 Okay. So now we're going to talk about excitation contraction coupling, which is the idea that your brain starts an action potential which travels all the way down and is going to excite your skeletal muscles to perform work. So what we're going to do is we're going to look at each step of that excitation contraction coupling in demo form. Ready? All right. Step number one is your brain starts an action potential, which means this action potential is coming all the way from the brain down a somatic motor neuron until it reaches the synaptic end bulb, which is going to be the end of your somatic motor neuron. At that point when the action potential gets there, it's going to open up voltage gated calcium channels, which is going to cause calcium to come in from the synaptic cleft and go into your synaptic end bulb. Now that the calcium has come in, it's going to stimulate the movement of vesicles, which contain your neurotransmitter acetylcholine to move toward the end of the synaptic bulb. And once they get there, they will perform exocytosis. Now on the other side of the synaptic cleft are going to be ligand gated sodium channels. So when you have exocytosis of your neurotransmitter, the ligand gated sodium channels are going to be activated. Now that they're activated, the sodium that was in the synaptic cleft is going to come through and go into the muscle cell, which is going to start a new action potential on your muscle cell, which will then travel all the way down the sarcolemma and further on into the transverse tubules. So once that new action potential has been created inside of the skeletal muscle cell, that new action potential is going to travel all the way down the sarcolemma and into the transverse tubules. At this point, it's going to cause calcium to be dumped from the sarcoplasmic reticulum. Calcium, as it's dumped from the sarcoplasmic reticulum, is going to come in and it's going to bind to this protein called troponin. Troponin is attached to another protein called tropomyosin, and when this binding happens, it changes the structure of troponin so that it pulls tropomyosin out of the way. And now that the wall of tropomyosin has been removed, you have myosin and actin, both, which are contractile proteins, can now reach each other, and the sliding filament theory can occur. So now we can begin to create force. And what happens when calcium and that action potential go away is that the wall simply slides back in place and the muscle relaxes. So now that we've moved tropomyosin out of the way, we now have to perform the steps of the sliding filament theory. There's only four steps. They are hydrolysis of ATP, attachment, power stroke, and detachment. So first step, hydrolysis, which is going to be where actin and myosin, they line up so that they can now attach. Attachment, you can see myosin is attached onto actin, power stroke, myosin pulls actin slightly toward the midline of the sarcomere. But before we can have detachment, there needs to be another myosin head that comes in, lines up, attaches so that the previous one can detach, right? The new one will power stroke. And then before this one detaches, we have to retouch again, right, power stroke. And now we have contraction. So it's almost like you're pulling in the myosin, or the actin. The myosin is hand over hand pulling in the actin toward the midline. And you can see how small the H-zone has gotten because the actin is sliding toward each other toward the center of the sarcomere. When the calcium goes away, right, when the calcium disappears and that wall slides back in place, the actin and myosin are no longer allowed to reach each other, and everything's going to slide back out to where it was at rest and the muscle will relax.