Muscle Types and Their Function in the Human Body

Muscle is a tissue that has the ability to contract. It is comprised fibers of that slide beside each other. These fibers are connected by bridges. When these bridges move the fibers move making the muscle shorter and it is this contraction that creates movement. Muscles do not lengthen unless they are pulled by an opposing muscle or by an external force.

There are over 600 separate muscles in the human body and these are broken into 3 types. Cardiac muscle which makes up the heart; smooth muscle which is found inside internal organs and around blood vessels; and skeletal muscle which is voluntary and is responsible for movement.

Cardiac Muscle

Cardiac muscle makes up the heart and has the ability to create, send, and conduct its own electrical system which is what keeps the heart beating, regardless of what the brain does.

At the top right of the heart, in the right atrium, is the Sinoatrial node (SA). This is a bundle of nerves that have the ability to release an electrical charge which is what causes the heart to contract. When this electrical impulse is released it travels through the cardiac muscle, first around the atriums causing them to contract which in turn pushes blood into the ventricles.

On the right ventrical is another bundle of nerves called the Atrioventricular node (AV) that collects the charge from the SA node and holds onto it for a split second (depending on the heart rate). Then it releases the charge so it can travel through the septum of the ventricles (the center). Then the charge travels through the Purkinje fibers (the outside of the ventricles) causing them to contract, which causes the blood to be squeezed out.

As the left ventricle squeezes, blood is sent through the aorta (biggest artery of the body) which takes oxygenated blood to all parts of the body. The right ventricle pushes blood through the right pulmonary arteries which takes deoxygenated blood to the lungs.

There are atrioventricular valves between the atriums and the ventricles to prevent back flow of blood when the ventricles contract. If these valves do not close properly then the person suffers from what's called a murmur. In some serious cases surgery is required to correct the valve or to replace it with an artificial one.

There are also pulmonary and aortic valves to prevent back flow into the ventricles.

Electro Cardiogram (ECG) is a method of measuring a heart's electrical activity.

Cardiac output is the amount of blood pumped through the heart in one minute. The average for a resting adult is 1.3 gallons (5 liters) per minute, but can be as high as 9.25 gallons (35 liters) per minute in an athlete during intense exercise. Cardiac output is determined by the heart rate and stroke volume (amount of blood that can fill the chambers). As heart rate increases so will cardiac output. However, if the rate becomes too, high cardiac output may decrease since there is now limited time for the blood to fill the chambers and therefore stroke volume decreases.

Carotid arteries branch off from the aorta as they leave the left ventricle. The carotid arteries are responsible for taking oxygenated blood back to the cardiac muscle, not through the chambers, but to feed the muscle so it can work effectively. During a heart attack one of these carotid arteries are blocked or ruptured, resulting in an area of the cardiac muscle not receiving oxygen. That part of the muscle, if not treated quickly, will die and that can result in death if a large enough area of the muscle is affected.

Ischemia is the term used to indicate lack of blood flow to part of the cardiac muscle, resulting in death of that tissue.

Skeletal Muscle

Skeletal muscle is what most of us are familiar with. It has several functions such as protection of organs and creating movement. It works by receiving signals from the brain. With stress, such as weight lifting, it becomes stronger and larger (hypertrophy). But becomes weaker and smaller (atrophy) with lack of use.

Unlike other cells in the body, muscle cells have very limited ability to reproduce/divide. However, embedded in the fibers are what's called satellite cells. These satellite cells do have the ability to create new cells in some cases, but will not restore a severely damaged muscle, and do not contribute to the increase in muscle strength seen during exercise.

With training you do not build more cells but rather you make those muscle cells you already have bigger.

There are also 3 different types of skeletal muscle:

• Slow oxidative fibers: this has the most endurance, and is the smallest in size. This muscle contracts the slowest, but can maintain the contraction the longest.
• Fast oxidative fibers: this has intermediate fatigue, and is intermediate in size.
• Fast glycolytic fibers: this is the thickest and strongest, but fatigues the quickest. This muscle has the ability to contract very quickly, but can not maintain the contraction for very long.

Depending on which muscle type one is predominant in may determine which activities they are best at. For example, someone with slow oxidative fibers may be a great endurance runner. Someone with fast glycolytic fibers may be a great short distance sprinter. Of course training is a factor.

Muscle is also very metabolically active and as such it requires large amounts of energy to work, even while at rest. For those looking to lose weight, this translates into the more muscle you have the more calories you burn.

Muscle is very flexible and fairly strong. It is attached to bones through a very strong and non-flexible tissue called a tendon.

Smooth Muscle

Smooth muscle is totally involuntary, meaning you can not control its contraction. It is found around internal organs (such as the stomach and bladder) and causes these organs to contract; blood vessels which causes them to constrict or dilate; and the intestines and esophagus which helps food pass through the body.

Smooth, unlike skeletal muscle, has the ability to divide. It is also able to stretch much more (e.g. urinary bladder) while still being able to contract quite forcefully.