MECHANISMS OF EXCITATORY POSTSYNAPTIC POTENTIALS AND INHIBITORY POSTSYNAPTIC POTENTIALS
A. Postsynaptic neuron at which several presynaptic afferent fibers terminate. Fibers colored in pink convey excitatory information across the synaptic cleft to the postsynaptic neuron, whereas the inhibitory fiber is blue and conveys inhibitory information to the postsynaptic neuron.
B. Excitatory fiber. At the excitatory synaptic cleft, glutamate is released. Glutamate passes across the and acts as an agonist at the AMPA and NMDA ionotropic receptor. The excitatory neurotransmitters signal the AMPA channel to open, permitting the inflow of Na+. This results in depolarization in the membrane potential so that the difference in potential across the membrane is shifted toward the positive, i.e., depolarization. With depolarization, there is a release of Mg2+ from the NMDA receptor, permitting Na+ and Ca2+ ions to enter the posts tic neuron. An excitatory postsynaptic potential (EPSP) is generated.
C. Inhibitory fiber. The inhibitory neurotransmitters, principally GABA, act on GABA receptors in the postsynaptic neuron membrane, permitting the entry of Cl– ions, shifting the membrane potential to a more negative potential, i.e., hyperpolarization. An inhibitory postsynaptic potential (IPSP) is generated. In normal synaptic transmission, there is a balance between excitatory and inhibitory neurotransmitters so that the summation of EPSP and IPSP maintains the polarization of the membrane at a level bursts of firing occur, termed the resting potential.
D. Increase in glutamate EPSP. With an increase in excitatory neurotransmitters, the postsynaptic neuron membrane becomes more positive, producing an increase in EPSP. The summation of the excitatory and inhibitory signals value, and an action potential occurs.
E. Decrease in IPSP. When there is a decrease in inhibitory neurotransmitters, the IPSP decrease and the postsynaptic neuron membrane becomes more positive. The summation of the excitatory and inhib oves across the threshold value and an action potential is fired.