How does a neuron transmit a signal across a synapse?

When an action potential from a pre-synaptic neuron approaches a synapse, the change in membrane potential causes voltage-gated calcium ion channels to open across the pre-synaptic bulb. Due to the high diffusion gradient, created by the active transport of Ca²⁺ ions out of the neurone, the ions rapidly diffuse into the pre-synaptic neurone. The increased concentration of Ca²⁺ causes vesicles in the pre-synaptic neurone to move towards the cell's membrane. These vesicles are filled with neurotransmitter molecules, and once the vesicles reach the pre-synaptic membrane they fuse to it and release their contents into the synaptic cleft. The influx of neurotransmitter molecules creates a diffusion gradient, and the molecules diffuse across the synaptic cleft towards the post-synaptic neurone. Here, they bind to complementary receptors on the post-synaptic membrane and cause gated sodium ion channels to open. This causes an influx of Na⁺ ions into the post-synaptic neurone, due to the concentration gradient created by Na+ ions being pumped into the synaptic cleft. The influx of Na⁺ ions causes a localised depolaristion across the post-synaptic membrane, as the inside of the cell becomes more positively charged. An excitatory postsynaptic potential (EPSP) is created, and if this is strong enough to reach the threshold voltage an action potential will be transmitted in the post-synaptic neurone.