Human muscular system
Muscular system is the system of all the muscles in the human body. This system is constitutes the major portion of the weight of human body and consists of about 650 different muscles. These 650 muscles are attached to various bones and by contraction and relaxation these muscles produce the movements of the body. Muscles can only pull, they cannot push. This statement describes the basic way of action of muscles. The muscles can only exert force on other structures when they are contracting. When the muscles have once contracted they cannot relax by exerting force on other structures to which they are attached. Instead, they need some other thing to help them come to their original size and this other thing is another muscle. This discussion enumerates that muscles no matter where they exist in the body, always occur in pairs and these pairs are known as antagonistic pairs of muscles because the muscles of a pair act opposite to one another and by acting opposite they actually help one another in their functions. The muscles may be voluntary or involuntary depending on which type of muscle is under consideration. Voluntary muscles are innervated by those centers of the nervous system that are under the control of the will of the animal and thus the voluntary muscles are under the control of the will of the animal. In contrast to the voluntary muscles, the involuntary muscles are innervated by those centers of the nervous system that are not under the control of the will of the animal.
Muscles are of different shapes and sizes. They may be long, short, round or flat. The muscular system enables us to move about and to manipulate our environment. In human body all the muscles are in a constant state of effortless tension that maintains the posture and shape of the human body.
Muscular tissue is composed of long celled muscle fibers and is the most abundant tissue of the human body. The muscular tissue is specialized in reacting to stimuli by counteracting. Four characteristics of muscular tissue highlight the functioning ability of the muscles. These characteristics are irritability, contractility, extensibility and elasticity.
Types of muscles:
Muscular tissue is classified into three main types on the basis of structures and functions. These muscle types are skeletal muscles, smooth muscles and cardiac muscles.
Skeletal muscles: Skeletal muscles attached to the bones by tendons are responsible for the movements of the body. They also make the moveable structures such as the lips, tongue and eyelids. The skeletal muscle fibers show alternate light and dark bands and for this reason, the skeletal muscle is also known as striated (or striped) muscle.
Smooth muscles: Smooth muscles lack striation and are known as non striated muscles. They are involuntary muscles and are found in blood vessels, digestive tract and urinary bladder etc. The cells are spindle shaped and exist as single cells arranged in bundles or in sheets. They contract slowly than the skeletal muscles but they can contract for a longer period of time without any signs of fatigue.
Cardiac muscles: Cardiac muscles constitute the contractile tissue of the heart. These are striated like the skeletal muscles but are involuntary like smooth muscles. The cells are branched and signals of contraction are carried from cell to cell during the heart beat. Their action is involuntary, automatic and rhythmic.