Overview of Neurons

The neuron is the basic structural and functional unit of the central nervous system (1). First, let us talk about the two types of cells in the CNS. Those include neurons, which are the nerve cells, and glia, which represent the supporting cells (1).

The task of the neurons is to create, guide, receive, and transmit nerve impulses. When it comes to the appearance, neurons significantly differ from other cells, but also one another.

They participate in the emergence and management of the action potential. They have voltage Na+ and K+ channels. They also have receptors, and we differentiate the resting potential, the postsynaptic potential, and the action potential (1).

The number of neurons is amazingly
high. Namely, the cerebral cortex contains 10 billion neurons. The size of the
neuron differs. As a result, we classify neurons as follows:

• Dwarf cells (granule cells of the
  3rd cortex layer - 7-8µm)
• Small neurons
• Medium-sized neurons
• Large neurons
• Giant (Purkinje cerebral cortex
  cells, Betz pyramid cells 120-150 µm) (1).

Neuron structure

There are four major morphological
sections of neurons. Those are:

• Dendrites,
• Soma,
• Axon,
• Presynaptic axon terminals (2).

Soma refers to the neuron nucleus and the surrounding cytoplasm. A single axon and several dendrites originate from one soma. Dendrites have thorns or spines. The axon starts at the axon hill and has sloping branches. There is a final branching (telodendrion) at its end.

Neuron's body shape differs. They
can be granular cells with a small round body, a dark nucleus and a small
cytoplasm. Also, their body can be elongated. Those are the neurons of the VI
cortex layer. Moreover, they can be pyramidal. Their bodies resemble pyramids
(in III and IV. cortex layers), and, finally, multipolar (alpha-motoneurons).

The neuron membrane plays a very important role in the neuron function. The electrical properties of neurons allow the transmission of information.

Cell membrane channels open and Na + ions enter and change the membrane potential in the thousandth part of a second. This property is extremely important and mediates the bio-electrical properties of the membrane.

Synapses

Synapse is the main site of interaction between neurons (2). Synapses can only be seen through an electron microscope. They are located on dendrites and dendritic thorns, and the soma of neurons.

Axons travel far through the brain to form proper bonds. Axons follow chemical bonds to reach their target location. Growing axons reach the target region following the chemical gradient towards which they are attracted or repelled by chemical substances.

When axons reach the target, they make links with more than one cell. Postsynaptic cells keep a stronger connection with some cells and eliminate the links with others. The formation or elimination of these bonds depends on the incoming signals.

The theory of the process of selection of neural connections is called Neural Darwinism. Namely, it explains it as a competition between synaptic connections. Initially, more connections than we need to develop. Then, the most successful axonal connections and combinations survive.

Neurotrophies are proteins that
promote the survival and activity of neurons. Axons that are not exposed to
neurotrophies after they form bonds go into apoptosis. Thus, a healthy adult
nervous system does not contain neurons that failed to make connections.

Early periods of brain development are critical for normal later development (2). Chemical abnormalities in the brain during early development can cause damage and developmental problems.

Therefore, it is very interesting to pay attention to the relation between neurons, synapses, and brain development, both before and after birth. As we have already said, the basic component of the nervous system, and especially the brain, is the network of specialized nerve cells called neurons.

These cells are interconnected by electro-chemical bonds - synapses, which are the physical basis for a person's complete psycho-physical development.

Neurons develop rapidly before birth and their number at birth is approximately equal to the number of neurons in the adult brain. After birth, brain development refers to making connections between neurons – building the synapses.

There are about one million neurons in each brain region that need to be connected. The number and organization of these synapses are extremely important. Namely, everything depends on the synapses, starting from the ability to recognize letters to maintaining social relations.

Connecting segments of our brain is
a dynamic process. From birth to the eighth month of life, synapses are made at
a very fast pace. Around the eighth month of life, there are about 1000
trillion synapses in the brain, which is double the number of synapses in the
adult brain.

After the first birthday, the synapse shutdown is accelerated. By the age of 10, a child has approximately 500 trillion synapses, which is the same as an average adult. The shutdown occurs until the age of twelve, after which the brain maintains its flexibility for future learning.

Early experiences, whether positive or negative, have a dramatic effect on the synapse formation. The brain works on a "use it or lose it" principle. Only those frequently used links and paths are kept.

The basic and most necessary experience in this early period is the interaction between the child and adults important for him/her. It is natural for a baby to laugh, growl, and thus attract the attention of adults.

Adults respond to these calls with speech, gestures, touch. In the following months of her life, in contact with adults, the baby listens carefully to speech, and the brain self-organizes itself through synapses to recognize only the voices it hears.

During early childhood, the brain still retains the ability to distinguish the voices it had rejected, and this is the so-called plasticity of the brain. That is why young children can learn a foreign language without accents. After ten years of age is reached, this plasticity is lost.

Development of neurons

The development of neurons in the brain happens in stages. Science has, so far, proved the existence of four key stages and processes. Those are: 

• Proliferation process
• Differentiation process
• Myelination process
• Synaptogenesis process/stage (2).

Proliferation refers to the creation
of new cells/neurons in the brain. It primarily occurs early in life. Some
cells become stem cells that continue to divide. The others remain where they
are or become neurons or glia and migrate to other locations.

Migration is the process of changing
place. Namely, the previously formed new neurons and glia travel to their new
locations. Migration can take place in different directions throughout the
brain. Migration can follow chemical pathways (immunoglobulins, chemokines).

Differentiation is the formation of axons and dendrites that give the neuron a specific appearance. The axon grows first either during the migration or once it reaches its destination and is accompanied by the development of dendrites. Neurons vary in shape and chemical components depending on location in the brain.

Myelination is the process through which the glia cells produce a fatty envelope that covers the axons of some neurons. Myelin interferes with neural impulse transmission. Myelination first takes place in the spinal cord, then in the midbrain. Then, it occurs in the forebrain. Myelination takes place gradually for decades.

Synaptogenesis is the final stage of
neural development and refers to the formation of synapses between neurons. It
happens throughout life as neurons constantly form new connections and shut
down the old ones. Synaptogenesis significantly slows down later in life.

Classification based on the number of cell extensions

There are five types of neurons
based on the number of their cell extensions. Those are:

• Unipolar neurons - have only one cell extension
• Bipolar neurons – they have two oppositely directed extensions; e.g. primary sensory neurons of the olfactory system, auditory, and vestibular systems, bipolar retinal cells.
• Pseudo unipolar neurons - neurons of the sensory ganglia of the cerebral nerves (ganglion spinale, ganglion trigeminale Gasseri).
• Multipolar neurons – they have several dendrites and one axon; they build sensory and motor structures.
• Amacrine cells – they have several dendrites but no axons (retinal amacrine cells) (2).

Projection neurons and Interneurons

The lengths of the axons are different depending on the function of the neurons. There are two main types based on this classification. Those are interneurons and projection neurons. 

The interneurons have the short axon
branches in the direct environment of the neuron. These are neurons of local
circuits. An interneuron is a neuron that mediates signal transmission. It is
usually an inhibitory neuron or the so-called "switch cell".

Projection neurons have long axons that end up in a remote area of gray matter. The axons of these neurons project themselves from one area to another. Such neurons can be afferent (those that bring - sensory) or efferent (those that take away – motor neurons).

Conclusion

Neurons are the key, core functional and structural unit of the central nervous system. The cerebral cortex contains around 10 billion neurons. Early years of age are extremely important for neuron, synapse, and brain development.

The interaction between early experiences and gene expression shapes the matured architecture of the brain. This heavily affects the interconnections or links between the neurons – the synapses, and everything we are, know and do.

References

1. Segev A, Curtis D, Jung S, Chae S. Invisible Brain: Knowledge in Research Works and Neuron Activity. PLoS One. 2016 Jul 20;11(7):e0158590. doi: 10.1371/journal.pone.0158590. PMID: 27439199; PMCID: PMC4954711. Found online on: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4954711/
2. Jeon H, Lee SH. From Neurons to Social Beings: Short Review of the Mirror Neuron System Research and Its Socio-Psychological and Psychiatric Implications. Clin Psychopharmacol Neurosci. 2018 Feb 28;16(1):18-31. doi: 10.9758/cpn.2018.16.1.18. PMID: 29397663; PMCID: PMC5810456. Found online on: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5810456/