Nerves transmit electrical impulses by changing the ionic distribution across the neuronal membrane (i.e. changing the membrane potential)

• Therefore, electrical signals are not able to be conducted when a semi-permeable membrane is absent
  

Synapses are the physical gaps that separate neurons from other cells (either other neurons or the receptor and effector cells)

• Neurons transmit information unidirectionally across synapses by converting the electrical signal into a chemical signal (called a neurotransmitter)
  

Synaptic Transfer 

The release of neurotransmitters from a pre-synaptic neuron involves a number of steps:

1. When an action potential reaches the axon terminal, it triggers the opening of voltage-gated calcium channels
   

2. Calcium ions diffuse into the cell and promote the fusion of vesicles (containing neurotransmitter) with the cell membrane
   

3. The neurotransmitters are released from the axon terminal by exocytosis and diffuse across the synaptic cleft
   

4. Neurotransmitters bind to specific receptors on the post-synaptic membrane and open ligand-gated ion channels
   

5. The opening of ion channels generates an electrical impulse in the post-synaptic neuron, propagating the pre-synaptic signal
   

   • The neurotransmitters released into the synapse are either recycled (by reuptake pumps) or degraded (by enzymatic activity)
     

Signalling Between Neurons