Multipolar Nerve Cell

When considering the structural features of a nerve cell, it is common to speak of a multipolar type of neuron. It possesses a nerve cell body, the perikaryon, from which, as mentioned above, a number of dendritic extensions and one axonal extension originate. As a result, this is the key feature of these nerve cells – a single axon and numerous processes.

Interestingly, these nerve cells are
the most widespread ones in our central nervous system. They are found both in
the brain and in the spinal cord. The axon is responsible for transferring the
impulses between the cells. The dendrites serve as receptive zones (1).

In this article, we will elaborate on the structure and functions of the multipolar nerve cells. We will also talk about their relations with other nervous system structures and elements.

Structure of the Multipolar Nerve Cell

Observation of the multipolar nerve cell preparations under the light microscope shows that a large, centrally located nucleus is observed in the body of the nerve cells. It is bright, euchromatic, and nucleolus stands out regarding its size and degree of coloration. In the cytoplasm, which is stained basophilically, one can observe the groupings that are extremely intensely colored by the base dyes. These groupings are called Nissl bodies or the Nissl material (2).

Numerous dendritic extensions originate from the cell body, which can have additional first, second, and third grade branches. On the dendritic branches, there are a number of discharges that are referred to as dendritic thorns. 

The axonal extension, which also originates from the perikaryon, may be externally enveloped by the myelin sheath. This sheath, however, never extends continuously along the axons, but in certain places, regions of discontinuity are observed.

In some cases, at the spots of these discontinuities, axons may be separated laterally at right angles by branches designated as collateral. They can branch, and each of the branches can make contact with the dendrite or the cell body of another neuron but also of the same neuron.

The ending of the axon is characterized by its branching and is called telodendria. In Greek, this literally means “end of tree”. Each of the branches of the telodendria ends with a small extension - a synaptic bud.

The nerve cell nucleus, located in the center of the perikaryon, shows all the characteristics that are specific for the nuclei of synthetically very active cells. It is large, spherical in shape, with very little condensed chromatin which is located especially along the nucleus sheath and appears to be the constitutive heterochromatin (1).

The nucleolus is usually large and shows all the structural features that indicate that the formation of ribosomal subunits has been intensively carried out there.

In the cytoplasm around the nucleus, numerous groups of the granular endoplasmic reticulum and polyribosomes are observed. On the preparations observed under the light microscope, these groupings are colored by base colors and correspond to the Nissl bodies.

Numerous dictyosomes of the Golgi apparatus, cisterns of the smooth endoplasmic reticulum and numerous mitochondria are also observed. The remaining bodies that may have the appearance of "myelin" figures or, in older individuals, may be in the form of lipofuscin granules are occasionally observed. The remaining bodies are proofs of the activity of the lysosomal system, especially in the autophagic processes.

Elements of the inner cell skeleton - microtubules, actin filaments and neurofilaments - are also present in the perikaryon. Proteins associated with microtubules are involved in the construction of microtubules and MAP-2 is particularly prominent in the perikaryon. Actin filaments are present in the peripheral part of the cytoplasm and form an actin peripheral network.

Neurofilaments are a type of the intermediate filaments that are found exclusively in the nerve cells.

In addition to the neurofilaments in nerve cells, there are also intermediate filaments built by peripherin, a vimentin-like protein. Filaments constructed from peripherin are localized in the peripheral region of the cell.

In the initial parts of the dendritic extensions, some of the organelles that are characteristic for the perikaryon - polyribosomes, cisterns of granular endoplasmic reticulum, as well as the elements of the Golgi apparatus - can be observed.

However, dendritic organelles are smooth endoplasmic reticulum, mitochondria and vesicles, as well as elements of the cytoskeleton, in the first place numerous microtubules but also actin filaments.

A network of the actin filaments has been specifically developed in the area of the dendritic thorns, and there are few or no neurofilaments.

Morphological Features of the Multipolar Nerve Cell

When it comes to the morphological features, the axonal extension is significantly different from both the dendritic extension and the nerve cell body. This difference is reflected in the place on the perikaryon from which the axon originates - in that part of the cytoplasm, namely, there are no cytoplasmic organelles involved in the synthetic processes (1).

This region of the perikaryon on the preparations observed under the light microscope is pale in color and is seen as a slight elevation and is therefore called the axonal hill. Along with the cytoplasmic surface of the cell membrane of the axonal hill, a moderately dark contrasted material is present. Its nature is not yet thoroughly researched.

Inside the axon, mitochondria, secretory vesicles, elements of the smooth endoplasmic reticulum, multivesicular bodies, sometimes multilamellar bodies as well as cytoskeletal elements - microtubules, actin filaments and neurofilaments are observed. The direction of the mitochondrial delivery is parallel to the direction of delivery of axons and cytoskeletal components, primarily microtubules.

The components of the inner cell skeleton in the axon are arranged in an ordered manner. Actin filaments, similar to the perikaryon and dendritic thorns, in the peripheral part of the axonal cytoplasm, just below the cell membrane, form a dense network that is connected to the cell membrane by proteins of fodrin. This network is often referred to as a submembrane cytoskeleton.

Microtubules and neurofilaments occupy the central part of the axon and are parallel to the direction of its orientation. The microtubules are usually single, their positive end is directed toward the axonal termination while the negative end faces the cell body. MAP-3 and tau protein are involved in their construction. The kinesin and cytoplasmic dynein molecules are located between the mitochondria as well as all categories of vesicles and microtubules.

Chinesin molecules allow their anterograde movement down the axon while cytoplasmic dynein allows retrograde movement of the vesicles toward the perikaryon. Neurofilaments are organized into bundles parallel to the microtubules. They are particularly prominent in the peripheral nervous system axons of nerve cells. Pectin, a protein associated with intermediate filaments, interconnects neurofilaments but also neurofilaments and microtubules.

Undoubtedly, cytoskeletal elements are very important for the nerve cell function. Microtubules, as in other types of cells, represent the necessary mechanical-motor supports for the movement of organelles, in this case mitochondria and all kinds of vesicles, down and along the axon.

With regard to the neurofilaments, it is assumed that their primary role is to stabilize the axon shape of these cells. The differences, however, at the chemical level, between the neurofilaments in the perikaryon and in the axon, indicate that they most likely still have some role that, for now, has not been discovered.

Perikaryon, as well as the dendrites and axonal extensions, have contact spots with the adjacent nerve cells and glia or have relations even with spatially distant ones. All this "forest" of extensions and branches has a common name – it is called “neuropil”.

It should be noted that the plasma membrane of the nerve cell is organized into several domains which, on the one hand, in the dendrite and perikaryon levels, provide information reception and integration and, on the other hand, in the axon level, information transmission in the form of an action potential or nerve impulse.

Ion pumps are present in all domains and are involved in maintaining a basic electrical gradient between the intracellular and extracellular environments. Voltage-dependent ion channels, which are involved in the membrane depolarization process, are also present mainly in all domains, whereas ligand-dependent ion channels distinguish only certain areas of the nerve cell plasma membrane.

Conclusion

Multipolar nerve cells or neurons are special types of nerve cells that have one peculiarity: they are characterized by numerous dendrites and a single axon. A large number of dendrites and dendritic branches plays the key role in handling a great deal of information. This is, at the same time, they key function of the multipolar nerve cells. Namely, they integrate information from other neurons.

Moreover, it is
important to say that these types of nerve cells are the most common ones in
the nervous system. In addition, they are found in the brain and the spinal
cord.

References

1. Doucet JR, Ryugo DK. Structural and functional classes of multipolar cells in the ventral cochlear nucleus. Anat Rec A Discov Mol Cell Evol Biol. 2006 Apr;288(4):331-44. doi: 10.1002/ar.a.20294. PMID: 16550550; PMCID: PMC2566305.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2566305/
2. Palay SL, Palade GE. The fine structure of neurons. J Biophys Biochem Cytol. 1955 Jan;1(1):69-88. doi: 10.1083/jcb.1.1.69. PMID: 14381429; PMCID: PMC2223597.  Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2223597/