A twitch is the muscle contraction in response to a single ap . an action potential is generated across the muscle membrane at the neuromuscular junction by motor neurons which synapse on muscle cells. the neurotransmitter secrete by the motor neuron is ach. the ach on the post synaptic membrane is selective for na+ and k+; this depolarization of the muscle membrane down the t-tubule leads to an ap generated in the muscle plasma membrane which allows for the opening caused by conformational change of dihydropyridine receptors which are voltage sensitive ca2+ channels. activated dhp receptors physically act on ry receptor ca channel. the release of ca2+ by the dhp receptors induces the increase of ca2+in the cytoplasm by the activation of the ryanodine receptors releasing calcium stores from the sarcoplasmic reticulum. other half of the ry receptor/sr ca channels not physically coupled to the dhp receptors are also thought to open during the excitation phase. in addition to stimulating contraction, ca also binds to the non-dhp coupled ry ca2+ channels and opens them causing an overall major increase in cytoplasmic ca2+. influx of ca2+ allows it to bind to troponin c after a certain latent period, which then allows the troponin complex to undergo a conformational change ultimately relieving the steric hindrance imposed by it and allowing for the atpase myosin head to bind to actin filaments myosin binding site.
the binding of free ca2+ to troponin c acts as a strong buffer for calcium so there is little free calcium floating around in cytoplasm. the binding of a myosin to a binding site on actin is known as a cross bridge. the number of cross bridges formed is directly proportional to the amount of tension generated. the generation of tension allows for a muscle contraction to occur, in the way of the shortening of the sarcomeres as the myosin heads are maximally cross-linked with the actin filaments. cross-bridge formation stimulates the removal of ca from cytoplasm by ca2+ pumps on the sr and plasma membrane. as free [ca] decreases, ca falls off troponin and tropomyosin covers myosin binding site on actin. \n