You attend a class in Psychology and plan to go back to your hostel room to take a nap in the one hour break before your next lecture. Not realizing, you tend to oversleep. Suddenly, you receive a call from your friend who shouts from the other end of the line “Wake up you lazy bones for the Sociology Class!”

You soon come out of the unconscious state the moment you realize what’s going on. And your immediate reaction is to get out of bed, collect your books and simply rush down the escalator, straight to your Sociology Class.

Now, this is something that happens to us in one way or the other everyday. What exactly triggered you to leave everything and rush for the class panting?

Well, your biology was responsible for that automatic reaction to your friend’s phone call. Precisely, it is the activity of neurons that triggered such an emotion or behavior.

This means whatever we do, feel or think in our everyday life has a biological base to it. From feeling hungry to experiencing stress or anxiety, there are a multitude of biological events and processes as well as the brain activity that influences such patterns of behavior.

Thus, the most fundamental component that a human brain is composed of is a collection of “Neurons”. In this article, you will learn what exactly are Neurons and what role they inside our Nervous System.

What is a Neuron?

‘Neurons’ are the cells inside a human brain that perform the function of receiving, carrying and processing information. These cells are the foundation on which the entire Nervous System is built.

Neurons help in communicating information from one Neuron to another Neuron or other cells which may include muscle cells, gland cells etc. Thus, these cells are responsible for everything that we do, feel and think in our everyday life.

As per an estimate by the scientists, the human brain is made up of more than a hundred billion neurons.

Each neuron is made up of basic parts:

• Dendrites
• Cell Body
• Axon
• Axon Terminals

Dendrites

Dendrites are structures that resemble branches of a tree. This part of Neuron is responsible for receiving information from other Neurons or cells and carrying such information towards the cell body of the given Neuron. Thus, the function of Dendrites of a given Neuron is to bring signals from other cells or neurons to the Neuron in question.

Cell Body

Cell Body of a Neuron is also known as the Soma. It is the circular part that contains the Nucleus. The Cell Body is connected with Dendrites on one side that bring information into the Neuron and the Axon on the other side that carry information from the cell body to the other Neurons.

Thus, Cell Body is the production house of a Neuron. In other words, the fundamental function of a Cell Body is to produce the necessary proteins needed by the Dendrites, Axons and Axon Terminals . Further, the Cell Body is made up of tiny organs that perform certain specialized functions that help the Cell Body to act as a production house of the Neuron.

These tiny organs include nucleus, nucleolus, endoplasmic reticulum, golgi apparatus, etc. The tiny organs function together to create, bundle and separating proteins needed by other parts of the cell. These proteins help in building fresh Dendrites and Axons that further connect with other neurons to create neurotransmitters necessary for communicating information.

Axon

The next component that forms part of the anatomy of a Neuron is the Axon. Axon is a projection coming out of the cell body that performs the basic function of sending information received from the cell body towards Axon Terminals.

Basically, the Axon is responsible for carrying the information away from the cell body. Further, the Axon is wrapped in a fatty covering known as Myelin. However, there are smaller gaps that exist along the Myelin wrapping where it is absent.

Both the Myelin Sheath as well as the gaps facilitate the ability of the Neuron to carry information. You must remember that there can be cases where this Myelin Sheath gets damaged which further seriously impacts the transmission of information through Axon.

Thus, continuous damaging of the Myelin Sheath leads to Multiple Sclerosis (MS). This is a disease in which the victim experiences shaky movements, tremors and lack of coordination.

Glial Cells

The Myelin Sheath is made up of yet another important type of cells known as Glial Cells. Apart from forming Myelin Sheath, Glial Cells perform other important functions in the Nervous System. For instance, they perform the caretaking or maintenance staff functions. These functions include acting as a Blood Brain Barrier – a Barrier that prevents certain substances present in the blood to enter your brain.

In other words, it keeps such substances outside the brain which if allowed to enter could be damaging. Further, these Glial Cells also help in cleaning the cellular debris that are left after the neuron dies.

Also, when one Neuron communicates with the other Neuron, it does so by transmitting chemicals called Neurotransmitters.

Once these Neurotransmitters pass on information or signals to a cell or a Neuron, they basically move about in the Synaptic region until they are taken back by the Neuron through which they were released. This process is known as the Reuptake process. Thus, Glial Cells help in this reuptake of the released Neurotransmitters.

Furthermore, the Glial Cells are more in number as compared to the Neurons. They exceed the number of Neurons by a ratio of 10 to 1.

Axon Terminal

The nerve fibre that comes out of the Cell Body further segregates into branch like structures towards its end. These branches further end in rounded structures known as Axon Terminals.

These Axon Terminals further contain structures known as Synaptic Vesicles which further help in Synaptic Transmission.

Synapse

Synapse is the area or the region where the Axon Terminal of a Neuron closely approaches but does not touch other Neurons or other type of cells such as muscle cells, gland cells etc.

To understand what exactly happens in the Synaptic Region, you need to understand how a Neuron transmits information within itself as well as to other Neurons or Cells in the Nervous System.

Nodes of Ranvier

These are the small gaps that exist in the Myelin Sheath that surrounds the Axons in a Neuron.

There are two things that you need to understand when it comes to a Neuron. The first deals with understanding of how a Neuron carries information within itself. And the second one relates to understanding how one Neuron transmits information or communicates to other Neuron or other Cells.

1. Communication Within Neurons

To understand how the information travels within Neurons, you need to understand how the neurons function. There exists a small amount of electrical charge all throughout the cell membrane. This charge is of the intensity of -70 millivolts.

Thus, the interior of a cell has a negative charge as compared to the outside of the cell. The reason such an electrical charge exists is that there are varying degrees of different types of positively and negatively charged particles. These particles are called ions and are present both inside and outside the cell.

Thus, as compared to the outside of the cell, the inner part of the cell membrane takes a net negative charge. This resting position of the Neuron is not something that happens by chance. Rather, it is the result of Neuron working continuously to maintain resting state by pumping out the positively charged particles if they happen to enter the cell. And at the same time allowing the negatively charged particles to enter in greater concentration inside the Cell relative to the negative charge present outside.

Now, any trigger caused directly by external stimuli like heat, sound, light or signals from other neurons produces graded potential within the cell.

What Are Graded Potentials?

Graded Potentials are nothing but the basic signals produced within the neurons by a stimulus like heat, sound, light or signals from other neurons . The strength of these graded potentials varies depending upon the size of the stimulus that produced such a signal at first place.

For example, a loud sound would produce a graded potential of greater magnitude as compared to the graded potential produced by a softer sound. Further, these graded potentials become weak in a small amount of time. As a result, they are capable of passing information over a shorter stretch, that is, from Dendrites to the Cell Body.

The moment the strength of the graded potentials approaching the Cell Body goes beyond the upper limit of the given Neuron, it generates Action Potential.

What Are Action Potentials?

When an Action Potential gets triggered within a Neuron, positively charged particles or ions are let into the Neuron more freely as compared to the earlier state of Graded Potential.

These positively charged ions entering the cell membrane take the Neuron out of the resting potential state.Thus, for a small amount of time, the interior part of the cell takes a net positive charge as compared to the outside.

This change of the electrical charge is what moves across the Cell Membrane and causes a disturbance within the Cell, known as the Action Potential. This is how the information travels within the Neuron.

Thus, after a brief time period, typically one to two milliseconds, these positively charged ions are pumped outside the Cell Membrane. And the negatively charged ions that were earlier pushed outside the Cell Membrane are allowed to re-enter.

This is how the Neuron attains resting potential once again and gets prepared to be fired once again. As you can see, Action Potentials are an all or none response. That is, either these occur at full magnitude or they do not occur at all.

This is pretty much in contrast to the graded potentials. Furthermore, the speed of transmission of these Action Potentials is very high in case of Neurons that have Myelin Sheath. This speed can be upto 270 miles per hour.

2. Communication With Other Neurons

The communication that takes place between Neurons or Neurons or other cells is known as Synaptic Transmission. When Graded Potentials go beyond the threshold limit of the Neuron, the Action Potential so generated travels along the Axon of the given Neuron towards the Axon Terminals.

Axon Terminals consist of tiny structures known as Synaptic Vesicles. The coming of the Action Potential makes these Synaptic Vesicles approach towards the Cell Membrane. Further, upon inching towards the Cell Membrane, the Synaptic Vesicles diffuse with the Cell Membrane and release their constituents into the Synapse.

These chemicals so released by Synaptic Vesicles into the Synapse are nothing but the Neurotransmitters. The Neurotransmitters keep moving in the Synaptic region till the time they find or locate the specialized receptor sites on the Cell Membrane of the other cells.

Furthermore, specific type of Neurotransmitters bind together with specific receptor sites only, thus lending accuracy to the Nervous System. Upon binding with the specific receptor sites, Neurotransmitters produce effects that are either excitatory or inhibitory in nature.

If the effect produced is excitatory in nature, these Neurotransmitters create higher odds for the Neuron to fire another Neuron. On the other hand, if the effect produced by the Neurotransmitter in a given is inhibitory, then the chances are less that such a Neuron fires the other Neuron.

So this is how the communication takes place between two Neurons or Neuron and other cells.

Finally, once the Neurotransmitters move from one Neuron to the other, one possibility is that they are taken back in the Axon Terminals of the Neuron from which they were released. This process is known as reuptake. The other possibility is these Neurotransmitters are deactivated within the Synapse region itself.

Thus, to sum it up, Neurons are the foundation of the brain’s Nervous System. These are the cells that our consciousness is made of. These are the channels through which we as humans receive information from the outside world, are able to understand or interpret the same and thus respond to the external events.

Key References

Psychology Robert A Baron and Girishwar Misra (Fifth Edition) Pearson Education India

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