okay in this video we're gonna talk about smooth muscle now we know that smooth muscle is found in the walls of hollow organs like your digestive tract or general urinary tract or respiratory tract except for the heart because the heart contains cardiac muscle in terms of the microscopic structure this felt of smooth muscle we know that it's kind of spindle shaped there's short thin cells that only have one nucleus and no striations now connective tissue is mostly lacking within smooth muscle and so we find that it actually contains endomysium just to nourish those cells now the smallest blood vessels of your body pretty much don't contains smooth muscle in their wall but pretty much the rest of your blood vessels do and it turns out though that the muscle that you find in hollow organs is usually arranged within two major layers and the layers here have cells that basically oppose each other in perpendicular directions we're gonna have a circular layer and a longitudinal layer and these actually allow for peristalsis where when one layer contracts in one direction another layer can just track contract on the other and that allows for a wave of muscular contraction that propels you know a substance in a particular direction so if we look at the two layers of smooth muscle within the wall of our digestive tract we find that there's a circular layer that actually surrounds the lumen or the space within this digestive organ then we have a longitudinal layer that goes along its length like lengthwise and longitudinally and so if these two layers can track together we actually get a peristaltic wave so imagine a circular layer that would squeeze in a longitudinal layer that kind of propels and pushes along its length or shortens taken together and actually actually is a wave-like motion and so get these Anjali motions which we call peristalsis that help to propel digestive contents along the length of your GI tract so we actually find out here that um smooth muscle doesn't have a neuromuscular junctions instead we actually have autonomic nerve fibers that innervate smooth muscle these actually contain what we call varicosities which are these little blebs or pouches of vesicles that actually released neurotransmitters directly on a smooth muscle and so what these varicosities look like is with our nerve fiber here we have these little blebs and that each bleb is a very casa T each varicosity is full of mitochondria which supply the ATP to help for exocytosis of these vesicles containing neurotransmitters now there are several different neurotransmitter types that can be released onto smooth muscle whereas with skeletal muscle it was always acetylcholine that's released at the neuromuscular Junction so smooth muscle doesn't contain sarcomeres it doesn't contain myofibrils either for that reason and it doesn't have t tubules instead what we find here is that the sarcoplasmic reticulum is smooth muscles far less developed we see that it doesn't really store a whole lot of calcium rather smooth muscle relies on the calcium from extracellular fluid and this calcium comes from these little unfoldings of the membrane called Calvillo lay now caviola contain numerous voltage-gated calcium channels that allow for a rapid influx of extracellular calcium so looking at a smooth muscle here we see that there's kind of a spindle shape we have one central nucleus these in foldings called caviola which actually have voltage-gated calcium channels and the gap junctions that allow for these cells to spread their action potentials from cell directly into the other cell now you find that there are no myofibrils within a smooth muscle cell because they don't contain sarcomeres and instead we find out of these dense bodies that are linked up with the intermediate cytoskeleton inside of the cell and is that these actually allow for contraction of the cell and because they crisscross it actually makes the whole cell kind of bunch up together so the thick filaments are fewer in number and have myosin heads along their entire length in fact the ratio of thick to thin filaments in smooth muscle is much lower than the skeletal muscle and smooth muscle it's about 1 to 13 and skeletal muscle is about one and two so thick filaments have heads along your entire length of the smooth muscle which means that they're still strong there's not as numerous now the troponin complex is actually missing in smooth muscle and it's kind of weird here where there is no troponin to bind with calcium rather calcium interacts with another protein called calmodulin which has a very different role than troponin within the sarcomeres of skeletal and cardiac muscle now thick and thin filaments are arranged diagonally within smooth muscle which allows for these cells to kind of contract in a corkscrew manner and because they're diagonally there's not really a sarcomere which means that smooth muscle cells don't have striations instead we have as this intermediate filaments dense body network which is a lattice like arrangement of intermediate filaments that resists tension and dense bodies you're kind of like the anchoring sites like the z-disks for these intermediate filaments and so that during contraction the sarcolemma actually starts to bulge outward between these dense bodies because the cell kind of bunches up and it makes the cell look kind of poufy so here's what what a contracted smooth muscle cell would look like we're basically the myosin filaments would pull on these intermediate filaments linked up with the dense bodies which allows for the whole cell to kind of bunch up and squeeze in a corkscrew manner now in terms of the three major types of skeletal cardiac and smooth muscle we're gonna compare and contrast these muscle types here we know that skeletal muscles attach to bones it's under voluntary control and the cells are these long straight slender fibers that are striated and contain mini nuclei that differs from cardiac muscle which is highly branched the cells are short and it's under involuntary control but also has striations like skeletal smooth muscle cells are short like cardiac but smooth muscle cells don't contain sarcomeres so there are no striations here and so we also find that in terms of the packaging of skeletal cardiac and smooth muscle you know skeletal muscles more highly packaged than anything similar like cardiac or smooth in fact cardiac and smooth really just have a lot of endomysium that packages these cell types now we do find myofibrils that are arranged in sarcomeres in both skeletal and cardiac muscle but smooth muscle doesn't have sarcomeres rather it's actin and myosin filaments are arranged in those criss crossing patterns that we saw earlier you do find sarcoplasmic reticulum in all three muscle types both scaled on cardiac and smooth but the sarcoplasmic reticulum is very scant within smooth muscle rather than in cardiac and smooth that's more abundant in fact the sarcoplasmic reticulum in skeleton cardiac muscle basically surround the myofibrils and but there are no myofibrils and smooth muscle in terms of the sarcoplasmic reticulum network yeah it's you find it all over you know in skeletal cardiac and smooth except for and smooth muscle it's a little bit less of a cell volume here you don't find gap junctions in skeletal muscle so skeletal muscle can't communicate with between cell the cell via gap junctions but that does differ from cardiac and smooth where there are gaps options that connect these cell types together and that actually allows for these cells to transmit their electrical impulses from cell to cell so cardiac and smooth muscle can actually spread their electrical currents all throughout the muscle now in terms of how these cells are excited skeletal muscle is excited by the neuromuscular Junction cardiac muscle cells can be stimulated but they're more rarely stimulated by your nervous system smooth muscle is stimulated by neuromuscular junctions but it's different because we actually have Calvillo leg which of those those expanded blebs of a nerve fiber in terms of where these nerve fibers come from your skeletal muscle cells are actually activated by the somatic motor neurons of the voluntary nervous system but both cardiac and smooth muscle actually are stimulated or inhibited by the autonomic neurons which are under involuntary control in terms of the source of calcium for scout all and cardiac muscle you find that it comes from the sarcoplasmic reticulum except cardiac muscle can also get its calcium from the extracellular fluid because it's part of the cardiac action potential and smooth muscle also gets its calcium from both SR and the exerciser fluid because skeletal cardiac muscle both have myofibrils they have the whole actin troponin tropomyosin complex but because smooth muscle have myofibrils it doesn't have troponin either it's got its own separate mechanism it what's kind of cool about cardiac and smooth muscle is actually contain their own pacemaker cells which means they can generate their own action potential spontaneously but skeletal muscle can't do this and so skeletal muscle can only be excited like you can't prevent skeletal muscle from contracting you can only excite it you can't inhibit it from contracting in fact the only way to prevent a skeletal muscle from contracting is just to not stimulate it but that differs from cardiac and smooth muscle because cardiac and smooth muscle can be both either excited or inhibited by hormones and neurotransmitters in terms of the twitch speed we see that in skeletal muscle we have fast twitch and slow twitch fibers whereas cardiac and smooth are slow - very slow and this relates to the speed of the myosin ATPase skeletal muscle cells don't have rhythmic contraction they're just going to contract once in one quick twitch whereas cardiac and smooth muscle can actually generate their own rhythms because of their pacemaker potentials or pacemaker electrical activities in terms of metabolism we see that skeletal muscle cells can be anaerobic whereas all three muscle types mainly rely on aerobic respiration so what we're going to do now is really get into the the mechanisms of smooth muscle contraction and we find that smooth muscle contraction is more slow and synchronized and this is actually because of these electrically coupled gap junctions where action potentials can spread from one smooth muscle cell directly into the next and what's cool too is that some of our smooth muscle cells are self excitatory and therefore can depolarize without any external stimuli like without neurotransmitters or hormones these cells can actually generate their own action potentials at their own pace and so that's what we call these pacemaker cells and the rate of intensity of contractions can be modified by you know neural or chemical stimuli so in terms of contraction of smooth muscle it's similar to skeletal muscle in a variety of ways you know we have acted in myosin the trigger is calcium we need ATP and the contraction stops when calcium is no longer available but that's the end of where they're similar we find is that smooth muscle actually differs quite a bit because most of the calcium and smooth muscle contraction actually comes from the extra souther fluid very few of that calcium is it actually comes from the sarcoplasmic reticulum also that calcium binds to a protein called calmodulin there is no troponin and smooth muscle and so that what happens is calcium activates calmodulin which activates this myosin kinase myosin kinase phosphorylates the myosin head which then activates the myosin head and allows it to form a cross bridge so stopping smooth muscle also allows requires rather calcium removal and smooth muscle calcium's pumped back into the sarcoplasmic reticulum but it's also pumped out of the cell so we need active transport of this calcium into SR and out of the cell and dephosphorylation of the myosin head needs to occur as well in order to relax the smooth muscle so to see all these steps of smooth muscle contraction laid out we have a about a five step process here where voltage-gated calcium channels open when the action potential rushes by calcium flows into the cell that triggers what we call calcium dependent calcium release which we get more calcium from the sarcoplasmic reticulum that calcium now floods the inside of them of the smooth muscle cell it interacts with a protein called calmodulin calmodulin activates another protein called myosin light-chain kinase and what the mouse and light-chain kinase does is puts a phosphate group onto the myosin head and that phosphate group actually comes from ATP and what this does is actually energizes the myosin head to be able to form a cross bridge with actin that way it can participate in the power stroke however this process is much more slow which explains why the smooth muscle contractions are slower than let's say skeletal and cardiac muscle now smooth muscles are actually very energy efficient not only because they are slower to contract but you know we actually can use less ATP for these muscles to contract and myofilaments can actually latch together to save energy and what's pretty interest to that smooth muscle maintains a constant steady state of contraction which we call smooth muscle tone and most of this ATP from smooth muscle contraction comes from Arabic respiration because it's slow enough that you won't really run out of ATP like you can rely on Arab pathways to maintain smooth muscle contractions now in terms of regulation or contraction is controlled by nerves hormones and chemical changes neuronal e we have neurotransmitters that bind to receptors on the smooth muscle cells that can cause graded potentials which may or may not cause an action potential these action potentials cause the release of calcium and from the sarcoplasmic reticulum which can you know lead to a muscle contraction but what's cool about suis muscle is that its response depends on the type of neurotransmitter that's released so that you don't always get the same response because we can get different types of neurotransmitters and different chemicals to bind to receptors we get varying responses by the smooth muscle because one neurotransmitter actually might have a stimulatory effect on that smooth muscle but it might also inhibit that smooth muscle if it's a different marrow transmitter so we also have hormones and other local chemicals that can you know interact with sweet muscle cells and this can cause me muscle to depolarize in a way that allows for muscles to contract but smooth muscle cells can also respond to factors like hormones carbon dioxide content pH as well as low oxygen levels and this makes smooth muscle more dynamic then let's say cardiac and and skeletal because it can respond to a variety of different stimuli now smooth muscles can respond to both neural and chemical stimuli which which kind of aids in the dynamic capability smooth muscle can also respond to stretch which is interesting so one of the responses here we can get our as a stretch relaxation response where smooth muscle responds to stretch only briefly but then adapts to a new length so this allows for smooth muscle to contract on demand it enables organs like your stomach and bladder to basically temporarily store contents and then contract only when necessary we also see that there's length tension changes in smooth muscle where smooth muscles contract between half in twice of its resting length this allows for organs to have huge volume changes without becoming flabby when relaxed so for instance your bladder can stretch to quite a bit and hold quite a bit of urine and when the bladder is empty it's not going to be flabby when it's empty rather because of the muscle tone and smooth muscle it maintains this contracted state that allows the organ to you know maintain some structure and shape and not be flabby when it's relaxed now in terms of the types of smooth muscle we find that there's unitary and multi-unit smooth muscle and this is gonna vary depending on you know different organs so it can vary based on the fiber arrangement innervation and responsiveness so unitary smooth muscle is commonly referred to as visceral muscle you only find this in hollow organs except for the heart and it possesses all common characteristics of smooth muscle like opposing circular and longitudinal sheets it's innervated by varicosities and exhibits spontaneous action potentials which are the pacemaker potentials and these cells are electrically coupled by gap junctions so that currents can spread from one cell to the next it can also respond to various chemical stimuli now multi-unit smooth muscles located in large areas of your lungs larger arteries as well as the erector pili muscle and the iris of your eye the iris is that pigmented part of your eye and it controls the diameter of your pupil now you find that there's very few gap junctions and spontaneous depolarizations are rare and for this reason multi-unit smooth muscle then depends on chemical stimuli from basically you know the the varicosities we saw earlier and these actually form motor units and because of the fact that these cells are more independently controlled by motor units it allows for a more directed and concerted more precise form of muscle contraction than you'd find in the visceral or you know the unitary smooth muscle and so what we find here then is that it's you know more precise which makes sense why you'd find it in places like the iris of your eye now it's different from a muscle because like unitary muscle is controlled by the autonomic nervous system and hormones which means that although multi-unit smooth muscle is regulated very similarly to skeletal muscle because there's motor units it's still under involuntary control