Cardiac & Smooth Muscle

Cardiac & Smooth Muscle


In previous videos we’ve been talking a lot about the structure of skeletal muscle, and that’s largely what we’re focused on in discussing the muscular system. But I do want to touch on cardiac muscle and smooth muscle, and we’re going to talk about these just in terms of how they compare to skeletal muscle. We’ll talk about cardiac muscle first in terms of microanatomy and stimulation, and then we’ll go in and talk about smooth muscle. Cardiac muscle is striated under the microscope like skeletal muscle. You can see these, like, thumbprint appearances of the alternating dark and light bands. But cardiac muscle differs from skeletal muscle in terms of you have branching of the muscle fibers. Skeletal muscle, remember, is just these long, cylindrical muscle fibers. Cardiac muscle fibers, the myofibrils can actually branch and so the cells themselves can actually branch. You can’t really see this clearly under the microscope, but cardiac muscle has a lot of mitochondria, very densely packed with mitochondria, which makes cardiac muscle very fatigue resistant. And I’m sure we can all agree that that is a good thing. You definitely don’t want your heart to hit the wall. Lastly, when you’re – when you are looking under the microscope with cardiac muscle, you see lots of these structures. They’re called intercalated discs, these dark lines perpendicular to the muscle fiber itself. Intercalated discs are where neighboring cardiac muscle fibers join together, right, and they show up more dense underneath the microscope because they are densely packed with protein. There are concentrations of desmosomes and gap junctions at the intercalated discs. Desmosomes hold the cardiac muscle fibers together so that the heart can contract as a unit without ripping apart; also something we can agree is a good thing. And gap junctions allow for ions to pass between cardiac muscle fibers, so the the cardiac – that the heart can contract as a whole and it’s electrically connected. This is a cartoon view of what the cardiac muscle fibers look like. You can see densely packed mitochondria. You can see the myofibrils, which are kind of branching and more variable in terms of diameter. The sarcoplasmic reticulum here in blue also looks different from what you saw with skeletal muscle. The Sarcoplasmic reticulum is less well-developed, it’s more simple, and there’s no terminal cisternae. So those big extensions, those big expansions of the sarcoplasmic reticulum alongside the T-tubules that you see in skeletal muscle, you don’t see those in cardiac muscle fibers, and this reflects the fact that cardiac muscle is less dependent on calcium from the terminal cisternae. Cardiac muscle uses calcium from the extracellular fluid when it contracts. The last thing that I want to point out here is the arrangement of the thick filaments in red and the thin filaments in blue. As I mentioned, cardiac muscle is striated like skeletal muscle and in skeletal muscle the striations come from the arrangement of thin and thick filaments. Cardiac muscle is striated because it has the same arrangements of thin and thick filaments. You have the I band with its Z disc and just the thin filaments present, and then you have the A band where the thick filaments are present, including the zone of overlap. When cardiac muscle contracts, it contracts through that same process of sliding filaments that we discussed with skeletal muscle. I don’t want to spend a huge amount of time getting into cardiac muscle stimulation, this isn’t meant to be a thorough explanation of contraction of cardiac muscle, this is just meant to be a brief comparison to skeletal muscle and this slide points out some of the differences. One really key difference is that cardiac muscle does not need to be stimulated by the nervous system. There are cells within your heart that depolarize spontaneously, they’re called pacemaker cells. They have a leaky membrane and periodically depolarize at a regular rate and these pacemaker cells are what stimulate the heart to contract. The nervous system adjusts the rate at which these pacemaker cells depolarize but the nervous system is not required to force these cells to depolarize. You may have heard the term pacemaker before because artificial pacemakers are often installed in patients whose pacemaker cells are not doing the job – whose pacemaker cells are not creating an adequate heart rate. An electronic pacemaker can be installed surgically. Secondly, cardiac muscle fibers are connected through gap junctions at the intercalated discs, so gap junctions allow for ions to pass directly from cardiac muscle fiber to cardiac muscle fiber, so that when the pacemaker cells depolarize that wave of depolarization passes through the entire heart and the heart contracts as a single unit. Third point that I want to make is that calcium comes from both the extracellular fluid and from the sarcoplasmic reticulum. This is why cardiac muscle does not need the terminal cisternae because it doesn’t need these enormous bags of calcium ions within the cell. But once that calcium does come into the cardiac muscle fiber it interacts with troponin and troponin rolls tropomyosin off of the actin thin filaments, activating the cross-bridge cycle. Last thing that I want to mention is that cardiac muscle gives a slow, prolonged contraction. The action potential is quite prolonged so the depolarization lasts about a hundred times as long in cardiac muscle as it does in skeletal muscle and then the tension, the contraction part of the actual muscle, lasts about ten times as long as the contraction of skeletal muscle from a single stimulation. Now I want to talk for a moment about smooth muscle. Smooth muscle is muscle that is under involuntary control by the nervous system and you find it in places like the digestive tract where smooth muscle controls the contractions of the digestive tract that move food through the digestive tract. You find smooth muscle around your blood vessels that control the diameter of blood vessels. You find smooth muscle in your eye, you find them in the male reproductive tract. You find smooth muscle in many different places throughout the body. Microscopically, smooth muscle is called smooth muscle because it doesn’t have the striations. And it doesn’t have the striations because the thin filaments and thick filaments are arranged differently; they don’t have that regular arrangement of the sarcomeres. Their thick and thin filaments are scattered around the cell. The thin filaments are anchored to the sarcolemma structures called dense bodies. And there are intermediate filaments called desmin that form a net around the cell so that when the thick filaments and thin filaments walk along each other, the thin Filaments pull at the dense bodies, which pulls at the desmin, and the whole smooth muscle cell contracts, wrings up like a sponge. The dense bodies also anchor the thin and thick filaments and the desmin net to the endomysium, the connective tissue surrounding smooth muscle fibers, and to neighboring cells. Sheets of smooth muscle cells are also connected by gap junctions and the connections through dense bodies. The connections through gap junctions allow smooth muscle cells to contract in a coordinated, synchronous manner. This is a diagram that shows the arrangement of myosin and actin filaments and the relationship to dense bodies and desmin and what happens when a smooth muscle fiber contracts. You can see the actin, actin and myosin along here, dense bodies anchoring the the thin filaments to the surface of the cell. When myosin moves the thin filaments, the thin filaments pull the dense bodies in towards each other and the whole cell – and that force is transmitted by desmin intermediate filaments around the cell and the whole cell squishes in. Smooth muscle can be found either as individual smooth muscle cells under individual control or as sheets of smooth muscle cells that are regulated together. Individual muscle cells are called multi-unit smooth muscle and In this type of smooth muscle the smooth muscle cells are innervated in motor units like skeletal muscle, only each smooth muscle cell can have more than one neuron interacting with it. Multi-unit smooth muscle tends to contract more slowly. You see multi-unit smooth muscle as the erector pili muscles, the ones that pull the hair on your body upright and give you goosebumps. Also in the iris of the eye controlling the pupil. Visceral smooth muscle is smooth muscle that’s arranged in sheets of cells connected together by gap junctions. Gap junctions allow for an action potential to spread directly through the entire sheet of smooth muscle cells and sometimes these this smooth muscle sheet contains pacesetter cells or pacemaker cells that may spontaneously depolarize. Smooth muscle is innervated by cells of your autonomic nervous system. The autonomic nervous system is basically your involuntary nervous system, it’s not under conscious control, and one of the ways that this innervation differs significantly from skeletal muscle innervation is that you can have either excitation or inhibition. Excitation means basically activation, telling the muscles to contract, and inhibition is inhibition. It’s blocking contraction or slowing down contraction. With skeletal muscle, when skeletal muscle receives a signal from the nervous system, in skeletal muscle it’s always an activating signal. It’s always excitation. Smooth muscle, there are nerves that can tell the smooth muscle to contract more and nerves that can tell the smooth muscle to contract less. Also, if you remember in skeletal muscle, you have a very defined relationship between the neuron and the muscle fiber, the neuromuscular junction. With smooth muscle, at least when you’re we’re talking about visceral smooth muscle now, where you have these sheets of smooth muscle cells, the Innervation is much broader and much less specific. It’s called a diffuse junction. And you just have the end of an innervating neuron, the end of that axon of the neuron, just widens into a series of expansions called varicosities and each varicosity can release neurotransmitter onto the sheet of smooth muscle, and it just kind of dumps it out like pouring water over the sheet of smooth muscle. So it’s not a specific relationship. It’s not – it’s not a specific this neuron goes to this cell. It’s all of the varicosities dump their neurotransmitter over the entire sheet of smooth muscle. When the smooth muscle does contract, there are some key differences from skeletal muscle. Listed here first, action potentials are transmitted directly from fiber to fiber by gap junctions. If you look back at that previous image, you may have a cell that gets an excitatory stimulus and that stimulus is passed on directly to a neighboring cell by gap junction. Smooth muscle also has much less sarcoplasmic reticulum, less even than the cardiac muscle. Calcium ions come primarily from the extracellular fluid so they don’t need the calcium from the terminal cisternae. They got it from the extracellular fluid. This is part of why keeping a steady calcium – a steady level of calcium in your blood is so critically important. The way that calcium enables contraction is also quite different. So in cardiac muscle and skeletal muscle, you have troponin and tropomyosin as the regulatory molecules. In smooth muscle, that influx of calcium activates a molecule called calmodulin. Calmodulin activation leads to activation of an enzyme called myosin light-chain kinase and myosin light-chain kinase acts directly on the myosin to activate it. And so it’s a very different pathway. It still requires calcium, but the calcium comes from a different location and what that calcium does within the cell is very different. Last thing is that some smooth muscle cells depolarize spontaneously, so this is similar to cardiac muscle, but very different from skeletal muscle. Smooth muscle cell contraction is much slower and much more energy efficient than skeletal muscle contraction as well, though I don’t have that listed. After studying this video, you should be able to do some comparing and contrasting between skeletal muscle, smooth muscle, and cardiac muscle. Think about what cells look like, both how they actually look under the microscope and in terms of cellular microanatomy, what do the structures within the cells look like? Think about the arrangements of the myofilaments. By myofilaments, I mean thin filaments and thick filaments, so the myosin and actin filaments. Also, think about how they contract and how they are excited. How they – how the nervous system controls these different types of muscle.

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