Summation at Higher Level: An AHL statement: how many inputs together decide whether a neuron fires.
Practise this as you read
- Explain why one input may not be enough.
- Explain the effect of inhibitory inputs.
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The big idea: One postsynaptic neuron receives synapses from many presynaptic neurons. Their excitatory and inhibitory effects add up: this is summation. If the total depolarization reaches the threshold, a full action potential follows; if not, nothing does. The result is all or nothing.
The points to remember
- A postsynaptic neuron has synapses from many presynaptic neurons.
- Excitatory inputs depolarize it a little; inhibitory inputs hyperpolarize it.
- Their effects add up: summation.
- Total reaches the threshold: an action potential follows.
- Below the threshold: no action potential. It is all or nothing.
Remember it as: Add up the votes: past the threshold, it fires.
Real example: motor neurons in the spinal cord. A single motor neuron in the spinal cord receives thousands of synapses. Whether it fires, and so whether its muscle fibres contract, depends on the sum of all the excitatory and inhibitory inputs at that moment.
Trap: A bigger total does not give a bigger action potential: once the threshold is reached, every action potential is the same size.
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How it is asked: Expect a question on why one presynaptic neuron may not be enough, or how excitatory and inhibitory inputs decide whether the postsynaptic neuron fires.
Explain why an impulse in one excitatory presynaptic neuron may not produce an action potential in the postsynaptic neuron.
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