Describe how an electrical impulse would travel across a synapse.

An electrical impulse arrives at the synaptic knob in the pre-synaptic neuron. This causes the voltage-gated calcium ion channels to open and calcium ions will diffuse into the synaptic knob, down the concentration gradient. Synaptic vesicles containing neurotransmitter will then undergo exocytosis, releasing the neurotransmitters into the synaptic cleft. Neurotransmitters will diffuse across the gap and attach to protein receptors on the post-synaptic membrane. This stimulates the voltage-gated sodium ion channels to open, and sodium ions will diffuse down the concentration gradient into the post-synaptic neuron. This will cause a change in potential difference across the membrane, setting up an excitatory post-synaptic potential (EPSP). If the EPSP is large enough, an action potential will be set up in the post-synaptic neuron.