Question

Which of the following statements about acetylcholine is true?

1) Acetylcholine is an excitatory stimulus to the muscle.
2) Binding of acetylcholine to its receptor triggers release of a second messenger, e.g., cAMP, inside the muscle.
3) These receptors have a corequisite binding of magnesium ions in order to function.
4) Binding of acetylcholine to the receptor protein converts the protein to an open ion channel.
5) The receptors ionize as a result of binding acetylcholine, and this directly alters membrane potentials.

Answer 1

The true statement about acetylcholine is that it binds to receptors on the muscle cell, leading to the conversion of these receptors into open ion channels and facilitating an action potential across the muscle fiber that ultimately causes muscle contraction. The sequence begins with acetylcholine binding and ends with calcium release from the sarcoplasmic reticulum.

Step-by-step explanation:

Regarding the effects of acetylcholine (ACh) on muscle, the true statement is that binding of acetylcholine to the receptor on the motor end plate converts the receptor into an open ion channel. When acetylcholine binds to its receptors on the sarcolemma of the muscle fiber, these receptors form an ion channel that opens. Positively charged sodium ions (Na+) then cross into the muscle cell, leading to depolarization, which reduces the voltage difference across the cell membrane. This local depolarization, known as an end-plate potential, initiates an action potential that spreads across the muscle fiber, leading to muscle contraction.

The process follows specific steps:

1. Acetylcholine is released from the presynaptic neuron and binds to ACh receptors on the postsynaptic membrane.
2. The binding of ACh opens ion channels, allowing sodium ions to enter the muscle cell and depolarize the membrane.
3. The action potential generated spreads down the sarcolemma and travels through the T tubules.
4. Ca²+ ions are released from the sarcoplasmic reticulum in response to the change in voltage.

The correct statement from the options provided is number 4, "Binding of acetylcholine to the receptor protein converts the protein to an open ion channel."