Answer: Acetylcholine is released into the synaptic cleft under the influence of calcium. The correct option is d.
Step-by-step explanation:
When an action potential reaches the end of a presynaptic neuron, it triggers the release of neurotransmitters into the synaptic cleft. Acetylcholine is one of the neurotransmitters involved in transmitting signals between neurons and across the neuromuscular junction.
Here's how the release of acetylcholine occurs:
1. Action potential: When an action potential reaches the end of the presynaptic neuron, it depolarizes the membrane, causing voltage-gated calcium channels to open.
2. Calcium influx: The opening of the voltage-gated calcium channels allows calcium ions (Ca2+) to enter the presynaptic neuron from the extracellular space. This influx of calcium is crucial for the release of neurotransmitters.
3. Vesicle fusion: The calcium ions bind to proteins within the presynaptic terminal called synaptotagmins. This binding triggers the fusion of synaptic vesicles containing acetylcholine with the presynaptic membrane.
4. Exocytosis: The fusion of synaptic vesicles with the presynaptic membrane leads to the release of acetylcholine into the synaptic cleft. Acetylcholine molecules diffuse across the synaptic cleft and bind to receptors on the postsynaptic membrane.
5. Postsynaptic response: Once acetylcholine binds to its receptors on the postsynaptic membrane, it initiates a response in the postsynaptic neuron or effector cell, such as a muscle fiber. This response can involve the opening of ion channels and the generation of a new action potential in the postsynaptic neuron or the contraction of the muscle fiber.
In summary, calcium plays a crucial role in the release of acetylcholine into the synaptic cleft. When an action potential reaches the presynaptic terminal, it causes an influx of calcium ions, which triggers the fusion of synaptic vesicles containing acetylcholine with the presynaptic membrane. This release of acetylcholine allows for the transmission of signals between neurons and across the neuromuscular junction. While serotonin, norepinephrine, and dopamine are neurotransmitters themselves, they are not directly involved in the release of acetylcholine.