Meat refrigerati Fig 3.3 Diagram of part of a muscle fibre in longitudinal section to demonstrate the effect of a nervous impulse. For abbreviations see text( source: Bendall, 1974). The effect of an impulse invading the muscle fibres is to cause release of 2+ from the cisternae of each fibril. The Ca+ then diffuses down its elec trochemical gradient, finally reaching the microfilaments of actin(thin )and myosin(thick) shown in the lower, stripped fibril in Fig. 3.3. Here the Ca2+ is temporarily absorbed and thereby triggers the contractile explosion. This consists first of the rapid splitting of ATP to ADP and Pi (inorganic phe phate) at active centres on the myosin filaments. Then there is transduction of some of the free energy released into relative movement of the two sorts of interdigitating filaments. The process is not unlike the explosion of the petrol/air mixture in a car cylinder, when the sparking plug fires. For a crude nalogy, the cylinder can be likened to the myosin filaments and the piston to the actin filaments. However, it is the upstroke which resembles a con- traction and not the downstroke (i.e. the piston is actively pulled or pushed cy Relaxation is the opposite process during which the status quo at the sarcolemma is restored, thereby enabling the lateral cisternae of the Sr to re-accumulate the Ca released during the contraction. They do this by an nctive pumping process, using the energy of ATP-splitting to push Ca up the now adverse electrochemical gradient. Meanwhile fresh ATP hasThe effect of an impulse invading the muscle fibres is to cause release of Ca2+ from the cisternae of each fibril. The Ca2+ then diffuses down its elec￾trochemical gradient, finally reaching the microfilaments of actin (thin) and myosin (thick) shown in the lower, stripped fibril in Fig. 3.3. Here the Ca2+ is temporarily absorbed and thereby triggers the contractile explosion. This consists first of the rapid splitting of ATP to ADP and Pi (inorganic phos￾phate) at active centres on the myosin filaments. Then there is transduction of some of the free energy released into relative movement of the two sorts of interdigitating filaments. The process is not unlike the explosion of the petrol/air mixture in a car cylinder, when the sparking plug fires. For a crude analogy, the cylinder can be likened to the myosin filaments and the piston to the actin filaments. However, it is the upstroke which resembles a con￾traction and not the downstroke (i.e. the piston is actively pulled or pushed into the cylinder). Relaxation is the opposite process during which the status quo at the sarcolemma is restored, thereby enabling the lateral cisternae of the SR to re-accumulate the Ca2+ released during the contraction. They do this by an active pumping process, using the energy of ATP-splitting to push Ca2+ up the now adverse electrochemical gradient. Meanwhile fresh ATP has 48 Meat refrigeration EP MN S SR (T) SR (L) TJ Fibril Fibril (stripped) ZZZZZ Fig. 3.3 Diagram of part of a muscle fibre in longitudinal section to demonstrate the effect of a nervous impulse. For abbreviations see text (source: Bendall, 1974)