Presynaptic neurons release neurotransmitters into the synapse to trigger graded potentials in post-synaptic neurons

  • Some generate excitatory post-synaptic potentials (EPSPs) and others produce inhibitory post-synaptic potentials (IPSPs)
  • EPSPs trigger depolarisation in the post-synaptic membrane, IPSPs trigger hyperpolarisation in the post-synaptic membrane
  • If the combination of signals reaches a threshold level, an action potential will be triggered in the post-synaptic neuron

The combination of graded potentials in the post-synaptic neuron is known as summation

  • Cancellation occurs when excitatory and inhibitory graded potentials cancel each other out (no threshold potential reached)
  • Spatial summation occurs when EPSPs are generated from multiple presynaptic neurons simultaneously to reach threshold
  • Temporal summation occurs when multiple EPSPs are generated from a single presynaptic neuron in quick succession

These summative effects determine which nerve pathways are activated and hence lead to alternate decision-making processes

Types of Summation