Since the 1st period of medical school, you are faced with the microscopy laboratory, where all the tissues of our organism are observed under the microscope, being possible to observe the cells and how the tissue is structured. And this structure changes in each tissue of our organism, it changes with the place where it is, it changes with the function, that is, there are several conditions that influence the tissues. Muscle tissue is no different.
If you’ve ever seen muscle tissue under light microscopy, you’ve realized that not all muscle tissue is the same. “But how so?”. In our body, we have different types of muscle tissue, for example, the biceps brachii muscle is a striated skeletal muscle tissue. The muscle present in the gastrointestinal tract, for example in the small intestine, is a smooth muscle. And yet, we have a large percentage of muscle in the heart, but this heart muscle is different from the two mentioned above, the heart muscle is a striated cardiac muscle tissue.
And you may think that this does not make sense, because “muscle is muscle”, but we will see that there are differences within the muscle tissue itself, and these differences are essential for the functions of such tissues to be fulfilled.
What is muscle tissue for?
Muscle tissue is basically the tissue that forms all the muscles of our body / organism and that guarantees us the most diverse fundamental actions.
Its diverse functions occur by two interconnected actions: contraction and relaxation.
For example, locomotion. We can only walk, run, and so on, because we have muscles that, when contracting, perform such movements. Another example very involved in our daily lives is chewing. That’s right, we don’t just chew with our teeth. The movement of the jaw, which allows chewing, occurs thanks to the action of a muscle group. Digestion is dependent on muscles as well, as peristalsis (contraction movement in progressive waves, from top to bottom) is by the action of muscle bundles. Even our speech depends on the action of muscles. The heart, when “beating”, needs muscle action, as it is the muscles that contract and pump blood.
I think you already realized the importance of muscle tissue in our body, right? !! And be sure, these actions that I mentioned are not even 1/3 of everything that is dependent on the action of muscles in our body.
As I said, there are different muscle types, and even those actions that I mentioned are of different types as well.
Types of muscle tissue
In our body, we have three muscle types: striated skeletal muscle, striated cardiac muscle and smooth muscle. Each with its specialty.
The skeletal striated muscle performs fast and vigorous contraction, it is voluntary, that is, its action depends on our will. This means that he only “acts” if you want to. Therefore, its action is discontinuous. Walking is a voluntary action, performed by the skeletal striated muscle. Another action that has already been mentioned, and that is of this muscular type, is chewing. You chew because you want to. Picking up a weight, or carrying any object, involves striated skeletal muscle action. When you work out, you also work on that musculature in order to hypertrophy it.
We also have the cardiac striated muscle, which is a specialized muscle, present only in the heart. Its contraction action is fast, rhythmic and continuous. He doesn’t stop. All cardiac musculature is striated cardiac muscle. And despite being a striated muscle, it is involuntary. Can you imagine if you consciously coordinate cardiac activity, a few seconds of distraction would bring big problems hahaha, impossible. Likewise, you cannot mentally order your heart to stop beating. This tissue, with some peculiarities, is what allows the heart, through electrical stimulation, to perform its function of pumping blood to nourish the tissues. And we will already see such peculiarities.
The other muscle type is smooth muscle tissue . It is an involuntary muscle. It is mainly present in the organ wall, such as the uterus, bladder, gastrointestinal tract, blood vessels, bronchioles, among others. Like the previous one, being involuntary is a very important functional characteristic, because what would it be like if you had to coordinate intestinal activity, blood vessel activity? It would certainly get out of hand!
Muscle tissue histology
Before talking about the histology of each type of muscle tissue, let’s remember the specific names that muscle cell structures are given.
The plasma membrane of the muscle cell is called a sarcolemma. The cytoplasm is sarcoplasm. And the smooth endoplasmic reticulum is known as the sarcoplasmic reticulum. It is good to get used to these names, so as not to get confused, or even not understand.
So, now that we remember these basic concepts, let’s go to the skeletal muscle structure.
Skeletal Striated Muscle
First thing, kind of obvious, that you need to know: this muscle contains transverse striations, so it receives the “striate” in its name. These streaks are formed, as there is an alternation between light and dark bands. The cells that form the skeletal striated muscle are cylindrical cells , very long, reaching up to 30 cm in length, containing several nuclei ( multinucleate ) that are located close to the sarcolemma, that is, on the peripheryof the cell. These long, multinucleated cells come together in bundles, and these bundles form the muscles. Skeletal muscle tissue also contains many filaments, myofibrils, which are essential for the contraction process. When we perform physical exercise, what occurs is the formation of new myofibrils, which then lead to an increase in the diameter of muscle fibers, and with this increase in cell volume, we have the known process of hypertrophy, increasing the musculature.
And who unites these various muscle bundles? The bundles containing the muscle fibers are surrounded by connective tissue. As with other fabrics, we have some layers of lining.
Each muscle fiber is covered by a layer of connective tissue, which is the endomysium . As I said, these fibers come together and form muscle bundles, and the perimysium covers these bundles . Finally, the outermost covering layer is the epimysium , which covers the entire muscle, joining all the bundles that form a given muscle, for example, covering the entire sternocleidomastoid muscle (ECOM). And remembering that in addition to covering, connective tissue is of paramount importance, even for vascularization and innervation of muscle tissue, as the vessels (arterial, venous and lymphatic) and nerves arrive through it.
Figure 1: Histology of the skeletal striated muscle
Striated Cardiac Muscle
The cardiac striated muscle also has transverse striations . Its cells are long and branched, with one or two nuclei , which are located in the central region of these cells. Record this, as it is a way to differentiate the skeletal striated muscle from the cardiac striated muscle: by the location of the nucleus .
An exclusive characteristic of the cardiac muscle, which can be observed under microscopy and is closely related to the function of the muscle, is the presence of intercalated discs , which are junctional connections present in the muscle, which unite muscle cells, increasing adhesion between them. These discs appear as straight lines or in the form of stairs, well colored. They are extremely important for the transmission of the action potential and the consequent contraction to occur in a synchronized manner throughout the muscle, so that, thus, the contraction is effective for cardiac activity.
Figure 2: Histology of the Striated Cardiac Muscle. Observe the presence of the insert discs at the tip of the arrows.
What about smooth muscle? What can we see in light microscopy?
The smooth muscle, unlike the previous ones, does not present transverse striations . It has a single nucleus , located in the center of the cell, a cell that is long and thick , tapering at its ends .
This muscle tissue sometimes undergoes hyperplasia (increasing the number of muscle cells / fibers) and hypertrophy (increased cell volume). One of these occasions is during the gestational period, when the uterus increases its size and, with that, the myometrium (uterine muscle layer) undergoes this increase as well, due to the processes of hyperplasia and hypertrophy.
And so we close the article, remembering that today we are talking about characteristics visible under the optical microscope. Ultrastructurally, these cells have many other characteristics.
Good study, don’t forget to use a histology atlas and, whenever possible, go to the microscopy lab to have fun with the histology slides !!