Synapses: Definition, Types, Postsynaptic Potentials, Signal Termination and Transmission in Chemical and Electrical Synapses

It is a region where nerve impulses are transmitted and received, encompassing the axon terminal of a neuron that releases neurotransmitters in response to an impulse.

Chemical synapses vs. electrical

Key points

  • Neurons communicate with each other at junctions called synapses. At a synapse, a neuron sends a message to a target neuron: another cell.
  • Most synapses are chemical; These synapses communicate using chemical messengers. Other synapses are electrical; At these synapses, ions flow directly between cells.
  • At a chemical synapse, an action potential activates the presynaptic neuron to release neurotransmitters. These molecules bind to receptors on the postsynaptic cell and make it more or less likely to trigger an action potential.
  • A single neuron, or nerve cell, can do a lot. It can maintain a voltage-resting potential across the membrane. It can trigger nerve impulses or action potentials. And it can carry out the metabolic processes necessary to stay alive.
  • However, the signaling of a neuron is much more exciting when its interactions with other neurons are considered. Individual neurons make connections to target neurons and stimulate or inhibit their activity, forming circuits that can process incoming information and carry out a response.

How do neurons communicate with each other?

Action occurs at the synapse, the point of communication between two neurons or between a neuron and a target cell, such as a muscle or gland.

At the synapse, the firing of an action potential in one neuron (the presynaptic or sending neuron) causes the transmission of a signal to another neuron (the postsynaptic or receiving neuron) that makes the postsynaptic neuron more or less likely to fire its own action potential.

Synaptic transmission scheme

An action potential travels down the axon of the presynaptic emitter cell and reaches the terminal of the axon. The axon terminal is adjacent to the dendrite of the postsynaptic recipient cell. This close connection point between the axon and the dendrite is the synapse.

In this article, we will take a closer look at the synapse and the mechanisms neurons use to send signals through it. To get the most out of this article, you may want to know the structure of neurons and action potentials first.

Electrical or chemical transmission?

In the late 19th and early 20th centuries, there was much controversy over whether synaptic transmission was electrical or chemical.

  • Some people thought that signaling across a synapse involved the flow of ions directly from one neuron to another: electrical transmission.
  • Other people thought that it depended on the release of a chemical from a neuron, causing a response in the neuronal transmission-receptor chemical.
  • We now know that synaptic transmission can be electrical or chemical, in some cases both at the same synapse.

Chemical transmission is more common and more complicated than electrical transmission. So, a look at the chemical transmission will be taken first.

Description of transmission at chemical synapses

Chemical transmission involves the release of chemical messengers known as neurotransmitters. Neurotransmitters carry information from the presynaptic sending neuron to the postsynaptic recipient cell.

Synapses generally form between nerve endings (axon terminals) in the sending neuron and the cell body or dendrites of the receiving neuron.

Synaptic transmission scheme

An action potential travels down the axon of the presynaptic emitter cell and reaches multiple axon terminals that branch off from the axon. The axon terminal is adjacent to the dendrite of the postsynaptic recipient cell. This close connection point between the axon and the dendrite is the synapse.

A single axon can have multiple branches, allowing it to synapse across multiple postsynaptic cells. Similarly, a single neuron can receive thousands of synaptic inputs from many different presynaptic sending neurons.

Within the axon terminal of a sending cell are many synaptic vesicles. These are spheres with membranes filled with neurotransmitter molecules.

There is a small space between the axon terminal of the presynaptic neuron and the membrane of the postsynaptic cell, and this space is called the synaptic cleft.

Voltage-gated calcium channels are on the outer surface of the axon terminal. Across the synaptic cleft is the postsynaptic cell surface covered with receptors (ligand-gated ion channels) for the neurotransmitter.

When an action potential, or nerve impulse, reaches the axon terminal, it activates voltage-gated calcium channels in the cell membrane, which is present at a much higher concentration outside the neuron than inside, rushing into the cell.

He allows synaptic vesicles to fuse with the axon terminal membrane, releasing neurotransmitters into the synaptic cleft. The neurotransmitter binds to receptors on the target cell (in this case, it causes positive ions to enter).

Neurotransmitter molecules diffuse through the synaptic cleft and bind to receptor proteins in the postsynaptic cell. Activation of postsynaptic receptors leads to the opening or closing of ion channels in the cell membrane.

This can be depolarizing, making the inside of the cell more positive or hyperpolarizing, making the inside of the cell more negative, depending on the ions involved.

In some cases, these effects on channel behavior are direct: the receptor is a ligand-controlled ion channel, as in the diagram above.

In other cases, the receptor is not an ion channel itself, but rather activates the ion channels through a signaling pathway.

Excitatory and inhibitory postsynaptic potentials

When a neurotransmitter binds to its receptor on a recipient cell, it causes ion channels to open or close. This can produce a localized change in the membrane potential voltage across the membrane of the receptor cell.

  • In some cases, the change makes the target cell more likely to fire its own action potential. In this case, the change in membrane potential is called the excitatory postsynaptic potential or EPSP.
  • In other cases, the change makes the target cell less likely to fire an action potential and is called a postsynaptic inhibitory potential, or IPSP.

An EPSP is depolarizing: it makes the inside of the cell more positive, bringing the membrane potential closer to its threshold to trigger an action potential.

Sometimes a single EPSP is not large enough to bring the neuron to threshold, but it can add together with other EPSPs to activate an action potential.

IPSPs have the opposite effect. That is, they tend to keep the membrane potential of the postsynaptic neuron below the threshold to trigger an action potential. IPSPs are important because they can counteract or nullify the excitatory effect of EPSPs.

Spatial and temporal sum

How do EPSPs and IPSP interact?

Basically, a postsynaptic neuron sums up, or integrates, all the excitatory and inhibitory stimuli it receives and “decides” whether an action potential is activated.

  • The integration of postsynaptic potentials that occur in different places, but at approximately the same time, is known as spatial summation.
  • The integration of postsynaptic potentials that occur in the same place, but at slightly different times, is called time summation.

A neuron has two synapses on two different dendrites, both of which are excitatory. No synapse produces a large enough excitatory postsynaptic potential, EPSP, when it indicates generating an action potential at the mound, the place where the axon attaches to the cell body and where the action potential begins.

However, when synapses fire around the same time, EPSPs add together to produce depolarization above the threshold, triggering an action potential.

This process is shown on a graph of voltage in millivolts versus time in milliseconds. The graph monitors the membrane potential-voltage-at the axon mound.

Initially, it is at -70 mV, the resting potential. Then a synapse fires, resulting in a small depolarization at about -60 mV. This is not enough to reach the -55 mV threshold. However, only a little later, the other synapse fires and is “added” to the first depolarization.

This results in a total depolarization that reaches -55 mV and triggers a depolarization action potential to +40 mV, followed by a repolarization and hyperpolarization below -90 mV, and then a gradual recovery to -70 mV, the membrane potential Resting.

On the other hand, if an IPSP is produced together with the two EPSPs, it could prevent the membrane potential from reaching the threshold and prevent the neuron from activating an action potential. These are examples of spatial addition.

What about the temporary sum?

A key point is that postsynaptic potentials are not instantaneous: instead, they last a bit before they dissipate.

If a presynaptic neuron fires rapidly twice in a row, causing two EPSPs, the second EPSP can arrive before the first has dissipated, knocking the membrane potential above the threshold. This is an example of a temporary addition.

Signal termination

A synapse can only function effectively if there is some way to “turn off” the signal once it has been sent. Termination of the signal allows the postsynaptic cell to regain its normal resting potential, ready for new signals to arrive.

For the signal to terminate, the synaptic cleft must be removed from the neurotransmitter. There are a few different ways to do this.

The neurotransmitter can be broken down by an enzyme, it can be absorbed by the presynaptic neuron, or it can simply disappear.

In some cases, the neurotransmitter can also be “cleaved” by nearby glial cells. Anything that interferes with the processes that terminate the synaptic signal can have significant physiological effects.

For example, some insecticides kill insects by inhibiting an enzyme that breaks down the neurotransmitter acetylcholine.

On a more positive note, drugs that interfere with the reuptake of the neurotransmitter serotonin in the human brain are used as antidepressants.

Chemical synapses are flexible

If you have learned about action potentials, you may remember that the action potential is an all or nothing response. That is, it either happens in full force, or it doesn’t happen at all.

Synaptic signaling, on the other hand, is much more flexible. For example, a sending neuron can “dial in” or “decrease” the amount of neurotransmitters it releases in response to the arrival of an action potential.

Similarly, a receptor cell can alter the number of receptors it puts on its membrane and how easily it responds to activation of those receptors. These changes can strengthen or weaken communication at a particular synapse.

Presynaptic and postsynaptic cells can dynamically change their signaling behavior based on their internal state or the signals they receive from other cells.

This type of plasticity, or capacity for change, makes the synapse a key site for altering the strength of neural circuits and plays a role in learning and memory. Synaptic plasticity is also involved in addiction.

Furthermore, different presynaptic and postsynaptic cells produce different neurotransmitters and neurotransmitter receptors, with different interactions and different effects on the postsynaptic cell.

Electrical synapses

In electrical synapses, unlike chemical synapses, there is a direct physical connection between the presynaptic neuron and the postsynaptic neuron.

This connection takes the form of a channel called a gap junction, which allows current ions to flow directly from one cell to another.

Electrical synapses transmit signals faster than chemical synapses. Some synapses are both electrical and chemical. At these synapses, the electrical response occurs before the chemical response.

What are the benefits of electrical synapses?

On the one hand, they are fast, which could be important, for example, in a circuit that helps an organism escape from a predator. Furthermore, electrical synapses allow the synchronized activity of groups of cells.

In many cases, they can carry current in both directions so that depolarization of a postsynaptic neuron leads to depolarization of a presynaptic neuron. This type of doubles the definitions of presynaptic and postsynaptic.

What are the drawbacks of electrical synapses?

Unlike chemical synapses, electrical synapses cannot convert an excitation signal in one neuron to an inhibitory signal in another. More generally, they lack the versatility, flexibility, and signal modulation capabilities seen at chemical synapses.