Due to the fact that one neurone consists of possibly thousands of synapses it can have a number of inputs. However, it only contains one output known as the Grand Postsynaptic Potential (GPP). This is where all the excitatory and inhibitory potentials are summed up: more excitatory than inhibitory potentials will cause the neurone to fire, but more inhibitory potentials will mean it doesn’t fire. Summation is the foundation of the nervous system’s processing power.

Synapse function

What are the reasons behind having gaps between neurones?

• They ensure unidirectionality: an impulse can only flow in one direction. The neurotransmitter containing vesicles are only contained in the presynaptic membrane while the receptor molecules are only found in the postsynaptic membrane.
• They permit integration: this means that an impulse can be dispersed from a synapse which is in contact with a number of post synaptic neurones. It’s also possible for a number of impulses to converge at a synapse.
• They let summation happen: in order for an impulse to be created in the postsynaptic neurone enough transmitter must be allowed into the cleft for it to bind to the postsynaptic receptors. Two types of summation are possible:
  • spatial summation is where a number of presynaptic neurones come together at one synapse which consists of one post synaptic neurone
  • temporal summation is where only one presynaptic and one postsynaptic neurone are involved and it’s the number of impulses getting to the synapse which counts

Both summation types still grade the response. However, if not enough presynaptic neurones are affected or the frequency isn’t high enough then the impulse won’t be carried across the synaptic cleft.

• Unimportant or unnecessary stimuli is filtered out: when a neurone is continuously being stimulated (when you wear your clothes for instance) then the neurotransmitter doesn’t have enough time to be renewed by the synapse and the impulse won’t be passed on. In other words, there’s a limit to the frequency of depolarisation after which the synapse becomes fatigued.