so in this video we're going to start to look at smooth muscles unlike skeletal muscle smooth muscle has a lot more variation there is no classical neuromuscular junction instead in many cases along the entire length of the axon you have these little bulbous areas called varicosities where you can bathe the entire skeletal muscle in neurotransmitter and the skeletal muscle excuse me the smooth muscle cells themselves can be interconnected using gap junctions where they're sharing cytosol so the signals that happen closest to the varicosity can penetrate into the deeper layers of the smooth muscle so you see this for example in the muscles of the small intestine to allow for that rhythmic contraction to move material down the length of the small intestine however when you look at the smooth muscle within the eye you have individual smooth muscle cells that are not connected by gap junctions instead the smooth muscle itself is penetrated by these varicosities so there's a huge amount of variation in the different types of smooth muscle cells there's variations in electrical properties and since you don't have a classical neuromuscular junction there are many ways to initiate both contraction and relaxation in smooth muscle so when we look at contraction in these three major muscles we talked about the muscle twitch which lasts about 100 milliseconds in skeletal muscle in cardiac muscle it's about 250 milliseconds in smooth muscle it's actually on the order of seconds so the contraction and the relaxation are going to be sustained for a much longer period of time and it's a much slower less energy intensive process so you're not developing tension as rapidly in smooth as opposed to skeletal and you're able to maintain that force of contraction for a longer period of time and this it primarily really has to do with the myosin atpase activity that we addressed earlier in skeletal muscle it's slow and it's steady now there's also variations in contraction and relaxation so the two main types of contractions that you see in smooth muscle are phasic versus tonic contractions and there are sort of many different types of phasic contractions but in this particular example with the esophagus the default state is relaxed so the muscle is usually relaxed it gets some sort of signal stretch something it doesn't it's not neural usually and you initiate contraction and then you follow it up with relaxation you could also have cyclical phasic contractions so again you see this in the intestines rhythmic contractions down the length of the axon to move material down the gi track so here again you're initiating contraction relaxation and as soon as you finish relaxation you initiate the next contraction with a tonic contraction the default is opposite a tonic contraction you're contracting and that's the default state and some sort of signal or stimulus causes the muscle to relax so we see this a lot with sphincters these are circular pieces of smooth muscle at important junctures so for example you look at the cardiac sphincter this is the set of circular muscles that will contract and prevent stomach acid from going up into the esophagus so the default is that you're contracting and you close it off to prevent things like heartburn and when food or drink impacts that smooth muscle it triggers relaxation of that smooth muscle allowing the food and drink to go into the stomach so the default for phasic is relaxation and a signal causes contraction for a tonic signal you're most you're going to be in a mostly contracted state and some sort of signal will cause relaxation now if we look at the histology and we look at the structure within the smooth muscle cell it actually is quite different than skeletal muscle we don't see the striations that we did for both cardiac and skeletal the organization of the thin and the thick filaments is different well that's because the contraction is different in smooth muscle in skeletal because you have insertions and origins you have contraction and relaxation along one plane right to pull the bones closer together or further apart for a smooth muscle you have sort of a three-dimensional contraction so just imagine if your stomach just contracted by increasing in length and decreasing in length you're really not manipulating and mechanically breaking food down if however you look at contraction in three dimensions where you're churning and breaking you're kind of uh how should i phrase this you're you're churning the the material you're sort of squeezing the material in three dimensions that's going to facilitate the mechanical breakdown of food so organization is different the mechanism is going to be a little bit different as well smooth muscle does not have troponin so mechanistically that we talked about in skeletal muscle contraction is going to be different because you don't have troponin you don't have to kick aside tropomyosin the calcium release from the sr is going to be a little bit different as well and so we're going to highlight that in the next video