A probe inserted into a postsynaptic neuron can monitor the membrane potential of the dendritic spines and soma and determine whether synapses in these regions are excitatory or inhibitory. Prior to a synaptic event, the dendritic spines and soma are polarized to approximately -70 mV. This value indicates that the inside of the membrane is slightly negative (or less positive) relative to the outside.
Neurotransmitters released at excitatory synapse open ion channels in the postsynaptic membrane that allow positive ions (Na+) to enter the cell. The influx of positive ions causes the membrane to temporarily depolarize (= move toward 0 mV). This voltage change, which is called an excitatory postsynaptic potential (EPSP), is small, local and will not elicit an action potential by itself. Instead, several EPSPs are required to reach the threshold point (-55 mV) to generate an action potential. Neurotransmitters released at inhibitory synapses open ion channels that allow negative ions (Cl-) to enter the postsynaptic neuron or positive ions (K+) to exit. The influx of negative ions and/or the efflux of positive ions causes the membrane to temporarily hyperpolarize. This is called an inhibitory postsynaptic potential (IPSP).
