Responses of The Postsynaptic Neuron
- When neurotransmitters reach the postsynaptic neuron of an excitatory synapse, these molecules can bind to two possible types of receptors that are clustered in a protein-rich portion of the postsynaptic cytoskeleton called the Postsynaptic density (PSD). Ionotropic receptors, which are also referred to as ligand-gated ion channels, contain a transmembrane domain that acts as an ion channel and can directly open after binding of a neurotransmitter. Metabotropic receptors, which are also called G-protein-coupled receptors, act on an ion channel through the intracellular signaling of a molecule called a G protein. Each of these channels has a specific reversal potential, Erev, and each receptor is selectively permeable to particular ions that flow either into or out of the cell in order to bring the overall membrane potential to this reversal potential. If a neurotransmitter binds to a receptor with a reversal potential that is higher than the threshold potential for the postsynaptic neuron, the postsynaptic cell will be more likely to generate an action potential and an excitatory postsynaptic potential will occur (EPSP). On the other hand, if the reversal potential of the receptor to which the neurotransmitter binds is lower than the threshold potential, an inhibitory postsynaptic potential will occur (IPSP).
- Although the receptors at an excitatory synapse strive to bring the membrane potential towards their own specific Erev, the probability that the single stimulation of an excitatory synapse will raise the membrane potential past threshold and produce an action potential is not very high. Therefore, in order to achieve threshold and generate an action potential, the postsynaptic neuron has the capacity to add up all of the incoming EPSPs based on the mechanism of summation, which can occur in time and space. Temporal summation occurs when a particular synapse is stimulated at a high frequency, which causes the postsynaptic neuron to sum the incoming EPSPs and thus increases the chance of the neuron firing an action potential. In a similar way, the postsynaptic neuron can sum together EPSPs from multiple synapses with other neurons in a process called spatial summation.
Read more about this topic: Excitatory Synapse
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