muscle
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(mŭs′ĕl )
[L. musculus, diminutive of mus, mouse]
A type of tissue composed of contractile cells. Each muscle cell is filled with parallel actin and myosin filaments. When activated by an internal release of calcium, the filaments use the energy in adenosine triphosphate to crawl along each other in opposite directions. This movement shortens the length of the cell, which then contracts.
The three classes of myocytes (muscle cells) are skeletal (striated), cardiac (striated), and smooth; most human muscle is skeletal. A typical muscle has a central portion (the belly) and two or more attachment ends with tendons. The more stationary of the attachments is called the origin; the more movable attachment is called the insertion.
SELECTED MUSCLES OF THE BODY 
MUSCLE FIBER 
MORPHOLOGICAL FORMS OF MUSCLE
Comparison of Properties of Three Types of Muscle
| Smooth | Cardiac | Striated | |
| Synonyms | Involuntary | Myocardial | Voluntary |
| Nonstriated | Skeletal | ||
| Visceral | |||
| Fibers | |||
| Length (in/m) | 50-200 | 25,000 | |
| Thickness (in/m) | 4-8 | 75 | |
| Shape | Spindles | Cylinders | |
| Markings | No striation | Striation | Marked striation |
| Nuclei | Single | Single | Multiple |
| Effects of cutting related nerve | Slight | Regulation of heart rate is lost | Complete paralysis |
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