Describe how an action potential spreads from one neuron to the next.

When an action potential reaches the the presynaptic cleft of the presynaptic neuron, it causes calcium ion channels ions to open, so Ca2+ ions diffuse across the membrane into the presynaptic knob. This causes vesicles in this neuron containing neurotranmitter to move to the presynaptic knob membrane. Then the vesicles fuse with the membrane and release neurotransmitter into the synaptic cleft by exocytosis. The neurotransmitter molecules diffuse across the synaptic cleft and bind to the complementary receptors on the membrane of the post synaptic knob. If enough neurotransmitter binds to the post synaptic membrane then this will cause sodium ion channels on the membrane on the post synaptic neuron to open, causing Na+ ions to move into the post synaptic neuron, triggering an action potential again in the post synaptic neuron.