Muscle tissue, which comprises about 40 percent of human body weight, consist of threads, or muscle fibers, supported by connective tissue. All living cells can move to some degree, but this ability is highly developed in muscles, which act by fiber contraction; the fibers can shorten to two thirds of their resting length.

Muscle vary greatly in structure and function in different organs and animals. Based on structure there are two types of muscles; smooth and s treated, "Involuntary", or smooth, muscles are found in the walls of all the hollow organs and tubes of the body (for example, blood vessels and intestines). There react slowly to stimuli from the autonomic nervous system. The "voluntary," or striated, muscles of the body attach mostly to the bones to move the skeleton, and under the microscope their fibers have a cross striped appearance. Striated muscle is capable of fast contractions. The heart wall is made up of special muscle fibers (cardiac muscle), a type of striated muscle. Some invertebrates have only smooth muscles. All arthropods have only striated muscles.

Structure and Function:
All muscle have basically the same structure. Each muscle has an attachment at both ends, called the origin and the insertion, and a fleshy contractile part, known as the muscle belly. Origins and insertions are usually noncontractile tendons; however, a great variety of muscle architecture is found that obtains the special mechanical advantages requred at specific joints.

Motor Units. The nerve control of the muscle fibers is organized in an economical fashion. Each motor nerve fiber running in a motor nerve to a muscle supplies a group of muscle fibers, which twitch each time an impulse is sent down the nerve fiber from the spinal cord. The frequency of these twitches can be increased and the number of motor units involved can be increased until all units are twitching rapidly. The blending of all the twitches fall to blend well but instead come in bursts an obvious tremor results.

Mechanics. A muscle produces a movement of a joint and can act only on the joints it crosses; muscles also steady joints, preventing movements in the direction opposite to those intended. For instance, the triceps brachii on the back of the arm not only in a powerful extensor straightener of the elbow joint. It also contracts to prevent straingtener of the elbow from bending when pushing with the hand. In many muscle of the lower limb it is fravity that must be counteracted, for example, muscles are just as important to keep a person erect as they are in raising the heels off the ground.

In mechanical term, the skeleton provides leverage, the muscles are the motors that act on the levers, and the joint provide fulcrum. The body employs all the typical leverage forms known to engineers. Most levers in the human body are levers of the first class, these are exemplified by the familiar seesaw and the crowbar. Levers of the third class are also common in the body; the ordinary hinged door is an example, the handle being the point at which the force is applied. In the body, this force is exerted by a tendon attached to a bone.


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