Final answer:
The neuromuscular phenomenon in which a synergist muscle compensates for a weak or inhibited muscle is called altered reciprocal inhibition. This often involves significant compensation by the surrounding muscles to maintain movement and posture.
Step-by-step explanation:
The neuromuscular phenomenon that occurs when a synergist muscle takes over for a weak or inhibited muscle is known as altered reciprocal inhibition. This is when one muscle is rendered inactive or weakened and its synergist muscles compensate by becoming more active. In certain neuromuscular disorders like myasthenia gravis, the neurotransmitter acetylcholine (ACh) is blocked from binding to its receptors, leading to muscle weakness. A proper balance and coordination of muscle contractions are essential for maintaining muscle tone and movement, as determined by motor neuron signaling and muscular responses such as the sliding filament process.