SYNAPTIC TRANSMISSION 

So far in our coverage of synaptic transmission we have kept things fairly simple by thinking about two neurones connected by a single synapse. In the nervous system however this is rarely the case. The image opposite is a scanning electron micrograph of a neuronal culture in which the soma of one neurone is contacted by a large number of axon terminals from other neurones.

In fact even this is probably an usually simple example because single neurones in the human spinal cord have been shown to receive over 10,000 synaptic inputs.

To complicate this story even further we must remember that some of these synapses will be excitatory and others will be inhibitory.

So what does a neurone do with all these inputs?

 

  

Well it simply adds up all the EPSPs and IPSPs and if the membrane potential reaches threshold then it generates and action potential. This adding together is referred to as summation and in effect it is the initial segment that does the arithmetic because, as you will recall from the excitable tissues lesson, this is where action potentials originate.

Because graded potentials only affect regions of the cell close to where they originate, synapses that are close to the initial segment have a much bigger impact on the neurone than synapses further away. For this reason excitatory or inhibitory effects on the soma or proximal dendrites affect the neurone to a much greater extent that those out on the distal dendrites.

Some examples of fairly simple synaptic interactions are described below.

The diagram on the right shows neurone 1 and neurone 2 forming an excitatory synapse with neurone 3. The membrane potential of each neurone is displayed below them.

If we stimulate neurone 1 or neurone 2 alone then you can see that this results in an EPSP in neurone 3 that does not reach threshold. So no action potential is produced in neurone 3.

However when you stimulate neurone 1 and 2 together you see that the two EPSPs add together, the membrane of neurone 3 reaches threshold and you get an action potential.

Because this type of interaction is from spatially distinct inputs it is referred to as spatial summation.

  

 

In the example on the left, two neurones are linked by an excitatory chemical synapse. The membrane potential of the two neurones is displayed below them.

If we stimulate neurone 1 once we get an EPSP in neurone 2 that is below threshold so no action potential occurs.

However if we apply two stimuli to neurone 1 in short succession then see that the EPSP caused by the second action potential arrives before the first one has completely decayed.

As a result these two EPSPs add together and consequently neurone 2 reaches threshold and we get an action potential.

Because this type of summation is a consequence of timing of the inputs it is known as temporal summation.

In the diagram opposite we have a neuronal circuit in which neurone 1 and neurone 2 synapse with neurone 3 and their respective membrane potentials are shown below.

If you stimulate neurone 1 alone you will see that in this instance the EPSP is big enough to reach threshold so we get an action potential in neurone 3.

If you stimulate neurone 2 alone 0 you will see that it must form an inhibitory synapse with neurone 3 because it produces an IPSP.

However when you stimulate neurone 1 and 2 together the IPSP negates the effect of the EPSP and so no action potential is elicited in neurone 3.

This is an example of excitatory and inhibitory synapses can interact.

  

These are just three simple examples of how neurones use summation to integrate information. But of course in real life, every second of every hour, each of the 100 billion or so neurones in your nervous system is using exactly the same principles to summate the thousands of EPSPs and IPSPs that affect them. The outcome of this summation determines what you choose for breakfast, when you will cross the road and the hundred of thousands of other decisions that you make every day.