Postsynaptic potentials are the potentials that typically take place due to the changes taking place around the postsynaptic membrane. It is very well known that synapses are the junctions either between two neurons or in-between a neuron and a muscle cell in case of a neuromuscular junction. In case of a chemical synapse, molecules known as neurotransmitters are released by the first neuron and after crossing the synapse or the junction, the neurotransmitters are taken up and a response is generated within the second neuron or muscle cell. In this pathway, the second neuron or the muscle cell is known as the postsynaptic terminal. Thus, the changes that take place around the membrane of such a cell or around the membrane of the postsynaptic terminal in case of a chemical synapse are said to be postsynaptic potentials.
Brief Account of Postsynaptic Potential
The changes in the membrane potential at the postsynaptic terminal of a chemical synapse is known as postsynaptic potential. These potentials vary in size and hence are said to be graded potentials. These postsynaptic potentials are not action potentials. Action potentials occur when there is a rapid increase in the membrane potential of a cell whereas the postsynaptic potentials are the membrane potential around the postsynaptic neuron or postsynaptic muscle cell. From this it can be said that the postsynaptic potential may regulate the action potential and also can be responsible for the initiation or inhibition of the action potential depending upon the signal which can be initiatory or inhibitory.
These potentials arise due to the release of the neurotransmitters from the terminal end of the first neuron in the synaptic cleft. These neurotransmitters then bind to certain receptors that are present on the postsynaptic terminal, known as postsynaptic receptors, which can be the membrane of a postsynaptic neuron or a postsynaptic muscle cell, in the scenario of neuromuscular junction. When these neurotransmitters are bound to the receptors, the receptors can respond in multiple ways. One of the ways in which the postsynaptic receptors respond is by opening up the ion channels that are present in the membranes of the postsynaptic cell. Due to the exchange of the ions across the membrane of the cell, there arises a potential difference which leads to further downstream processes such as action potentials. The cellular arrangement and the transfer of neurotransmitters through the synaptic cleft to the receptors is shown in the image given below:
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The ions present around the cell membrane experience two types of forces - one is the diffusion force and another one is electrostatic repulsion. The general property of the ions is to reach a state of equilibrium across the membrane of the cell. This leads to the membrane potential reaching an equilibrium potential. As the membrane reaches equilibrium potential the diffusion of the ions across the membrane from higher to lower concentration cancels out the electrostatic repulsion between the ions across the membrane potential. This leads to the net movement of ions being zero across the postsynaptic membrane. Such changes in the postsynaptic membrane potential lead to the postsynaptic potential triggering a response which can be either excitatory or inhibitory for a signal to be transmitted across the neuron or the muscle cell.
The postsynaptic potential develops because of the attachment of the neurotransmitter to its respective receptor on the postsynaptic neuron or muscle cell. Hence, when the neurotransmitter detaches itself from the receptor the postsynaptic potential is terminated. This initiates another cascade of events such as the receptor returning to its original structural position and the closing of the ion channels that had been opened by the receptor when the neurotransmitter was attached to it. This stops any flow of ions across the membrane and hence there are no depolarization events taking place due to the diffusion of the ions. As the flow of ions stops, the membrane gets restored to its previously held polarization state. Under such conditions the postsynaptic potential is thus said to be terminated and there is no further change until the neurotransmitters are released from the presynaptic cell. And this continues as the neurotransmitters then cross the synapse and bind to the postsynaptic receptors after crossing the chemical synapse. Thus, the postsynaptic potential regulates further transmission of signals and action potentials and hence, any disruption in this potential can lead to drastic effects on nerve conduction and signal transmission.
Postsynaptic Potential and Action Potential
The neurons are typically said to have a resting potential of -70mV. A resting potential can be said to be the static membrane potential that exists across the membrane of the postsynaptic neuron or muscle cell. This membrane potential is the potential across the membrane when there is no flow of ion across the membrane by any means. When the flow of ions increases, the membrane can either undergo depolarization i.e. the potential reaching towards net zero value or hyperpolarization i.e. the potential further becoming more and more negative. This of course depends on the type of influx of ions inside the cell.
When the neurotransmitters bind to the receptors it results in the opening of ion channels. If the ion channel that is open allows influx of positive ions i.e. there is a net gain of positive charge across the membrane due to the opening of ion channels, the membrane as known from above undergoes depolarization. Due to the depolarization the potential across the membrane starts to increase and it reaches closer to the firing threshold of the action potential i.e typically -55mV. This is known as the excitatory postsynaptic potential (EPSP) as it leads to further transmission of the action potential.
On the contrary, when the binding of neurotransmitters to the receptors on the postsynaptic membrane results in the opening of ion channels that allow the influx of negative ions, a net gain of negative charge occurs across the membrane due to the open ion channels, a phenomenon known as hyperpolarization. Because of hyperpolarization the potential across the membrane grows further negative and thus the polarization value moves further away from the firing threshold of the postsynaptic cell. Owing to this movement of the value away from the firing threshold value such a kind of potential is known as inhibitory postsynaptic potential (IPSP). This potential thus inhibits any further transmission of signals or action potentials.
A noteworthy point is that the neurotransmitters themselves are not inherently excitatory or inhibitory in nature. It is possible that the same neurotransmitter might bind to different receptors. Hence, depending on the type of receptor the neurotransmitter activates by binding, the corresponding ion channels open which further open and allow the inflow of particular types of ions across the membrane. Thus, the combination of the receptors and the ion channels that open up on activation of the receptor is the main factor that determines the excitation or inhibition of the signal within the postsynaptic cell.
Both the excitatory and inhibitory postsynaptic potentials, the EPSPs and IPSPs, are potential changes that last only for a very short time. For a signal to be transmitted by the action of EPSPs due to the transmitter release from a single neuron is far too small along with being transient for generating any significant spike in the postsynaptic cell. For an EPSP to be significant enough to generate an action potential, more quantities of depolarization events need to take place. This is what happens daily in the functioning of the nervous system. A neuron usually receives many synaptic inputs from hundreds if not thousands of other neurons. These signals from hundreds of other neurons can provide varying amounts of inputs simultaneously causing the combined activity of the EPSPs in the afferent (conducting) neurons to create a large fluctuation in the membrane potential which are all together known as subthreshold membrane potential oscillations. When these potential oscillations are significant enough to cause the required amount of depolarization, the afferent neuron will generate an action potential. An example of this is the low-threshold spikes of depolarization that occur because of the T-type calcium channel. The simultaneous yet additive input of the low negative membrane depolarizations from these channels helps the neuron to reach the required firing threshold. Thus, in this manner the action potential is generated and then a signal is passed onto another neuron or muscle cell.
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One thing to keep in mind regarding the action potentials is that the action potentials are not graded like the postsynaptic potential. The postsynaptic potentials may vary in magnitude and response but the action potential is an all or none response. Once the firing threshold is reached then and only then the action potential will be generated.
Addition of Postsynaptic Potentials
The postsynaptic potentials are subject to summation which means that they can be added. They can summed up spatially i.e. in a particular region or temporally i.e. during a particular time. The summations are explained as follows:
Spatial Summation: This type of summation usually comes into effect when the synaptic potentials arise from two or more nearby synapses. If the postsynaptic potential is excitatory as received from the two synapses, the two potentials are added together to give a resultant depolarization of the membrane. Similarly, if the postsynaptic potential is inhibitory then they are added to give a resultant hyperpolarization of the membrane. And if the cell receives one excitatory postsynaptic potential and on inhibitory then they both cancel each other in addition.
Temporal Summation: This type of summation is utilised when cells receive the stimulus that are close together in a given time period. They are added together even if they are from the same synapse. For example, if a presynaptic neuron fires a stimulus causing an excitatory postsynaptic potential in the afferent neuron or muscle cell and within a very short time, the same presynaptic neuron fires again resulting in EPSP, then there occurs a membrane depolarization due to the sum of the two EPSPs.