Human muscular system or muscles is often thought as amount of strength we possess. Muscles work with each other to perform various functions. Human muscles (Figure 1) not only provide strength, but are also associated with movement, posture, and balancing the whole body.
Muscular system is associated with movement of body parts, such as hand and leg movements for positioning, eye movements, facial expression etc.
The skeletal muscles are composed of water (75%), proteins (actin and myosin), carbohydrates, inorganic salts (mainly potassium phosphate, calcium, sodium, manganese, iron etc.), pigments (myoglobin, cytochrome), and enzymes.
They are developed from mesoderm (myotomes), an exception being the head muscle, which developed from loose mesenchyme during embryonic development. The cells, which give rise to the muscular tissue, are called myoblasts. The plasma membrane of the muscle fiber is called sarcolemma.
The sarcolemma (Figure 2) conducts electrochemical signals, in turn stimulating muscle cells. Mitochondria are present in abundance in muscle cells that help to break down sugars and provide energy in the form of ATP to active muscles.
Types of Muscle Tissues
The muscles constitute 40-50% of total body weight. These muscles are made of muscle tissues. Each tissue is composed of cells called muscle fibers (Figure 3). Each muscle fiber contains cytoplasmic filaments (actin, myosin, and other proteins), that provide contractility to the muscle. Myofibrils (contractile fibers forming muscle fiber) are made up of many proteins fibers arranged into repeating subunits called sarcomeres.
Sarcomeres are functional unit of muscle fibers. These are composed of thick fibers (made of protein myosin) and thin fibers (made of actin, troponin, and tropomyosin).
Depending upon their location, function, and appearance muscles are divided into three main types, described below (Figure 4 and 5):
Skeleton muscles are also known as striated, striped, or voluntary muscles, they perform all activities that we perform consciously. They are multinucleated cylindrical structures with longitudinal and cross striations (Figure 6A and B).
Skeleton muscles can perform rapid, powerful contraction as well as slow sustained tonic contraction. They are found in upper as well as lower limbs, tongue, pharynx etc. Most skeletal muscles are attached to two bones across a joint and are innervated or supplied with nerves.
Cardiac muscles are also known as myocardium or heart muscles, they are found in the walls of the heart (Figure 7). These are involuntary muscles responsible for pumping blood throughout the body. They possess characteristics of both skeleton and smooth muscle fibers. Each fiber is a uninucleate, long and cylindrical structure, which lacks a definite sarcolemma.
Smooth muscles are also known as unstrained, involuntary, or visceral muscles, they are found in the gastrointestinal tract (stomach, intestine, part of esophagus), lungs, urinogenital tract, urinary bladder, blood vessels, iris of eyes, dermis of skin and hair pilli (Figure 8). These are elongated, spindle-shaped and lack sarcolemma. Since they controlled by autonomic nervous system, they cannot be controlled by our will.
Muscles, which act in opposition to other muscles are called antagonistic muscles. For example, biceps, bends or flexes the arm and is called a flexor. Its antagonist, triceps straightens or extends the arm and is called extensor.