What happens when an impulse arrives at a synapse?

Answer 1

The electrical impulse causes chemicals called neurotransmitters to be released from the axon terminals of the pre-synaptic neuron which diffuseacross the synaptic cleft and fit into receptors on the post-synaptic neuron.

In an excitatory synapse, the presence of the neurotransmitters in the receptors of ligand-gated ion pores cause those pores to open and allow sodium ions into the post-synaptic neuron, which results in an electrotonic signal being conducted down the dendrite and soma to the axon hillock, which may initiate an action potential in the axon if enough signals are summed up at the axon hillock to reach a trigger value.

Answer 2

When a neural impulse reaches the end of a neuron, an axon terminal, it causes voltage-gated calcium ion pores to open, allowing calcium ions into the axon terminal, where a sequence of chemical actions causes vesicles (small bags) containing neurotransmitters to move into contact with the cell membrane, and fuse with it, releasing the neurotransmitters into the synaptic cleft (gap).

The neurotransmitters diffuse across the synaptic cleft, where they fit into receptor sites which are part of ligand-gated sodium ion pores, causing those pores to open, allowing sodium ions into the neuron. The sodium ions push one another down the dendrites and cell body (soma) to the axon hillock, in a process know as electrotonic conduction, which is a graded response since it can vary from weak to strong.

If a combination of many such signals, either from many dendrites or quickly from one or more, sums up to reach a trigger voltage, then an action potential will be triggered in the axon.

(Note that this is a description of what happens at an excitatory synapse; a similar but different process occurs at an inhibitory synapse. At an inhibitory synapse, the resulting membrane potential may serve to inhibit the action of other excitatory inputs.)