Question

Let’s say the synapses of a neuron’s dendrites are filled with glutamate-gated channels which, when activated by glutamate, cause an excitatory postsynaptic potential (EPSP) in the neuron. In one instance, glutamate is released at all of the synapses simultaneously and this leads to an action potential in the neuron.

A) What type of summation of PSPs is this? Why?

B) List the sequence of steps that occur starting from binding of glutamate to the glutamate-gated channels at the synapses and ending with the membrane potential at the axon hillock returning to resting potential at the end of the action potential. (Include all the changes in voltage-gated channels underlying the action potential.)

Answer 1

A).

If the neurotransmitter is excitatory and causes depolarization of the postsynaptic membrane, then EPSP (excitatory postsynaptic potential) is generated. Or if the released neurotransmitter is inhibitory and causes hyperpolarization of the postsynaptic membrane, then IPSP (inhibitory postsynaptic potential) is generated.

EPSP’s produced simultaneously by different synapses, but on the same postsynaptic neuron also add up, and this is called “spatial summation.”

In the given case, glutamate is released at all of the synapses simultaneously and this leads to an action potential in the neuron. Thus, it is an example of spatial summation.

B).

Binding of glutamate to the glutamate-gated channels at the synapses --> Influx of sodium ions --> Depolarization (membrane potential becomes positive) --> Action potential generation at the axon hillock (excitatory) --> Action potential propagation towards the axon terminals --> As the action potential propagates forward, the depolarized part of neuron becomes repolarized (potassium efflux), and then return to the normal resting potential (becomes negative).