Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command.
(2) A motor neuron in the ventral horn of the spinal cord is activated, and an action potential passes outward in a ventral root of the spinal cord. (3) The axon branches to supply a number of muscle fibers called a motor unit, and the action potential is conveyed to a motor end plate on each muscle fiber. (6) At the opening of each transverse tubule onto the muscle fiber surface, the action potential spreads inside the muscle fiber. (7) At each point where a transverse tubule touches part of the sarcoplasmic reticulum, it causes the sarcoplasmic reticulum to release Ca ++ ions. Muscle contraction is the activation of tension-generating sites within muscle fibers. In physiology, muscle contraction does not mean muscle shortening because muscle tension can be produced without changes in muscle length such as holding a heavy book or a dumbbell at the same position. The termination of muscle contraction is followed by muscle relaxation, which is a return of the muscle fibers to their low tension-generating state. Muscle contractions can be described based on two variables: length and tension. A muscle contraction is described as isometric if the muscle tension changes but the muscle length remains the same. In contrast, a muscle contraction is isotonic if muscle length changes but the muscle tension remains the same Let's take a look at what occurs within a skeletal muscle, from excitation to contraction to relaxation 1  : An electrical signal (action potential) travels down a nerve cell, causing it to release a chemical message (neurotransmitter) into a small gap between the nerve cell and muscle cell. 2  This gap is called the synapse. to contraction in skeletal muscle. The trigger for a muscle contraction is an electrical impulse. The electrical signal sets off a series of events that lead to crossbridge cycling between myosin and actin, which generates force. The series of events is slightly different between skeletal, smooth and cardiac muscl Through the process of muscle contraction, tension is developed within muscle tissue, which may or may not lead to movement of a part of the body. The term contraction often means to shorten; however, during a muscle contraction the tension may cause muscles to stay the same, get longer or become shorter.  All muscles exert their force by pulling between at least two points of attachment. The movement that results from contraction is called the action of the muscle. Typically, one attachment remains stationary and is called the origin and the other attachment moves and is called the insertion. The sliding filament theory is the explanation for how muscles contract to produce force. As we have mentioned on previous pages, the actin and myosin filaments within the sarcomeres of muscle fibres bind to create cross-bridges and slide past one another, creating a contraction.  Muscle action can be classified as being either voluntary or involuntary. Cardiac and smooth muscles contract without conscious thought and are termed involuntary, whereas the skeletal muscles contract upon command. Skeletal muscles in turn can be divided into fast and slow twitch fibers. 1 Smooth muscle or involuntary muscle is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair). Muscle is a soft tissue found in most animals. Muscle cells contain protein filaments of actin and myosin that slide past one another, producing a contraction that changes both the length and the shape of the cell. Muscles function to produce force and motion. 1 Smooth muscle or involuntary muscle is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, blood vessels, and the arrector pili in the skin (in which it controls erection of body hair).
http://meat.tamu.edu/ansc-307-honors/muscle-contraction/ https://en.wikipedia.org/wiki/Muscle_contraction http://health.howstuffworks.com/human-body/systems/musculoskeletal/muscle3.htm http://www.wisegeekhealth.com/what-is-muscle-contraction.htm http://droualb.faculty.mjc.edu/Course%20Materials/Elementary%20Anatomy%20and%20Physiology%2050/Lecture%20outlines/muscle_anatomy.htm http://www.ptdirect.com/training-design/anatomy-and-physiology/skeletal-muscle-the-physiology-of-contraction https://en.wikipedia.org/wiki/Muscle https://en.wikipedia.org/wiki/Muscle
what is muscle action