What initiates the synaptic transmission at the presynaptic neuron?

Synaptic transmission at the presynaptic neuron is primarily initiated by the occurrence of an action potential. When an action potential travels along the axon of a neuron and reaches the synaptic terminal, it causes a significant depolarization of the membrane. This depolarization is crucial because it leads to the opening of voltage-gated calcium channels in the presynaptic membrane.

Once these calcium channels open, calcium ions (Ca²⁺) flow into the presynaptic neuron due to the concentration gradient and the electrical gradient across the membrane. The influx of calcium ions is a critical step in the process, as it triggers the movement of synaptic vesicles filled with neurotransmitters toward the presynaptic membrane, ultimately leading to the release of these neurotransmitters into the synaptic cleft. This release allows the neurotransmitters to bind to receptors on the postsynaptic neuron, thereby propagating the signal.

The other options do not initiate synaptic transmission. Potassium ions diffusing out would typically be associated with repolarization after an action potential, rather than initiating it. The release of neurotransmitters occurs as a result of an action potential and the opening of calcium channels, rather than being an initiating factor. While the opening