 Dear students, in this topic we shall discuss the excitation-contraction coupling. You know that the skeletal muscles contract in response to a nerve impulse that comes from the brain and arrives through motor neurons at the neuromuscular junction. At the neuromuscular junction, acetylcholine is released as a neurotransmitter from the motor neurons. This acetylcholine binds to specialized receptors which are ligand gated ion channels present in the post-synaptic muscle fibers. These ligand gated ion channels open and cause the movement of sodium and potassium ions. The movement of these ions result in change in the membrane potential of the muscle membrane which results in membrane excitation. The membrane potential of excited muscle fiber is known as end plate potential. The membrane excitation of the muscle fiber results in triggering of an all or none action potential in the muscle fiber membrane. This action potential then propagates away, exciting the entire membrane of the muscle fiber. When this excitation spreads, it sets in motion all the sequence of events that lead to contraction of the muscle. The sequence of all these events which convert an action potential to a muscle contraction is known as excitation-contraction coupling. Dear students, when an action potential arrives, it takes several milliseconds to begin contraction. Any action potential ke bil ko saath immediately contraction happen nahi ho jati. There is a latent period. This latent period is because of the large size of skeletal muscle fibers which cannot contract unless the action potential spreads deep into the fiber to the vicinity of each myofibril. During this latent period, action potential is transmitted through the transverse tubes or transverse tubules deep within the fiber. Transverse tubules is the one which takes the action potential to the deeper parts of the cell to the myofibrils. Dear students, the transmission of action potential through the T-tubules results in the release of calcium ions from the stores of sarcoplasmic reticulum. Sarcoplasmic reticulum has a calcium ion store and when the T-tubules transmit action potential, then the sarcoplasmic reticulum affects it and the calcium ions are released. As a result, the concentration of calcium ions increases inside the muscle fiber in the immediate vicinity of myofibrils. These calcium ions then cause the contraction to begin. We can see that the net effect of excitation-contraction coupling is that the action potential generated in the plasma membrane is linked to the contraction with the help of free calcium ion concentration in the cytosol. This initiates the contraction.