Which term refers to the smooth muscle usually arranged in two layers that is responsible for the motility that propels food and residue through the digestive tract?

Smooth muscle tissue anatomy review: In my previous lectures, I’ve discussed skeletal muscle tissue and cardiac muscle tissue. In this review, I’m going to cover the third and final type of muscle tissue: smooth muscle tissue.

Smooth muscle is quite a bit different from the other two types of muscle tissue, but it also shares a few similarities. Here’s a quick rundown of the key concepts you’ll need to know about smooth muscle tissue.

Smooth Muscle Location

Whereas cardiac muscle is only located in the heart, and skeletal muscles mostly attach to bones, smooth muscle tissue is found throughout the body. To remember the main locations, I created a simple mnemonic to help you.

Remember the word “STOVE”:

• Skin (arrector pili muscles that cause goosebumps)
• Tracts found in the reproductive, respiratory, and urinary systems
• Organs that are hollow (such as the intestines, bladder, uterus, and stomach)
• Vessels (smooth muscle helps blood vessels constrict)
• Eyes (iris constriction/dilation, as well as the lens movement)

Smooth Muscle Shape and Orientation

Smooth muscle has a fusiform shape, which resembles a football or spindle. This is different from cardiac muscle tissue, which develops into an irregular branched pattern, or skeletal muscle tissue, which consists of fibers that are long and cylindrical. However, like skeletal and cardiac muscle, smooth muscle is also surrounded and separated by a connective tissue called endomysium.

Smooth Muscle Nucleus

Smooth muscle cells have only one nucleus, which is located in the central portion of the cell. In contrast, skeletal muscle tissue has multiple nuclei around the peripheral portion, whereas cardiac muscle usually has one or two nuclei centrally located.

Smooth Muscle Layers

Smooth muscle will often develop in layers within an organ to help it move in different directions. For example, in most of the digestive tract, smooth muscle cells are formed in two layers with different orientations, which work together to propel food down the digestive tract, a process known as peristalsis.

• Longitudinal layer – This word starts with “long,” and that will help you remember that these cells run along the length of the organ as the outermost smooth muscle layer, helping it become shorter during contraction.
• Circular layer – This layer is deep to the longitudinal layer and runs in a perpendicular direction to it, forming around the organ’s circumference, hence the word “circular.” This layer narrows (or constricts) the organ during contraction.
• Oblique layer – The stomach is unique in that it has a 3rd layer of smooth muscle, an oblique layer, which helps break down food before it reaches the intestines.

Smooth Muscle Control

Like cardiac muscle tissue, smooth muscle tissue is controlled involuntarily via the autonomic nervous system. This means that we do not consciously control it. Remember, skeletal muscle tissue is the only muscle tissue type that is voluntary (under our conscious control).

Smooth Muscle Structure

Smooth muscle tissue has a different structure compared to cardiac and skeletal muscle tissue. Smooth muscle does not contain sarcomeres, the organized contractile units that are found in cardiac and skeletal muscle tissue, nor does it contain the myofibrils, which are those rod-like structures made up of the repeating segments of sarcomeres.

Because smooth muscle lacks myofibrils and sarcomeres, it does not contain the striations (or striped pattern) that characterizes both skeletal and cardiac muscle tissue. And that’s an important point to remember for exams: smooth muscle is the only muscle tissue type that does not contain striations, and that’s why it’s called “smooth.”

However, smooth muscle tissue does consist of the same thin (actin) filaments and thick (myosin) filaments found in both skeletal and cardiac muscle tissue, which work to contract the muscle fiber via a sliding filament mechanism.

Smooth Muscle Anatomy

As you look at this diagram of a smooth muscle fiber, you’ll notice the single nucleus in the center. There is a net-like structure running throughout the muscle fiber. The little dots that connect the net structure are called dense bodies.  Dense bodies attach to the sarcolemma, which is the smooth muscle cell’s outer sheath, and they work like the Z-disc in a sarcomere, allowing the thin filaments to attach to them.

The dense bodies also allow for the attachment of intermediate filaments such as desmin and vimentin, which run throughout the cell in a networked fashion, adding strength and stability to it.

Smooth Muscle Contraction

Smooth muscle contracts via a sliding filament mechanism, which is similar to that of skeletal and cardiac muscle. During contraction, calcium ions initiate a reaction that causes the phosphorylation of myosin, causing the heads on the myosin filaments to rise up and bind to the actin filaments, pulling them forward in the process.

As the myosin filament heads slide the actin filaments forward, they also pull on the dense bodies to which the actin filaments attach, which then pulls on the network of intermediate filaments running throughout the cell. Thus, the entire smooth muscle fiber contracts, or shortens.

Single-unit vs Multi-unit Smooth Muscle

It’s important to note that there are actually two sub-types of smooth muscle tissue: single-unit and multi-unit smooth muscle. Single vs multi primarily refers to the number of nerve fibers required to activate the smooth muscle tissue.

• Single-unit smooth muscle, also called unitary smooth muscle, is innervated by only one (or very few) nerve fibers per bundle. There is no need for many nerve fibers, because one nerve fiber can contract the entire sheet of smooth muscle in unison due to the presence of gap junctions, which allow the electrical signal to spread rapidly to all of the adjacent smooth muscle cells. This would be analogous to a string of Christmas lights. A single plug could power all the lights in a strand because they are electrically coupled. Single-unit smooth muscle is found primarily in the hollow organs such as the intestines, which is why it is sometimes called visceral smooth muscle (viscera refers to organs or guts).
• Multi-unit smooth muscle, however, contains fewer (or no) gap junctions, so each cell requires its own electrical impulse (hence, there are “multiple” nerve fibers found in this type of smooth muscle). This would be analogous to separate lamps. Each lamp would require its own power source, hence, there would be “multiple” power sources needed due to the lack of electrical connection between the individual lamps. Multi-unit smooth muscle is found in the skin, eyes, blood vessels, and so on.

Free Quiz and More Anatomy Videos

Take a free smooth muscle anatomy quiz to test your knowledge, or review our smooth muscle video. In addition, you might want to watch our anatomy and physiology lectures on YouTube, or check our anatomy and physiology notes.