Neurons

Introduction

Neurons (or nerve cells) and the glial cells are the central nervous system’s main constituents. A neuron is a single nerve cell that permits communication between the cortical structures and the body. Despite being less numerous than the glial cells, the neurons play a more important role in the brain.

 

What Is the Main Function of Neurons?

These basic units of the cerebral cortex and the central nervous system receive the sensory information from external stimuli and relay it via electrical or chemical messengers. The corresponding brain areas then process it as information, reaction, feeling, or sensation to all other parts of the body.

 

The afferent (sensory) neurons carry the sensory information toward the central nervous system. In contrast, the efferent (motor) neurons transmit the sensory impulses from the central nervous system to the glands and muscles.

 

In this regard, neurons are also involved in the transportation of neurotransmitters, the chemicals that support the neurons in the transmission of sensory information to the brain’s dedicated structures.

 

What Does the “All-Or-None” Principle Mean?

When a neuron relays the sensory information via its axon, an action potential occurs. When this electrical signal reaches the end of a neuron, the soma secretes neurotransmitters that activate the neighboring cells.

 

The ‘all-or-none’ principle refers to the fact that this action potential either triggers the neuron to fire or doesn’t reach the firing threshold. In short, the nerve either fully responds to the stimuli or doesn’t respond at all.

 

What Are the Parts of a Neuron?

Each neuron consists of a cell body (the soma), an axon, dendrites, and synapses. However, a more detailed analysis includes the following neuron parts.

 

• Soma (the cell body)

• Axon

• Axon terminal

• Dendrite

• The nucleus of the neuron

• Mitochondrion

• Golgi apparatus

• Telodendria

• Synapses

• Myelin Sheath

• Nodes of Ranvier

• Nissl bodies

 

Soma (The Cell Body)

The cell body of a neuron is called a soma. It contains the nucleus of the neuron, as well as many specialized organelles. This neuron core possesses the genetic material of our body and stores the energy required to transmit sensory information among the other neuron cells.

 

Axon

The axon of the nerve cell is also known as a nerve fiber. It’s a part of the neuron cell that relays the electrical or chemical impulses away from the cell body. A neuron connects to the other cells via this neuronal structure.

 

Axon Terminal

The axon terminal (or the synaptic bouton) is the furthest part of the neuronal cell body. It’s directly involved in the communication with other neurons and target cells by creating synaptic connections.

 

Dendrite

A dendrite is a point where the neuronal cell receives sensory stimuli. Due to their nature to branch while creating their neuronal projections, the neuron’s dendrites are also called ‘tree branches’. Their function is to conduct the electrical or chemical impulses to the cell body when stimulated by other information-carrying neuron cells.

 

 

The Nucleus of a Neuron

This neuronal structure is located in the body of the neuron. Composed of the cells’ DNA, i.e. the genetic material contained in chromosomes and the nucleolus, it supports the regulation of all processes within a neuron cell.

 

 

Mitochondrion

The mitochondrion is an organelle located in all neuron cells. It supports the excitability of the neuronal membrane, which is very important for neurotransmission.

 

 

Golgi Apparatus

The Golgi apparatus is an organelle located near the nucleus of a neuron cell. It refers to a network of membranes that support protein modification, but this organelle is also involved in the production of some hormones.

 

 

Telodendria

Telodendria is the name of the ending branches of the neuronal axon. The axon terminals are located at their ends. Their primary function is the transmission of sensory information via the synapses they form.

 

 

Synapses

The synapses (or neuronal junctions) refer to the point where the electric or chemical impulses transfer from one neuron to another, or between a neuron and a muscle.

 

 

Myelin Sheath

The protein layer that forms around the nerve fibers is labeled as a myelin sheath. It supports the fast neurotransmission between neurons. When the myelin sheath is damaged, it triggers neurological diseases such as multiple sclerosis.

 

 

Nodes of Ranvier

The myelin sheath mentioned above is marked with periodic gaps known as the Nodes of Ranvier. Their primary role is to speed up the transmission action potentials that are relayed along the neurons’ axons.

 

 

Nissl Bodies

This large granular body found in the cytoplasm of the neuron’s soma synthesizes the proteins within a neuron cell.

 

 

How Are Neurons Classified by Function?

According to their function and depending on their location in the cerebral cortex, neurons are divided into 3 large groups:

 

• Sensory neurons,

• Motor neurons,

• Interneurons.

 

Sensory Neurons

The sensory neurons are in charge of detecting and translating the sensory impulses from the surrounding environment, gained via receptors. After they receive these sensory signals, the sensory neurons assign them meaning that they relay to the processing brain areas utilizing electrical or chemical impulses.

 

 

Motor Neurons

This type of neuron is located in the motor cortex, brain, or spinal cord. They transmit sensory signals from the cerebral cortex to the hormone-secreting glands or our muscles via their cell body’s axons.

 

 

Interneurons

Interneurons serve as mediators between the neurons since they do not possess sensory nor motor function. In this way, they provide communication between the central nervous system with the sensory or motor neurons.

 

 

How Are Neurons Classified by Structure??

According to their structure, the neurons are classified as:

• Unipolar neurons,

• Bipolar neurons,

• Multipolar neurons,

• Pseudounipolar neurons.

 

Unipolar Neurons

A unipolar neuron is a sensory neuron that consists of only one cell body extension, labeled as a neurite. The neurite is where the dendrites and axons begin to branch out with their fibers.

 

 

Bipolar Neurons

This type of neuron has one axon and one dendrite that support the relay of the sensory stimuli gained via the senses of smell, taste, hearing, sight, balance, proprioception, and touch.

 

 

Multipolar Neurons

The multipolar type of neurons consist of one axon and multiple dendrites. They support the transmission of information (collected by the sensory organs) to the other neurons.

 

 

Pseudounipolar Neurons

The cell body of the pseudounipolar neuron extends only one axon that splits into two axonal branches. Each of these branches connects a neuron with the central nervous system and the peripheral nervous system.

 

 

How Does a Neuron Know Where to Migrate?

Some neurons migrate following the path of the cell fibers. These fibers protrude the inner layers of the cerebral cortex to reach the other surface of the brain. That’s how these neurons supply the sensory information to the outer cortical structures.

 

But, this is not the only way that neurons migrate. Namely, they also travel with the help of the chemical or electrical impulses. Whereas the chemical impulses lead the neurons in only one direction, the neurons’ electrical impulses go in both directions.

 

 

How Do Glial Cells Guide the Neurons to Their Destination?

The glial cells are a type of cell in the brain responsible for the coordination of neuron cell activities. These cells help neurons reach their final destination, giving them a significant role in neurotransmission.

 

However, some of the neurons never reach their designated destination due to gene mutation. Such misguided neurons are believed to be the triggers for the development of neurological diseases such as schizophrenia or dyslexia.

 

 

What Are the Most Common Neurological Disorders?

The dysfunctionality of neuronal processes or the genetic mutation of neuronal cells can lead to many neurological disorders. They can range from common medical conditions, such as headaches (migraines, tension headaches, cluster headaches), to some more serious diseases, namely Charcot–Marie–Tooth disease or myasthenia gravis.

 

 

How Are Neurological Disorders Classified?

Neurological disorders can be classified according to the type of dysfunction (A) and the affected location or cause (B):

 

A.

• Aphasia

• Agnosia

• Dysgraphia

• Amnesia

• Dysarthria

B.

• Temporal lobe damage

• Parietal lobe damage

• Occipital lobe damage

• Frontal lobe damage

• Spinal cord disorders

• Cranial nerves disorder

• Epilepsy (Seizure disorders)

• Disorders of the autonomic nervous system

• Parkinson’s disease

• Tourette’s Syndrome

• Tremors

• Multiple sclerosis

• Obsessive-compulsive disorder

• Huntington’s disease

• Tumors

• Delirium

• Dementia

• Vertigo

• Asperger Syndrome

• Autism

• Narcolepsy

 

What Is Neuroregeneration?

The human brain can regenerate its nervous tissue and neuron cells. The process of neuroregeneration generates new synapses, axons, neurons, glial cells, or the myelin sheath of the neurons.

 

After an injury, the neuroregeneration occurs mostly in the peripheral nervous system. Still, this process is not so extensive when damage occurs in the central nervous system area.

 

Some of the aforementioned neurological diseases result from the dying out of the neurons. Such are the following cases of:

 

• Parkinson’s disease – This disease occurs due to the dying out of the basal ganglia neurons. This cortical structure is involved in the body’s motor movements, so when the neurons seize in order to produce dopamine in the basal ganglia, the movement of the affected person becomes inhibited.
• Huntington’s disease – Due to the mutation of genes, the neurons begin to produce glutamate in substantial quantities. This neurotransmitter destroys the neurons located in the brain’s basal ganglia, which triggers uncontrollable twisting and squirming in the affected individual.

 

Fun Facts

Did you know?

1. The brain consists of around 100 billion neurons – there are almost as many stars in the Milky Way.
2. Watching TV harms the lifespan of neurons. If a person spends more than 3 hours in front of the TV, it can lead to the obstruction of cognitive abilities.
3. Neural coding is a field in neuroscience studying neuronal processing of sensory information.
4. The list of cells that do not have mitochondria organelles isn’t long. It contains red blood cells, the prokaryotic organism, and some bacteria.
5. Eukaryotic cells are cells with clearly defined nuclei.
6. The transmission of neuron impulses slows down when the myelin sheath is damaged.
7. All neurons possess a high degree of excitability, which is very important for the fast relay of sensory information to the cortical structures.
8. Interneurons can be found only in the central nervous system.
9. A nerve refers to a bundle of neuron cell fibers that are linked by connective tissue.
10. Glial cells support the metabolic processes by supplying neurons with nutrition and oxygen.