The Structural Classification of Neurons - Essay Example

STRUCTURAL CLASSIFICATION OF NEURONS Name of Student: Student ID: Lecturer: Course: Assignment: Date of Submission: Structural Classification of neurons. Introduction. A neuron is a special kind of a cell that is most in the bodies of majority of animals and in fact all the members of Eurmetazoa group have neurons. Just sponges and a small number of simpler organisms do not have the neuron. The compulsory characteristics to define a neuron are the presence of a synapse and essence of electrical excitability that are sophisticated junctions of membranes that transport impulses from one cell to the other. The glial cells and the neuron body provide them with the metabolic and structural support and all these processes form a nervous system. A larger portion of neurons are in the Central nervous system in vertebrates though some can be found existing in the peripheral ganglia. Most of the sensory nerve cells are located within the sensory organs like the cochlea and the retina. Neurons are highly diverse and there are limitations to almost of the rules, it’s only appropriate to start with a schematic illustration of the structure of a typical neuron. The billions of neurons existing in the nervous system have various forms; therefore can be categorized by their structures or functions. The key component of the nervous system is the neuron. The neuron is also known as the nerve cells and it forms the pathway or channels through which information is transmitted and processed from the nervous system. The nerve cells or the neuron come in a lot different sizes, shapes and many types depending on the role they play. A number of them are quite small in size. In fact they are so tiny they can only be visible under the help of an electron microscope in order to observe them. Some of these nerve cells are long in structure. In some cases their length can go beyond one meter stretch. Regards less of their sizes, shapes and varieties, they normally have a common form. In general, neurons usually have the following structures. a) Dendrites b) The cell body or the soma c) The axon d) The axon terminals or the terminal buttons. Usually the cell body is compact in shape and appearance, the dendrites and the axon are filaments that protrude from it. A typical dendrite has profusion branches getting gradually slender with every branching and can extend their branches a couple of hundred micrometers from the cell body. The axon hillock is the site where the axon leaves the cell body and it forms a swelling and this can go to several further distances generating many branches. The axon in most cases maintains its diameter as it grows longer unlike the dendrites. The cell body produces many dendrites but not more than one axon. The synaptic impulses coming from other neurons are normally received by the dendrites and the cell body while impulses to other nerve cells are transported through the axon. A normal synapse will then be found at the contact between the a dendrite of neuron and the axon of another neuron.. to more than one axon. Synaptic signals from other neurons are received by the soma and dendrites; signals to other neurons are transmitted by the axon., Discussion Structural classification of Neurons. Neurons are categorized as bipolar, anaxonic,or multipolar or unipolar with respect to the relationship of the cell body to the dendrites and the axon. a) Anaxonic neurons. The anaxomic nerve cells are really small. They do not have any anatomical features to distinguish axons from the dendrites. And the whole cell process looks similar. Anaxonic neurons are found in the special sense organs and in the brain. The functions are not been well understood and its still a subject on research. b) The Unipolar Neurones. They have two distinct processes. The first one has a dendrite process that has numerous braches at the distal tip with a single axon, and the cell body is positioned right in the middle between the them. The Bipolar neurons are not many and are most found in the special sense organs. They key function is to transmit information about smell, sight and hearing from the receptor cells to other nerve cells. They are the smallest among the multipolar neurons and the unipolar neurons. From end to end measurement the longest in this group will be about 30 mm. C) The unipolar Neurons.(Pseudounipolar neurons) Here the axons and the dendrite are continuous, technically fused and the cell body is lying off on one side. The first segment is at the site where the dendrites converge. And their axon is found in the rest of the process that basically carries the action potential. The unipolar neurons are mostly found as sensory neurons at the peripheral nervous systems.Unipolar neurons may be one or more meters long and terminate at the synapse in the central nervous system. Multipolar Neurons. They have one axon with two or more dendrite processes. Multipolar nerve cells are the most common type of neurons in the central nervous system. A good example is, all the motor nerve cells that regulate the skeleton muscles are multipolar nerve cells. The length of their axons is almost the same as those of uinipolar nerve cells. Cell Body or Soma: The protruded head of a nerve cell is known as the cell body or the soma if you like. The cell body or the soma is housed by the cell membranes and the nucleus of the nerve cell is located there. Soma is the front part of the neuron. It utilizes nutrients and oxygen to manufacture energy to perform the role of the neuron. The shape of the cell body varies significantly in different types of neurons.. Dendrites: The word dendrite was borrowed from Greek word Dendron meaning tree and was because the nerve cells network resembles that of a tree. The cell body extension arms are dendrites. The cell body has these numerous extensions of dendrites. The neural impulses from the many adjoining nerve cells and the receptors are received by the dendrites, and this is very strict no any other section of the nerve cell receives impulses apart from the dendrites. Axon: The cell body has two types of extensions. The numerous shorter extensions from the soma are known as the dendrites. The axon is a long one branched extension from the cell body. The soma has only a single axon which looks like a trunk extending from the cell body. Axon hillock is the part of the axon next to the cell body. Axons are very long but thin. Axons could also have some branches just like tree-trunks, and these branches are referred to as axon collaterals. All axons of all neurons have double coverings, 1. The memnbrane- It is the outer layer of the neuronis . The membrane has a protective role to act as a barrier to the neuron. Certain axons of some neurons have myelin sheath which is normally a white fatty sheath. The axons that have myelin sheaths within their bodies are known as myelinated axons and the ones lacking the myeilin sheaths are called unmyelinated axons. The myelin sheaths do not exist in any other part of the nerve cell apart from the axons and that where they are associated with their roles. Mylein sheaths are composed of an array of proteins and fats of specialized cells. The thickness of the myelin sheath is not uniform across the axon. Its role is to insulate the impulse while being transmitted via the axon. And also act to facilitate the velocity at which the electric message is transmitted along the axons. Though not all axons are exciting in the central nerves system are not myelinated, the process of not covering completely the unmyelinated axons is neuroglia. These axons are rampant in locations where collaterals and short axons create synapse with heavily packed neuron cell bodies. These sites normally have a grey dusky color basically due Nissl bodies’ high concentrations within this spot and generally compose the gray matter of the central nervous system. And between the myelinated portions, are tiny gaps known as the nodes of ranvier or (nodes). The myelinated axons when dissected, they look glossy white, majorly because of the presence of lipids in them, and the parts serious occupied with the myelinated axons compose the white matter of the central nervous system. Demyelination. It’s the process through which myelin sheath within the central nervous system are progressively destroyed. The outcome is loss of the motor regulation and sensation that makes the affected parts paralyzed and become numb. Most of the unrelated situations that can end up in the myelin destruction can also cause some symptoms of demyelination. Most common significant demyelination disorders are diphtheria , extensive metal poisoning and also multiple sclerosis. The .second covering of the axon is called the Neurilemma. Neurilemma. It’s a covering existing in axons. The neuilemma are mostly found within the axons of the nerves located mainly outside the spinal cord and the brain as well. Neuralemma is responsible for regeneration and it’s an extremely thin covering. This indicates that if a nerve cell not within the spinal cord and the brain suffers a damage, neurilemma can regenerate them. This is because the axons of the neurons located within the spinal cord and the brain to not have the neurialemma. They are highly sensitive and specialized cells, and in case they are damaged, they are dead completely. Terminal Buttons: Axons terminate in tiny branching model known as the terminal buttons or axonterminals. They are enlarged bulb-like models positioned at the end of the axon to enable stimulations transmissions pass to adjacent muscles, glands and other nerve cells. There are also synaptic vessels which appear bag-like structures within the tips of the terminals of the axons. They perform the function of conveying the information to another nerve cell. Therefore the delivering part of a neuron is the terminal end of the axon, hence axon terminal deliver while dendrite receives. The transmission of information in neurons is systematic and is unidirectional; starts from the dendrites, a long the soma, moving to the axon until the terminal buttons. This information transmission along the neurons is referred to as forward induction. Conclussion NEUROGLIA OF THE PERIPHERAL NERVOUS SYSTEM The soma in the Peripheral nervous system form masses known ganglia. (ganglion). Neurons and their axons in PNS are entirely insulated from the periphery by the process called neuroglia. The satellite cells and Schwann cells are the two neurons involved in the process of neuroglia. Satellite cells (Amphicytes) The encompass the cell bodies in ganglia. And most they control the environment of the nerve cells just like astrocytes do in the case of the central nervous system. Schwann cells (neurilemmocyets or neurilemmal cells) Whether the axon is myelinated or not myelinated, these cells form a sheath on the periphery of each axon and each time the outer membrane of the Schwann cell in covers the axon, the cell name transforms to neurilemma. Microglia They are the smallest and least numerous neurogalia in the CNS. They have many fine branches within their slim cytoplasmic process. The cells have the potential of migrating along the neural tissue. Microgalia appears during the early stages of the development of the embryo. Their origin is the messodermal stem cells that are closely associated with the stems cells of that produce macrophages and monocytes. Their movement in the nervous is systematic and most rest isolated at the neutral tissue. They behave like police force by wandering and engulfing pathogens, waste products and cellular debris. Ependymal Cells. They are extensions along the longitudinal axis of the brain and the spinal cord which are fluid filled central pathways. The walls diameter and thickness vary depending on which region they are a located at. These narrow pathways in the spinal cord are known as the central canal. The pathway develops an enclosed chamber referred to as ventricles. The ventricles and the central canal are lined by a layer of cellular epithelial cells called the ependyma and is normally filled with a fluid called the cerebrospinal fluid. This fluid usually environs the spinal cord and the brain. These fluid transports dissolve nutrients, waste, dissolve oxygen and other substances. Ependymal cells make up Ependyma , which range from columnar to cuboidal structures when viewed over through their sections. During early child development stages and at the embryonic stage, cilia are covering the free surfaces of the ependymal cells. At the ventricles of the brain, the cilia persist to adult stages of development where they help in CSF circulation. The ependymal cells in other regions virtually have scattered microvilli. There are specialized ependymal cells in small parts of the brain where they perform secretion of the CSF functions. Other areas of the epyndyma can have some functions such as sensory role e.g. supervising the composition of the CSF. The ependymal cells unlike typical epithelial cells, consists of thin processes that form extensive branch networks and get direct contact to the neuroglia in the periphery of the neural tissue. The functions of these connections have not been established and research on their function is underway. The stem cells line the ventricles and the central canal at the initial stages of the embryonic developments. When these stems cells later differentiate, they give rise to all CNS neuroglia and neurons apart from the microglia. Studies show that the ependyma in adults has stem cells which can differentiate producing more neurons. The particular mechanism for the regulation is still under investigation. Surprisingly if scientists were able to control this, then the common debilitating nervous systems malfunction disorders and treatment of strokes would been completely revolutionized. Astrocytes. These are the most numerous and the largest forms of neuroglia commonly found in the central nervous system. They have many functions, though most of these functions are poorly understood. Among them are: 1. Keeping the blood-brain barrier. Ensuring that there is no access of the dissolve compounds in circulating blood to the fluids of the central nervous system. Mainly because other chemicals in the blood and hormones can change their neuron functions. The chemical exchange between the interstitial fluids and the blood is regulated by the endothelial cells that line the Central nervous system capillaries. The blood-brain barrier is formed by these cells that controls and isolates the central nervous system from the mainstream general circulation. Wrapping are the capillaries, are the cytoplasmic astrocytes extension processes. A full blanket is formed around the capillaries through astrocytes processes and can only get interference where the capillary walls come into contact with other neuroglia. The permeability characteristic of the endothelial cells is somehow controlled by the chemical secretions of the astrocytes. 1. Creating a three-dimensional structure for the Central Nervous System. Astrocytes are fitted with microfilaments which cut across the width of the cell and its processes. These networks create a cytoskeleton backup helping them in giving a structural outfit for the nerve cells of the spinal cord and the brain. 3. Repairing the damaged tissues of neural in the central nervous system. Damaged tissues of the neural hardly recapture their normal functions. Any way the astrocytes heading to the damaged site can initiate structural repairs within the damaged tissues to normalize and stabilize the injured tissue and can also help evade further damage. The details on the neural damage and further repair actions are not in this assignment. 4. Directing neuron development. The embryonic brain has astrocytes that appear like participating in directing both the interconnection of developing neurons and its growth. 5. Regulating the interstitial environment. Despite big work still remaining to be studied on the astrocytes physiology, research shows that astrocytes control the interstitial composition by a number of ways. Such as ( i) controlling the concentration of potassium ions, sodium ions and carbon dioxide . ii) Offering a quick-transport system for the transit of ions, dissolved gases and nutrients between the neurons and the capillaries. ii) Regulating the volume of and the capacity of blood flow within the capillaries. iv) Recycling and absorbing other neurotransmitters. Oligodendrocytes. Just like astrocytes, oligodendrocytes have thin cytoplasmic extensions. The soma of the oligodendrocytes is smaller in size compared to astroccytes and the cell body processes are not many. Most of their cell body processes are in contact with the surfaces of neurons which are exposed. The roles of these cell body extension processes that terminate at the nerve cell are yet to be established and documented. Only the ones that their process terminates at the surfaces of the axons have their functions known. The central nervous system has majority of axons sheathed through a process known as oligodendrocytes. . References. 1 Schwartz, J.H., .Kandel E.R., Jessell, T.M. 2000. Principles of Neural Science, 4th ed., McGraw-Hill, New York. 2. Bennett, M.V.L., Bullock, T.H., Josephson, R., Marder, E., Fields R.D. 2005. The Neuron Doctrine, Redux, Science, V.310, p. 791-793. 3. Cajal, S Ramón y. 1933 Histology, 10th ed., Wood, Baltimore. 4. Bush B.M.H, Roberts A.,. 1981. Neurones without Impulses. Cambridge University Press, Cambridge. 5. Palay, S.L., Peters, A., Webster, H, D.,. 1991 The Fine Structure of the Nervous System, 3rd ed., Oxford, New York Read More