Final answer:
The threshold stimulus is the minimum strength of stimulation required to contract a muscle fiber. It is necessary to reach this threshold to generate an action potential that will lead to muscle contraction. The correct option is D.
Step-by-step explanation:
The question asks to identify the definition of a threshold stimulus. The correct answer is D. minimum strength of stimulation required to contract a muscle fiber.
Muscle fibers are stimulated to contract by a signal called an action potential, which is triggered by the neurotransmitter acetylcholine (ACh) affecting the muscle fiber at the neuromuscular junction.
For a muscle fiber to contract, a minimum level of stimulus is necessary — this is referred to as the threshold stimulus. If the stimulus is below this threshold, no contraction will occur.
Once the threshold is reached or surpassed, an action potential is generated, leading to muscle fiber contraction.
The strength of the contraction will not increase with more stimulus strength; each fiber contracts maximally in response to a threshold stimulus in an all-or-nothing fashion. The correct option is D.