Choose events that occur at the neuromuscular junction following release of acetylcholine from the axon terminus:
a) A Na+ ion channels opens and Na* exits the cell depolarizing the membrane
b) A non-specific ion channel opens, and a net influx of Na depolarizes the membrane
c)The nicotinic cholinergic receptor on the presynaptic membrane binds acetylcholine
d)The nicotinic cholinergic receptor on the motor end plate binds acetylcholine
e) a non-specific ion channel opens, and a net influx of K+ depolarizes the membrane
f) a K+ ion channels opens and K* enters the cell depolarizing the membrane
The correct answer and explanation is:
The correct answer is d) The nicotinic cholinergic receptor on the motor end plate binds acetylcholine.
Explanation:
At the neuromuscular junction, the process of muscle contraction begins with the release of acetylcholine (ACh) from the motor neuron. ACh is released from the presynaptic axon terminus into the synaptic cleft. This neurotransmitter then binds to receptors on the motor end plate, which is a specialized part of the muscle cell membrane. These receptors are nicotinic cholinergic receptors, which are ligand-gated ion channels.
When acetylcholine binds to these nicotinic receptors, the ion channels open, allowing for the passage of sodium ions (Na+) into the muscle cell. This influx of Na+ leads to a local depolarization of the muscle membrane, called the end plate potential (EPP). The depolarization generated by the opening of these channels spreads along the muscle fiber and triggers the action potential that eventually leads to muscle contraction.
The key point is that the nicotinic receptor is located on the motor end plate of the muscle fiber, not on the presynaptic membrane (as stated in option c). The presynaptic membrane releases acetylcholine, but the receptors are located on the motor end plate of the muscle cell.
Options a) and b) mention ion channels opening and ions exiting or entering the cell, but they do not correctly describe the sequence of events at the neuromuscular junction. Sodium (Na+) enters the muscle cell, not exits it, and the channel is specifically a non-selective ion channel that allows the influx of Na+.
In summary, the correct sequence begins with acetylcholine binding to the nicotinic cholinergic receptor on the motor end plate, which opens ion channels and leads to the depolarization of the muscle membrane, setting off the chain reaction for muscle contraction.