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The term muscle refers to individual, discrete organs of the body. However, the structure and actions of tissues are often quite different in detail from the structure and actions of organs.
Shivering is a state in which the muscles of the body are primarily concerned with producing heat. Contraction means the production of tension through the interaction of the muscle tissues
Smooth muscle tissue consists of elongated cellular elements. Striated muscle tissue is composed of fibers. These fibers represent the fusion of many cells into a single functioning fiber. Cardiac muscle tissue is also composed of banded fibers. However, its fibers have a branched character.
The fiber, as a whole, is surrounded by a membrane known as the sarcolemma. The fiber is filled with a type of intracellular fluid called sarcoplasm. Within the sarcoplasm is a tubular system called the sarcoplasmic reticulum that stores calcium, which is necessary for the muscle activation and contraction.
Myofilaments are found in the sarcoplasm.
MYO = muscle
FIL = thread
There are two main types of myofilaments: actin and myosin. Great numbers of well-developed mitochondria are found in striated muscle fibers.
The Sliding Filament Theory involves energy provided by the mitochondria in the form of ATP. With this energy, the myosin bridges swing and draw the actin filaments over the myosin filaments. The length of the striated muscle fiber is thus shortened.
When stimulated to contract by a nervous impulse, a striated muscle fiber contracts totally or not at all.