Neuron : Structure and its types

 

What is Neurons :   

A neuron, also known as a neurons and nerve cell, is an electrically excitable cell that receives, processes, and transmits information through electrical and chemical signals. These signals between neurons occur via specialized connections called synapses. Neurons can connect to each other to form neural networks. Neurons are the primary components of the central nervous system, which includes the brain and spinal cord, and of the peripheral nervous system, which comprises the autonomic nervous system and the somatic nervous system. 

 Structure Of Neurons: 

1. Cell body
2.  Dendrite
3.   Axon 
4.  Cell membrane
5.   Nodes of Ravine
6.   Glial cells 
7.  Myelin sheath
8.   Axon terminals
9.   Synaptic vesicles 
10.  Synapse
11.   Neurotransmitter

  1. Cell Body:

                        The (soma) is the factory of the neuron. It produces all the proteins for the dendrites, axons and synaptic terminals and contains specialized organelles such as the mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes and polysomes to provide energy and make the parts, as well as a production line to assemble the parts into completed products.

 In Cell body some Part are in consist: 

a) Cytosol

b) Nucleus

c) Golgi Apparatus

d)  Polymerizes

e)   Mitochondria 

 a) Cytosol:  

              Is the watery and salty fluid with a potassium-rich solution inside the cell containing enzymes responsible for the metabolism of the cell. 

b) Nucleus: 

                   

 Derived from the Latin word for "nux", nut, the nucleus is the archivist and the architect of the cell. As archivist it contains the genes, consisting of DNA which contains the cell history, the basic information to manufacture all the proteins characteristic of that cell. As architect, it synthesizes RNA from DNA and ships it through its pores to the cytoplasm for use in protein synthesis. 

c) Golgi Apparatus:

                             A membrane-bound structure that plays a role in packaging peptides and proteins (including neurotransmitters) into vesicles.

d)  Polymerizes: 

                       There are several free ribosomes attached by a thread. The thread is a single strand of mRNA (messenger RNA, a molecule involved in the synthesis of proteins outside the nucleus). The associated ribosomes work on it to make multiple copies of the same protein.

e) Mitochondria : 

                            This is the part of the cell responsible for the supply of energy in the form of ATP (adenosine triphosphate). Neurons need an enormous amount of energy. The brain is one of the most metabolically active tissues in the body. In man, for example, the brain uses 40 ml of oxygen per minute. Mitochondria use oxygen and glucose to produce most of the cell's energy. The brain consumes large amounts of ATP. The chemical energy stored in ATP is used to fuel most of the biochemical reactions of the neuron. For example, special proteins in the neuronal membrane use the energy released by the breakdown of ATP into ADP to pump certain substances across the membrane to establish concentration differences between the inside of the neuron and the outside. 

Dendrite : 

            These structures branch out in treelike fashion and serve as the main apparatus for receiving signals from other nerve cells. They function as an "antennae" of the neuron and are covered by thousands of synapses. The dendritic membrane under the synapse (the post-synaptic membrane) has many specialized protein molecules called receptors that detect the neurotransmitters in the synaptic cleft. A nerve cell can have many dendrites which branch many times, their surface is irregular and covered in dendritic spines which are where the synaptic input connections are made. 

Axon: 

          Usually a long process which often projects to distant regions of the nervous system. The axon is the main conducting unit of the neuron, capable of conveying electrical signals along distances that range from as short as 0.1 mm to as long as 2 m. Many axon split into several branches, thereby conveying information to different targets. Many neurons do not have axons. In these so-called amacrine neurons, all the neuronal processes are dendrites. Neurons with very short axons are also found. 

Cell Membrane : 

     The plasma membrane of the neuron is semipermeable, being highly permeable to K+ and slightly permeable to Cl− and Na+. In the extracellular fluid, electroneutrality is preserved by a balance between a high concentration of Na+on the one hand and a high concentration of Cl−, as well as small quantities of impermeant anions such as bicarbonate, phosphate, and sulfate, on the other. In the cytoplasm, where K+ concentration is high, the concentration of Cl− is much below that necessary to balance the sum of the positive charges. Electroneutrality is maintained there by negatively charged impermeant proteins and phosphates. Osmotic balance is maintained between the extracellular fluid and the cytoplasm by movement of water through the plasma membrane when the total concentration of particles on one side is not equal to that on the other.

 Nodes Of Ranvier : 

                             The Node of Ranvier is the 1-2 micrometer gap between the glial cells of the myelin sheath. These glial cells are called Schwann cells, and they help to electrically insulate the neuron. The Nodes of Ranvier are only present when the axon of a neuron is myelinated. Myelination allows for an increased rate of action potential transmission due to action potentials "jumping" between Node of Ranvier, this is called saltatory conduction.

The movement of sodium ions to depolarize the membrane can only occur at the Node of Ranvier, as the sodium voltage-gated channels are found only at the nodes of Ranvier. The Schwann cells of the myelin sheath block the movement of sodium ions elsewhere along the axon.

Glial Cells : 

                   Neuroglia, also called glial cells, or simply glia are non-neuronal cells that maintain homeostasis, form myelin, and provide support and protection for neurons in the central and peripheral nervous systems.

 As the Greek name implies, glia are commonly known as the glue of the nervous system; however, this is not fully accurate. Neuroscience currently identifies four main functions of glial cells:

1. To surround neurons and hold them in place
2. To supply nutrients and oxygen to neurons
3. To insulate one neuron from another
4. To destroy pathogens and remove dead neurons.

   

 Myelin : 

             Myelin is a fatty white substance that surrounds the axon of some nerve cells, forming an electrically insulating layer. It is essential for the proper functioning of the nervous system. It is an outgrowth of a type of glial cell.

The production of the myelin sheath is called myelination or myelinogenesis. In humans, myelination begins early in the 3rd trimester,  although little myelin exists in the brain at the time of birth. During infancy, myelination occurs quickly, leading to a child's fast development, including crawling and walking in the first year. Myelination continues through the adolescent stage of life.

Axon terminal : 

                        Axon terminals (also called synaptic boutons or terminal boutons) are distal terminations of the telodendria (branches) of an axon. An axon, also called a nerve fiber, is a long, slender projection of a nerve cell, or neuron, that conducts electrical impulses called action potentials away from the neuron's cell body, or soma, in order to transmit those impulses to other neurons, muscle cells or glands.

Neurons are interconnected in complex arrangements, and use electrochemical signals and neurotransmitter chemicals to transmit impulses from one neuron to the next; axon terminals are separated from neighboring neurons by a small gap called a synapse, across which impulses are sent. The axon terminal, and the neuron from which it comes, is sometimes referred to as the "presynaptic" neuron.

Synaptic Vesicle:

                                  In a neuron, synaptic vesicles (or neurotransmitter vesicles) store various neurotransmitters that are released at the synapse. The release is regulated by a voltage-dependent calcium channel. Vesicles are essential for propagating nerve impulses between neurons and are constantly recreated by the cell. The area in the axon that holds groups of vesicles is an axon terminal or "terminal bouton". Up to 130 vesicles can be released per bouton over a ten-minute period of stimulation at 0.2 Hz. In the visual cortex of the human brain, synaptic vesicles have an average diameter of 39.5 nanometers (nm) with a standard deviation of 5.1 nm.

Synapse : 

          In the nervous system, a synapse is a structure that permits a neuron (or nerve cell) to pass an electrical or chemical signal to another neuron or to the target efferent cell. Santiago Ramón y Cajal proposed that neurons are not continuous throughout the body, yet still communicate with each other, an idea known as the neuron doctrine. The word "synapse" – from the Greek synapses, meaning "conjunction". 

Neurotransmitter :

                             Neurotransmitters, also known as chemical messengers, are endogenous chemicals that enable neurotransmitter. They transmit signals across a chemical synapse, such as a neuromuscular junction, from one neuron (nerve cell) to another "target" neuron, muscle cell, or gland cell.  Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by neurotransmitter receptors on the target cells. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available from the diet and only require a small number of biosynthetic steps for conversion. Neurotransmitters play a major role in shaping everyday life and functions. Their exact numbers are unknown, but more than 100 chemical messengers have been uniquely identified.