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Introduction to the muscular system

Overview of the anatomy, function and main structures of the muscular system.

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Hey everyone! This is Nicole from Kenhub, and in this tutorial, we'll be looking at the muscular system.

As you know when studying anatomy, we absolutely are going to come across several muscles whose names, function, origin, insertion and innervation you will have to learn and memorize. However, have you ever stopped to ask yourself, what is the muscular system? Well, you might not know this but the muscular system is actually an organ system. It can be broken down into three types of muscles – cardiac muscle which of course is the muscle that is found within the heart, smooth muscle such as the detrusor muscle of the bladder, and skeletal muscle such as this image of the latissimus dorsi muscle on the right.

Before we discuss these three types of muscles that make up the muscular system, however, let's first begin with the basics and describe exactly what a muscle is.

So the term muscle comes from the Latin word "musculus" which means "little mouse". Like any organ, muscles are made up of cells known as muscle cells or myocytes. These myocytes or muscle cells contain protein filaments of actin and myosin. The actin and myosin protein filaments slide past one another to produce contractions that produce movement in body parts and even in internal organs. Muscle fibers are bound together into bundles or fascicles by associated connective tissue that also conveys blood vessels and nerve fibers to the muscle cells. It should also be noted that muscle tissue has four main properties – excitability or the ability to respond to stimuli, contractibility which is basically a muscle cell's ability to contract, extensibility which is the ability for a muscle to stretch out without tearing, and elasticity which is the muscle cell's ability to return to its normal shape after stretching.

So now that you have a basic description of what a muscle is, let's move on to look at the types of muscles that make up the muscular system starting with the cardiac muscle. Cardiac muscle makes up the myocardium or muscular wall of the heart which you can see in this ventral view of the heart. As you can see in this ventral image of an isolated heart, cardiac muscle is also found in the walls of the aorta which is the major artery of the body and as you can see, arises directly from the left ventricle, the pulmonary veins which are the veins that carry oxygenated blood from the lungs to the left atrium, and the superior vena cava which is the major vein bringing deoxygenated blood from the superior half of the body back to the right atrium. In addition, the cells of cardiac muscle are shorter and broader and this broadness coupled with interdigitations increases the surface area for impulse conduction of the cells.

The muscle fibers of cardiac muscle are striated or striped in appearance and they're connected end to end by cell junctions formed by intercalating discs. Cardiac muscle is innervated by the autonomic nervous system and the heart rate is intrinsically regulated by the sinoatrial node. The sinoatrial node, also referred to as the pacemaker, is made up of special cardiac muscles that are also innervated by the autonomic nervous system.

The second type of muscle found in the muscular system is known as smooth muscle. Smooth muscle is found mainly in the middle layer of the walls of most blood vessels otherwise known as the tunica media which you can find in the walls of the femoral arteries on the right. It's also found in the muscular layer of the walls of the digestive tract such as this inner circular layer of muscle in the stomach, and also in the eyeball where it controls lens thickness and pupil size. And you can see an example of smooth muscle in this image of the ciliary body on the right.

Like the myocytes of cardiac muscle, the smooth muscle cells contain one nucleus, however, they have a narrow spindle-shaped appearance and lie parallel to one another. When the cell membranes of adjacent cells lie really close together, they are able to send contractile impulses from one cell to another through gap junctions or nexuses. The fibers of smooth muscle which are non-striated are arranged in longitudinal and circular fashion such as can be seen in the muscular layer of the alimentary canal. In this image, you can see part of the esophagus which is part of the alimentary canal highlighted in green. These fibers are innervated by the autonomic nervous system which is why smooth muscle is an involuntary muscle.

Now let's look at the last type of muscle of the muscular system – skeletal muscle. Skeletal muscle, like cardiac muscle, is striated. Unlike the two types of muscle that we've already looked at, however, skeletal muscles are multinucleated. Because the muscle fibers of skeletal muscle can be several centimeters long, it is possible to find that one muscle fiber has several thousands of nuclei. The fibers of skeletal muscle are also non-branching fibers and they abound by loose areolar tissue.

The reason skeletal muscle are classed as such is because their contraction produces movements of the skeleton or other body parts. These muscles are attached either directly or indirectly to bone, cartilage, ligaments, fascia or a combination of these structures via tendons. Skeletal muscle groups or fascicles can be arranged into four groups based on their basic structural pattern. These groups are circular, parallel, convergent and pennate.

Now let's look at these groups of skeletal muscles in a bit more detail beginning with the circular muscles. Circular muscles as the name describes contain fibers of skeletal muscle that are arranged in a circular pattern in concentric rings and is typically found surrounding the external body openings as you can see in this illustration here of the orbicularis oris muscle which surrounds the mouth. The contraction of these muscles close openings. Circular muscles are often referred to as sphincters.

The second group of skeletal muscle is the parallel group whose muscle fibers are either arranged in a strap shape such as the sartorius muscle seen here on the right or in a spindle shape like the biceps brachii muscle seen here again on the right. Note that the spindle-shaped fibers of skeletal muscle also has been known to be classified by some scientists as fusiform muscles.

The third group of skeletal muscles are convergent muscles. A good example of such muscles is the pectoralis major muscle seen here highlighted in green on the right. This group of muscles have a broad origin but converge and insert as a single tendon. Such muscles can appear triangular or fan-shaped.

The final group of skeletal muscles are muscles whose fibers are arranged in a pennate pattern otherwise known as a bilaterally symmetrical pattern with short fascicles like the fibers on the vastus lateralis muscle which you can see on the right. These fibers attach obliquely to a central tendon that runs the length of the muscle.

Pennate muscles can be further divided into three forms. Unipennate muscles – a good example of which is the extensor digitorum muscle and as you can see the unipennate muscles insert their fascicles into only one side of the tendon. Bipennate muscles, for example, the rectus femoris muscle, insert their fascicles into the tendon from the opposite side giving the muscles a feather shape as the tendon is located in the center. Multipennate muscles – a good example of which would be the deltoid muscle and multipennate muscles appear to look like many feathers side by side whose quills all insert into one large tendon.

As we come to the end of this tutorial, bear in mind that even though skeletal muscles are often referred to as voluntary muscles as many of them can be controlled at will, this is not always the case. Some actions of some skeletal muscles can be automatic. For example, the movement of the diaphragm when we take a deep breath.

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