Video: Introduction to the muscular system

Wave your hand. Wiggle your toes. Now smile. Pat your stomach while rubbing your head. Thank goodness for our muscles! These fleshy fellows allow us to move around. But did you know that's not all they're good for? Or that some muscles can be controlled and others do their jobs without you having to think about it?

Imagine how exhausting and dangerous it would be if we had to keep reminding our hearts to beat or respiratory muscles to take in a breath of fresh air. There's a lot to know when it comes to muscles; however, don't frown. It actually takes more muscles to frown than to smile. Save your energy. You'll be smiling once you've completed an introduction to the muscular system.

Before we start, let's take a quick overview of what we'll cover in this video. First, we'll learn what is the muscular system and what is a muscle. Then we'll identify the three types of muscle and their locations: Cardiac muscle which composes the muscular walls of the heart, smooth muscle found in the walls of blood vessels and organs of several body systems such as the gastrointestinal tract and urinary tract, and skeletal muscle which as the name suggests is generally attached to the skeleton and provides voluntary movement.

After that, we're going to look at the four major regional groups of skeletal muscles based on where in the body they're found. These are the muscles of the head and neck, the muscles of the trunk, the muscles of the upper limb, and the muscles of the lower limb. Within each of these body regions, the muscles are grouped taking into account their function and location. So we'll take a look at the main groups of muscles within each region while also mentioning examples of the most relevant muscles within each group.

Finally, we'll finish up with some clinical notes and talk about common problems affecting the muscular system, specifically, muscle paralysis and atrophy.

Let's first begin with the basics. Have you ever stopped to ask yourself what exactly is the muscular system? Well, to give it a definition, it's an organ system composed of specialized contractile tissue called muscle tissue which, on average, makes up about 40 percent of total body weight and includes all types of muscles in the body. The term muscle comes from the Latin word musculus, which means little mouse.

Like any organ, muscles are made up of cells. In this case, they are known as muscle cells or myocytes, often also called muscle fibers because they are long and narrow when relaxed. Muscle fibers are bound together into bundles, or fascicles, by associated connective tissue that also convey blood vessels and nerve fibers to the muscle cells. These are specialized contractile cells that contain protein filaments of actin and myosin. These filaments slide past one another to produce contractions that create movement in body parts and even in internal organs.

It should also be noted that muscle tissue has four main properties: Excitability or the ability to respond to neural stimuli, contractibility which is basically a muscle's ability to contract, extensibility which is the ability for a muscle to stretch out without tearing, and elasticity which is the muscle's ability to return to its normal shape after stretching.

So now that we have a basic description of what a muscle is, let's move on to look at the different types of muscles and their location.

The muscular system is most often associated with one main type of muscle – skeletal muscle – the one that acts on body joints to produce movements. However, there are two other types of muscle tissue found in the body – cardiac muscle and smooth muscle. These two types of muscles are important components of other systems.

Based on whether they appear striped or unstriped when viewed under a microscope, muscles are also classified into striated and non-striated muscles with the skeletal and cardiac muscles being grouped as striated while the smooth muscle is non-striated.

Starting with the cardiac muscle, it makes up the myocardium or muscular wall of the heart. Although contractions of cardiac muscle are less powerful than those of skeletal muscle, this type of muscle is resistant to fatigue. It is the hardest working muscle of your body because it keeps your heart beating every single day of your life.

Staying alive! Staying alive! Staying alive! Staying alive!

Cardiac muscle consists of a branching network of individual cells linked electrically and mechanically to work as a unit in the contraction of the heart. The muscle fibers or cells of cardiac muscle, also called cardiomyocytes, are striated or striped in appearance and they're connected end to end by cell junctions formed by intercalating discs. Cardiac muscle is not under voluntary control and its activity is influenced by the autonomic nervous system.

The second type of muscle found in the muscular system is known as smooth muscle. It is called smooth because it is non-striated when viewed under a microscope. Smooth muscle is found in the middle layer of the walls of most blood vessels, otherwise known as the tunica media. It's also found in the muscular layer of the walls of various structures associated with the gastrointestinal, respiratory, and urogenital systems.

At this histological slide, you can see smooth muscle cells highlighted. Like the cardiomyocytes, smooth muscle cells contain only one nucleus. They have a long, spindle-shaped appearance and lie parallel to one another closely arranged in bundles. These cells are capable of slow and sustained contractions. When the membranes of adjacent cells lie really close together, they are able to pass contractile impulses from one cell to another through gap junctions. Sheets of smooth muscle cells can be orientated in longitudinal and circular fashion such as can be seen in the muscular layer of the gastrointestinal tract.

Smooth muscle is responsible for several functions in the body. In the tubular structures and hollow organs, smooth muscle is responsible for moving substances through them by coordinated sequential contractions called peristaltic contractions. In storage organs such as the urinary bladder, contraction is slow and sustained and brings about elimination of its contents. Smooth muscle can also have the opposite effect – a sphincteric action which prevents elimination such as is the case of the internal urethral sphincter.

In the walls of the blood vessels, the smooth muscle fibers are arranged circularly and serve to modify the caliber of the lumen of the vessel leading to vasoconstriction or vasodilation which, in turn, increases or decreases blood pressure.

Depending on the organ, smooth muscle fibers may be stimulated to contract by local stretching of the fibers by nerve impulses from the autonomic nervous system or by hormonal stimulation which is why smooth muscle contraction is involuntary.

Now let's look at the last type of muscle of the muscular system – skeletal muscle. Skeletal muscle, like cardiac muscle, is striated. The fibers of skeletal muscles are non-branching and organized into parallel bundles of long multi-nucleated muscle fibers wrapped in connective tissue sheaths. Skeletal muscles are the main functional units of the muscular system comprising the majority of muscle tissue in the body. Together with our bones, they comprise the musculoskeletal system that provides voluntary movement.

Besides movement production, the skeletal muscles have other important roles like providing support and giving form to the body, maintaining posture, and stabilizing joints. Many muscles in their tendons pass over joints and thereby stabilize the articulating bones by holding them in position. Besides that, approximately 85 percent of the heat produced in our body is the result of muscle contraction which makes our muscles essential for maintaining normal body temperature. Find yourself shivering when your body gets cold? These rapid muscle contractions help to heat us up.

Generally, this type of muscle is under voluntary control, but this is not always the case. Some actions of skeletal muscles can be involuntary; for example, the movements of the diaphragm during breathing.

Now that we know the main three types of muscle and have explored the most important feature of skeletal muscles, it's time to describe the major groups of skeletal muscles based on body regions.

There are more than 600 muscles in the human body. They vary greatly in shape and size with the smallest one being the stapedius muscle in the inner ear and the largest being the gluteus maximus in the buttocks. To make it easier to learn, the skeletal muscles of the human body can be organized into four groups – one group for each region of the body. These groups are the muscles of the head and neck, the muscles of the trunk, the muscles of the upper limb, and the muscles of the lower limb.

Within each region, we can also group the muscles based on their function or location. Using this logic, the muscles of the head and neck can be divided into several groups that we're going to describe next. The muscles of facial expression which you can see here, also called the mimetic muscles, are a group of about 20 flat skeletal muscles lying underneath the skin of the face and scalp. The specific location and attachments of the facial muscles enable them to produce movements of the face such as smiling, grinning, and frowning.

When looking at the muscles of facial expression, two circular muscles come to our attention – the orbicularis oculi muscle, a paired muscle that surrounds the eyes, and the orbicularis oris muscle surrounding the mouth. These muscles act like sphincters closing the orifices they encircle.

The masticatory muscles attach to the mandible and thus produce movements of the lower jaw at the temporomandibular joint to enable functions such as chewing and grinding. One of these muscles, the masseter, is the strongest muscle in the body. The extraocular muscles, also referred to as the muscles of the orbit, mainly control eye movement and also eyelid elevation.

The tongue is a muscular organ situated in the oral cavity whose main functions include sensation of taste, mastication or chewing, deglutition or swallowing, speech, and clearing the oral cavity. The tongue has several intrinsic and extrinsic muscles that help in these actions.

The pharynx is a muscular column that runs between the oral cavity and the esophagus. The actions of its muscles are important in the process of swallowing. Located in the neck, the larynx has several functions such as proper air conduction, speech, movements of the epiglottis, and airways protection. The larynx has several intrinsic and extrinsic muscles that produce movements of the larynx and its cartilages thus enabling the performance of these functions.

The muscles of the neck, as its name implies, are muscles that cover the area of the neck. These muscles are mainly responsible for the movement of the head and neck in all directions. Within the neck, we'll mention two important members of this group – the platysma muscle, a thin sheet-like muscle that lies superficially within the anterior aspect of the neck, and the sternocleidomastoid muscle.

Now let's move on to another region of the body – the trunk – which includes the back on the posterior surface and the thorax, the abdomen, and the pelvis on the anterior surface. The muscles of the back are a group of strong paired muscles that lie on the posterior aspect of the trunk. They provide movements of the spine, stability to the trunk as well as coordination between the movements of the limbs and trunk.

The back muscles are divided into two large groups. The extrinsic, or superficial back muscles, lie most superficially on the back. These muscles are related to the upper limb in terms of function and embryonic origin. The trapezius muscle and the latissimus dorsi muscle are two very large muscles that belong to this group. In this image here, we can also see a little bit of the latissimus dorsi muscle from an anterior perspective.

The intrinsic, or deep back muscles, which are also called the true back muscles are located deep to the extrinsic muscles. Their primary function is to produce movements of the vertebral column. A very prominent member of this group is the erector spinae, a massive muscle group composed of three muscular columns that are located on either side of the spine.

The muscles of the thorax include muscles that attach the upper limb to the thoracic wall, and hence, the axial skeleton and muscles that support the thoracic wall itself. The first are mostly involved in producing upper limb movements and include the pectoralis major muscle and the serratus anterior muscle. The latter are responsible for moving the ribs during ventilation and include the intercostal muscles.

The abdominal wall surrounds the abdominal cavity providing it with flexible coverage and protecting the internal organs from damage. This is a complex structure that consists of numerous layers which include a muscular layer formed by several paired muscles that include the rectus abdominis muscle.

The muscles of the pelvis are located more inferiorly in the trunk. These muscles along with the nearby ligaments and their investing fascia make up the pelvic floor – a basin-shaped muscular diaphragm that helps to support the organs of the pelvis composed mainly of urinary and reproductive organs.

Now it's time to move to the limbs starting with the upper limb.

The upper limb includes different regions – the shoulder, the arm, the forearm, and the hand. Each of these regions has its own group of muscles. The scapulohumeral muscles, as the name implies, connect the scapula located at the posterior aspect of the thorax to the humerus which is the bone of the arm. These muscles are mainly involved in producing movements of the arm at the shoulder joint as well as stabilizing this joint. They include the deltoid muscle – the muscle responsible for the contour of the shoulder region.

The muscles of the arm are located in the region between the shoulder and elbow joints. They are divided into two distinct compartments. The anterior, or flexor compartment, contains muscles that are primarily responsible for the flexion of the shoulder or elbow joints. This group includes the biceps brachii muscle, which is responsible for the anterior contour of the arm.

The posterior, or extensor compartment, contains the muscles that allow the extension of the forearm at the elbow joint. This group is mainly represented by the triceps brachii muscle.

Just like the arm, the forearm is divided into anterior and posterior compartments. The anterior compartment contains the flexor muscles of the wrist and hand. One of these muscles is the flexor digitorum superficialis muscle involved in flexing the fingers. The posterior compartment contains the extensor muscles of the wrist and hand. It includes the brachioradialis muscle found in the lateral aspect of the forearm and the extensor digitorum muscle responsible for extending the fingers.

The muscles of the hand are very important for gripping and manipulating objects. One example of these muscles are the dorsal interossei muscles. The intrinsic hand muscles are only partially responsible for all its range of motion. The other major contributors are actually the forearm muscles, which project tendons towards the hand through the wrist.

Now it's time to take a look at the muscles of the lower limb. These muscles can also be classified according to the region they belong to or the function they have into muscles of the gluteal region, muscles of the thigh, muscles of the leg, and muscles of the foot.

The gluteal muscles are essential for stabilizing the pelvis over the femur which allows bipedalism and an erect posture. That's the reason why the prominence of the gluteal region is unique to humans. This prominence is mainly caused by this huge muscle – the gluteus maximus muscle. The tensor fasciae latae muscle also belongs to the gluteal muscles and is the most anterior of the group.

Deeper in the gluteal region, we find the inner hip muscles running from the medial features of the pelvis laterally to insert onto the proximal end of the femur which is the bone of the thigh. These muscles produce movements of the thigh at the hip joint and also stabilize this joint maintaining the head of the femur in place. The thigh can be split up into three compartments.

The anterior compartment contains muscles that mainly extends the leg at the knee joint. The quadriceps femoris muscle is the main component of this group. The medial compartment of the thigh consists of muscles that mainly adduct the thigh at the hip joint bringing the lower limb closer to the midline. The adductor magnus muscle is the largest and most posterior muscle of the medial compartment of the thigh. The posterior compartment of the thigh contains muscles that mainly extend the thigh at the hip joint and flex the leg at the knee joint. We can see one of these muscles highlighted here on this image – the biceps femoris muscle.

The leg muscles produce different movements of the ankle and foot important for many activities such as walking, running, and dancing. There are also three muscle compartments in the leg. The anterior compartment consists of muscles that mainly produce dorsiflexion of the foot at the ankle joint. This is the movement of the foot upwards towards the tibia. The tibialis anterior muscle is the main foot dorsiflexor. It lies superficially in the leg and is easily palpable lateral to the anterior border of the tibia. Deep to it, we find the extensor digitorum longus muscle which also dorsiflexes the foot and extends the toes.

The lateral compartment consists of muscles that mainly produce an eversion of the foot which is the movement of the foot laterally.

The posterior compartment contains muscles that primarily produce plantarflexion of the foot at the ankle joint – the movement of the foot downwards away from the tibia. In this compartment, forming the bulk of the back of the calf, we have a muscle called triceps surae. This is a three-headed muscle which actually consists of two muscles – the gastrocnemius and the soleus. The latter is very important for maintaining a steady posture when standing on the ground.

Intrinsic muscles of the foot originate and insert in the foot. These muscles mainly modify the actions of the long tendons from the leg muscles and generate fine movements of the toes. In this group, we find the dorsal interossei muscles of the foot are four feather-like muscles found interposed between the five metatarsal bones of the foot.

And with that, we concluded our revision of the skeletal muscle of the different body regions. Now it's time to get clinical.

Muscle strength testing is an important component of the physical exam that can reveal information about neurologic deficits. There are two common testing methods: The patient performs movements that resist those of the examiner or the examiner performs movements that resist those of the patient.

Electromyography – the electrical stimulation of muscles – is another method for testing muscle action. Muscle paralysis is the inability to move a specific muscle or muscle group. It may be associated with other neurological abnormalities including loss of sensation. Paralysis may be due to damage to the brain, spinal cord, or the nerves supplying the muscles. Major causes of paralysis include stroke, trauma with nerve injury, infections, and autoimmune diseases.

Paralysis may also be caused by drugs that affect the neurotransmitters at the nerve endings and the action upon the muscle themselves. In the long term, muscle paralysis will produce secondary muscle wasting and overall atrophy of the region due to disuse.

Wasting of a muscle may also result from a primary disorder of the muscle or caused by immobilization of a limb such as with a cast. In the case of the limbs, remember that a muscle on the opposite side of the body can be used for comparison.

Muscle atrophy is an important problem in patients who have undergone long-term rest or disuse; for example, patients that are in the ICU for long periods of time. These patients require extensive rehabilitation and muscle building exercises to regain normal activities of daily living.

Before we finish, let's briefly review what we learned in this video.

First, we talked about the muscular system which is an organ system composed of specialized contractile tissue called the muscle tissue. Then we explored the three types of muscle tissue – cardiac muscle, smooth muscle, and skeletal muscle. Skeletal muscles constitute the great majority of named muscles. We described the four major groups of skeletal muscles based on body regions – the muscles of the head and neck, the muscles of the trunk, the muscles of the upper limb, and the muscles of the lower limb.

We then saw that within each body region, the main skeletal muscles can be grouped based on their function or location and gave some examples of the most relevant muscles.

First, we talked about the muscles of the head and neck which comprise the muscles of facial expression or mimetic muscles which include the orbicularis oculi muscle and the orbicularis oris muscle; the masticatory muscles which include the masseter muscle, the strongest muscle in our body; the extraocular muscles or muscles of the orbit; the muscles of the tongue; the pharyngeal muscles; the laryngeal muscles; and the muscles of the neck including the platysma and the sternocleidomastoid muscle.

Then we move to the muscles of the trunk – a big region that includes several groups of muscles. The muscles of the back are divided into a superficial or extrinsic group and a deep or intrinsic group. The muscles of the thorax comprise muscles related to the upper limb like the pectoralis major muscle and the serratus anterior muscle and muscles responsible for moving the ribs during ventilation like the intercostal muscles; the anterolateral abdominal muscles that protect and support the contents of the abdominal cavity; and the muscles of the pelvis that help to support the pelvic viscera.

We then turned into the limbs starting with the muscles of the upper limb. The scapulohumeral muscles connect the scapula to the humerus. The arm contains one group of anterior or flexor muscles and a group of posterior or extensor muscles. The forearm is also divided into an anterior or flexor compartment and a posterior or extensor compartment. And finally, we had the numerous muscles of the hand.

Finally, we described the muscles of the lower limb which include the gluteal muscles which make up the buttocks. The most superficial and prominent of this group is the gluteus maximus muscle. Deeper in the gluteal region, we can also find the inner hip muscles. The thigh can be split up into three compartments. The muscles of the anterior compartment extend the knee joint and include the quadriceps femoris muscle. The muscles of the medial compartment adduct the thigh at the hip joint and the posterior compartment of the thigh contains muscles that flex the knee.

The muscles of the leg are also divided into three compartments. The anterior compartment consists of muscles that produce dorsiflexion of the foot, the lateral compartment consists of muscles that evert the foot, and the posterior compartment contains muscles that plantarflex the foot. And finally, we looked at the much smaller muscles of the foot.

Finally, in the clinical notes, we looked at the common problems affecting the muscular system, specifically, muscle paralysis and atrophy.

And that's it for now! Hope you enjoyed this video. See you next time and happy studying!