Introduction

Brain is the central control system of the body. It is the main component of the central nervous system. It controls all the voluntary activities performed by a person. In addition, it also has control systems for the regulation of involuntary processes like respiratory rate, blood pressure, etc. It is also responsible for higher functions such as thought processing, memory formation, behavior, thinking, etc.

All these functions in the brain are performed by neurons. Neurons are the basic structural and functional units of the nervous system. different types of neurons are present in the brain. These neurons are connected via special links called synapses. In addition to the neurons, supporting cells called the neuroglial cells are also present in the brain.

In this article, we will talk about different types of neurons present in the brain, the structure, and functions of these neurons as well as the way they are connected. We will also discuss the role of glial cells in the brain. Besides, we will be discussing different types of synapses and their role in the brain.

Structure of Neuron

Although different types of neurons are present in the brain, the basic structure of all the neurons is always the same. In this section, we will talk about the basic structure of neurons found in the brain.

A neuron can be divided into three basic parts; cell body or perikaryon, axons, and dendrites.

Cell Body

The cell body of a neuron serves as the synthetic or trophic center for the entire cell. It is the region that contains the nucleus and the surrounding cytoplasm. The major organelles are also present in the cell body or perikaryon. These include the Golgi apparatus and the endoplasmic reticulum.

A brief detail of the major organelles found in the cell body is as follows.

Nucleus

The nucleus occupies is present in the central portion of the cell body of neurons. Most neurons have a large spherical central nucleus having a prominent nucleolus. In most of the cells, the nucleus is pale-staining that indicates the euchromatic nature of chromatin. The fine chromatin threads can also be visualized within the nucleus.

Rough Endoplasmic Reticulum

The cell body of the neurons is responsible for protein synthesis. It has a highly developed system of rough endoplasmic reticulum for making proteins. A large number of parallel cisternae of the reticulum are present close to the nucleus associated with polyribosomes (a group of ribosomes attached to a single copy of mRNA). They can make multiple copies of a polypeptide at the same time.

The cell body appears to be highly basophilic in the regions containing rough endoplasmic reticulum and associated polyribosomes. They are present in the form of clumps of basophilic material known as Nissl substance or Nissl bodies.

Golgi Apparatus

Recall that the Golgi apparatus is responsible for the packaging of the proteins. In the cell body of neurons, they are present adjacent to the Nissl bodies. They pack the proteins made by this system of the rough endoplasmic reticulum.

The Golgi apparatus can pack proteins in vacuoles to be transported to other organelles within the neuron or the extracellular fluid. The Golgi apparatus is exclusive to the cell body and is not found in other parts of neurons.

Mitochondria

Mitochondria serve as the powerhouse of neurons. They are responsible for the synthesis of ATP. ATP is needed for the conduction of nerve impulses as well as other cellular processes such as intracellular transport. Mitochondria are found abundantly in the cell body of neurons. They are also present in dendrites and axons of the neuronal cell.

Cytoskeleton

The cytoskeletal framework provides structural support to the cell body as well as cellular processes of neurons. This framework is made up of intermediate filaments and microtubules. The intermediate filaments found in the neurons are called neurofilaments. The filament can be viewed under a light microscope as thin threads upon silver staining after treatment with some fixatives.

The cytoskeleton not only maintains shape but is also responsible for intracellular transport of various substances.

Inclusion Bodies

Inclusion bodies are the residual bodies left in the cell body after lysosomal degradation. These appear as pigmented bodies within the cell body when viewed under a light microscope. These bodies are not harmful as they do not interfere with the cellular functions of neurons.

Dendrites

These are the cellular processes that carry nerve impulses towards the cell body of neurons. They function as an antenna of the neuron as they receive neuronal signals and transmit them to the cell body of a neuron.

Dendrites show abundant branching. They do not have a constant diameter. The diameter goes on decreasing as they divide into more and more branches.

The arborization pattern of dendrites is specific for different types of neurons. Tree-like arborization of dendrites is seen in most of the inter-neurons found in the brain.

Dendritic spines are small blunt processes that emerge from dendrites at specific points. These spines are the sites for synapse formation.

The cytoplasm of dendrites has the same composition as the cytoplasm found in the cell bodies of neurons. However, the cytoplasm of dendrites has abundant cytoskeletal components.

Axons

These are the cellular processes that carry nerve impulses away from the cell body of neurons. These are the cylindrical processes having a constant diameter throughout their length. In the case of the motor neurons present in the brain, they can be as long as one meter.

Axons originate from the cell body via a pyramid-shaped structure called the axon hillock. The portion of axon just beyond the axon hillock is called the initial segment. It is where the nerve impulses coming to the cell body are processed and a decision is made whether to conduct the impulse of not.

The cytoplasm present in the axons is called axoplasm. It is abundant in mitochondria and cytoskeletal filaments. However, ribosomes and RER are absent. Thus, it depends on the cell body for protein synthesis. The synaptic vesicles containing neurotransmitters are also abundantly present in the axoplasm of neurons. These vesicles are made in the cell body are then transported to the axon.

Axons do not show branching as seen in dendrites. However, the terminal end of axons, called the axon terminal, forms multiple branches called terminal arborization.

Types of Neurons

Based on the functions performed by neurons, they are divided into three basic types; sensory neurons, motor neurons, and interneurons.

Sensory Neurons

These neurons are responsible for identifying an environmental stimulus or a stimulus arising within the body. They undergo depolarization and give rise to a nerve impulse when exposed to the specific stimulus. This nerve impulse is then carried by the axon to the other neurons of the nervous system.

The dendrites of sensory neurons serve as the sensory organs or receptors in most of the cases. However, they may also be associated with other non-neuronal cells that serve as the sensory receptors. In the latter case, the stimulus generates a nerve impulse in the receptor cell which is then transmitted to the neuron.

The brain does not contain sensory neurons in a true sense. However, it does contain sensory processing neurons as present in the auditory cortex and visual cortex. The neurons in these areas are responsible for processing the information receives via auditory or visual pathways.

Motor Neurons

Motor neurons are responsible for exciting the skeletal muscles to perform the desired action. They receive information from the sensory of interneurons and pass it to the muscle cells.

Motor neurons have small dendrites and a large single axon. The axon is mostly myelinated to increase the speed of nerve impulse conduction. They are stimulated by nerve impulses coming from other neurons.

The true motor neurons that directly supply the muscles are also absent in the brain. however, motor processing neurons are present. These include the pyramidal cells in the cerebral cortex, the Purkinje cells in the cerebellum, and many more. These cells receive the sensory information regarding the state of muscle, process this information, and then stimulate or inhibit the motor neurons present in the spinal cord.

The axons of these neurons form large descending tracts that run throughout the length of the spinal cord. These tracts maintain the motor tone and facilitate or inhibit the activity of motor neurons present in the spinal cord.

Interneurons

These neurons receive information and process it. After processing the information, they either store it in the form of memory or pass the analyzed information to the next neurons.

Most of the neurons present in the brain are interneurons. They have abundant dendrites that show complex arborization patterns.

These interneurons also communicate among themselves by forming complex neuronal circuits in the brain. These circuits increase the processing power and can handle a large amount of information.

It is not easy to classify all the neurons present in the brain. The basic classification of neurons that we just discussed fits perfectly for the spinal cord but not the brain. The human brain has more than 86 billion neurons. These neurons perform different functions based on the part of the brain they are located in. however, most of these neurons can be regarded as the interneurons as they are involved in processing the sensory or motor information.

Arrangemet of Neurons

The neurons have different arrangements in different parts of the brain. In this section, we will talk about the neuronal arrangement in the cerebral cortex.

The cerebral cortex can be divided into grey matter and white matter.

Grey Matter

The grey matter is present in the outer portion of the cerebral cortex. It contains the cell bodies of neurons and the unmyelinated neuronal fibers. the axons of neurons leave the grey matter and move inwards.

White Matter

It is the inner portion of the cerebral cortex. It contains myelinated axons or nerve fibers. Cell bodies of some neurons are also present in the whiter matter. The collection of cell bodies inside the white matter of the cerebral cortex is called a nucleus. Examples of nuclei are the red nucleus, nuclei of the cranial nerves, etc.

Synapse

Neurons present in the brain and other parts of the nervous system communicate via synapses. A synapse is a site where the nerve impulse is transmitted from one neuron to another neuron or a non-neuronal cell.

Functional Classification

Based on the mechanisms by which they transmit a nerve impulse, synapses can be divide into two types; electrical synapses and chemical synapses.

In an electrical synapse, the cytoplasm of two or more cells is interconnected via gap junctions. Thus, the ions can freely move from one cell to the other. In this way, depolarization in any one cell causes depolarization of all the connected cells. These synapses are seen in cardiac myocytes.

In a chemical synapse, the nerve impulse is transmitted from one cell to the other via chemical substances called neurotransmitters. Most of the neuronal synapses re chemical synapse. The structure of such a synapse is mentioned below.

Structure

There are three components of a neuronal synapse; a pre-synaptic terminal, synaptic cleft, and a post-synaptic neuron.

An axonal terminal forms the pre-synaptic terminal of a synapse. When a nerve impulse arrives at a presynaptic terminal, it causes synaptic vesicles to fuse with the plasma membrane. In this way, the neurotransmitters present in the vesicle are dumped into the synaptic cleft.

The synaptic cleft is a space between the pre-synaptic and post-synaptic neurons. The neurotransmitters released into the cleft diffuse through it to reach the post-synaptic neuron. Neurotransmitters remain in cleft for only some time. They are either taken up by the presynaptic neuron or are broken down by the enzymes present in the synaptic cleft.

The post-synaptic neuron contains several receptors on its plasma membrane. The neurotransmitters bind to these receptors and initiate a nerve impulse by causing depolarization. In this way, a nerve impulse is transferred from one neuron to the other via a synapse.

Structural Classification

Based on the structural components participating in the formation of a chemical synapse, they can be classified as follows;

Axosomatic synapse, between an axon and a cell body

Axoaxonic synapse, between two axons

Axodendritic, between dendrite and axon

Neuroglial cells

Neuroglial cells are the supporting cells present in the brain. They are ten times more abundant than the neuronal cells in the human brain. The glial cells provide support and nutrition to the neuronal cells. They also play a role in the survival or protection of the neurons present in the brain. They can be considered as a substitute for connective tissue in the brain.

Types

Different types of glial cells are present in the brain. The most important glial cells are mentioned below.

Oligodendrocytes: They form the myelin sheath around the axons present in the brain. One oligodendrocyte can form a myelin sheath around axons of more than 10 different neurons.

Astrocytes: These are the most abundant glial cells. astrocytes are the star-shaped cells having slender processes. They provide structural and metabolic support to the neurons. They are also a component of the blood-brain barrier.

Ependymal cells: These cells line the ventricles of the brain and the central canal of the spinal cord. They play a role in the synthesis and movement of the cerebrospinal fluid (CSF).

Microglial cells: These cells protect neurons from the attack of pathogens. These glial cells have defense and immune-related activities. Their function is similar to macrophages present in other parts of the body.

Summary

The human brain consists of billions of neurons that are interconnected via neuronal synapses. Cells other than neurons called the glial cells are also present to proivide support to the neurons.

A neuron consists of a cell body and cellular process.

The cell body contains the nucleus in the center and other organelles for the synthetic needs of the cell. The complex system of rough endoplasmic reticulum and polyribosomes is also present called the Nissl granules. They are exclusive to the cell body of neurons.

Dendrites are the slender processes that show abundant branching and carry nerve impulses to the cell body of neurons. Dendritic spines are the sites for synapse formation on dendrites.

Axons are the cylindrical processes that carry nerve impulses away from the cell body of neurons. They do not show branching and have a constant diameter throughout their length. They are the myelinated nerve fibers.

Neurons can be divided into sensory, motor, and interneurons. Most of the neurons present in the brain are interneurons.

The neurons in the brain are arranged into gray matter and white matter.

These neurons communicate via chemical synapses.

In a chemical synapse, neurotransmitters are released by the pre-synaptic cells that diffuse through the synaptic cleft and excite the post-synaptic neuron.

The synapses present in the brain might be axoaxonic, axosomatic, or axodendritic.

Glial cells replace the connective tissue in the brain. They provide support, nutrition, and protection to the neurons. They are involved in myelin sheath synthesis, providing metabolic support, regulating the flow of CSF providing protection from pathogens.

Reference

Squire, Larry R.; Floyd Bloom; Nicholas Spitzer (2008). Fundamental Neuroscience . Academic Press. pp. 425–6. ISBN 978-0-12-374019-9 .

Bear, Conners, Paradiso (2007). Neuroscience: exploring the brain. Philadelphia, PA: Lippincott Williams & Wilkins. pp. 113–118

Van Spronsen M, Hoogenraad CC. Synapse pathology in psychiatric and neurologic disease. Curr Neurol Neurosci Rep. 2010;10(3):207–214. doi:10.1007/s11910-010-0104-8