Kinds of muscle
Humans have over 650 muscles which differ in size according to the jobs they do.  These muscles constitute 40% of body weight.  The special function of muscle tissue is contraction.  There are three kinds of muscle tissue: striated muscle, smooth muscle, and cardiac muscle.  Most of the body's muscle consists of striated muscle which is the skeletal muscle.  It is also called voluntary muscle because it can be consciously controlled via the central nervous system.  Smooth muscle is the muscle of the internal organs and is called involuntary because it is not under voluntary control.  Cardiac muscle is a special type of muscle found only in the heart.  It consists of linked fibers that contract in unison producing the heartbeat.

The striated muscle cells, which comprise about 40% of the body weight, are voluntary.  They are mostly attached to the bones to move the skeleton and are fast acting and powerful.  The voluntary muscles are of three series: those more or less arranged around the axial skeleton (head, neck, and trunk), and those nonsegmentally arranged around the appendicular skeleton (arms and legs), and those associated with the visceral skeleton (brachiometric muscles).  All muscles have basically the same structure.  Each muscle has an attachment at both ends, called the origin and insertion, and a fleshy contractile part, called the muscle belly.  The point of origin is the point of attachment where the muscle is anchored to the bone.  The point of insertion is the attachment of the muscle to the bone it moves.  These muscles are attached either directly or indirectly (via tendons) to the bones, and work in opposing pairs (one muscle in the pair contracts, while the other relaxes) to produce body movements (The muscles work together to produce movement of a joint, to steady a joint, and to prevent movement in the direction opposite to those intended.).  As a rule, only the insertion bone moves.  The shortening of the muscle as it contracts pulls the insertion bone
toward the origin bone.  The origin bone stays put, holding firm while the insertion bone moves toward it.  These muscles always tire with continued use and require rest.

Because of their cross-striped appearance under a microscope, these muscles are called striated.  There are two types of striated muscle: dark fibers and light fibers.  The dark fibers are a deep red color and predominantly produce slow, tonic movement.  The light fibers are lighter in color and predominantly produce quick and contracted motions.  Each muscle fiber is encased in a thin, transparent membrane called the sarcolemma.  The fibers are subdivided longitudinally into minute fibrils and myofibrils encased in a fluid called sacroplasm.  The muscle cells are elongated tubular structures with as many as several hundred nuclei and are actually fusions of cells (syncytia).  The muscles are bound together in bundles of white fibrous connective tissue called perimysia.  Striated muscles not directly under voluntary control include vocal cord muscles and the diaphragm.

Smooth-muscle cells are not attached to the skeleton, but are found in the walls of the blood vessels, the digestive tract, and in the dermal layer of the skin.  They react slowly to stimuli from the autonomic nervous system and perform actions such as forcing food through the intestines, transporting urine to the kidneys and pumping blood through blood vessels.  The muscle is
nonstriated (lacks the striped appearance) and consist of spindle-shaped, uninuclear cells that are not bound together, as in skeletal muscle.  Like skeletal muscle, smooth muscle has fibrillae but without cross striations. The muscles are involuntary, and are slow-acting, untiring, and weak in action.

Cardiac muscle is red-colored involuntary muscle that contracts automatically and rhythmically, like a smooth muscle, but is striated (striped) and multinucleated, like skeletal muscle.  The muscle is fast-acting and powerful.  It is under the control of the autonomic nervous system and continuously contracts and relaxes throughout life.

Skeletal muscles contract rapidly in response to messages from the central nervous system.  Each group of several fibers receives a nerve supply that allows voluntary contraction of the muscle.  Muscles can move some body parts in several directions and others in only two directions.  The direction the body part is moved depends largely on the shapes of the bones at the joints.  The stimulus for the muscle contraction begins in the cerebral cortex and passes down the spinal cord and the nerve root to the junction between the nerve fiber and the muscle surface.  This gap, called the end plate, acts as a kind of amplifier, increasing the effect of the tiny current coming down the nerve fiber to stimulate the much larger muscle fiber.  On the arrival of the nerve impulse, a chemical called acetylcholine is released from the motor nerve ending and passes across the gap to stimulate the membrane of the muscle fiber.  This stimulation is in the form of an electric current which passes along the surface of the muscle, causing it to contract.  It takes one millisecond (1/1000th of a second) for the current to pass along the surface of the muscular fiber.

Cardiac muscle differs slightly from skeletal muscle because it has a built-in mechanism to maintain the necessary rhythmical contraction independently of any nervous connections.  Smooth muscles react much more slowly to stimulation than skeletal muscles.  The nerves, when present, alter the activity of the muscle rather than initiating it.  This action is somewhat similar to cardiac muscle.  The contractions take place rhythmically without direct control from the central nervous system.  The impulses for contraction come from within the muscle itself.

Muscles work by contracting and relaxing.  During contraction they shorten their length to bring the bone closer to their points of attachment on two different bones.  Every muscle movement, therefore, is a pull.  This pulling action is accomplished by the fibers and fibrils of the muscle.  All skeletal muscles are made up of small fibers.  The fibers are cylindrical in shape and are several centimeters long, with regular bands (striations) dividing them into sections.  The fibers are made up of many cylindrical subunits called fibrils.  These are the structures that actually contract.  Muscular fibers are able to shorten 30% to 40% in length during muscle contraction.  Fibrils are made of two types of protein: actin and myosin.  These proteins are in the form of long filaments.  The filaments made of myosin are thicker than the filaments made of actin.  These filaments interlock and are able to slide over each other, shortening the length of the muscle.  When the muscle is stretched the filaments tend to be pulled apart.  During shortening (contraction), they shorten by sliding into one another.  It appears that during contraction several cross-links are made between the actin and myosin filaments.  By the process of making and breaking these cross-links, the two filaments move towards one another and the whole muscle shortens.  This process is very rapid.  Cardiac muscle has a similar appearance to skeletal muscle.  It has striations and is thought to contract in the same manner as the skeletal muscle.  Smooth muscle has no striations and is composed of small spindle-shaped cells totally lacking in filaments.  Researchers still do not
understand the mechanism of smooth muscle contraction.

The muscles are biological machines which convert chemical energy into force and mechanical work.  The energy for contraction comes from the mechanical reaction between the food we eat and the oxygen we breathe.  Almost all the sugars in food are converted into glucose, the fuel of muscles.  Muscles, therefore, need a good blood supply to bring nutrients and oxygen and to remove chemical waste products.  The actual chemical process involves the breakdown of glucose to carbon dioxide and water.  This process releases energy which is used by the muscle proteins to cause contraction.  This process is known as aerobic glycolysis and involves first the production of a 3-carbon substance, pyruvate, which when combined with oxygen, is broken down further to water and carbon dioxide by a cycle of enzymes in the mitochondria, the Krebs cycle.  This chemical reaction requires a great supply of oxygen, which is often not available during intense exercise.  During exercise, a
muscle requires about fifty times more oxygen per minute than at rest.  To overcome this low level of oxygen the muscles are able to convert the glucose into a substance called lactic acid, without the use of oxygen, which still gives the necessary release of energy.  For a short period of time, this process, called anaerobic glycolysis, is a highly efficient means of harvesting energy.  Unfortunately, an accumulation of lactic acid from intense exercise causes the energy processes within a cell to cease, which eventually fatigues the muscles and causes cramps.  Removal of lactic acid requires oxygen.  To acquire the large amount of oxygen required, the body causes panting after exercise.  The lactic acid is removed as normal circulation is re-established.  Muscles are also able to store glucose.  It is stored in the form of glycogen (a carbohydrate) granules.  This store is used during
exercise.

Muscular system

The human body contains more than 650 individual muscles anchored to the skeleton, which provide pulling power so that you can move around.  These muscles constitute about 40% of your total body weight.  The muscle's points of attachment to bones or other muscles are designated as origin or insertion.  The point of origin is the point of attachment to the bone to which the muscle is anchored.  The point of insertion is the point of attachment to the bone the muscle moves.  Generally, the muscles are attached by tough fibrous structures called tendons.  These attachments bridge one or more joints and the result of muscle contraction is movement of these joints.  The body is moved primarily by muscle groups, not by individual muscles.  These groups of muscles power all actions ranging from the threading of a needle to the lifting of heavy weights.
 

Links:
Muscular System
Muscle types
 
 

Muscles of the:

feet
lower leg
upper leg
abdominals
back
torso
neck
shoulder
arm
hand
 

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