Final answer:
The release of acetylcholine from motor neurons triggers the opening of mechanically gated calcium ion channels in the sarcolemma, leading to muscle contraction.
Step-by-step explanation:
The event that triggers the opening of mechanically gated calcium ion channels in the sarcolemma of muscle cells is indeed A) the release of acetylcholine (ACh) from motor neurons. The process begins at the neuromuscular junction (NMJ), where the motor neuron communicates with the muscle fiber. When ACh is released into the synaptic cleft, it binds to receptors on the sarcolemma, initiating the opening of sodium ion channels. This results in the generation of an action potential that propagates along the sarcolemma and into the T-tubules.
The change in voltage caused by the action potential triggers the sarcoplasmic reticulum, an organelle within the muscle cell, to release stored calcium ions (Ca2+). These calcium ions play a pivotal role in muscle contraction by binding to troponin, ultimately allowing the actin and myosin filaments to interact and generate force. Thus, the release of ACh sets in motion a sequence of events leading to the activation of mechanically gated calcium ion channels and the initiation of muscle contraction.