What happens to the acetylcholine in the synaptic cleft once an action potential has been triggered in the postsynaptic neuron? (A) It is broken down by enzymes. (B) It binds to acetylcholine receptors on the presynaptic membrane. (C) It dissolves into the cytoplasm of the neuron. Or (D) it diffuses out of the cleft and into the bloodstream.
Let’s start by reviewing the structure of a synapse and how it works. Here, we have a synapse. A synapse is the junction between two neurons, or a neuron and an effector, such as a muscle or a gland. There exists a small space between the two neurons or between the neuron and its effector. And this small space is known as the synaptic cleft. Chemicals known as neurotransmitters are used to send messages across the synaptic cleft, from the presynaptic neuron or the neuron before the synaptic cleft, which is sending the message, to the postsynaptic neuron or the neuron after the synaptic cleft, which is receiving the message.
The terminal of the presynaptic neuron contains vesicles filled with neurotransmitter. In this case, the neurotransmitter would be acetylcholine. And on the surface of the postsynaptic neuron, we have these receptors. When an action potential traveling down the axon of a presynaptic neuron reaches the terminal, the vesicles filled with neurotransmitter will fuse with the presynaptic membrane, releasing the neurotransmitter into the synaptic cleft. The neurotransmitter will then travel across the synaptic cleft and bind to the receptors on the postsynaptic membrane. This will then trigger an action potential in the postsynaptic neuron.
So, if the binding of the neurotransmitter, such as acetylcholine, triggers an action potential in the postsynaptic neuron, what then happens to the acetylcholine? It needs to be removed from the receptors or it will continue stimulating and triggering action potentials in the postsynaptic neuron. So there are also enzymes in the postsynaptic membrane that can help break down the neurotransmitter that remains in the synaptic cleft. In the case of a cholinergic synapse, where acetylcholine is used as the neurotransmitter, acetylcholinesterase is the enzyme that breaks down the acetylcholine. We can recognize acetylcholinesterase as an enzyme because of the ending A-S-E.
So, just based on its name, we can recognize acetylcholinesterase as the enzyme that breaks down acetylcholine. So acetylcholinesterase will take the acetylcholine and break it down into choline and ethanoic acid parts. These parts will be absorbed back into the presynaptic membrane, where they can be used to create more acetylcholine to potentially be used again to transmit more messages over the synaptic cleft.
So let’s return to our question. What happens to the acetylcholine in the synaptic cleft once an action potential has been triggered in the postsynaptic neuron? We said that the acetylcholine is broken down by the acetylcholinesterase enzymes in the postsynaptic membrane and then reabsorbed into the presynaptic neuron for potential reuse later. We can eliminate some of these answer choices because, for example, there are not acetylcholine receptors on the presynaptic membrane, only on the postsynaptic membrane. And likewise, there is not a direct blood supply to the synapse. So, while some of the neurotransmitter might defuse out of the cleft, it will not be diffusing into the bloodstream. The correct answer is that the acetylcholine in the synaptic cleft is broken down by enzymes.