Understanding:
• A nerve impulse is only initiated if a threshold potential is reached
Neurotransmitters bind to neuroreceptors on the post-synaptic membrane of target cells and open ligand-gated ion channels
- The opening of these channels cause small changes in membrane potential known as graded potentials
A nerve impulse 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
Graded versus Action Potentials
