 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. A 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 past 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. That is, a grunt that produces because of an ion flow is very little. It cannot produce a significant effect on the post-synaptic cell membrane. Actually, grunts through many channels are summed up. That is, they are collected as a result of a post-synaptic cell, which generates 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 an action potential. The neurotransmitters, which are most often associated with producing the excitatory post-synaptic potential in central nervous system, is the glutamate. While, the neurotransmitter, which is most important in generating the excitatory post-synaptic potential in the peripheral nervous system, is acetylcholine. Dear students, now we shall discuss the inhibitory post-synaptic potentials. One such synaptic potential, which decreases the probability of generating action potential in the post-synaptic neuron, 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. The inhibitory post-synaptic potentials can occur in all chemical synapses that reduce the probability of generating action potential. These inhibitory post-synaptic potentials can occur in all chemical synapses that release the inhibitory neurotransmitters. These inhibitory 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. In order to produce these neurotransmitters, we need to produce depolarization. Therefore, 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. If a neurotransmitter opens a channel through which the chloride ions are transferred, that will become inhibitory. So, mainly the neurotransmitters which are responsible for 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. It affects the potassium selective channels and prolongs the stage of hyperpolarization. Keep in mind that any inhibitory or excitatory effect is an important phenomenon for cellular regulations and if this inhibition is done, then if the action potential is stopped, then this is also a regulatory mechanism.