When the acetylcholine binds to the receptor sites
Multiple Choice
a ligand is formed.
the sodium channels open.
the sodium channels remain closed.
sodium ions diffuse through and enter the cell.
sodium ions diffuse through and leave the cell.
The Correct Answer and Explanation is:
Correct Answer:
sodium ions diffuse through and enter the cell.
When acetylcholine, a neurotransmitter, binds to its receptor sites on the postsynaptic membrane—specifically, the nicotinic acetylcholine receptors—a crucial step in synaptic transmission occurs. These receptors are ligand-gated ion channels, meaning they open in response to the binding of a chemical messenger (in this case, acetylcholine).
Once acetylcholine binds to the receptor, the receptor undergoes a conformational change that opens the sodium (Na⁺) channels embedded in the cell membrane. This opening allows sodium ions to diffuse through the channel and enter the cell, moving down their electrochemical gradient. Sodium ions are more concentrated outside the neuron than inside, and the inside of the neuron is negatively charged compared to the outside. This difference in concentration and charge creates a strong drive for sodium to move into the cell.
The influx of positively charged sodium ions causes a depolarization of the postsynaptic membrane. If the depolarization reaches a threshold, it can trigger an action potential in the postsynaptic neuron, continuing the transmission of the neural signal.
To clarify why the other options are incorrect:
- “A ligand is formed” is incorrect because acetylcholine itself is the ligand; it binds to the receptor but is not “formed” during the process.
- “The sodium channels remain closed” is incorrect because they open upon acetylcholine binding.
- “Sodium ions diffuse through and leave the cell” is also incorrect because sodium ions enter, not exit, the cell in this context.
In summary, acetylcholine binding to its receptor facilitates the opening of sodium channels, allowing sodium ions to enter the postsynaptic cell, initiating a key step in the transmission of a nerve impulse.
