Nervous System

Neuron and Neuroglia: Definition, types and their functions

Biology of Neuron and Neuroglia

The objective of this post

To know how the neurons communicate with each other and what are the effects of drugs and injuries on them.

What is a Neuron?

A neuron is an excitable nerve cell with their process so-called axons and dendrites. These axons and dendrites are specialized in the conduction of information to the cell body and from the cell body of the neuron respectively.  In spite of having similar basic structures, these neurons are different in terms of shape and size. Axon and dendrites are collectively known as nerve fiber whose primary function is conduction of nerve impulse.

 

 

Different types of Neurons

Based on the morphology, there are different types of neurons classified mainly according to the mode of branching and their length. The different types of neurons are as follows:

  1. Unipolar Neuron – In the unipolar neuron, the cell body has only one neutrality. This neutral has two further branches, one of the branch going to the central nervous system and the other to the peripheral nervous system. They have characteristics similar to that of the axon and each of the terminal ends has a fine structure called dendrites that carry information to the peripheral nervous system.
  2. Bipolar Neuron – It has one neurite at each end of the cell body that’s two in total which further justifies the name bipolar. Bipolar Neurons are found in retinal cells, sensory cochlear cell and that of vestibular ganglia.
  3. Multipolar Neuron – Multipolar neurons have a large number of neurites branching out from the cell body. Most of the branches except the long axons are referred to be dendrites. Examples of such neurons are the neurons of the brain and spinal cord.

Structure of the Neuron and their cell organelles

Structurally, the neuron has centrally located cell body with a long branch so-called axon going towards it and a number of fine structures called dendrites going outside of the cell body that carries information to the peripheral nervous system from the cell body. The cell body of a neuron is composed of a mass of cytoplasm with nucleus embedded in it. The cell body has an external covering called plasma membrane which provides semi-permeability and protection from the external threats. Here is an interesting fact to be noted that the total volume of cytoplasm in the cell body is much less than that of the cytoplasm in the nutrite

Cell organelles of Nerve cell Body

  • Nucleus – A centrally located large, round cell organelle with pale appearance and widely scattered chromatin throughout the nucleoplasm. It has a single prominent nucleus. Its main function is to control the entire cell activity and that’ s must be the reason why it is often called as master of the cell body.
  • Mitochondria – A double membrane, spherical, rod-shaped structure found scattered in the cytoplasm. It functions in the production of energy required for the normal growth and functioning of the cell. It is also called the powerhouse of cells because it actively participates in power generation.
  • Microfilament – It forms a dense network beneath the plasma membrane. It helps in the formation and retraction of the cell process and in cell transportation.
  • Lysosome – It is a vesicle shaped structure found throughout the cell that acts as a cleaning agent of a cell. It digests and destroys the debris of the cell. It is often called a suicidal bag of a cell as it helps in killing and destroying many of the foreign, unwanted agents in the cell.

 

  • Golgi Apparatus – AN organelle appearing like smooth endoplasmic reticulum with the wavy threadlike structure that helps mainly in the packaging and transportation of the molecules like carbohydrates and proteins.

 

  • Nissl Substances- These are the granules of the rough endoplasmic reticulum that helps in the production of protein. Nissl substances are not present in the axon hillock ( Axon hillock is a region close to axon which appears to be swollen).

 

How does the excitation of the plasma membrane of the Nerve Cell Body occur?

Whenever the nerve cell is stimulated by any medium ( Let it be chemical, mechanical or electrical), there occurs rapid membrane permeability to the sodium ions ( Na+) leading to its diffusion through the plasma membrane to the cytoplasm of the cell. Due to this sudden diffusion of sodium ions, the membrane becomes depolarized. Again, there occurs an influx of sodium ions (Na+)) followed by altered polarity and this produces an action potential of +40Mv that lasts for about 5 milliseconds.

After this short period of 5milliseconds, the membrane permeability for sodium ions ceases and the membrane becomes permeable to potassium ions (K+). Thus potassium ions start to flow from the cell to the cytoplasm.

Once the action potential is produced, it spreads all over the cell membrane. However, the next elicitation of action potential does not take place immediately and there is a duration of non–excitation which is called refractory period. The strength of the initial stimulation determines the extent of the area in the surrounding to which it will spread.

 

Neuroglial Cells

Neuroglial cells are non-excitable cells of the nervous system whose function is to support the central nervous system. There are four types of neuroglial cells. They are:

  1. Astrocytes
  2. Oligodendrocytes
  3. Microglia
  4. Ependyma

Let us discuss each of them in detail

  1. Astrocyte: Astrocytes are of two types. They are:
  2. Fibrous Astrocytes: It has a small cell body with the long slender process, cytoplasmic filaments, and perivascular feet. It is found in white matter.

The functions of fibrous astrocytes are as follows:

  • Provides supporting framework
  • Limits spread of neurotransmitter
  • Acts as an electrical insulator

 

 

  1. Protoplasmic Astrocytes: It has a small cell body, short thick process, many branches, few cytoplasmic filaments, and perivascular feet. It is found in gray matter.

The functions of protoplasmic astrocytes are as follow:

  • Stores glycogen
  • Have a phagocytic function
  • Takes place of dead neurons

 

  1. Oligodendrocytes: These are the microglial cells found in rows along myelinated nerves surrounding the neuron cell body in the central nervous system. They have small cell bodies, few delicate processes, and no cytoplasmic filaments.

Its function is to form the myelin sheath in the central nervous system.

  1. Microglial Cells: These are the smallest among the neuroglial cells with wavy branches and spines, scattered throughout the central nervous system. These are usually inactive in the normal central nervous system and start to grow their number and influence only if there are any diseases or need to phagocyte any debris or foreign substances.
  2. Ependyma: Ependyma is those microglial cells that line the ventricles of the brain and the central canal of the spinal cord. Ependyma are classified into three groups they are as follows:
  3. Ependymocytes: These are the types of ependyma that are found in the ventricles of the brain and the central canal of the spinal cord and are in direct contact of the cerebrospinal fluid.
  4. Tanycytes: They line the floor of the third ventricles and functions in transporting substances from cerebrospinal fluid to the hypophyseal – portal system.
  5. Choroidal Epithelial Cells: This group of ependymal cover the surface of the choroid plexuses and helps in the production of cerebrospinal fluid. Their location and functions fully justify its name. Therefore it would be easy to relate for easy memorization.