Neuron (A detailed guide)

In this article, we will discuss what neurons are, what the structure of a neuron is, and how neurons communicate with each other to carry out the essential functions of our brains and body. 

What is a neuron? 

Cells found in the nervous system are called neurons. They are the primary unit of the brain.

Neurons are specialized cells designed to transmit information to other nerve cells, tissues, and glands.

Proper transmission of this information is essential in order for your body to function properly.

This article will give an overview of what a neuron is and its function in the human body.

What is the structure of a neuron? 

There are three basic parts of the neuron: dendrites, cell body (or soma) and axon.

While all neurons have the same basic makeup, they all tend to vary in size, composition, and properties.

Some neurons have more dendritic branches than others, for example, and are able to transmit more information at a faster speed.

Some neurons have short axons, while others have longer axons.

The longest axon in the human body extends from the spinal cord to the big toe, and it is approximately 3 feet long!

The neuron resembles a tree. In the picture below, you can see the neuron’s three main components (dendrites, soma, and axon/axon terminals).

Observe how those components come together to resemble parts of a tree: the dendrites resemble the branches of a tree, the cell body resembles the trunk, and the axon and axon terminals resemble the roots.

Dendrites (the branches of the tree) are where the neuron receives input from other cells.

The input signal is passed along the dendrite branches, to the cell body (the trunk), and an output signal is sent out along the axon, to the axon terminals (the roots). 

How do neurons transmit information?

For neurons to communicate with other parts of the body, or for neurons to transmit information from one neuron to another, a process involving electrical signals and chemical messengers is used.

This process involves what is called an action potential. The action potential will be described further.

As stated before, dendrites receive informational input for the neuron, either from sensory receptors in your body or from other neurons.

This information is then transmitted to the cell body. The soma (the tree trunk) is where the nucleus of the cell is located.

It is where the DNA of the neuron is stored. It is here that the signal from the dendrites is processed and then sent out along the axon.

Once the information is received by the axon, the information moves down the length of the axon in the form of an electric signal.

At the end of the axon, more specifically, at the axon terminals, the signal can be passed on to another neuron’s dendrites, and the two neurons will have communicated with each other.

Axons also contain what is called the myelin sheath, which is a special fatty coating on the outside of the axon.

This layer provides a type of insulation so that the electric signal that travels down the axon is contained.

Each periodic gap within the myelin sheath are what are called the nodes of Ranvier. 

How do neurons communicate with each other?

Once the electric signal reaches the tip of the axon, information must be transmitted to the dendrites of the adjacent neuron across what is called the synapse– the gap between one neuron’s axon and another neuron’s dendrites.

In most cases, neurotransmitters are considered the messengers.

They are required to transmit information from one neuron to another across this synapse.

Neurotransmitters are chemical agents released from the ends of an axon at the sign of an electric signal.

When signaled to transmit messages, neurotransmitters are released from their synaptic vesicles on the presynaptic side of the synapse (at the end of one neuron’s axon), travel across the synapse, and bind to the adjacent neuron’s postsynaptic membrane receptors (on the other neuron’s dendrites).

It is here that the other neuron will have received the message.

In a process called reactivation, the neurotransmitters that were just released become re-attached to the presynaptic receptors of the original neuron.

This way, the neuron can reuse the same neurotransmitters.  

How is a message transmitted down a neuron?

If neurons receive input from neurotransmitters, or chemical messengers, how is this message then converted into an electric signal that can travel down the length of the neuron?

It utilizes an action potential- a large wave of electrical activity after the cell reaches more positive charge.

Neurons always have an electric charge inside the cell wall, as well as outside the cell wall.

This electric charge is maintained by a steady influx and outflow of ions, or charged particles; more specifically, the ions Potassium (K+), Sodium (Na+), and Chloride (Cl-).

Notice that K+ and Na+ both contain positive charges, while Cl- contains a negative charge. While a neuron is at rest, the K+, Na+, and Cl- are constantly moving in and out of the cell via a passive process called passive diffusion.

They move in and out of the cell through ion channels- passageways along the membrane specific to each ion.

The balance of these charges inside and outside the cell leaves an overall negative charge inside the cell membrane.

In fact, the overall charge inside a neuron while it is at rest is approximately -70 mV. This is called the resting potential.

Once neurotransmitters reach the dendrites, they signal the Na+ channels along the cell membrane to open.

This allows an influx of Na+ into the cell, causing the charge in the cell to become more positive. Once the charge reaches approximately -55mV, it is considered to reach the threshold potential.

It is here that the cell membrane is positive enough to open voltage gated ion channels- these are channels that open in response to a more positive charge.

Once more of these channels open, more Na+ rushes into the cell membrane, making the voltage inside the neuron increasingly more positive until it reaches approximately +30mV.

Once the voltage reaches +30 mV, the voltage gated Na+ channels become inactivated. K+ ion channels open, allowing K+ ions to flow out of the cell membrane.

During this process, the inside of the neuron becomes less positive. In fact, it may even become as negative as -90mV before the channels restore the neuron to its resting -70mV.

Once the signal has passed through a portion of the axon, the membrane in that area will remain at -70mV until another signal arrives and the next action potential commences. 

Neurons are formed in the womb and throughout development.

Normally, when neurons die, they are not replaced, although neurogenesis or subsequent neuronal formation may occur in certain parts of the brain.

Studies show that new connections between neurons can last a lifetime.

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Frequently Asked Questions (FAQs) about neurons: 

What are the types of neurons?

There are three types of neurons.

There are Sensory Neurons, which are responsible for detecting sensory input and for converting external stimuli, such as light, smell, or taste, into information that will be passed to the brain. 

There are also Motor Neurons, which are responsible for sending signals from the brain to muscles in order to activate movement.

The third type of neuron is the Interneuron, which is a neuron that solely receives information from, and sends information to, other neurons.

They are intermediary neurons, and are as such, are not considered sensory, nor motor. 

What are the types of neurotransmitters?

Acetylcholine

The cholinergic system is a specific neurotransmitter system that uses the neurotransmitter acetylcholine (ACH).

This technique arises from the Autonomic nervous system. It is used in higher cognitive processing and attention.

Alterations in this system may be associated with Alzheimer’s Disease.

The cholinergic system consists of two types of receptors: the nicotinic receptor and the acetylcholine receptor, also known as the muscarinic receptor.

Those two chemical receptors interact with the neurotransmitter acetylcholine.

Nicotine, a common chemical in tobacco, may bind to the nicotinic receptor in the same way acetylcholine does.

It therefore mimics the action of acetylcholine.

Mascarin, a chemical product found in  some mushrooms, interacts with the muscarinic receptor in the same way acetylcholine does as well. 

Dopamine

Dopamine is a major neurotransmitter that is predominantly involved in reward and reinforcement.

Dopamine is released when reward-inducing behavior occurs.

It trains the brain to repeat these activities that feels rewarding.

It is also increased when certain drugs  are taken, contributing to the “high” feeling and subsequently, the addiction to these drugs.

Dopamine is also involved in movement and speech.

A decreased amount of dopamine is linked to Parkinson’s disease, Schizophrenia, and Attention Deficit Hyperactivity Disorder.  

Amino acids

One class of neurotransmitters is amino acids. It includes Glutamate (Glu), GABA (Gamma Aminobutyric Acid) and Glycine (Gly).

These amino acids contain amino and carboxyl in their chemical structures.

Glutamate is one of the 20 amino acids that do not make proteins.

Each aminoalkanoic acid neurotransmitter has its own system, namely the glutamatergic, GABAergic and glycinergic system.

Each has its own receptors and they do not interact. Aminoalkanoic acid neurotransmitters move away from synapses by recycling.

The skin pump of the presynaptic element cell, or sometimes the neighboring neurogliacetate, removes aminoalkanoic acid from the synaptic cleft, regenerates, recycles the vesicles, and re-releases it.

Biogenic amines

Another class of neurotransmitters consists of biogenic amines. It includes dopamine, norepinephrine, and epinephrine.

It also includes histamine and serotonin.

Biogenic amines are in fact derived from amino acids; however, they are not considered amino acids. a goggle of neurotransmitters synthesized from amino acids.

They require amino groups, but they do not contain carboxyl groups and are therefore not classified as amino acids.

Neuropeptides

Neuropeptides are another class of neurotransmitters.

They are much larger than other neurotransmitters.

They usually contain 3-36 amino acids. 

Want to learn more about neurons? Try these recommended readings!

The Neuron: Cell and Molecular Biology by Irwin B. Levitan and Leonard K. Kaczmarek

This book delves deeply into the cellular and molecular biology of neurons.

The book contains detailed descriptions of nerve cells and the mechanisms behind neuronal activity.

Included are great images and figures to help supplement the text.

From Neuron to Brain: A Cellular and Molecular Approach to the Function of the Nervous System by John G. Nicholls, A. Robert Martin, Bruce G. Wallace, and Paul A. Fuchs

This book discusses the nervous system in great depth.

It provides an in depth description of everything about the central nervous system, from nerve cells to the brain.

The authors discuss experiments and research that have been performed, as well as research that is being conducted today. 

Single Neuron Studies of the Human Brain: Probing Cognition by Itzhak Fried, Ueli Rutishauser, Moran Cerf, and Gabriel Kreiman

This book delves into the study of neurons and the human brain.

It discusses neuronal activity, as well as experiments and discoveries about nerve cells that led to further discoveries about the human brain and cognition.

This book also describes various techniques for researching cognitive functions, such as memory and decision making ability.

 References

Neurons. Boundless Psychology. 2020. 

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