 There everybody, Dr. O here. In this video I want to talk about excitation and contraction coupling. Muscle fiber contraction, muscle contraction is a complicated business and you're going to hear cross bridge formation and lots of troponin and tropomyosin, lots of tricky terminology. So I kind of want to just do a high level overview and then we'll have to mine in deeper on some of these parts as we move forward. But excitation contraction coupling says that a muscle contraction is coupled with excitation from the nervous system. So for a muscle fiber to contract, its membrane has to be excited. And what excites the membrane of a muscle cell or muscle fiber is going to be the nervous system. So you see here at the top, an action potential arrives at a neuromuscular junction. So a motor nerve or a motor unit, which is a motor nerve and whatever muscle fibers it's going to, it's going to send a signal. It's going to branch and branch until it reaches an individual neuromuscular junction. So like the name implies, it's where a nerve ends or a nerve and a muscle come together. So here you see the neuromuscular junction. So what happens here at the end or axon terminal, the end of the axon, it's going to release a chemical messenger, a neurotransmitter. In this case, and with muscle contraction, it's always acetylcholine, which you see here as ACH. So it actually takes calcium, it requires calcium for neurotransmitters to be released. So we need calcium here and you'll see we need a lot of calcium moving forward as well. So acetylcholine, let's just read, this acetylcholine or ACH is released. It binds to acetylcholine receptors. So acetylcholine is released from the axon of the motor nerve. It's going to bind to receptors that's on the sarcolemma or the plasma membrane of a muscle fiber. So it's released, binds to receptors, and it opens sodium ion channels. So as we're learning, or as you learn with the nervous system, sodium depolarizes and sodium is going to turn signals on. So calcium is needed to release the neurotransmitter acetylcholine. Acetylcholine is used to open up sodium channels leading to an action potential in the sarcolemma, which is the cell membrane or plasma membrane of a muscle fiber. So this action potential is going to travel. So it's the acetylcholine travel through the synapse and it triggered this response. And this action potential is going to travel down in through your muscle fibers. And that's going to be here. So this sodium is being released. Here's a picture of the T-tubule. So the T-tubule we was talking about is just it's a way to get this action potential deep into muscles. So what happens here, so this sodium depolarization causes a wave through this muscle fiber. And that's going to lead to the release of calcium. And that's the really, really important thing. So you see the T-tubules are how this wave gets deep into muscle fibers and muscles. The sarcoplasmic reticulum is going to be the fancy terminology for the endoplasmic reticulum of a muscle cell. And it's going to release the calcium. So what do we need this calcium for? We had the action potential traveled down the T-tubules. Now you see all this calcium being released. So the reason that's so important is the calcium is actually what's going to make it possible for muscle fibers to contract. So calcium is going to bind, as you see there in the middle, to something called troponin, which is one of the shielding proteins that usually shields the binding sites. So act in your thin filament and myosin your thick filament cannot actually connect and contract. So as long as there's no calcium, these shielding proteins stay in the way and it doesn't work. So calcium is going to initiate the contraction of a muscle fiber at this level here by binding the troponin and moving it. And when it changes its shape and moves it, that's going to allow the myosin heads on the thick filament to actually bind to act in and lead to muscle contraction. So as long as we have calcium, as long as this signal is still being sent from the nervous system, and as long as we have calcium, then your muscles can contract. And that's what all this has led to at this point. So as you see there, troponin and now has calcium on it, thick and thin filaments can now interact and that leads to a muscle contraction. And as long as we have that calcium and that signal, muscles will continue to shorten and contract. The technical terminology here, troponin, the shielding protein moves what's called tropomyosin out of the way and that's what exposes the myosin binding sites on actin. So I think we've covered that well. Now what happens at the end? How does this muscle fight? How does a muscle stop contracting? How does muscle relaxation occur? Well, the signal from the nervous system will disappear and that's gonna start this process. And then the acetylcholine that's in the synapse will either be reabsorbed by the neuron or an enzyme called acetylcholinesterase will come in and break it down. So the motor neuron signal has stopped, the acetylcholine's been destroyed or resorbed so it's now gone and that's gonna lead the muscle fiber to repolarize or it's going to turn off. And then as you can see here, calcium's gonna be resorbed, it's gonna be reabsorbed and that's gonna start this relaxation process. Without calcium, the calcium comes off of that troponin. Troponin and tropomyosin will actually go back and cover the myosin binding site on actin. They're gonna do their job as shielding protein so they'll reshield this. Now myosin can no longer grab onto actin, they'll have to separate and that's gonna cause your muscles to relax and lengthen. Okay, so that's the 10,000 foot view of what's called excitation contraction coupling, how the nervous system tells your muscle fibers what to do and then also just as a bonus at the end, how muscle fibers, muscle cells relax. I hope this helps, have a wonderful day, be blessed.