Human Muscular System

Human Body

Over 600 skeletal muscles function for body movement through contraction and relaxation of voluntary, striated muscle fibers. These muscles are attached to bones, and are typically under conscious control for locomotion, facial expressions, posture, and other body movements. Muscles account for approximately 40 percent of body weight. The metabolism that occurs in this large mass-produces heat essential for the maintenance of body temperature.


Almost every muscle constitutes one part of a pair of identical bilateral muscles, found on both sides, resulting in approximately 320 pairs of muscles, as presented in this article. Nevertheless, the exact number is difficult to define because different sources group muscles differently, e.g. regarding what is defined as different parts of a single muscle or as several muscles.

The muscles of the human body can be categorized into a number of groups which include muscles relating to the head and neck, muscles of the torso or trunk, muscles of the upper limbs, and muscles of the lower limbs.

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Cardiac Muscles

Cardiac Muscles

Cardiac muscle is a type of involuntary striated muscle found only in the walls of the heart. This is a specialized muscle that, while similar in some fundamental ways to smooth muscle and skeletal muscle, has a unique structure and with an ability not possessed by muscle tissue elsewhere in the body. Cardiac muscle, like other muscles, can contract, but it can also carry an action potential (i.e. conduct electricity), like the neurons that constitute nerves. Furthermore, some of the cells have the ability to generate an action potential, known as cardiac muscle automaticity.

As the muscle contracts, it propels blood into the heart and through the blood vessels of the circulatory system. For a human being, the heart beats about once a second for the entire life of the person, without any opportunity to rest. It can adjust quickly to the body's needs, increasing output from five liters of blood per minute to more than 25 liters per minute. The muscles that contract the heart can do so without external stimulation from hormones or nerves, and it does not fatigue or stop contracting if supplied with sufficient oxygen and nutrients.

Cardiac muscle is only in the heart and makes up the atria and ventricles (heart walls). Like skeletal muscle, cardiac muscle contains striated fibers. Cardiac muscle is called involuntary muscle because conscious thought does not control its contractions. Specialized cardiac muscle cells maintain a consistent heart rate.

Cardiac

The muscular tissue of the heart is known as myocardium. The myocardium is composed of specialized cardiac muscle, which consists of bundles of muscle cells, technically known as myocytes. A myocyte, or muscle fiber, is a single cell of a muscle. These muscle fibers contain many myofibrils, the contractile units of muscles. Myofibrils run from one end of the cell to the other and are alternating bundles of thin filaments, comprising primarily actin, and thick filaments, comprising primarily the protein myosin. Like smooth and skeletal muscle, cardiac muscle contracts based on a rise of calcium inside the muscle cell, allowing interaction of actin and myosin.


Cardiac and skeletal muscle are similar in that both appear to be "striated" in that they contain sarcomeres. In striated muscle, such as skeletal and cardiac muscle, the actin and myosin filaments each have a specific and constant length on the order of a few micrometers, far less than the length of the elongated muscle cell (a few millimeters in the case of human skeletal muscle cells). The filaments are organized into repeated subunits along the length. These subunits are called sarcomeres. The sarcomeres are what give skeletal and cardiac muscles their striated appearance of narrow dark and light bands, because of the parallel arrangement of the actin and myosin filaments. The myofibrils of smooth muscle cells are not arranged into sarcomeres. Striated muscle (cardiac and skeletal) contracts and relaxes in short, intense bursts, whereas smooth muscle sustains longer or even near-permanent contractions.



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Smooth Muscles

Smooth Muscles Cells

Smooth Muscles

 The muscle fibers making up the single-unit muscle are gathered into dense sheets or bands. Though the fibers run roughly parallel, they are densely and irregularly packed together, most often so that the narrower portion of one fiber lies against the wider portion of its neighbor. These fibers have connections, the plasma membranes of two neighboring fibers form gap junctions that act as low resistance pathway for the rapid spread of electrical signals throughout the tissue. The multi-unit smooth muscle fibers have no interconnecting bridges. They are mingled with connective tissue fibers.

Except the heart, any action that the body performs without conscious thought is done by smooth muscle contractions. This includes diverse activities such as constricting (closing) the bronchioles (air passages) of the lungs or pupils of the eye or causing goosebumps in cold conditions.

Skeletal Muscles

Skeletal

Skeletal muscles move and support the skeleton. They make up fifty percent of your body weight. There are 640 individually named skeletal muscles. A skeletal muscle links two bones across its connecting joint. When these muscles contract or shorten, your bone moves. Muscles are arranged in layers over the bones. Those nearest to the skin are called superficial muscles. Those closest to the inside of the body are called deep muscles. Skeletal muscles are voluntary muscles. These are muscles that we can consciously control.



The actin and myosin myofilaments align evenly, producing dark and light bands on the myofibril. Each dark band depicts an area where the myofilaments overlap, causing the striated appearance of skeletal muscle.

All dark and light bands of the myofilaments have names. At the Z-line, actin strands interweave. The region between two Z-lines is a sarcomere, the functional unit of skeletal muscle. Muscle contraction occurs when overlapping actin and myosin myofilaments overlap further and shorten the muscle cell. The myofilaments keep their length. The overlapping of myofilaments is the basis for the sliding filament theory of contraction.

Skeletal muscle is a system of pairs that relax and contract to move a joint. For example, when front leg muscles contract, the knee extends (straightens) while back leg muscles relax. Conversely, to flex (bend) the knee, back leg muscles contract while front leg muscles relax. Some muscles are named for their ability to extend or flex a joint; for example, extensor carpiradialis longus muscle and flexor digitorum brevis muscle.

Muscle Fiber

Tendons attach most skeletal muscles to bones. Tendons are strong sheets of connective tissue that are identical with ligaments. Tendons and ligaments differ in function only: tendons attach muscle to bone and ligaments attach bone to bone. Physical exercise strengthens the attachment of tendons to bones.

Skeletal muscles have muscle tone (remain partly contracted), which helps maintain body posture. Ongoing signals from the nervous system to the muscle cells help maintain tone and readiness for physical activity.

Skeletal muscle aids in heat generation. During muscle contractions, muscle cells expend much energy, most of which is converted to heat. To prevent overheating, glands in the skin produce sweat to cool the skin; and, the body radiates heat from the blood and tissues through the skin. When the body is chilly, shivering causes quick muscle contractions that generate heat.

Muscle fibers and exercise

Skeletal muscles have two types of muscle fibers: fast-twitch and slow-twitch. Anaerobic exercise uses fast-twitch fibers. Such exercise includes activities that are fleeting and require brief high-energy expenditure. Weightlifting, sprinting, and push-ups are examples of anaerobic exercise. Because all cells require oxygen to produce energy, anaerobic exercise depletes oxygen reserves in the muscle cells quickly. 


The result is an oxygen debt. To repay the debt, humans breathe deeply and rapidly, which restores the oxygen level. Anaerobic exercise creates excess lactic acid (a waste product). By increasing oxygen intake, the liver cells can convert the excess lactic acid into glucose, the primary food molecule used in cellular metabolism.


Aerobic exercise uses slow-twitch muscle fibers. Such exercise includes activities that are prolonged and require constant energy. Long distance running and cycling are examples of aerobic exercise. In aerobic exercise, the muscle cell requires the same amount of oxygen that the body supplies. The oxygen debt is slashed and lactic acid is not formed.