The Central Nervous System and it’s Functioning

Nerve cells and neuroglia cells form regions of the central nervous system that are anatomically referred to as the forebrain, midbrain, hindbrain, and spinal cord (1). Simply put, the central nervous system or the CNS consists of the brain and spinal cord (medulla spinalis).

Histological and Anatomical Review of the CNS

Histologically, the neurons and glia cells are the elements that form two "masses", gray and white, which differ in the representation of neuroglial components as well as in the presence of certain types of nerve cells and their regions. The gray mass contains nerve cell bodies and dendritic extensions while the white mass is characterized by axons (2).

The distribution of gray and white mass in the central nervous system is not the same. In all anatomical parts of the brain, the gray mass occupies a superficial position, while the white mass lies below it.

In the extended and spinal cord (medulla oblongata and medulla spinalis), the gray mass does not form the outer layer but forms a structure that can be compared with the Latin letter H. we can say that it occupies the inner, central position, and the white mass "fills" the spaces between the dorsal and ventral parts of the gray mass – the segments of the previously mentioned letter H.

In addition to the neuroglia cells - astrocytes and oligodendrocytes, often referred to as macro-glial cells and microglial cells that participate in the structuring of gray and white mass, the accompanying cells of the central nervous system are the ependymal cells, as well as cells that form the choroidal plexus.

The surface of the central nervous
system is protected by connective membranes labeled as brain envelopes or
membranes, scientifically called meninges, which are formed from loose and
dense connective tissues (1).

Spinal Cord (medulla spinalis)

The back of the nerve tube, the spinal cord, is a cylindrical column of nerve tissue, about 40 cm long, extending through the entire length of the body. Going towards the back of the body it narrows down, and it has a central canal in the middle.

The arrangement of gray and white mass in the spinal cord is reversed to that in the brain - white mass is on the outside and gray is on the inside. The white mass consists of ascending and descending nerve fibers, which stimulate the brain, as well as impulses from the brain to various organs.

The spinal cord is, therefore, a conduit of stimuli in the direction of the brain (ascending nerve fibers) as well as impulses sent from the brain (descending fibers). In addition, it is the center for many simple reflexes, that is, the spinal cord controls simple reflex actions.

This is achieved by neurons whose fibers extend for short distances along and down the spinal cord, as well as by interneurons that transmit messages directly between the sensory and motor neurons.

Some of the messages are immediately
transmitted by neurons to motor neurons that control the movements of the
muscles. Other messages go along the spinal cord and are transmitted by neurons
to motor neurons that control neck movements. The following messages are sent
to the brain and cause a conscious feeling of, for example, heat or pain.

Brain membranes

Between the cranial bones and the nerve tissue and between the spinal vertebrae and the brain, three brain membranes or protective envelopes are present. Those are the dura mater, arachnoidea mater, and pia mater. They play the most important protective role in the CNs. Also, they have a metabolic role.

The first of these membranes or
sheaths, one that, in certain ways, represents the continuation of the dense
connective tissue adjacent to the periosteum of the bone, is the “hard brain
sheath” or the dura mater. It is made of thick connective tissue.

The subdural space is found between
the hard membrane and the arachnoidea mater. This is a space that is filled
with a very small amount of tissue fluid.

The arachnoid brain structure is not a compact structure. It includes the arachnoid membrane and arachnoid trabeculae that extend within the subarachnoid space that is filled with cerebrospinal fluid.

The arachnoid membrane, another structure that incorrectly got a name of a membrane, is composed of five to eight layers of flattened fibroblasts. It resembles the spider web. That’s why it is often referred to as the web membrane.

The soft brain membrane, pia mater, directly adheres to the nerve tissue of the central nervous system and is made up of several components. On the surface facing the subarachnoid space, cells are similar to the mesotheliomas and form an epithelium lying on mutually parallel collagen fibers.

Below the collagen fiber layer, there is a layer of connective tissue made up of collagen fibrils, flattened fibroblasts as well as stellar-shaped macrophages. On the nerve tissue surface, in other words, there is a layer of soft brain membrane formed by the flattened cells lying on the lamina that is facing the glia limitans.

Auxiliary (supporting) Cells of the Central Nervous System

Central nervous system supporting cells - astrocytes, oligodendrocytes, and microglial cells, as well as ependymal cells and epithelial-forming cells of choroid plexuses, not only play a supporting role but actively participate in metabolic processes that characterize and are important for the nerve tissue.

Ependymal cells

The ependymal cells form the boundary "line" between the nerve tissue and brain chambers in the area of the brain, that is, nerve tissue and the central canal in the area of the brain, cavities that are filled with cerebrospinal fluid.

Although these cells have the same embryonic origin as the central nervous system neurons, i.e. they differentiate during organogenesis from the neuroectodermic material, they can be morphologically significantly different from nerve and neuroglial cells. The ependymal epithelium, which is formed by these cells, is constructed of two types of cells.

The bulk of the cells of the ependymal epithelium are the real ependymal cells that resemble the epithelial cells with their features - it is possible to distinguish their top and sides. The base, from which the cytoplasmic extensions may originate, relies predominantly on the glial cells, although there are also those that reach deeper, down to the capillaries.

The height and shape of the ependymal cells are not uniform. Some have a cylindrical shape while others are cuboidal. Because of their different height on preparations observed under the light microscope, the ependymal epithelium gives the impression of a multilayer structure, so it is often classified as the false multilayered, pseudostratified epithelium.

The apex surface of the largest number of these cells is differentiated into numerous cilia as well as micro-elements. With their movements, they help the overall flow and movement of the cerebrospinal fluid. Due to the presence of the cilia, the epithelium that these cells form is called the pseudo-structured ciliated epithelium.

Cells of the Choroid Plexus

Cerebrospinal fluid fills the subarachnoid space, cerebral ventricles, and the central cerebral canal, and is also present in the area of the parenchymal brain sheath.

It is vital for the functioning of the nervous tissue of the central nervous system, since, on the one hand, it protects the nervous tissue from possible mechanical damage that might happen due to the proximity of the cranial bones, and on the other hand, it participates in its metabolism.

Namely, this fluid is responsible for the chemical stability of the environment in which the nerve tissue is located, and it is also rich in nutrients. Cells that produce cerebrospinal fluid and, in association with fenestrated capillaries, maintain its chemical stability, forming structures called choroid plexuses.

The choroid plexuses are located in the third and fourth brain chambers. When it comes to their shape, they resemble branched clusters that descend from the soft brain membrane into the chamber cavities. Although the embryonic origin of the cells that form choroid plexuses is not different from other cells of the central nervous system - they come from the neuroectoderms, they are closest to the ependymal cells.

Moreover, a number of ependymal cells differentiate into choroidal cells. They form a single-layered cubic epithelium beneath which lies a thin layer of loose disorganized connective tissue - lamina propria, as well as numerous capillaries.

The choroid plexus cells are characterized by a centrally located ball nucleus. In the cytoplasm, numerous mitochondria of the rod-shaped, relatively short cisterns of the granular endoplasmic reticulum and a small Golgi apparatus are observed. Cells are tightly bound together by adhesive-type bonds.

On the top surface, they are supplied with numerous micro-fissures, and amphibians, for example, are present with cilia. At the base of these cells, the plasma membrane forms numerous gaps between which the majority of the mitochondria are located.

Conclusion

The central nervous system or, shortly, the CNS, is the key part of the nervous system built by neurons concentrated in the nerve centers and located in the spinal canal and the cranial cavity.

It extends along the longitudinal axis of the body. In vertebrates, this system consists of brain, located in the cranial cavity and the spinal cord, located in the spinal canal. Parts of the human CNS are medulla spinalis, medulla oblongata, pons, cerebellum, diencephalon, telencephalon, and mesencephalon.

References

1. Ludwig PE, Varacallo M. Neuroanatomy, Central Nervous System (CNS) [Updated 2019 Feb 4]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2019 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK442010/
2. Sousa AMM, Meyer KA, Santpere G, Gulden FO, Sestan N. Evolution of the Human Nervous System Function, Structure, and Development. Cell. 2017 Jul 13;170(2):226-247. doi: 10.1016/j.cell.2017.06.036. PMID: 28708995; PMCID: PMC5647789. Available from: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5647789/