Contraction Of A Striated Muscle Fiber

a. "Sliding Filament" Theory. The current consensus of opinion of how a striated muscle fiber contracts is known as the "sliding filament" theory (Figure 5-1). This theory emphasizes the role of the myosin bridges. Energy is 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.

b. "All-or-None" Phenomenon. When stimulated to contract by a nervous impulse, a striated muscle fiber contracts totally or not at all. This is the "all-or-none" phenomenon. The striated muscle fiber has a threshold of stimulation. Below this threshold, the fiber will not act. When stimulated at or above this threshold, the fiber will contract totally every time.

iff y

"RELAXED'

CONTRACTED'

MYOSIN FILAMENT

- ACTIN FILAMENT

- MYOSIN "BRIDGE'

Figure 5-1. Schematic diagram of the "sliding filament" theory.

c. Length-Tension Curve. The contraction of a striated muscle fiber produces tension (force). The amount of this tension varies with the length of the fiber at each moment of contraction. This tension is greatest when the fiber is at its resting length. The tension is proportionately less when the fiber is shorter or longer than its resting length. These variations in tension according to differences in fiber length may be plotted. The resulting curve is called the length-tension curve of the striated muscle fiber.

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