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Neuron structure


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neuron structure

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Neuron structure

  1. 1. Structure of neurons Aswathi K S 1st MSc BCM Roll No: 04 Central University of
  2. 2. CELLS OF THE NERVOUS SYSTEM Two types of cells make up the nervous system: 1. Neurons • Exitable cells that conduct impulses • ‘wiring’ of nervous system’s information circuits
  3. 3. 1 1. Glia • aka neuroglia • discovered accidentally by Camillo Golgi, upon dropping a piece of brain tissue in a bath of silver nitrate •Appeared as wast network of various kinds of darkly stained cells surrounding neuron •Glia means ‘glue’ •Unlike neurons, retain capacity for cell division throughout adulthood
  4. 4. •Funtion as supporting roles of nervous system Types of neural glia 1. Astrocytes 2. Microglia 3. Ependymal cells 4. Oligodendrocytes 5. Schwann cells First 4 types are found in the CNS and 5th type is found in the PNS
  5. 5. 1. Astrocytes • Star shaped glia (greek: astron-star) • Found only in CNS • Largest and most numerous type of glia • Tiny delicate points extend through brain tissue attaching neurons and tiny blood capillaries • ‘Feed’ neurons by taking up glucose from blood, converting to lactic acid and giving to neurons
  6. 6. • The BBB is a double barrier made up of astrocyte feet and endothelial cells of blood capillary walls • Small molecules (oxygen, carbon dioxide, water) diffuse rapidly to reach brain neurons and other glia, larger molecules penetrate slowly or not at all
  7. 7. 2. Microglia • Small, usually stationary cells found in CNS • Also known as Brain’s immune cells • In inflamed or degenerating brain tissue, microglia enlarge, move about, and carry on phagocytosis • they engulf and destroy micro organism and tissue debris • Although named as glia, are functionally and developmentally unrelated to nervous system cells
  8. 8. 3. Ependymal cells • Resemble epithelial cells • Thin sheets that line fluid filled cavities in brain and spinal cord. • Some take part in producing fluid that fills these spaces • others have cilia that helps keep fluid circulating within cavities.
  9. 9. 4. Oligodendrocytes • Smaller than astrocytes and have fewer processes • Oligo-few, dendro-branch, cyte-cell : meaning cells with fewer branches • Some lie clustered around nerve cell body, some arranged in rows between nerve fibre in brain and spinal cord •Help hold nerve fibre together • Important function : produce fatty myelin sheath around nerve fibre of CNS
  10. 10. Oligodendroc yte Axon Nucleus Myelination in the Central Nervous System
  11. 11. 5. Schwann cells • Found only in PNS • Also called as ‘Neurolemmocytes’ •Sometimes called satellite cells •Functional equivalents of oligodendrocytes • Support nerve fibre and form myelin sheath around them • Many schwann cells wrap around single neuron • Myelin sheath is formed by layers of
  12. 12. • Microscopic gaps in the sheath, between adjecent Schwann cell- Node of Ranvier or myelin sheath gaps •The myelin sheath and gaps in nerve fibre are important for transfer of nerve impulses • Schwann cell’s nucleus and cytoplasm are squeezed to perimeter to form neurilemma. • neurilemma important for regeneration of injured nerve fibres.
  13. 13. Node of Ranvier
  14. 14. • Nerve cells with many Schwann cells attached and having thick myelin sheath -White fibre or Myelinated fibre • Several nerve fibres held by single Schwann cell and does not wrap around to form thick myelin sheath – Grey fibre or Unmyelinated fibre
  15. 15. 2. Neurons • The human brain estimated to contain about 100 million , or 10% of total neurons of nervous system • Neuron consist of: i. Cell body ii. Axon iii. Dendrites iv. Axon ends
  16. 16. i. Cell body • Largest part of a nerve cell • Contain nucleus, cytoplasm and various organells like mitochondria and golgi apparatus • also called as perikaryon ; meaning surrounding the nucleus •Plasma membrane encloses the whole neuron • RER and attached ribosomes provide proteins for neuron
  17. 17. • Neurotransmitters are proteins packaged in vesicles that aid neurotransmission • other proteins used for repair of neuron • Mitochondria replicate themselves in the cell body: some transported to end of axon to provide energy for signal transduction • Nissl granules are
  18. 18. ii. Dendrites • Branch extensively from cell body like tiny tree • Greek word : tree • Distal end of sensory neurons called receptors : receive stimuli that initiate nerve signals • some dendrites in brain have knoblike dendritic spines: serve as connection point of other
  19. 19. iii. Axon. • Extend from tapered portion of cell body called axon hillock • Conduct impulse away from cell body. • Axon has side branches called Axon collaterals • Distal ends of axon form branches called telodentria and each terminate in a synaptic knob
  20. 20. • Some axons have vericosities or swellings, and act as point of contact with other cells or smooth muscle fibres • Axons vary in size from a meter long to just millimeters long • Axon diameter also vary: larger the diameter, greater the conduction • Myelination of axon also affects conductance • Only axons have myelin and not dendrites
  21. 21. • Neurofibrills arebundles of intermediate filaments called neurofilaments extending through cytoplasm of the neuron • Microtubules and microfilaments also present
  22. 22. • Along with providing structural support, act as railway for rapid transport of small organelles • Small motor molecules attach to vesicles and mitochondria , carry them to end of axon • Used vesicles and transmitters returned by same process to the cell body : Axonal transport
  23. 23. • There are 4 functional regions of neuron based on their role in receiving and conducting impulsea. Input zone: receive and initiating impulse in response b. Summation zone: adding together all impulse received c. Conduction zone: conduct impulse to end of axon d. Output zone: release
  24. 24. Classification of Neuron Structural classification 1. Multipolar 2. Bipolar 3. Unipolar Functional Classification 1. Afferent neurons 2. Efferent neurons 3. Interneurons
  25. 25. Structural Classificationl Classified according to number of extensions from cell body 1. Multipolar neurons: one axon but several dentrites, most of the neurons in brain and spinal cord are multipolar
  26. 26. 2. Bipolar neurons: one axon and one heavily branched dentrite, least nemerous kind and found in retina , inner ear and olfactory pathway
  27. 27. 3. Unipolar neuron : single process extending from the cell body, which branches into central process (towards CNS) and peripheral process (away from CNS), process forms a single axon and conducting impulse away from dentrite, they are always sensory neurons.
  28. 28. Functional Classification Classified based on direction of conduction of nerve impulse. 1. Afferent neuron: to spinal cord or brain 2. Efferent neuron: away from brain or spinal cord to muscle or gland 3. Interneurons: from afferent
  29. 29. Thank You!!!!