Final answer:
Directly after the formation of choline substance in the synaptic cleft—a byproduct of ACh breakdown by AChE—the correct answer is the entry of sodium ions into the muscle fiber (option C), which initiates depolarization and subsequent muscle contraction.
Step-by-step explanation:
The formation of choline substance in the synaptic cleft is indicative of the presence of acetylcholine (ACh), a neurotransmitter that plays a crucial role in muscle contraction. When ACh is released into the synaptic cleft, it binds to receptors on the muscle fiber's membrane. This binding leads to the opening of ion channels, allowing sodium ions (Na+) to enter the muscle fiber, ultimately causing depolarization of the membrane, which is the crucial step in muscle contraction. Following the action of ACh, acetylcholinesterase (AChE) rapidly degrades ACh, preventing continuous stimulation of the muscle fiber.
Therefore, After the formation of choline in the synaptic cleft, which results from the breakdown of acetylcholine by acetylcholinesterase, sodium ions enter the muscle fiber. This process leads to the depolarization of the muscle fiber membrane, a fundamental step in muscle contraction.