Synapse Definition and Function

How Neurons Communicate With Each Other

By Olivia Guy-Evans, published Feb 21, 2021

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What is a Synapse?

Neurons do not touch each other, but where the neuron does come close to another neuron, a synapse is formed between the two.

Neurons essentially communicate with each other through synapses. When signals have traveled through neurons to the endpoint, they cannot simply continue onto the next neuron.

It must trigger the release of neurotransmitters which then carry the signals across the synapse in order to reach the next neuron. Terminal buttons belong to the presynaptic endings of the neuron and have vessels containing neurotransmitters. These are responsible for transmitting signals to other neurons.

When a nerve impulse has triggered the release of these neurotransmitters from the terminal buttons, these chemicals are then released into the synaptic cleft and are then taken up by receptors on the next cell.

The neuron that then receives the neurotransmitters is the postsynaptic neuron. Neurons receive messages from many terminal buttons and in turn, terminal buttons form synapses with many other neurons.

A synapse is a combination of:

• Presynaptic endings – which contain the neurotransmitters (chemical messengers).
• Synaptic clefts – which is the gap between the two neurons.
• Postsynaptic endings – which contains the sites for receptors (molecules which receives signals for a cell).

Synapses also have the ability to communicate a change in the message being passed on. Postsynaptic neurons can also send communications back to the presynaptic neurons telling them to change how often or much a neurotransmitter is released. Due to this, we can say that the synapses are able to communicate bi-directionally.

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Electrical or Chemical Synaptic Transmission?

Synapses can be either chemical or electrical and are essential to the functioning of neural activity. Neuroscientists understand that synapses play a vital role in a variety of cognitive functions, including learning and memory formation.

Excitatory and Inhibitory Postsynaptic Potentials

Chemicals released from the presynaptic neuron may either excite or inhibit the postsynaptic neuron, telling it to release neurotransmitters or to slow down or stop signaling.

When the axon fires and the terminal buttons release a neurotransmitter that excites the postsynaptic neuron, this is excitatory postsynaptic potential (EPSP). This effect of excitation makes it more likely that the axons of the postsynaptic neuron will also fire.

The inhibitory postsynaptic potential (IPSP) has the opposite effect. Inhibition is caused by inhibitory neurotransmitters. When the neurotransmitter binds with the post-synaptic receptor, it results in a IPSP and the cell is less likely to fire.

The rate at which the axon fires is determined by the activity of the synapses on the dendrites and soma of the neuron. If the excitatory synapses are more active then the axons fire at a high rate, compared to firing at a low rate, or not at all when inhibitory synapses are active.

An EPSP is depolarising, meaning it makes the inside of the neuron more positive, which causes more action potential. IPSPs however, bring the potential down, meaning it will be less likely to cause action potential and can cancel out the excitatory effect of the EPSPs.

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Spatial and Temporal Summation

Summation is the process that determines whether/how frequently the neuron will fire by the combined effects of excitatory and inhibitory signals, both from multiple simultaneous inputs (spatial summation), and from repeated inputs (temporal summation).

EPSPs and IPSPs interact with each other when a postsynaptic neuron combines (or summates) all the excitatory and inhibitory signals it receives and then makes a decision as to whether to fire an action potential. There are two types of summation that can occur at this stage:

For example, if there are two single excitatory signals (EPSPs) that arrive at the postsynaptic neuron from two different dendrites, they cannot reach the threshold for action potential on their own. However, they can sum up together to reach the threshold and cause an action potential of the postsynaptic neuron.

If an inhibitory signal (IPSP) also enters through another dendrite, this can counteract the two EPSPs and could prevent the neuron from firing action potential. This is an instance of spatial summation.

If the same two EPSPs arrive from the presynaptic neuron to the postsynaptic neuron, but at slightly different times, they can still cause an action potential to fire. This is because postsynaptic potentials are not instantaneous, and they can survive in a neuron for a while before dissipating.

Therefore, if one EPSP arrives first and has not dissipated yet, when the second EPSP arrives, they can then reach the threshold for action potential. This is an instance of temporal summation.

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Reuptake of Neurotransmitters

For a synapse to function effectively, they must be shut off once the signal is sent. This signal termination allows the postsynaptic neuron to return to its resting potential state, ready for new signals. When neurotransmitters get released into the synaptic cleft, not all of them are able to attach to the receptors of the next neuron.

The synaptic cleft must be cleared of all neurotransmitters at signal termination, which can be done by either being broken down by enzymes, diffused away, or re-uptake occurs.

Re-uptake is when neurotransmitters get reabsorbed back into the presynaptic neuron from which they came from. Transporter proteins from the presynaptic membrane remove the neurotransmitters from the synaptic cleft, carrying the neurotransmitter back into the presynaptic neuron. The neurotransmitter then either gets re-packaged into the synaptic vesicles and stored until it is next needed again, or they are broken down by enzymes.

Serotonin is a type of neurotransmitter which is associated with a variety of psychological and bodily functions such as mood, sexual desire, appetite, sleep and memory. If there are imbalances in the way serotonin is transmitted between neurons, through too much reuptake of this neurotransmitter, then this has implications for contributing to mood disorders, specifically depression.

Selective serotonin reuptake inhibitors (SSRIs) are a type of medication, also known as antidepressants, which work in a way to increase the amount of serotonin being transmitted between cells. SSRIs essentially aid in blocking the reuptake of serotonin into the presynaptic cell, meaning there is more serotonin in the synaptic cleft.

If there is more serotonin in the synaptic cleft, it is more likely that serotonin will reach the receptors of the postsynaptic cell. As SSRIs allow more serotonin to pass along between neurons, they have been shown to alleviate mood disorders, making them a common therapy for depression.

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Synaptic Plasticity

Plasticity refers to how much something can be changed or adapted through growth and reorganization. It was once believed that once synapses were formed, they remain the same forever, never changing.

However, it is now understood that activity, or lack of activity, can affect the strength of synapses, or even change the number and structure of synapses in the brain. Therefore, the more the synapses are used, the stronger they can become and the more influence they can have over postsynaptic neurons.

Likewise, not fully using synapses can weaken them and can have a detrimental impact over the long-term.