When an action potential reaches the end of one neurone there must be a way to start an action potential in the next neurone.

The two neurons will not be in direct contact and action potentials cannot jump across the gap, called a synapse (or synaptic cleft), so another method is employed...

As you can see above, the electrical impulse cannot cross the synaptic cleft, so a chemical called a neurotransmitter is released at the end of the first neurone out of the presynaptic membrane. It diffuses across the synapse, binds with the second neurone on the postsynaptic membrane and generates an action potential.

Two examples of neurotransmitters are acetylcholine (ACL) and noradrenaline. They are synthesised in vesicles, which requires energy, so the synaptic knobs have many ATP-producing mitochondria in them.

As the action potential reaches the end of the first neurone, Ca2+ channels are also opened. Ca2+ flows into the cell and this induces several hundred vesicles containing the neurotransmitter to fuse with the presynaptic membrane. The neurotransmitter is released into the synaptic cleft.

The molecules of neurotransmitter bind with complementary receptors (similar to an enzyme and substrate fitting together) in the postsynaptic membrane. This makes the Na+ channels open and depolarisation occurs in the postsynaptic membrane thus starting an action potential.

Generation of a new action potential

To stop the neurotransmitter continually generating action potentials either the neurotransmitter is actively absorbed back into the presynaptic neurone or an enzyme is released to break it down before reabsorption.

Synapses break up the flow of action potentials and so slow down the transmission of impulses but they are useful...

  • they ensure that the impulses travel only in one direction.
  • they allow neurons to connect via neurotransmitters with many, many other neurons. This increases the range of possible responses to any particular stimulus or group of stimuli.

Many drugs act by affecting the events at synapses:

Nicotine: Lowers the threshold for activation of neurons by mimicking the action of acetylcholine on the post-synaptic membrane because it is a similar shape.
Caffeine: Causes the release of calcium ions from cell stores, thereby making firing easier.
Organophosphate insecticides: Prevent the enzyme breaking down acetylcholine after it has produced an action potential. This allows acetylcholine to produce a continuous stream of action potentials, leading to an uncoordinated response in the effectors.
Curare: (Used on the tips of arrows by some tribes) blocks the acetylcholine at the junction between neurone and muscles. This means that the victim is paralysed. Also used medically as a reversible muscle relaxant during heart surgery.