moving on now to the functional differences between sceletal and smooth muscle and this relates to how the two different muscle types contract so we'll begin with a bit of a recap on the contraction of scall muscle and I've listed these steps out in dot points but you can learn and remember this content in any way you would like whether that be using the images that we looked at in the previous lectures writing out your own step-by-step process or just generally understanding how things work but for me muscle contraction starts with an action potential traveling along a sematic motor neuron to the axon terminal calcium enters the axon terminal which triggers the release of neurotransmitter which in the case of sceletal Muscle is acetal choline into the synaptic Clift at the neuromuscular Junction the neurotransmitter or aceto choline binds to the Lian gated sodium channels on the Sara and sodium enters the muscle cell the action potential then runs along the Sara and down into the muscle fiber through the T tubules in response to the action potential calcium is released into the psychop from the psychop reticulum and the terminal systems the calcium in the psychop binds to troponin moving tropomyosin out of the way and exposing the mein binding sides on actin the me heads bind to actum forming cross Bridges and the me heads rotate performing the power stroke moving the thin filaments toward the center of the saramia short in the saramia and producing muscle contraction now moving on to muscle contraction in smooth muscle and don't stress I won't ask you to write out these whole 10 steps it's just for me the best way to highlight the differences between sceletal and smooth muscle contraction which is the part that I want you to know so in smooth muscle the first step in muscle contraction is an action potential traveling along an autonomic motor neuron instead of acetal choline being released at a neuromuscular Junction like in sceletal muscle either acetal choline or nor epinephrine will be released by The varicosities at those diffuse Junctions which remember lie along a sheet of smooth muscle tissue the neurotransmitter which as I said will either be a SEO choline or nor epinephrine will bind to the Liang gated sodium channels on the sarala sodium enters the muscle cell and the action potential spreads along the sheet of muscle cells via Gap Junctions just like in sceletal muscle the spread of the action potential causes calcium to be released from the psychop plasmic reticulum but because of the underdeveloped psychoasfxck instead of calcium binding to troponin it will bind to calmodulin the Cal modulin activates enzymes called mein light chain kisee enzymes these enzymes re-energize and reorientate the heads so that they can now bind to Acton and form cross Bridges the mice and heads can now perform the power stroke which instead of Simply shortening the sarer and the muscle cause a bit of a a spiraling or a twisting contraction as it brings all of the dense bodies in the region closer together so think about it how you ring out a towel and you kind of twist each end that is how a smooth muscle will contract rather than simply shortening like sceletal muscle so as I said I don't need you to know these 10 exact steps in how smooth muscle contracts but I want you to know the major differences in how scal muscle and smooth muscle contracts so these are the differences we have the differences in the types of neuromuscular Junctions and the varicosities we have that the action potential spreads via Gap Junctions instead of T tubules that some of the calcium will come from the extracellular fluid and not just the sarcoplasmic reticulum that calcium binds to calmodulin instead of chonin and that the power stroke causes a spiraling contraction rather than simply shortening the muscle now lastly before we finish there are a few other special features or unique characteristics of smooth muscle that are important to note firstly that smooth muscle is slow to contract and slow to relax it produces low levels of tension relative to scal muscle but it does so with very little energy cost and this is important for the everyday functions that it has things like breathing things like digesting our food smooth muscle also has a stress relaxation response what this means is that the smooth muscle can be stretched and then instead of just automatically Contracting or returning back to its normal shape it can actually relax and remain in that stretch state so this is important for the bladder or the stomach where these organs are going to be filled with urine or food and they need to remain relaxed to actually contain their contents smooth muscle also have some cells that are self excitatory meaning that the action potentials that innovate the muscle actually arise within the muscle cells themselves so they don't need an external stimulus and lastly some smooth muscle can be regulated by hormonal or chemical input and not just an electrical stimulus or an action potential