Final answer:
The smallest stimulus strength that triggers a contraction in a muscle fiber is called threshold stimulus It is the minimum stimulus needed to induce an action potential that leads to muscle contraction, operating on an 'all-or-none' principle.
Step-by-step explanation:
The smallest stimulus strength that triggers a contraction in a muscle fiber is called a threshold stimulus.
This concept is integral to understanding how muscle fibers function within the body.
When a motor neuron sends an electrochemical signal in the form of the neurotransmitter acetylcholine (ACh) to a muscle fiber at the neuromuscular junction,
It initiates the depolarization of the muscle fiber's membrane as sodium ions enter.
This depolarization results in an action potential that travels along the membrane, which includes T-tubules, and this in turn triggers the release of calcium ions from the sarcoplasmic reticulum (SR).
The presence of calcium then allows for the contraction of the muscle fiber, facilitated by the sliding filament model of contraction, where myosin heads form cross-bridges with the actin filaments, effectively causing the muscle fiber to contract as the sarcomeres shorten.
Muscle fibers behave according to the 'all-or-none' principle, which means they will either contract fully or not at all in response to a stimulus.
The threshold stimulus is the minimum stimulus required to induce an action potential in a muscle fiber and produce a contraction.
When a stimulus does not reach this threshold, no contraction will occur. Once the threshold is reached or exceeded, a muscle fiber will respond with a full contraction.
The distinguishing feature of muscle response is the ability for graded responses; this is due to the phenomenon of recruitment, where more and larger motor units are activated to increase the strength of muscle contraction when necessary.