Smooth muscle

Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be discussing the tissue that keeps our bodies moving without us even having to think about it which is the very special tissue known as smooth muscle.

As you may already know, muscle is one of the four major tissue types found in the human body. The other three being connective tissue, epithelial tissue, and nervous tissue. All muscle tissue is comprised of specialized cells known as myocytes, like the ones seen here highlighted in green in this micrograph. These are the elongated cells that contain the contractile proteins – actin and myosin – in their cytoplasm. The cells are arranged in distinct bundles and can easily be distinguished from surrounding tissue.

Not all muscle is created equally though and in fact there are a few easily distinguishable subtypes. Muscle tissue is characterized according to its functional properties and the contractility of muscle cells into one of three major types – skeletal muscle, cardiac muscle, and smooth muscle. Let’s begin by talking briefly about the skeletal muscle.

Skeletal muscle which you can see in the micrograph is also known as voluntary muscle and is attached to bones and tendons. It’s under the voluntary control of the somatic nervous system or SNS.

The second type of muscle found in the human body is the cardiac muscle tissue, an example of which is seen here in this micrograph. As the name suggests, this type of muscle tissue is found in the heart. The highly synchronized muscle contraction of the cardiac muscle cells transforms the heart into a pump which sends blood throughout the body.

The third muscle tissue type is what we’ll be focusing on in this tutorial and this is, of course, smooth muscle. We can see an example of smooth muscle tissue in this micrograph here. This type of tissue can also be referred to as involuntary muscle because its activity is neither initiated nor monitored consciously.

In this tutorial, we’ll be looking of course in more detail at smooth muscle, looking particularly at where it can be found in the body, its structure and composition, and finally focusing in more detail on how they’re structured in hollow organs such as the digestive tract. So, without further ado, let’s get into it by looking at some examples of where we can find smooth muscle tissue in the body.

As a general rule of thumb, we can find smooth muscle where the involuntary contraction is to take place and one example of this is the walls of the gastrointestinal tract. Here, the smooth muscle layers function to support normal gastrointestinal motility through a combination of tonic contractions and forceful contractions. We’ll be going into more detail later on about how smooth muscle functions in hollow organs such as those seen here to move matter through the digestive system.

In addition, we can also find smooth muscle in the respiratory tract, specifically in the bronchi and the bronchioles, which are these structures just here. Here, smooth muscle helps to regulate air flow into the lungs. We can also find smooth muscle tissue in the uterus, specifically in the layer of the uterine wall known as the myometrium. Here, smooth muscle functions to induce uterine contractions – for example, during menstruation or labor.

We can also find a small ring of smooth muscle in the eye known as the ciliary muscle. This controls lens accommodation when viewing objects at varying distances and also regulates the flow of aqueous humor into the canal of Schlemm.

The final example of smooth muscle I’ll be mentioning today is that found in the blood vessels. Here, smooth muscle functions to regulate the caliber of the blood vessels by controlling the amount of vasodilation and vasoconstriction. This has a direct effect on blood pressure whereby too much vasoconstriction can lead to high blood pressure and too much vasodilation can cause the blood pressure to drop too low.

Now that you have an idea of where smooth muscle tissue can be found around the body, let’s dive into the histology and look in more detail at its structure and composition.

Smooth muscle cells, also known as muscle fibers, are generally arranged in bundles. These elongated fusiform cells have tapered ends and are connected to each other via specialized communication junctions called gap junctions. Smooth muscle cells range in size from 0.2 micrometers to ten micrometers in diameter and from fifty micrometers to two hundred micrometers in length. Of course, as you might expect, the shape of smooth muscle cells and their nuclei do vary depending on whether the muscle is relaxed or contracting.

The smooth muscle cells in this micrograph have been cut in longitudinal sections. We can see that the nuclei are elongated and stained basophilic, which means that it’s stained a bluish color when stained with a hematoxylin dye. We can also see that the cytoplasm stains evenly eosinophilic – that is, a pinkish color in this combined hematoxylin and eosin staining, which is another kind of stain. The even eosin staining of the cytoplasm of these muscle fibers is due to the concentration of actin and myosin within the cytoplasm which stain pink.

Smooth muscle fibers are smaller than those of striated muscle. It’s important to remember that smooth muscle cells can align with their long axis parallel and staggered longitudinally so that the white central portion of one cell lies next to the tapered end of another. Such an arrangement achieves both close packing and a more efficient transfer of force from cell to cell. This appearance contrasts with that of skeletal muscle cells which show a consistent diameter in cross-section and peripherally placed nuclei throughout their length.

Each of the muscle fibers is surrounded by the sarcolemma which is basically a plasma membrane and external lamina. In this micrograph, you can see the sarcolemma highlighted in green. The term sarcolemma is derived from the Greek word “sarco” which means flesh and “lemma” which means husk or sheath. Aside from being the membrane that surrounds each muscle fiber, the actin-containing thin filaments found in the cytoplasm of smooth muscle cells also anchor to the sarcoplasm via cytoplasmic densities that are also known as dense bodies.

The attachment of thin filaments to the sarcolemma via the dense bodies is important for the transmission of the contractile forces that are produced inside the cell to the surface of the cell. Here in this micrograph, we see the nuclei of fibroblasts present in smooth muscle tissue. Fibroblasts are cells that secrete collagen, glycoproteins and glycosaminoglycans – all of which helps maintain the extracellular matrix.

Here we see the endomysium highlighted in green which comprises a network of reticular connected tissue. The endomysium surrounds individual muscle fibers or myocytes and its main role is to provide an appropriate chemical environment in order to concentrate the forces generated by each muscle fiber so as to facilitate the contraction of the smooth muscle such as during peristalsis. This is in addition to the sarcolemma we looked at earlier which is a plasma membrane that also surrounds muscle fibers.

As we’ve already mentioned, actin and myosin are contractile proteins found in the cytoplasm of smooth muscle cells. The cytoplasm of muscle cells is known as the sarcoplasm. Contraction of smooth muscle cells occurs when the thick myosin filaments slide along the thin actin filaments. It’s useful to note that these contractile proteins do not form sarcomeres which are a feature of other muscle tissue types.

If we take a look at our micrograph of skeletal muscle tissue, we can clearly see these pale transverse striations and these are the sarcomeres. I mentioned this because the major distinguishing feature of smooth muscle is that it does not form sarcomeres, and so it does not appear striated hence why we call it smooth muscle.

Earlier on in the tutorial, I mentioned that smooth muscle can be found on the walls of hollow organs. This is a particularly important function of the smooth muscle so let’s take a look into some more detail about how smooth muscle works in these types of organs.

We’ve seen that smooth muscle is an involuntary muscle that has a strong contractile force and lines hollow organs such as the organs of the gastrointestinal tract. In these organs, the smooth muscle is arranged in two layers – an inner circular muscle layer and an outer longitudinal muscle layer. First, let’s look at the circular muscle layer.

Here we see a micrograph of circular muscle which makes up the inner layer of the tunica muscularis or muscular layer of organs such as the intestines, the stomach, the bladder, and other hollow organs. This circular layer of smooth muscle forms sphincters at certain locations in the GI tract. It should be noted that the tunica muscularis is situated deep to the submucosal layer or the tunica submucosa of these hollow organs. Contraction of the circular muscle layer reduces the width of the lumen.

The outer layer of the tunica muscularis is comprised of longitudinal smooth muscle fibers. The longitudinal muscle is usually thicker than the circular muscle layer of the tunica muscularis and also serves a different function due to the orientation of the fibers. For example, in the intestines, contraction of the circular muscle fibers reduce the width of the lumen while the contraction of the longitudinal muscle fibers shortens the segment of the intestine subsequently widening the lumen.

In between the two layers of the tunica muscularis is a layer of connective tissue. This thin layer of connective tissue contains blood vessels and lymphatic vessels.

In the intestinal wall, we also find the myenteric plexus seen here in green in this micrograph. This is a plexus of neuronal cells also found in the connective tissue layer that separates the two layers of the tunica muscularis. The axons of the neuronal cells spread caudally mainly to innervate the circular muscle. This nerve plexus controls motility of the intestine’s wall.

The myenteric plexus is also known as the Auerbach’s plexus. Degeneration of the cells of this plexus can lead to several disorders, the most known of which is achalasia of the cardia. This is a motor disorder of the esophagus in which there is a failure of relaxation of the cardiac-esophageal sphincter and loss of peristalsis in the esophageal body due to the degeneration of myenteric plexus neurons.

Now that we’ve covered all the main points of this tutorial, let’s take a quick look at a useful clinical note relating to the smooth muscle tissue.

Quire a rare disease that affects smooth muscle cells is known as multisystemic smooth muscle dysfunction syndrome. This is caused by a mutation in the ACTA2 gene. This gene codes the smooth muscle alpha-two actin and this as we’ve seen is one of the proteins that forms the contractile components of muscle tissue. The other one being myosin.

Because this mutation affects smooth muscle, individuals suffering from multisystemic smooth muscle dysfunction syndrome exhibit a number of symptoms including weakening of the bladder, abnormalities in the walls of the blood vessels, gastrointestinal problems due to weakness of the smooth muscle of the GI tract, and pupils that are fixed and dilated or that have a decreased response to light, just to name a few.

This brings us to the end of this tutorial. Let’s quickly recap.

First, I introduced smooth muscle and we saw that it was an involuntary type of muscle tissue. We then saw that smooth muscle can be found in several places throughout the body. For example, lining the walls of hollow organs of the gastrointestinal tract, the respiratory tract, in the myometrium of the uterus, in the ciliary muscle of the eye, as well as in tubular structures such as the blood vessels.

Next, we looked in more detail at the structure and composition of smooth muscle tissue. We saw that it is mainly comprised of bundles of elongated fusiform cells which are tapered and contained elongated nuclei. These are surrounded by a membrane known as the sarcolemma as well as fibroblast nuclei highlighted in green here.

Next, we looked at the endomysium which surrounds individual myocytes to facilitate contraction of the smooth muscle and then we looked at the sarcoplasm which is the cytoplasm of muscle cells. Remember, that smooth muscle gets its smooth nonstriated appearance from the fact that it does not contain sarcomeres. We also looked at the layers of smooth muscle found in the walls of hollow organs which are the circular muscle and longitudinal muscle. Finally, we talked about the myenteric plexus which controls the motility of the intestines wall.

And that concludes our tutorial on smooth muscle tissue. See you next time and happy studying!