The opening of ligand gates on the sarcolemma is directly caused by
The correct answer and explanation is:
Correct Answer: Acetylcholine
The opening of ligand-gated channels on the sarcolemma is directly caused by the neurotransmitter acetylcholine. The sarcolemma is the plasma membrane of a muscle fiber. It plays a crucial role in initiating muscle contraction. The process begins when a motor neuron releases acetylcholine into the synaptic cleft at the neuromuscular junction. This neurotransmitter diffuses across the cleft and binds to specialized receptors on ligand-gated sodium channels located on the motor end plate of the sarcolemma.
These channels are called ligand-gated because they open in response to the binding of a chemical ligand, in this case, acetylcholine. Once acetylcholine binds to the receptor, the channel undergoes a structural change that opens the gate and allows sodium ions (Na⁺) to flow into the muscle cell. This influx of positive sodium ions generates a local depolarization called the end plate potential. If the end plate potential reaches a threshold, it triggers an action potential that travels along the sarcolemma and into the T-tubules, ultimately leading to the release of calcium ions from the sarcoplasmic reticulum and the initiation of muscle contraction.
It is important to distinguish ligand-gated channels from voltage-gated channels. Ligand-gated channels respond to chemicals like acetylcholine, while voltage-gated channels open in response to changes in membrane potential. The first event that starts the cascade of muscle contraction is the binding of acetylcholine to the ligand-gated channels. Without this step, the muscle fiber would not become excited, and contraction would not occur.
Therefore, the direct cause of the opening of ligand-gated channels on the sarcolemma is the binding of acetylcholine released from the motor neuron. This event is fundamental to the communication between the nervous system and the muscular system, allowing voluntary muscle movement.