Types of Muscle

Skeletal Muscle

Of the three types of muscle, skeletal is used as the example to explain muscle mechanics.  Skeletal muscle is voluntarily controlled. For example, you may be taking notes right now. You are voluntarily moving your hands to do so.  Skeletal muscle is striated, which is something we have to dive into deeper in another minilecture.  But, for now, know that the arrangement of proteinsLarge molecules made of amino acids with various functions in the body. in muscle fibers leads to the creation of stripes. We can see these stripes under a compound microscope. 

They are subtle, but they are there.  We also already know that skeletal muscle cellsThe basic structural and functional units of life. are cylinder shaped.  Pictured here is a skeletal muscle at two very very different magnifications.  The top picture is a low magnification and we can see all the cylinder shaped muscle fibers squished together.  That’s what all those little circles are – the cylinder-shaped muscle fibers.  This picture on the bottom has a higher magnification. It also shows a longitudinal cut of the muscle fibers. 

Remember that challenge of taking 3D structures and representing them as 2D for the microscope?  What would happen if I took this bundle of skeletal muscle fibers here? Then, cut them this way?  That would look like this down here.  You can see the sarcolemma of each fiber. They pull away from each other when the tissue dries.

Smooth Muscle

Smooth muscle is one of the two types of involuntary muscle. This means that you are not in conscious control of your smooth muscle. Now make your small intestine contract. You can’t command your small intestine to contract because you cannot voluntarily control the smooth muscle around it. Although it has actin and myosin, smooth muscle is not striated. This is because the actin and myosin are not exactly in the same orientation. They differ in orientation compared to skeletal or cardiac muscle.

Smooth muscle cells are said to be spindle shaped. Many students don’t make a connection to this anatomical shape and it is also not well represented in 2D pictures. The individual muscle fiber at the bottom right corner of this slide illustrates this. This cell could also be interpreted as a simple squamous cell. However, a spindle is much like the flame on those cheap plastic holiday candles. You can plug these candles in and put them in your windows.

Take a look at the shape just above the individual muscle fiber which is a sheet of these muscle fibers. If we collected many spindle-shaped muscle fibers and cut across them, we would see different diameters of each cell. We would also see different parts of the nucleusThe control center of the cell that contains DNA and directs cellular activities. in each cell. And that’s what would produce this figure up top which is a transverse section A cut or slice of the body or an organ for study. of the smooth muscle.

Smooth muscle is found in sheets. It usually is found in two sheets. These sheets surround a lumenThe inside space of a hollow organ or structure. or the opening of a tube shaped organ. For example 2 sheets of smooth muscle cover every single part of your gastrointestinal tract. One of these sheets wraps around the tube and when tightens reduces the diameter of the tube. The other sheet runs long ways along the tube. When it contracts, it can shorten the length of the tube, somewhat like an inchworm does. These two sheets create a longitudinal and a circular sheet. This difference in cell orientation can be seen in the histological picture to the left.

Cardiac Muscle

Cardiac muscle, similar to skeletal muscle has actin and myosin in the same orientation and is thus striated. Cardiac muscle fibers  are a branching cylinder shape. This branching cylinder shape is easily shown in the picture on the bottom right hand side of this slide. However, those branching cylinders can sometimes be very difficult to see in cardiac muscle. There is a branching cylinder shape here on the bottom left corner of this histological picture.

Similar to skeletal muscle the striations run perpendicular to the sarcolemma. However in cardiac muscle there are additional structures called intercalated discs Structures in cardiac muscle that allow electrical connectivity. that also run perpendicular to the sarcolemma. Intercalated discs are where each branching cylinder muscle fiber connects to the muscle fiber next to it. So like this area here and this area over here would result in an intercalated disc. Over here on the histological picture these intercalated discs stand out more prominently than striations. They are often confused for striations.

There are few interpolated discs over here on this area. Cardiac muscle is found in only one place in the body. It is in the muscle layer of the heart, called the myocardium. It’s important to realize that there are many layers of tissue in the heart. The myocardium is strictly the contractile muscle tissue. Much like smooth muscle, you can try really hard to control your cardiac muscle. However, it won’t work. It’s only involuntarily controlled by centers in your brain stem rather than in the part of your brain that produces conscious thought.