Muscle is a type of tissue that is responsible for movement in the body. Most of us think of muscles by its outer appearance in terms of physique and the movement of different body parts. However, there are muscles in various different internal structures and organs which are constantly working but are not under our control. We may not be able to see these muscles in action or even feel its activity.
In fact these small and unseen muscles are working even when we are at rest. For example, the breathing muscles work throughout life to allow us to breathe. The heart muscles as well do not stop until death. If it were not for these muscles in vital organs and other structures, human life would not be possible. However, not all muscles are the same and it is important to understand the different types.
What are Muscles?
Muscles are collections of myocytes (muscle cells) that contract and relax to facilitate movement. The myocytes are also referred to as the muscle fibers and contain proteins known as actin and myosin which make movement possible. These bands of protein within each cell are responsible for this contractility. These protein bands are known as myofibrils.
In order for a muscle to contract, thousand of these myofibrils within each muscle cell has to slide in a coordinated manner. This contraction happens when the nerve is stimulated by a nerve impulse. When the stimulation ceases, the myofibrils slide back to its normal position. Some of these muscles are under voluntary control and nerve impulses can be initiated at will. Others occur involuntarily and the body controls the action of these muscles on its own.
Types of Muscles
There are three types of muscles in the body – skeletal, smooth and cardiac. Skeletal muscles are under voluntary control and responsible for movement of the body or parts of the body. Smooth muscles are not under voluntary control. It is present throughout the body – in the walls of the blood vessels, airways, bowels and bladder. However, the heart has a different type of muscle that is not under voluntary control. This is known as cardiac muscle.
Skeletal muscles comprise almost 40% of the body weight. It is these muscles, when well conditioned and large, that are responsible for the physique that is associated with being muscular. It is also the muscles that are visible under our skin. The skeletal muscles are under voluntary control but sometimes it is active even when a person is not consciously contracting a specific skeletal muscle.
For example, when we stand or sit there are many skeletal muscles that help us maintain our posture. These muscles are constantly working to oppose the force of gravity although it is not being consciously contracted. When moving, we are able to select which muscle or muscles will contract and relax . For example when moving the arm we voluntarily control the muscles of the shoulder and upper arm.
Skeletal muscles attach to bones by bands of connective tissue known as tendons. When the muscles contract, the tendons pull against the bone to which it is attached. This allows for movement at the joint between the bones.
Read more on skeletal muscles.
Slow Twitch and Fast Twitch Skeletal Muscles
There are two types of skeletal muscle cells – slow twitch and fast twitch muscle fibers. Some muscles have more than one type of muscle fiber.
Those muscles that have a larger number of slow twitch muscle fibers are also referred to as red muscles (type I skeletal muscle). It is better suited for sustained physical activity of low intensity like walking or jogging for a long period of time.
Muscles with more fast twitch muscle fibers are known as white muscle (type II skeletal muscle). It is better suited for high intensity activities over a short period. For example when a heavy load is lifted it is the white muscle fibers that are utilized to a greater extent.
Many small muscles are located throughout the body and are constantly functioning to allow for various processes to occur apart from movement of parts of the body. Many of these processes are necessary for life but are not under voluntary control. The muscles responsible in these cases are known as smooth muscle.
The autonomic nervous system controls these muscles as and when is necessary. Most of the time a person is unaware on this muscle activity under normal circumstances. Smooth muscles are abundant in various organs and structures such as the walls of the digestive tract, airways, urinary tract and blood vessels.
These muscles contract and relax for different purposes such as moving the contents within a hollow organ, example when urine is pushed out of the bladder or stool is passed out of the rectum. These muscles may also widen or narrow some structures like the airways or blood vessels.
Even the tiny hairs on the skin surface has small smooth muscle fibers which control it. When these muscle fibers contract the hairs stand up, usually in response to cold but may also be elicited by emotions such as fear.
The heart is a muscular vital organ. The muscles of the heart wall known as the myocardium are constantly working throughout life. This type of muscle is known as cardiac muscle and the fibers are similar to skeletal muscle in that it is a striated muscle. However, unlike skeletal muscle the muscles of heart are under involuntary control. In fact cardiac muscle activity is regulated by structures in the heart itself.
The cells that comprise the cardiac muscles are known as cardiomyocytes. Nerve impulses generated by the natural pacemaker located in the heart (known as the sinoatrial node) is conducted throughout the heart muscle to cause it to contract. This is ongoing throughout life and the rhythmic contraction and relaxation of the heart muscle is what we refer to as the heart beat.
When the cardiac muscle contracts, blood is forced out of the heart and into the large arteries which either send the blood to the lungs or circulate it to the rest of the body. When the muscle relaxes, the heart fills with blood.