the L-type channel protein.
• The released calcium ions undergo allosteric binding to the regulatory protein troponin and initiate mechanical activity, by removing the inhibitory action of the troponin-tropomyosin complex on the actin-myosin interaction. This causes conformational change in the thin (actin) filament. When calcium binds to troponin, this moves the tropomyosin threads sideways into the groove between the two F-actin chains, thereby uncovering the active binding sites on actin.
• Uncovering of active sites on thin (actin) filaments allows attachment of the myosin heads of the cross-bridges. When the previously activated cross-bridges attach to actin they undergo conformational change. A power stroke causes pulling (sliding) of the thin filaments over the thick filaments.
• Attachment of fresh ATP allows the cross-bridges to detach from actin, with dissociation of the actin-myosin complexes, and to repeat the contraction cycle as long as Ca2 + remains attached to troponin.
• Relaxation results from calcium reuptake into the sarcoplasmic reticulum by active transport through a membrane Ca2+-ATPase. The ATPase is activated by the binding of actin to myosin. The thin filament returns to a configuration £
i in which further cross-bridge cycles are inhibited. t
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