Temporal summation and Muscle tension

MANY PROCESSES MUST TAKE place within the skeletal muscle fibres in order for muscle contraction to occur. The cross bridge cycle, process by which the muscle length is shortened as myosin heads, extending from the myosin filaments, interact cyclically in a rowing motion with the actin filament (Rayment et al. 1993), is one of the crucial mechanical events required. The mechanism is initiated when an ATP molecule is bound to a myosin head. An enzyme within the head. known as ATPase, hydrolyzes the ATP molecule into ADP and phosphate (Rayment et al. 1993), causing a release of energy. This energy is used to rotate the myosin head into an extended, high-energy position facing towards the tail of the myosin filament (Mann 2011, p 14-9). The repositioned myosin head attaches to the actin filament to form a cross bridge between the two filaments. Upon to formation of the actin-myosin complex, ADP and phosphate molecules are released from the myosin head, which subsequently returns to a bent, low-energy position, pulling the still attached actin filament with it. This is referred to as the power stroke, as this is the movement that results in the shortening of the sarcomere (Saladin 2012). When a new ATP molecule is bound to the myosin head, the cross bridge is compromised and the bond between the filaments is broken. This is expressed in a quick cycle of contraction and relaxation known as a twitch (Saladin 2011). The myosin head will then again, hydrolyze the ATP and repeat the process to produce subsequent power stokes to further shorten the sarcomere (Mann 2011, p 14-10). Active sites on the actin filament available for bonding with myosin heads only become exposed in the presence of calcium ions (Ca2+) (Orentlicher & Gersho 1977). Hence sarcoplasmic concentration of Ca2+ must increase in order for the cross bridge cycle to commence. This is achieved through excitation-contraction coupling – a series of events that link the depolarisation of the muscle surface