the final module for the muscular system is on smooth muscle and although we do not cover smooth muscle in as great detail as scal muscle there are a few unique structural and functional characteristics of smooth muscle that I want you to be aware of so starting with our structural characteristics and unlike sceletal muscle which is found attached to bone smooth muscle is found throughout the body mostly in the walls of Hol organs so the stomach intestines bladder and uterus in the walls of passageways such as blood and lymph vessels and in the tracks of our respiratory urinary and reproductive systems smooth muscle is found in the cery muscles of the eyes a muscle that can dilate and contract the pupil and and alter the shape of the lens as well as the muscles that produce goosebumps in response to cold or fear smooth muscle also makes up many of the involuntary sphincter that we find throughout the body the fibers that make up smooth muscle are spindle shaped which we've mentioned before so this is more like a long football shape tapered at the ends and fatter in the middle rather than parallel cylindrical like scal muscle fibers where scal muscle has many nuclei per muscle cell smooth muscle only has one centrally located nucleus smooth muscle is non-striated whereas sceletal muscle has obvious striations due to the way in which the thick and the thin filaments line up along the scal muscle fiber our smooth muscle does not actually contain sers but it does contain thick and thin filaments they're just arranged in a slightly different way in smooth muscle the thin filaments are anchored to what we call dense bodies which are scattered throughout the smooth muscle it's the dense bodies which are pulled closer together when the muscle contracts rather than the Z discs in scal muscle so taking a look and at an illustration of a smooth muscle cell and going through some of the differences that we just listed and firstly smooth muscle cells are spindle shaped as you can see here so tapered at the ends and then fatter in the middle our smooth muscle cells have a single centrally located nucleus our smooth muscle cells do contain thick and thin filaments but they're not arranged in parallel in saram like in scalo muscle so they're spread out all along that muscle cell instead of being anchored to the zisk of a saramy they anchored to these yellow structures called dense bodies and it's these dense bodies which get pulled closer together when that muscle contracts or shortens now not that you can see it in this image here but I also wanted to point out that smooth muscle is typically arranged in two layers or sheets of muscle throughout the body we typically have an inner circular layer which forms things like our sphincters and then an outer longitudinal layer that when it's combined with an inner circular layer is important for things like peristalsis which is that wav likee contraction or the ability to to move substances down a tube or a hollow organ like the gastrointestinal tract now still focusing on structural differences between smooth and scalo muscle but on the next few slides we're going to focus on the structures that are important for the contraction of smooth muscle so when we consider how a muscle cell is innovated with our sceletal muscle we have a single motor neuron innovating a single motor unit and a single branch of that axon extends down to form a neuromuscular Junction with a single muscle fiber smooth muscle is innovated by an autonomic motor neuron which instead of forming NE neuromuscular Junctions with the individual muscle fibers they contain varicosities that form duse Junctions so to show you what these structures look like and here is our sceletal muscle which is innovated by our sematic motor neurons here we have a single sematic motor neuron extending from the spinal cord it then branches that axon branches and each branch of that axon forms a single neuromuscular Junction with a single muscle fiber as we spoke about previously a single motor neuron and all of the muscle fibers that it innovates is called a motor unit this is then our smooth muscle which is innovated by an autonomic motor neuron and these contain varicosities which form duse Junctions so let's say this is our autonomic motor neuron here along the neuron are these swellings which we call varicosities and varicosities are just swellings in the neuron that contain VES filled with neurotransmitter so instead of synapsing with a single muscle fiber these neurons will lie on top of a sheet of smooth muscle so that that whole region of muscle tissue can be in ated and therefore hopefully contract at exactly the same time now these Junctions are called defuse Junctions because the sinapse between the neuron and the muscle cell is more broad or diffuse and not as well defined or structured as in our scal muscle now as well as being innovated by a different kind of modor neuron an action potential doesn't travel along smooth muscle via T tubules like it does in sceletal muscle rather an action potential travels along smooth muscle via Gap Junctions so when an autonomic motor neuron is stimulated and releases neurotransmitter from those varicosities the neurotransmitter binds to Liang gated sodium channels on the smooth muscle cells sodium enters the cell and the action potential can move through that smooth muscle directly via those Gap Junctions remembering from our nervous system content that a gap Junction is like a little tunnel between two cells that allow those charged ions to pass directly through so that that action potential can spread really easily and really quickly so again as a bit of a visual recap this is our scal muscle so an action potential runs along the sarala and then down into the scal muscle fiber via our T tubules so these are our T tubules here with our smooth muscle once that autonomic mon neuron has been stimulated and we have The Binding of the neurotransmitter to the smooth muscle cell and sodium's going into the smooth muscle that action potential runs through that sheet of smooth muscle tissue through these Gap Junctions now after an action potential travels along the cyle Emma and either down the T tubules in sceletal muscle or through the Gap Junctions in smooth muscle The Next Step In muscle contraction is that the calcium is released into the psychop in sceletal muscle calcium is released from the psychop plasmic reticulum and the ter terminal systs which are those swellings that sit at the end of the pyop plasmic reticulum just next to the two t tubules in smooth muscle the psychop plasmic reticulum is less developed than in sceletal muscle it can't hold as much calcium so the smooth muscle can also receive calcium from the extracellular fluid or the fluid outside of that muscle cell lastly the reason that calcium is released into the psychop is so that in sceletal muscle it can bind to chipon which moves the tropomyosin out of the way so that the mein and the actin can bind and form a crossbridge in smooth muscle we don't have troponin and tropomyosin instead we have calmodulin so when calcium binds to calmodulin in smooth muscle it activates an enzyme that leads to The Binding of mein and actin and therefore the cross bridg is forming