SOMA: FEATURES PARTS AND FUNCTIONS - BIOLOGY - 2025

The soma, cell body, soma or perikaryon is the central portion of neurons, where the nucleus, cytosol, and cytosolic organelles are located. Neurons are made up of four fundamental regions: the soma, the dendrites, the axon, and the presynaptic terminals.

Therefore, the neuronal body is a part of the neuron and from this derive the dendritic processes and the axon.

Photograph of a neuron from a chicken embryo stained and observed by confocal microscopy (Source: Xpanzion at the English language Wikipedia / CC BY-SA (http://creativecommons.org/licenses/by-sa/3.0/) via Wikimedia Commons)

The soma or cell body comes in various sizes and shapes. Neurons of the central nervous system, for example, have polygonal cell bodies and concave surfaces that separate multiple cell processes, whereas neurons in the dorsal root ganglion have round bodies.

Soma, dendrites and axon

Basic shape of a neuron

The soma or cell body is the metabolic center of a neuron. It is the voluminous area of ​​neurons and the one that contains, proportionally, more cytoplasm. Dendrites and an axon project from the soma.

The dendrites are thin extensions and branched specialized functions is receiving stimuli from axons of other neurons, sensory cells or other dendrites. This information received in the form of electrical stimuli is transmitted to the cell body.

The axon is a single branching process of varying diameter and length, which can be up to one meter (1 m) long, like the axon of the motor neurons that innervate the muscles of the feet. The axon conducts information from the perikaryon to other neurons, muscles, or glands.

Connection representation between neurons

Soma characteristics

In vertebrate organisms the body of the nerve cells or soma is found in the gray matter of the central nervous system or in the ganglia. The white matter of the nervous system is made up of nerve fibers, which are extensions of the body of neurons.

There are different types of neurons and different shapes and sizes of neuronal bodies or bodies. Thus, bodies are described:

- spindle-shaped

- crashed

- pyramidal and

- round

Neurons establish connections with each other and with various organs and systems. These connections have no anatomical continuity and are called "synapses."

The connection between neurons is made by contact of the axon of a neuron with the body of another neuron, with the dendrites and, in some cases, with the axon of another neuron. Hence, these connections are named axosomatic, axodendritic, or axoaxonic, respectively.

The soma integrates all the electrical signals and emits a response through the axon that, depending on the type of neuron, will go to another neuron, to a muscle or to a gland.

Parts of soma

Graphic representation of a neuron pointing to the cell body, axon and dendrites (Source: Ajimonthomas / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0), via Wikimedia Commons, modified by Raquel Parada)

- The neuronal body has a membrane similar to the membrane of other cells in the body, a nucleus and the perinuclear cytosol (around the nucleus).

- The nucleus is large and round and is generally located in the center of the soma. It has dispersed chromatin and a well-defined nucleolus.

- In the cytosol there are inclusions such as melanin granules, lipofuscin and fat droplets. There is also the rough endoplasmic reticulum, with abundant cisternae arranged in parallel groups and scattered polyribosomes, and some lysosomes and peroxisomes.

When the rough endoplasmic reticulum cisterns and polyribosomes are stained with basic dyes, they are observed under light microscopy as "basophilic clusters" called Nissl bodies .

These are observed in the soma, except for the area where the axon or axon mound arises, and in the dendrites .

- Numerous fragments of the smooth endoplasmic reticulum that form the hypolemmal cisterns are found throughout the body, in the dendrites and in the axon. These cisternae are continued with the rough endoplasmic reticulum in the cell body.

- A fairly prominent juxtanuclear Golgi complex is also found in the soma, with typical cisternae of protein-secreting cells.

- The cytosol of the soma, dendrites and axon also contains many mitochondria, however, these are more abundant at the axon terminal.

When neurons are prepared with silver impregnation, the neuronal cytoskeleton is observed with the light microscope.

This is formed by neurofibrils of up to 2 µm in diameter that cross the soma and extend in its processes. Neurofibrils are made up of three different structures: microtubules, neurofilaments, and microfilaments.

Features

Cytoplasmic inclusions

Melatonin is a derivative of dihydroxyphenylalanine or methyldopa. It gives a blackish color to certain neurons, especially the neurons of the "nucleus coeruleus" and of the substantia nigra, where these cytoplasmic inclusions are very abundant.

It is also found, although to a lesser extent, in the dorsal motor nuclei of the vagus and spinal cord, as well as in the sympathetic ganglia of the peripheral nervous system.

The function of these cytoplasmic inclusions is not very clear, as they are believed to be an accessory product of the synthesis of two neurotransmitters, dopamine and norepinephrine, which share the same precursor.

Lipofuscin is a yellowish pigment that appears in the neuronal cytoplasm of the elderly adult. It increases with age and its accumulation can affect cell function.

Fat droplets do not appear very frequently in the neuronal cytoplasm, but they may be the product of a metabolic defect or they may be used as an energy reserve.

Core

Cell nucleus

The nucleus contains chromatin, which is the cell's genetic material (DNA, deoxyribonucleic acid). The nucleolus is the center for RNA synthesis and the nucleoplasm, which includes macromolecules and nuclear particles that are involved in the preservation of the neuron.

The nucleus has all the information necessary for the synthesis of all the substances that the neuron needs to manufacture for its function and maintenance, especially for the synthesis of all functional and structural proteins.

Organelles

The smooth endoplasmic reticulum has functions related to calcium management. The rough endoplasmic reticulum, together with the Golgi complex and polyribosomes, has functions related to the synthesis of proteins, both structural and those that must go to the cytoplasm.

In the rough endoplasmic reticulum, post-transcriptional modifications of proteins also occur, such as folding, glycosylation and the addition of different functional groups, etc. In addition, the integral lipids of the membranes are synthesized.

Lysosomes are polymorphic organelles that contain at least about 40 different types of acid hydrolases. These enzymes help digest macromolecules, phagocytosed microorganisms, cellular debris, and even senescent organelles.

Mitochondria are the organelles responsible for oxidative phosphorylation for the production of ATP (adenosine triphosphate), a high-energy molecule that the cell uses for its function. It is the site where cellular respiration occurs, where oxygen extracted from the environment is consumed.

Illustration of mitochondria

Cytoskeleton

The proteins that make up the neurofibrils have structural and transport functions allowing the transport of substances from the soma to the axon terminal and from this to the soma. In other words, it is the neuron's vial system.

Thus, from the previous lines it is understood that the soma or cell body is, like any cell, a complex interconnected system of organelles, membranes, proteins and many other types of molecules, whose fundamental function has to do with the transmission and reception of stimuli nervous in vertebrates.

References

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