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Synapses and Neurotransmission

How do neurons interact with each other to create us?


Overview



One of the most fundamental questions about our brain is about how it works to create who we are as a human being. Specifically, how do the microscopic cells communicate amongst themselves to create the emotions and thoughts that exist within our minds? In this article, we will explore the process of neurotransmission between the synapses of neurons, along with the upcoming medical technique of neuromodulation.


A Review of the Neuron



The standard neuron consists of three major components: the axon, cell body, and dendrites. The cell body, which is also known as the soma, is the spherical body of a neuron. Given that a neuron is a cell, it contains all the usual subcellular structures (or organelles) within this body. These are structures such as the nucleus, endoplasmic reticulum, mitochondria, and the Golgi apparatus. The axon and dendrites, however, are the distinct features that separate the neuron from other eukaryotic cells. The axon is specialized in the transfer of information over large distances. It branches off from the axon hillocks on the cell body, and then ends at the axon terminal. This is the location where the axon is in contact with the dendrite of the other neuron, and the topic for the next portion of the article will be about this crucial point of interaction. Any additional branches extending from the axon are known as axon collaterals. Moving on, the dendrite is the receiving end of a neuron. Some dendrites are covered in blobs that hang off the surface of the dendrite, which are known as dendritic spines. All the dendrites of a neuron are referred to as a dendritic tree.




Figure 1. Diagram of a Neuron and its Parts

https://www.sciencefacts.net/parts-of-a-neuron.html


The Synapse



As mentioned previously, the location where the axon of one neuron meets the dendrite of another neuron is a noteworthy area of investigation. How information is transferred from one neuron to a second neuron occurs at the synapse. A synapse contains two sides: the presynaptic side and the postsynaptic side. In this case, the prefixes pre and post refer to the flow of information across the synapse. There are two primary types of synapses: chemical and electrical. Due to this classification system, there are numerous basic variations between the two types:


  1. The two neurons at an electrical synapse are also connected by an intercellular mechanism known as a gap junction. Gap junctions contain meticulously aligned channels that form pores, whereas chemical synapses only contain ion channels.

  2. Electrical synapses are instantaneous because of gap junctions. On the contrary, chemical synapses include a delay.

  3. The space between the presynaptic neuron and the postsynaptic neuron is called the synaptic cleft. However, the space of gap junctions in electrical synapses are exceptionally close to each other (usually 3.8 nanometers apart), while the synaptic clefts of chemical synapses are typically 20 - 40 nanometers apart.



Figure 2. Left: an electrical synapse with an enlarged image of the gap junction. Here, the ions are freely flowing through the gap junction channels. Right: a chemical synapse with an enlarged image of the synaptic cleft. Here, neurotransmitters are released via synaptic vesicles



Neurotransmission



Regardless of these contrasting elements between the two synapses, the flow of information across synapses in general is called synaptic transmission, or neurotransmission. Just like with synapses, there are two different types of neurotransmission: chemical and electrical. Each form of neurotransmission occurs at their respective synapse types.


The structures behind electrical neurotransmission, such as gap junctions and pores, are positioned in a way that allow the ionic currents, along with ATP or other metabolites, to easily pass through from the presynaptic neuron to the postsynaptic neuron. This only occurs between dendrites of two neurons.


On the other hand, chemical neurotransmission utilizes a different set of biological tools. Within the synaptic cleft of the chemical synapse, there exists extracellular fluid—fluid which envelops all the cells in the brain. As an electrical signal travels to the axon terminal, synaptic vesicles will release the corresponding neurotransmitters into this synaptic cleft. Neurotransmitters, put simply, are the chemical runners of our bodies. Within the nervous system, these molecules help transmit messages in the nervous system. In turn, those neurotransmitters will bind to receptors on the postsynaptic neuron. This is why this type of neurotransmission is referred to as “chemical”.


As a result of the particular type of neurotransmitter released, specific ions will travel through channels on the postsynaptic neuron. The ions and the neurotransmitters binding to their receptors will then convert the signal back to its original, electrical form. The clear distinction between the two forms of neurotransmission is that the latter does not employ neurotransmitters.


Neurotransmitters



There are 3 types of neurotransmitters: excitatory, inhibitory, and modulatory. Excitatory neurotransmitters encourage the target cell to increase in activity, while inhibitory neurotransmitters do the opposite and decrease the chances of activity in the target cell. Modulatory neurotransmitters are simply able to influence multiple cells, and even other neurotransmitters. Below are several examples of common neurotransmitters present in human brains:



Neurotransmitter

Function


Acetylcholine

Controls heartbeat and causes muscle contractions

Dopamine

Released during pleasurable activities, and is responsible for memory, learning, behavior, and movement coordination

Endorphins

Inhibits pain signals and creates exhilarating sensations

Epinephrine

Involved in the “fight and flight” responses (also known as adrenaline)

Gamma-aminobutyric acid (GABA)

Inhibits neurons from becoming “overexcited”

Serotonin

Controls mood, appetite, blood clotting, sleep, and the Circadian Rhythm


As you can see, all of these neurotransmitters play a prominent role in human bodies. Because of this, any irregularities can cause significant harm. For instance, a lack of dopamine is responsible for a disorder known as Parkinson’s disease. Similarly, a shortage of GABA will cause anxiety, irritability, and restlessness. There is no concrete method in which we can fix these types of cellular issues in our bodies, but the best we can do is to live a healthy lifestyle.



Conclusion



The manner in which neurons communicate amongst each other is complicated, to say the least. A plethora of interactions occur at a cellular level, which is also dependent on the type of synapse. The neurotransmitters that cross these synapses also vary, and each specific type produces different effects on the human mind and body. All of this constitutes the process known as neurotransmission.



Term Dictionary:


Synapse

Where the axon of a neuron meets the dendrite of another neuron. This is where neurotransmission occurs.

Gap Junction

Contain meticulously aligned channels that form pores, whereas chemical synapses only contain ion channels

Synaptic cleft

The space between the axon and dendrite at a chemical synapse.

Neurotransmission

The movement of information across a synapse. Can be electrical or chemical, depending on the synapse.

Neurotransmitters

The chemical messengers of the brain present during chemical neurotransmission.

Gap junction

An extracellular structure present at electrical synapses, which have aligned channels that allow electrical neurotransmission to occur.

ATP

An energy-carrying molecule found in cells.

Extracellular Fluid

A term for the fluid outside of the cell but within the body cavities and vessels.






Works Cited


Berry, J. (2019, October 11). What are neurotransmitters?. Retrieved from https://www.medicalnewstoday.com/articles/326649


Purves D, Augustine GJ, Fitzpatrick D, et al., editors. (2001). Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates.


Stufflebeam, R. (2006). Neurons, Synapses, Action Potentials, and Neurotransmission. Retrieved from https://mind.ilstu.edu/curriculum/neurons_intro/neurons_intro.html#:~:text=

Neurotransmission%20(or%20synaptic%20transmission)%20is,electrical%20signals%

20across%20a%20synapse.&text=At%20electrical%20synapses%2C%20two%20neurons

,one%20another%20through%20gap%20junctions


Sukel, K. (2019, August 1) What Happens at The Synapse?. Retrieved from https://www.dana.org/article/qa-neurotransmission-the-synapse/


The University of Queensland. (2017, November 9). What are neurotransmitters?. Retrieved from https://qbi.uq.edu.au/brain/brain-physiology/what-are-neurotransmitters


The University of Queensland. (2017, November 9). Action potentials and synapses. Retrieved from https://qbi.uq.edu.au/brain/brain-physiology/what-are- neurotransmitters


Wikipedia. (2021, 16 December). Electrical synapse. Retrieved from https://en.wikipedia.org/wiki/Electrical_synapse#:~:text=An%20electrical%20synapse

%20is%20a,known%20as%20a%20gap%20junction.&text=In%20many%20animals%

2C%20electrical%20synapse,co%2Dexist%20with%20chemical%20synapses












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