Postsynaptic potential
Encyclopedia
Postsynaptic potentials are changes in the membrane potential
of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials
, and should not be confused with action potentials although their function is to initiate or inhibit action potentials. They are caused by the presynaptic neuron releasing neurotransmitters from the terminal button at the end of an axon
into the synaptic cleft. The neurotransmitters bind to receptors
on the postsynaptic terminal, which may be a neuron
or a muscle cell
in the case of a neuromuscular junction
. These are collectively referred to as postsynaptic receptors, since they are on the membrane of the postsynaptic cell.
and electrostatic repulsion
. Ions will tend towards their equilibrium potential, which is the state where the diffusion force cancels out the force of electrostatic repulsion. When a membrane is at its equilibrium potential, there is no longer a net movement of ions. Two important equations that can determine membrane potential differences based on ion concentrations are the Nernst Equation
and the Goldman Equation
.
of about -70mV. If the opening of the ion channel results in a net gain of positive charge across the membrane, the membrane is said to be depolarized
, as the potential comes closer to zero. This is an excitatory postsynaptic potential
(EPSP), as it brings the neuron's potential closer to its firing threshold (about -50mV).
If, on the other hand, the opening of the ion channel results in a net gain of negative charge, this moves the potential further from zero and is referred to as hyperpolarization
. This is an inhibitory postsynaptic potential
(IPSP), as it changes the charge across the membrane to be further from the firing threshold.
Neurotransmitters are not inherently excitatory or inhibitory: different receptors for the same neurotransmitter may open different types of ion channels.
EPSPs and IPSPs are transient changes in the membrane potential, and EPSPs resulting from transmitter release at a single synapse are generally far too small to trigger a spike in the postsynaptic neuron. However, a neuron may receive synaptic inputs from hundreds, if not thousands, of other neurons, with varying amounts of simultaneous input, so the combined activity of afferent neurons can cause large fluctuations in membrane potential. If the postsynaptic cell is sufficiently depolarized, an action potential
will occur. Action potentials are not graded; they are all-or-none responses.
Spatial summation: If a cell is receiving input at two synapses that are near each other, their postsynaptic potentials add together. If the cell is receiving two excitatory postsynaptic potentials, they combine so that the membrane potential is depolarized by the sum of the two changes. If there are two inhibitory potentials, they also sum, and the membrane is hyperpolarized by that amount. If the cell is receiving both inhibitory and excitatory postsynaptic potentials, they can cancel out, or one can be stronger than the other, and the membrane potential will change by the difference between them.
Temporal summation: When a cell receives inputs that are close together in time, they are also added together, even if from the same synapse. Thus, if a neuron receives an excitatory postsynaptic potential, and then the presynaptic neuron fires again, creating another EPSP, then the membrane of the postsynaptic cell is depolarized by the total of the EPSPs.
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are graded potentials
Membrane potential
Membrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
, and should not be confused with action potentials although their function is to initiate or inhibit action potentials. They are caused by the presynaptic neuron releasing neurotransmitters from the terminal button at the end of an axon
Axon
An axon is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses away from the neuron's cell body or soma....
into the synaptic cleft. The neurotransmitters bind to receptors
Receptor (biochemistry)
In biochemistry, a receptor is a molecule found on the surface of a cell, which receives specific chemical signals from neighbouring cells or the wider environment within an organism...
on the postsynaptic terminal, which may be a neuron
Neuron
A neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
or a muscle cell
Myocyte
A myocyte is the type of cell found in muscles. They arise from myoblasts.Each myocyte contains myofibrils, which are long, long chains of sarcomeres, the contractile units of the cell....
in the case of a neuromuscular junction
Neuromuscular junction
A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface, ultimately causing the muscle to contract...
. These are collectively referred to as postsynaptic receptors, since they are on the membrane of the postsynaptic cell.
The role of ions
One way receptors can react to being bound by a neurotransmitter is to open or close an ion channel, allowing ions to enter or leave the cell. It is these ions that alter the membrane potential. Ions are subject to two main forces, diffusionDiffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
and electrostatic repulsion
Electrostatics
Electrostatics is the branch of physics that deals with the phenomena and properties of stationary or slow-moving electric charges....
. Ions will tend towards their equilibrium potential, which is the state where the diffusion force cancels out the force of electrostatic repulsion. When a membrane is at its equilibrium potential, there is no longer a net movement of ions. Two important equations that can determine membrane potential differences based on ion concentrations are the Nernst Equation
Nernst equation
In electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
and the Goldman Equation
Goldman equation
The Goldman–Hodgkin–Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the equilibrium potential across a cell's membrane taking into account all of the ions that are permeant through that membrane.The discoverers of this are David E...
.
Relation to action potentials
Neurons have a resting potentialResting potential
The relatively static membrane potential of quiescent cells is called the resting membrane potential , as opposed to the specific dynamic electrochemical phenomena called action potential and graded membrane potential....
of about -70mV. If the opening of the ion channel results in a net gain of positive charge across the membrane, the membrane is said to be depolarized
Depolarization
In biology, depolarization is a change in a cell's membrane potential, making it more positive, or less negative. In neurons and some other cells, a large enough depolarization may result in an action potential...
, as the potential comes closer to zero. This is an excitatory postsynaptic potential
Excitatory postsynaptic potential
In neuroscience, an excitatory postsynaptic potential is a temporary depolarization of postsynaptic membrane potential caused by the flow of positively charged ions into the postsynaptic cell as a result of opening of ligand-sensitive channels...
(EPSP), as it brings the neuron's potential closer to its firing threshold (about -50mV).
If, on the other hand, the opening of the ion channel results in a net gain of negative charge, this moves the potential further from zero and is referred to as hyperpolarization
Hyperpolarization
Hyperpolarization has several meanings:* Hyperpolarization occurs when the strength of the electric field across the width of a cell membrane increases...
. This is an inhibitory postsynaptic potential
Inhibitory postsynaptic potential
An inhibitory postsynaptic potential is a synaptic potential that decreases the chance that a future action potential will occur in a postsynaptic neuron or α-motoneuron...
(IPSP), as it changes the charge across the membrane to be further from the firing threshold.
Neurotransmitters are not inherently excitatory or inhibitory: different receptors for the same neurotransmitter may open different types of ion channels.
EPSPs and IPSPs are transient changes in the membrane potential, and EPSPs resulting from transmitter release at a single synapse are generally far too small to trigger a spike in the postsynaptic neuron. However, a neuron may receive synaptic inputs from hundreds, if not thousands, of other neurons, with varying amounts of simultaneous input, so the combined activity of afferent neurons can cause large fluctuations in membrane potential. If the postsynaptic cell is sufficiently depolarized, an action potential
Action potential
In physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
will occur. Action potentials are not graded; they are all-or-none responses.
Termination of postsynaptic potentials
Postsynaptic potentials begin to be terminated when the neurotransmitter detaches from its receptor. The receptor is then free to return to its previous structural state. Ion channels that had been opened by the receptor when the neurotransmitter was bound to it will now close. Once the channels are closed, ions return to their equilibrium states, and the membrane is returned to its equilibrium potential.Algebraic summation
Postsynaptic potentials are subject to summation, spatially and/or temporally.Spatial summation: If a cell is receiving input at two synapses that are near each other, their postsynaptic potentials add together. If the cell is receiving two excitatory postsynaptic potentials, they combine so that the membrane potential is depolarized by the sum of the two changes. If there are two inhibitory potentials, they also sum, and the membrane is hyperpolarized by that amount. If the cell is receiving both inhibitory and excitatory postsynaptic potentials, they can cancel out, or one can be stronger than the other, and the membrane potential will change by the difference between them.
Temporal summation: When a cell receives inputs that are close together in time, they are also added together, even if from the same synapse. Thus, if a neuron receives an excitatory postsynaptic potential, and then the presynaptic neuron fires again, creating another EPSP, then the membrane of the postsynaptic cell is depolarized by the total of the EPSPs.
See also
- Action potentialAction potentialIn physiology, an action potential is a short-lasting event in which the electrical membrane potential of a cell rapidly rises and falls, following a consistent trajectory. Action potentials occur in several types of animal cells, called excitable cells, which include neurons, muscle cells, and...
- ElectrophysiologyElectrophysiologyElectrophysiology is the study of the electrical properties of biological cells and tissues. It involves measurements of voltage change or electric current on a wide variety of scales from single ion channel proteins to whole organs like the heart...
- Goldman equationGoldman equationThe Goldman–Hodgkin–Katz voltage equation, more commonly known as the Goldman equation is used in cell membrane physiology to determine the equilibrium potential across a cell's membrane taking into account all of the ions that are permeant through that membrane.The discoverers of this are David E...
- Membrane potentialMembrane potentialMembrane potential is the difference in electrical potential between the interior and exterior of a biological cell. All animal cells are surrounded by a plasma membrane composed of a lipid bilayer with a variety of types of proteins embedded in it...
- Nernst equationNernst equationIn electrochemistry, the Nernst equation is an equation that can be used to determine the equilibrium reduction potential of a half-cell in an electrochemical cell. It can also be used to determine the total voltage for a full electrochemical cell...
- NeuronNeuronA neuron is an electrically excitable cell that processes and transmits information by electrical and chemical signaling. Chemical signaling occurs via synapses, specialized connections with other cells. Neurons connect to each other to form networks. Neurons are the core components of the nervous...
- NeurotransmissionNeurotransmissionNeurotransmission , also called synaptic transmission, is the process by which signaling molecules called neurotransmitters are released by a neuron , and bind to and activate the receptors of another neuron...
- Postsynaptic
- SynapseSynapseIn the nervous system, a synapse is a structure that permits a neuron to pass an electrical or chemical signal to another cell...
- End-plate potentialEnd-plate potentialEnd plate potentials are the depolarizations of skeletal muscle fibers caused by neurotransmitters binding to the postsynaptic membrane in the neuromuscular junction. They are called "end plates" because the postsynaptic terminals of muscle fibers have a large, saucer-like appearance...