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The neuron: morphology and structure

The neuron: morphology and structure


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Our nervous system (SN) is made up of billions of cells that establish a large number of connections with each other.

In the SN there are two types of cells: Neurons and glial cells.

A neuron is a highly specialized and electrically excitable nervous system cell that conducts nerve impulses between different parts of the body.

Neurons can process and transmit information from internal and external environments, communicating this information through chemical or electronic impulses through a synapse (the union between cells) and using the action potential, an electrical signal that is generated by means of the electrically excitable membrane of the neuron. In vertebrate animals, neurons are the central components of the brain, spinal cord and peripheral nerves.

Content

  • 1 Parts of neurons
  • 2 Soma or cell body
  • 3 axon
  • 4 dendrites
  • 5 Organelles and cytoplasmic particles
  • 6 myelinated axons

Parts of the neurons

Neurons, like other cells, have a cell body called soma. The nucleus of the neuron is in the soma. Neurons need to produce many proteins, and most neuronal proteins are also synthesized in the soma.

Various appendages or bumps extend from the cell body. These include many short ramifications, known as dendrites, and a branch that is generally longer than dendrites, known as the axon.

There are neurons of different shapes and sizes, but they all share common structural characteristics.

Soma or cell body

The soma is the bulbous body of a neuron (nerve cell)from which dendrites and axon come off. It is the part of the neuron that contains the cell nucleus that encloses a nucleolus.

The soma includes many organelles, including Nissle granules, predominantly composed of rough endoplasmic reticulum and free polyribosomes. The machinery of proteins and membranes of free ribosomes and endoplasmic reticulum in the soma is probably the most active in the human body. The Golgi apparatus is also well developed and the mitochondria are widely dispersed. Nevertheless, The key feature of the soma is the cell nucleus, where most of the RNA is produced. In general, the complex coordination between the various parts of the soma, between the soma and its processes (axons and dendrites), and the interactions between an individual neuron and other neurons, reflects a remarkable intracellular and intercellular harmony, which allows it To perform maintenance functions for herself and for the rest of the nervous system and the body.

Axon

The axon is one of two types of protoplasmic extensions that extend from the body or soma of neuronal cells. The axon is a long arm-shaped projection It can extend in tens, hundreds or even tens of thousands of times the diameter of the soma and, in general, conducts electrical impulses away from the cell body of the neuron.

The function of the axon is to transmit information to different neurons, muscles and glands. In certain sensory neurons (pseudounipolar neurons), such as touch and heat, the electrical impulse travels along an axon from the periphery to the cell body and from the cell body to the spinal cord along another branch of the same axon. No neuron has more than one axon. The axon is specialized for the conduction of the particular electrical impulses known as action potentials.

Dendrites

The dendrites are branches that leave the cell body or soma. They contain dendritic spines, which are small bumps. They are cellular extensions with many branches and are specialized to receive chemical signals from the axon terminals of other neurons.

Its main function is to receive information from other neurons. The dendrites convert these signals into small electrical impulses and transmit them to the soma. Electrical stimulation is transmitted to dendrites by neurons through synapses, which are found at various points along the dendritic tree. Dendrites play a critical role in the integration of these synaptic inputs and in determining the degree to which the neuron produces action potentials.

Dendrites differ from axons by several characteristics, such as shape (dendrites tend to narrow while axons tend to maintain a constant radius), length (dendrites are restricted to a small region around the cell body while axons can be much longer) and generally receive signals while axons generally transmit them.

Of course, all these rules have exceptions. For example, although the axon is usually involved in the output of information, this region may also receive information from other neurons. The output of information from dendrites to other neurons can also happen. And axons can be very short (and even absent) in some types of neurons. In the case of neurons without axons, dendrites can fulfill the function of transmitting signals from the cell body.

Organelles and cytoplasmic particles

Neurons, like any cell in our body, have a cytoplasmic membrane that separates them from the outside and allows them to maintain an orderly relationship with their environment. The membrane allows the neuron to retain inside (the cytoplasm) liquids (mainly water), dissolved substances and various organelles responsible for different functions.

The cytoplasmic organelles found in neurons are the same as in other cells, although their distribution is different in the soma, dendrites and axon. Throughout the neuron we can find mitochondria, smooth endoplasmic reticulum and lysosomes. In addition, in the soma and dendrites, we also find ribosomes and rough endoplasmic reticulum. Other organelles, such as the Golgi apparatus and the Nissl substance, are only found in the soma.

Along with these organelles, neurons also have a "skeleton," the cytoskeleton, with two main functions, which are as follows:

  • Structural: gives rigidity and shape to the neuron.
  • Transport: Participates in the transport of substances and vesicles along the dendrites and especially the axon.

The cytoskeleton of neurons is made up of protein filaments: microtubules, microfilaments and intermediate neurofilaments or filaments.

The transport of substances along the axon can be done in two directions: antegrade or retrograde.

  • Transport in the antegrade direction involves the movement of particles from the soma to the terminal buttons.
  • Retrograde transport involves the movement of particles from the axon terminal to the soma.

There are two types of axons: axons or myelinated fibers and axons or unmyelinated fibers.

Myelinated Axons

Myelinic axons are coated by a fatty substance called myelin. Myelin is formed mainly of lipids. Being the insulating lipids, myelin does not conduct the electric current. This myelin sheath is called myelin sheath. The myelin sheath is not continuous, it has several interruptions.

The areas of the axon that are not surrounded by myelin are called Ranvier nodules and are the only areas that are not isolated and where the axon is exposed to the extracellular environment. The area surrounded by myelin, between nodule and nodule, is called the internodule.

In the central nervous system (CNS) the myelin sheath is formed by oligodendrocytes. In the peripheral nervous system (SNP) the myelin sheath is formed by Shawnna cells.

Unmyelinated axons

They are partially coated with myelin. A single glia cell, Schwann or oligodendrocyte, half surrounds different axons of different neurons, so part of the axon is coated and part is not.

References

Carpenter, M.B. (1994). Neuroanatomy Fundamentals Buenos Aires: Panamerican Editorial.

Delgado, J.M .; Ferrús, A .; Mora, F .; Blonde, F.J. (eds) (1998). Neuroscience Manual. Madrid: Synthesis.

Diamond, M.C .; Scheibel, A.B. i Elson, L.M. (nineteen ninety six). The human brain Work book. Barcelona: Ariel.

Guyton, A.C. (1994) Anatomy and physiology of the nervous system. Basic Neuroscience Madrid: Pan American Medical Editorial.

Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.

Martin, J.H. (1998) Neuroanatomy. Madrid: Prentice Hall.

Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.

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