Question

what is the meaning of the term threshold stimulus?
the concentration of ca ions in the sarcoplasm of an isolated muscle fiber that stimulates contraction
the minimum strength of an electrical impulse that can stimulate contraction in an isolated muscle fiber
the minimum number of action potentials per second that an isolated muscle fiber must generate in order to contract
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Explanation:

In the context of muscle - fiber physiology, the threshold stimulus is defined as the minimum strength of a stimulus (such as an electrical impulse) that can trigger a contraction in an isolated muscle fiber. It is not related to calcium ion concentration or the number of action potentials per second for the definition of threshold stimulus.

Answer:

The minimum strength of an electrical impulse that can stimulate contraction in an isolated muscle fiber