Nerve impulses arise from the differential distribution of ions across a membrane and hence cannot occur when no membrane is present

• Electrical signals must be converted into chemical signals for communication to continue across a synaptic cleft
  

Neurotransmitters are the chemicals that are released into the synapse to bind to receptors on the post-synaptic membrane of target cells

• Neurotransmitters open ligand-gated ion channels and cause small changes in membrane potential known as graded potentials
  

An action potential is only initiated if a threshold potential is reached, so as to open the voltage-gated ion channels within the axon

• Excitatory neurotransmitters (e.g. noradrenaline) cause depolarisation by opening ligand-gated sodium or calcium channels
  

• Inhibitory neurotransmitters (e.g. GABA) cause hyperpolarisation by opening ligand-gated potassium or chlorine channels

The combined action of all neurotransmitters acting on a target neuron determines whether a threshold potential is reached

• If overall there is more depolarisation than hyperpolarisation and a threshold potential is reached, the neuron will fire
  

• If overall there is more hyperpolarisation than depolarisation and a threshold potential is not reached, the neuron will not fire
  

For a typical neuron, the threshold potential (required to open voltage-gated ion channels) is approximately –55 mV