Question

c) skeletal muscle contraction: explain the role of Ca2+ in the development of a twitch and explain the relationship between action potential frequency, twitches and tetanus

Answer 1

Normal Muscle Contraction

when a motor nerve innervate the the muscle fibre(myofibrils) , pre synaptic nerve endings releases acetylcholine neurotransmitter into the synapse -------> it bind with the postsynaptic receptor on the muscle membrane -------> depolarises the sarcolemma(muscle membrane) ------->depolarisation of the T tubules -------->Calcium ion are released by the sarcoplasmic reticulum into the sarcoplasm------> it bind with the Troponin c present on the tropomyosin - actin complex------> change in spatial configuration of the tropomyosin result in the exposure of actin binding site -------> Myosin filament bind with the exposed actin binding site ----> Actin filament(thin) slide over the myosin(thick) filament ----> muscle contract.

Twitch refers to Contraction of the muscle in response to stimuli.

[In Relaxed muscle,Tropomyosin masks the Myosin binding Site on actin molecule.But During Stimulus,When action potential Reach the Motor End plate,Ca2+ is Released.This ca2+ Binds to Troponin molecules.this troponin molecules are in contact with Tropomyosin molecule.Binding of Ca2+ on troponin molecule induces Change in shape of Troponin molecule so that Tropomyosin gets moved from the myosin binding sites on Actin molecule.This exposes Myosin binding sites on actin ,So that Myosin heads binds to Action------Cross bride formation --Cycle of cross bridge formation and Breakdown occur---skeletal Muscle contraction Occur.]

**When skeletal muscle recieves second stimulus after Completion of Relaxation to 1st stimulus,there is normal muscle contraction.

**When Skeletal muscle recieves Second stimulus,Before the muscle has relaxed completely in response to second stimuli,which results in Greater Tension----Muscle Contraction occur with more force.(There is greater force of COntraction)---This response to muscle Twitch is called Twitch Summation.

[At the molecular level-- summation occurs because the 2nd stimulus causes the release of more Ca2+ ions So that additional sarcomeres(Motor Units) are activated while the muscle is still contracting due to the first stimulus. So Motor units are Involved in the Contraction.Hence Greater tension occur that Leads to Greater Contraction]

**When skeletal muscle recieves Second stimulus before the muscle has a chance to relax at all,Maximal Sustained Contraction occur.This response to a Muscle twitch is called Tetanus.Repeated stimulation like this leads to Fatigue.