Organization and Structural Classification OF Neuron PDF

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A neuron is made up of three parts: the nerve cell body, the dendrite, and the axon (Fig 2). The cell body, also known as the perikaryon or soma, is made up of a bulk of cytoplasm called neuroplasm. A big nucleus, neurofibrils, nissl bodies, mitochondria, and the Golgi apparatus are all found in neu...

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ORGANIZATION AND STRUCTURAL CLASSIFICATION OF NEURON The brain, the most complicated organ of the body, communicates with all other body parts via the nervous system, which includes both the central nervous system (CNS) and the peripheral nervous system (PNS) (PNS). The central nervous system is made up of the and spinal cord, with the PNS connecting the CNS to various effector organs of the body. It is separated into two subdivisions: somatic nervous system (SNS) and autonomic nervous system (ANS) (ANS). In addition, new studies have highlighted the importance of the enteric nervous system (ENS), also known as the intrinsic nervous system, as one of the nervous system's primary divisions. It contains a mesh-like system of neurons that mostly controls the gastrointestinal tract's function. The ENS contains two plexuses, the submucosal and the myenteric, each with its own set of functions. Nerve cells, also known as neurons, and supporting cells, known as glial cells, are the basic building blocks of the entire nervous system. Nearly 100 billion electrically excitable neurons and ten times as many glial cells make up the human brain (Herculano-Houzel S. 2009). Neurons are specialised for long-distance electrical and chemical signaling. Glial cells, unlike nerve cells, are incapable of transmitting electrical signals, making the neuron the nervous system's only functional unit. According to current research, supportive glial cells play a critical role in neuronal function and developmental processes.

Neuron as the Basic Unit of Nervous System A neuron is made up of three parts: the nerve cell body, the dendrite, and the axon (Fig 2). The cell body, also known as the perikaryon or soma, is made up of a bulk of cytoplasm called neuroplasm. A big nucleus, neurofibrils, nissl bodies, mitochondria, and the Golgi apparatus are all found in neuroplasm. Only neurons have neurofibrils and nissl bodies, which are not present in other cells. The lack of a centrosome in the mature neuron prevents it from dividing. After birth, only a few nerve cells are created. Although new research suggests that several transcription factors, such as A to h7, have important roles in many aspects of early neurogenesis in the vertebrate retina (Jusuf et.al. 2012). The head of the neuronal cell is called the soma. It has a nucleus and cytoplasm, commonly known as neuroplasm. The mitochondria, Golgi apparatus, endoplasmic reticulum, secretory granules, ribosomes, and polysomes are among the organelles found there. Is the watery and salty fluid inside the cell with a potassium-rich solution containing enzymes important for the cell's metabolism. The nucleus aids in the formation of RNA, which can then be translated into proteins. Dendrites are extensions from the soma that assist in the reception of messages from other neurons. It has numerous receptors on its membrane that can interact with other receptors. Rough endoplasmic reticulum and ribosomes generate Nissl bodies, which are granular entities seen in neurons. Protein synthesis takes place here. Neuro fibrils are cytoskeleton elements such

as intermediate filaments, microtubules, and microfilaments that assist the neuron maintain its shape and transport neurotransmitters. Axons are long or short projections from the soma that convey messages away from the neuron's head. Myelinated or non-myelinated axons are found in the brain. The myelin sheath is made up of concentric layers of oligodendrocytes in the CNS and Schwann cells (discovered by Theodor Schwann) in the PNS. Glycolipids abound in them. They contribute to the rapid conductance of electric activity. The nerve fibers outermost layer is formed by it. The neurolemma is the outermost layer of the peripheral nervous system. It protects peripheral nerve fibers from damage during regeneration. The neurolemma forms a regeneration tube through which the growing axon reestablishes its original connection after a nerve lesion. The spaces between these coverings are known as Nodes of Ranvier. The axon hillock is where the axon connects to the soma. Synapses are the points of interaction between two neurons. There are two types of synapses: electrical and chemical. Electrical synapses use ions to transmit electrical activity from one cell to another. In a chemical synapse, on the other hand, the message is carried on by the transmission of neurotransmitters released by the pre-synaptic membrane and acting on the postsynaptic membrane. They have the ability to either stimulate or inhibit activity.

Structural classification of neuron: Neuronal cells have an astounding variety of forms and sizes, which is responsible for functional differentiation among the cells. Dendrites, which are similar to antennas that pick-up signals from other neurons, branch from the cell bodies of neurons. The Axon is a lengthy projection of a neuron that transmits electrical impulses away from the cell body. The quantity of dendrites present on the cell body determines the structural classification of neurons. Unipolar neuron: Unipolar neurons have only one axon and no Dendron. Unipolar neurons are found in the brains of insects, where the neuron's cell body is placed in the peripheral part of the brain and is dormant. Another is the pseudo unipolar form, which has two branches on the axon. Pseudo unipolar neurons are bipolar neurons that have two processes that fuse into one short common axon during development. Pseudo unipolar neurons are primary sensory neurons found in the dorsal root ganglions of spinal nerves and the semilunar ganglions of the trigeminal nerves. Bipolar neurons are made up of a single axon and a single dendrite. Bipolar cells primarily function as sensory neurons, transmitting information from the senses of smell, taste, and hearing. Multipolar neuron: A multipolar neuron is a type of neuron that has several dendrites. Interneurons and motor neurons are among the most prevalent types of neurons found in the CNS. Pyramidal neurons are a type of multipolar neuron found in the hippocampus, cerebral cortex, and amygdala, among other places in the brain. The neuron's key structural traits include a conic-shaped cell body, after which it is named. The corticospinal tract contains the majority of pyramidal neurons.

The mechanism that leads to the formation of these four distinct morphological characteristics is unknown. Although it is considered that several transcription factors are important in this regard. According to recent Drosophila study, the transcription factor Dar1 is responsible for the shape of multipolar neurons. (Wang and colleagues, 2015). When Dar1 is present, multipolar neurons develop, however when Dar1 is absent, multipolar neurons gradually become bipolar or unipolar.

Functional classification of neuron: Sensory neurons, also known as afferent neurons, are nervous system cells that transform external stimuli from the environment into internal electrical impulses that serve as sensory information (e.g. the dorsal root ganglion cell). A peripheral sensory neuron (a first-order sensory neuron) generates an electrical impulse in response to a sensory input that travels down the nerve fibre to the central nervous system. An external stimulation in sensory neurons can change the permeability of cation channels in nerve terminals, resulting in a depolarizing current (receptor potential). The sensory neuron generates an action potential if the receptor potential is large enough. The conduction speed of an action potential is determined by the axon diameter of a sensory neuron. A second- or third-order sensory neuron, as well as a motor neuron, can be activated by a first-order sensory neuron. Sensory information from the "special senses" of touch, smell, hearing, taste, and sight is carried to the CNS through the sensory division. It also carries pain perception, bodily position (proprioception), and a range of other visceral sensory data. Neurotrophins are a family of polypeptide growth factors that play a vital role in sensory neuron development. Parasympathetic, sympathetic, proprioceptive, enteroceptive, and cutaneous sensory neurons all benefit from GDNF.

Efferent neuron vs. motor neuron: The nerve cell along which electrical impulses move from the brain or spinal cord to a gland, muscle, or any other target location is known as a motor neuron (or motoneuron) (for example neurons in the autonomic nervous system). Somatic motor neurons (which deliver their axon to skeletal muscles), specific visceral motor neurons (which innervate branchial muscles), and general visceral motor neurons are the three major types of motor neurons (that innervate cardiac muscle and smooth muscles). Motor neurons are divided into two groups based on their position: upper motor neurons and lower motor neurons. Upper motor neuron electrical impulses flow from the cerebral cortex to the spinal cord. The nerve impulse is then carried from the spinal cord to the neuromuscular junction by the lower motor neuron. In motor neuron signalling, acetylcholine is a key neurotransmitter.

Interneurons are the neurons that connect different neurons in the brain and spinal cord, and they play a crucial role in neural circuits. Neuronal oscillations and adult brain neurogenesis are both reliant on interneurons in the circuit. GABAergic interneurons with axons located near the subgranular zone (SGZ) neurogenic niche may have a functional impact on adult brain neurogenesis.

Classification based on neurotransmitter secretion: A specialised structure known as a synapse in the nervous system transmits an electrical or chemical communication from one neuron to another. Endogenous chemical messengers called neurotransmitters enable neurotransmission by delivering messages across a chemical synapse. Synaptic vesicles in presynaptic neurons store and release neurotransmitters, which are then secreted in the synaptic cleft. Otto Loewi, a German pharmacologist, discovered acetylcholine (ACh) in 1921, making it the first recognised neurotransmitter. After interacting with the postsynaptic ligand binding ion channel receptors, neurotransmitters operate immediately. Acetylcholine, glutamate, gamma-aminobutyric acid, dopamine, serotonin, norepinephrine, and epinephrine are all significant neurotransmitters. Cholinergic neuron, Glutamatergic neuron, GABAergic neuron, Dopaminergic neuron, Serotonergic neuron, Noradrenergic/Norepinephrinergic neuron, and Adrenergic/epinephrinergic neuron are the different types of neurons.

Cholinergic neuron: A cholinergic neuron is a nerve cell that sends messages mostly through its own neurotransmitter, acetylcholine (ACh). During both waking and rapid eye movement sleep, cholinergic neurons play a critical role in CNS functioning. Cholinergic neurons are also involved in memory and cognitive functioning in the brain. During the early stages of Alzheimer's disease, there is malfunction as well as loss of basal forebrain cholinergic neurons (AD). Cholinergic neurons are also significant in normal ageing, since there are beadlike swellings and axon thickening within the cholinergic fibres, which typically form clusters (Nyakas et al. 2011). The cholinergic innervations of the suprachiasmatic nucleus (SCN) of hypothalamus are vital in control of mammalian circadian system. Glutamatergic neuron: A neuron that employs glutamate as a neurotransmitter is known as a glutamatergic neuron. It is a crucial excitatory neuron in the central nervous system. Memory and learning are aided by glutamatergic neurons. AMPA receptors, N-methyl-D-aspartate receptor (NMDA) receptors, and metabotropic glutamate receptors are the three types of glutamate receptors that glutamate binds to. In some abnormal circumstances, glutamate can act as a neurotoxic, impairing cognition. GABAergic neuron: The main inhibitory neuron in the mammalian central nervous system is the GABAergic neuron, which secretes gamma-Aminobutyric acid (also known as GABA). It reduces neuronal excitability and so protects the nervous system as a whole. The main function of GABAergic neurons is to regulate muscular tone (Watanabeet al. 2002). Purkinje cells, or Purkinje neurons, are a type of GABAergic neuron found in the cerebellum region of the brain. Simple spikes and complicated spikes are two types of electrophysiological properties seen in Purkinje neurons.

Dopaminergic neuron: Dopaminergic neurons are small neurons that secrete dopamine. However, these neurons are involved in motor control, motivation, and arousal. Lower-level activities such as nursing and sexual satisfaction are also influenced by these neurons. Serotonergic neuron: A neuron that utilises serotonin as its neurotransmitter is known as a serotonergic neuron. Serotonergic neurons, which are the primary source of serotonin release in the brain, are found in the Raphe nuclei. According to recent studies, a low level of serotonin can cause severe depression, hence it plays a function in mood and mental health maintenance. Adrenergic neuron: Adrenergic neurons are sympathetic nervous system secondary neurons. Adrenergic nerve fibres control the sweat glands in the dermal layer of the skin and cause an increase in heart rate and a decrease in digestion. Noradrenergic neurons, which make up the postganglionic neurons of the sympathetic nervous system, are another significant type of neuron. Noradrenergic neurons are involved in the regulation of bodily fluid metabolism (Antunes-Rodrigues, et al. 2004). Many current medications exert their effects without interacting with the brain or body's epinephrine systems.

Classification of neurons based on Electrophysiological property (Modern idea about classification): Tonic firing type: The majority of neurons tested exhibit spontaneous electrical activity. Tonic or regular spiking neurons are those that are always firing. Tonic firing, which occurs without presynaptic input, is particularly common in sensory and cerebral neurons. It can be thought of as a kind of background activity. Na+ and K+ channels may play a key part in this type of action. Increased tonic firing may also be induced by very high catecholamine levels. The phasic or bursting kind of neuron is another essential form of neuron that fires in bursts. The NMDA receptor facilitates the burst of electrical spikes that are mainly seen in dopaminergic neurons. The size of fire bursts is reduced when the NMDA receptor is removed from dopaminergic neurons. Fast-firing type: Some neurons, such as those in the vestibular and auditory circuits, as well as some cortical and basal ganglia neurons, have a fast firing rate. The firing rate of such a neuron is 30–100 Hz at rest. These neurons have been found to have a high level of sodium channels, according to research.

Neuronal Stem Cells (NSCs): The adult brain contains one type of stem cell, whereas the embryo contains the other. Embryonic stem cells are pluripotent, meaning they have the ability to differentiate into any cell type. Adult brain cells are not pluripotent, meaning they can only become neurons or glial cells. They are assumed to be the cause of memory and learning plasticity since they are discovered to

replace missing or injured neurons and glial cells. Plasticity is defined as an increase in the number and interconnections between cells in response to a practise or condition. Neurons in the adult brain mostly come from proliferating neural progenitors in the lateral ventricles' subventricular zone and the hippocampus's subgranular zone....