the synapse

• The synapse is a junction between two neurons or a neurone and an effector cell.
• the presynaptic neurone is swollen and known as the presynaptic knob.
• The cell after the synapse is the postsynaptic nerve
• the gap between each cell is the synaptic cleft
• The neurotransmitter is released from the presynaptic membrane and diffuses across the synaptic cleft. only the presynaptic cell contains the neurotransmitter.
• Signal transmission:

  1. An action potential arrives at the end of the presynaptic neurone.
  2. This depolarizes the membrane and stimulates voltage-gated calcium ion channels to open and an influx of Ca+ ions to enter the synaptic knob.
  3. The influx of Ca+ ions causes vesicles containing ACh to fuse to the presynaptic membrane releasing ACh into the cleft.
  4. ACh diffuses across the cleft and binds to specific receptors on the postsynaptic membrane. this causes a slight delay in the impulse.
  5. The neurotransmitter binds to Na+ ion channels which opens and an influx of sodium depolarises the membrane.
• Spatial summation:

  where two or more presynaptic neurons releases neurotransmitter at the same time into the post neuron. The combination of small amounts of neurotransmitter released from each can be enough to reach threshold.
• Temporal summation:

  Two or more nerve impulses arrive at the same presynaptic neuron in quick succession.