Once an action potential reaches the end bulbs of a neuron, the first event in synaptic transmission is the release of neurotransmitters from the presynaptic neuron.
When an action potential reaches the end bulbs, it causes the opening of voltage-gated calcium channels in the presynaptic membrane. This influx of calcium ions into the presynaptic neuron triggers the release of neurotransmitters stored in vesicles at the synapse.
The neurotransmitters then diffuse across the synaptic cleft, the small space between the presynaptic and postsynaptic neurons, and bind to receptors on the postsynaptic membrane. This binding causes a change in the electrical or chemical properties of the postsynaptic neuron, leading to the initiation or inhibition of an action potential in the postsynaptic neuron.
It's worth noting that, this process is called synaptic transmission, it's the process by which an action potential in one neuron causes an electrical or chemical signal in another neuron, muscle cell or gland cell.