Postsynaptic potentials are changes in the

membrane potential Membrane potential (also transmembrane potential or membrane voltage) is the difference in electric potential between the interior and the exterior of a biological cell. That is, there is a difference in the energy required for electric charges ...

of the postsynaptic terminal of a chemical synapse. Postsynaptic potentials are

graded potentials Graded potentials are changes in membrane potential that vary in size, as opposed to being all-or-none. They include diverse potentials such as receptor potentials, electrotonic potentials, subthreshold membrane potential oscillations, slow-wave p ...

, and should not be confused with

action potentials An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

although their function is to initiate or inhibit action potentials. They are caused by the presynaptic neuron releasing

neurotransmitters A neurotransmitter is a signaling molecule secreted by a neuron to affect another cell across a synapse. The cell receiving the signal, any main body part or target cell, may be another neuron, but could also be a gland or muscle cell. Neurot ...

from the terminal bouton at the end of an

axon An axon (from Greek ἄξων ''áxōn'', axis), or nerve fiber (or nerve fibre: see spelling differences), is a long, slender projection of a nerve cell, or neuron, in vertebrates, that typically conducts electrical impulses known as action p ...

into the synaptic cleft. The neurotransmitters bind to receptors on the postsynaptic terminal, which may be a

neuron A neuron, neurone, or nerve cell is an electrically excitable cell that communicates with other cells via specialized connections called synapses. The neuron is the main component of nervous tissue in all animals except sponges and placozoa ...

or a

muscle cell A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a mus ...

in the case of a

neuromuscular junction A neuromuscular junction (or myoneural junction) is a chemical synapse between a motor neuron and a muscle fiber. It allows the motor neuron to transmit a signal to the muscle fiber, causing muscle contraction. Muscles require innervation ...

. 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,

diffusion Diffusion is the net movement of anything (for example, atoms, ions, molecules, energy) generally from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in Gibbs free energy or chemical ...

and electrostatic repulsion. Ions will tend towards their

equilibrium potential In a biological membrane, the reversal potential is the membrane potential at which the direction of ionic current reverses. At the reversal potential, there is no net flow of ions from one side of the membrane to the other. For channels that are pe ...

, 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 In electrochemistry, the Nernst equation is a chemical thermodynamical relationship that permits the calculation of the reduction potential of a reaction ( half-cell or full cell reaction) from the standard electrode potential, absolute tempe ...

and the Goldman Equation.

Relation to action potentials

Neurons have a

resting potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as op ...

of about −70 mV. 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 In neuroscience, an excitatory postsynaptic potential (EPSP) is a postsynaptic potential that makes the postsynaptic neuron more likely to fire an action potential. This temporary depolarization of postsynaptic membrane potential, caused by the ...

(EPSP), as it brings the neuron's potential closer to its firing threshold (about −55 mV). 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 or subthreshold membrane potential oscillations. If the postsynaptic cell is sufficiently depolarized, an

action potential An action potential occurs when the membrane potential of a specific cell location rapidly rises and falls. This depolarization then causes adjacent locations to similarly depolarize. Action potentials occur in several types of animal cells ...

will occur. For example, in

low-threshold spikes Low-threshold spikes (LTS) refer to membrane depolarizations by the T-type calcium channel. LTS occur at low, negative, membrane depolarizations. They often follow a membrane hyperpolarization, which can be the result of decreased excitability or ...

depolarizations by the T-type calcium channel occur at low, negative, membrane depolarizations resulting in the neuron reaching the threshold. Action potentials are not graded; they are all-or-none responses.

Termination

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 Summation, which includes both spatial summation and temporal summation, is the process that determines whether or not an action potential will be generated by the combined effects of excitatory and inhibitory signals, both from multiple simultan ...

: 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.

External links

References