 Dear students, the post-synaptic potentials may be excitatory or inhibitory. In this module, we shall discuss these two types of post-synaptic potentials. First we shall discuss the excitatory post-synaptic potentials. The synaptic potential that makes the post-synaptic neuron more likely to fire an action potential is called the excitatory post-synaptic potential. It results from the flow of positively charged sodium or calcium ions into the post-synaptic cell. It happens when ligand-gated ion channels open due to the binding of neurotransmitter molecules produced in fast chemical transmission. Dear students, the flow of ions that causes excitatory post-synaptic potential is known as the excitatory post-synaptic grunt. The grunt through a single ion channel is too small, i.e., a grunt that is produced due to an ion flow from one channel is very small. It cannot produce a significant effect on the post-synaptic cell membrane. Actually, grunts through many channels are summed up i.e., as a result post-synaptic cell may generate excitatory post-synaptic potential. Dear students, because of summation, large excitatory post-synaptic potentials result in greater membrane depolarization. That increases the likelihood of post-synaptic cells to reach threshold to fire and action potential. The neurotransmitters, which are most often associated with producing the excitatory post-synaptic potential in central nervous system, is the glutamate. Meanwhile, the neurotransmitter, which in peripheral nervous system generates excitatory post-synaptic potential on neuromuscular junctions, is the acetylcholine. Dear students, now we shall discuss the inhibitory post-synaptic potentials. Post-synaptic assasynaptic potential, which in post-synaptic neuron generates the probability to generate action potential, is called inhibitory post-synaptic potential. Such potentials generate because of the inflow of negative ions or the outflow of positive ions. The inhibitory synaptic currents are carried by channels that are permeable to potassium ions or chloride ions. Either the chloride ions are permeable for them, then the chloride ions will go into the cell or if the potassium ions are permeable for them, then the potassium ions will come out. What will be the result of this? The result of this is that the post-synaptic potential cannot be generated. These inhibitory post-synaptic potentials can occur in all chemical synapses that release the inhibitory neurotransmitters. These neurotransmitters bind to the receptors which induce change in the permeability of post-synaptic membrane to particular ions that is chloride or potassium ions. The mechanism of inhibition due to these neurotransmitters includes that the ionic currents cause the post-synaptic membrane to become more negative than the resting membrane potential. When the resting membrane becomes negative, it means that it has hyperpolarization. To generate action potentials, we have to produce depolarization, so the neurotransmitters or the neurotransmitters that produce hyperpolarization instead of depolarization are not excitatory but inhibitory. Dear students, actually these are not the neurotransmitters which are inherently excitatory or inhibitory. Actually these are the properties of those channels and the specificity of those channels with certain neurotransmitters that causes the ions to flow through them. So, mainly the neurotransmitters which are responsible for the activating such channels are act as inhibitory. Dear students, there are some neurotransmitters which act both as inhibitory as well as excitatory. A best example of such neurotransmitters is acetylcholine which acts as an excitatory neurotransmitter at the neuromuscular junction where it opens the sodium ion channels and cause the inflow of sodium and outflow of potassium. The same acetylcholine is inhibitory in the parasympathetic neurons in the heart. There, it affects potassium selective channels and prolongs the stage of hyperpolarization. You will keep in mind that any inhibitory or excitatory effect is an important phenomenon for cellular regulations and if this inhibition is done, then the action potentials are stopped and this is also a regulatory mechanism.