Cells of Nervous System

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Cells of Nervous System

  1. 1. Cell Structure in the Nervous System
  2. 2. Cells & the Brain <ul><li>All info comes from sensory receptors </li></ul><ul><li>This information is transformed in the brain into perceptions & commands for movement. </li></ul><ul><li>The complexity of the brain results from the number, not the variety, of cells </li></ul><ul><li>Brain contains ~ 100 billion nerve cells </li></ul>
  3. 3. Types of Cells <ul><li>2 classes of cells in nervous system </li></ul><ul><ul><li>Neurons or nerve cells </li></ul></ul><ul><ul><li>Glial cells (glia) </li></ul></ul><ul><li>Neurons not initially recognized as single cells because they differ from other cells: </li></ul><ul><ul><li>much larger (axon can be 0.1 mm to 2 m) </li></ul></ul><ul><ul><li>different in shape </li></ul></ul><ul><ul><li>don't touch each other directly </li></ul></ul><ul><ul><li>Golgi stain </li></ul></ul><ul><li>Glia surround neurons – from the Greek for ‘glue’ </li></ul>
  4. 4. Glial Cells <ul><li>There are about 1 trillion glial cells in the brain </li></ul><ul><ul><li>10 - 50 x more than there are neurons </li></ul></ul><ul><li>Glial cells have perform important functions including: </li></ul><ul><ul><li>surround & support neurons </li></ul></ul><ul><ul><li>separate & insulate groups of neurons </li></ul></ul><ul><ul><li>some produce myelin </li></ul></ul><ul><ul><li>some are scavengers </li></ul></ul><ul><ul><li>maintain concentration of K + in extracellular space </li></ul></ul><ul><ul><li>some take up & remove chemical transmitters </li></ul></ul><ul><ul><li>some guide migration of neurons during development </li></ul></ul><ul><ul><li>some help form an impermeable lining in capillaries & venules, creating the blood-brain barrier </li></ul></ul>
  5. 5. Types of Glial Cells <ul><li>Astrocytes </li></ul><ul><ul><li>Most numerous in brain </li></ul></ul><ul><ul><li>Fill spaces between neurons </li></ul></ul><ul><ul><li>Serve to regulate composition of the extracellular space </li></ul></ul><ul><li>Oligodendroglia </li></ul><ul><ul><li>Wrap axons with myelin sheath in brain and spinal cord (Not all axons are myelinated) </li></ul></ul><ul><ul><li>Each oligodendrocyte wraps several axons </li></ul></ul><ul><li>Microglia </li></ul><ul><ul><li>Phagocytes - clean debris from dying neurons & glia </li></ul></ul>
  6. 6. More Glial Cells <ul><li>Schwann Cells </li></ul><ul><ul><li>Wrap axons with myelin sheath outside the brain and spinal cord </li></ul></ul><ul><ul><li>Each Schwann cell wraps only one axon </li></ul></ul><ul><li>Ependymal Cells </li></ul><ul><ul><li>Line ventricles </li></ul></ul><ul><ul><li>Involved with secretion and absorption of cerebral spinal fluid </li></ul></ul><ul><ul><li>Also play a role in routing embryonic cells during development  </li></ul></ul>
  7. 7. Neurons <ul><li>Basic cells of the nervous system </li></ul><ul><li>There are about 100 billion neurons in the brain </li></ul><ul><ul><li>more in the spinal cord, peripheral nervous system and sensory organs </li></ul></ul><ul><li>Neurons derive form the neural tube during development </li></ul><ul><li>Neurons signal information electrically with nerve impulses </li></ul>
  8. 8. The Neuron Doctrine <ul><li>Golgi stain – A reduced silver method using silver chromate that stains a few, isolated neurons in their entirety </li></ul><ul><ul><li>Invented by Camillo Golgi </li></ul></ul><ul><ul><li>Santiago Ramon y Cajal used it to survey the variety of structures in vertebrate nervous systems </li></ul></ul><ul><li>Led Cajal to propose the neuron doctrine: </li></ul><ul><ul><li>nerve cells are the structural and functional basis of the nervous system and they must communicate with one another </li></ul></ul><ul><ul><li>confirmed by electron microscopy 50 years later </li></ul></ul><ul><ul><li>Golgi and Cajal shared a Nobel Prize </li></ul></ul>
  9. 9. The Structure of a Neuron <ul><li>4 morphologically distinct regions: </li></ul><ul><li>• Cell Body ( soma ) </li></ul><ul><li>• Axon </li></ul><ul><li>• Dendrites </li></ul><ul><li>• Presynaptic Terminals </li></ul>
  10. 10. The Cell Body <ul><li>Also called soma ( somata ) - perikaryon </li></ul><ul><li>Metabolic center of the cell </li></ul><ul><ul><li>Assembly of new membrane </li></ul></ul><ul><li>Gives rise to dendrites & axon </li></ul><ul><li>Contains the nucleus </li></ul><ul><ul><li>Site of DNA in chromosomes </li></ul></ul><ul><li>Contains rough & smooth endoplasmic reticulum </li></ul><ul><ul><li>Rough ER: Stacks of flattened membrane compartments with ribosomes attached </li></ul></ul><ul><ul><li>Smooth Endoplasmic Reticulum: connects RER with Golgi apparatus and serves to further process membrane proteins; serves to sequester and release calcium to control cytoplasmic composition </li></ul></ul>
  11. 11. Other Elements of the Cell Body <ul><li>Site of protein synthesis (ribosomes) </li></ul><ul><ul><li>Free ribosomes and polyribosomes loose in cytoplasm </li></ul></ul><ul><li>Contains the Golgi apparatus </li></ul><ul><ul><li>Stacks of membrane compartments </li></ul></ul><ul><ul><li>Packaging of secretory products (neurotransmitter peptides) </li></ul></ul><ul><li>Mitochondria </li></ul><ul><ul><li>Site of conversion of energy from chemical bonds in food to ATP, the common energy currency of the cell </li></ul></ul>
  12. 12. The Cytoskeleton <ul><li>Superstructure of the cell – 3 components: </li></ul><ul><li>Microtubules </li></ul><ul><ul><li>Largest - 20 nm thick-walled tubes </li></ul></ul><ul><ul><li>Spirals of alpha and beta tubulin molecules </li></ul></ul><ul><ul><li>Railroad tracks for movement of organelles, etc. </li></ul></ul><ul><li>Neurofilaments </li></ul><ul><ul><li>10 nm twisted cables </li></ul></ul><ul><ul><li>Tend to be the most static structures of the three </li></ul></ul><ul><li>Microfilaments </li></ul><ul><ul><li>5 nm double helix of actin </li></ul></ul><ul><ul><li>Especially common in neurites along with microtubules </li></ul></ul><ul><ul><li>Also found in thick meshes associated with some regions of the cell membrane </li></ul></ul>
  13. 13. The Neuronal Membrane <ul><li>A continuous sheet covering the neuron </li></ul><ul><li>Separates the cytoplasm from the extra-cellular fluid </li></ul><ul><li>Lipid bilayer with many embedded proteins </li></ul><ul><li>The embedded proteins are channels critical to neuronal function </li></ul>
  14. 14. Dendrites <ul><li>Neurites which receive signals from other nerve cells (message IN ) </li></ul><ul><li>Postsynaptic membrane in dendrites have molecular receptors for neurotransmitters (chemical messengers) </li></ul><ul><li>May have little bulges or pegs called dendritic spines where incoming synapses connect </li></ul><ul><li>Carry impulses from other neurons or receptors toward the cell body </li></ul><ul><li>Most neurons have several dendrites </li></ul><ul><li>Have polyribosomes suggesting some local protein synthesis in dendrites </li></ul>
  15. 15. The Axon <ul><li>Main conducting unit of the neuron </li></ul><ul><li>Each neuron has only one axon </li></ul><ul><li>Axon collaterals – many axons do have branches </li></ul><ul><li>Carries impulses away from the cell body to other neurons or to effectors (message OUT ) </li></ul><ul><ul><li>Efferent - axon going away from reference neuron </li></ul></ul><ul><ul><li>Afferent - axon coming toward reference neuro </li></ul></ul><ul><li>Axon hillock = the conical region at the beginning of an axon where it joins the cell body </li></ul><ul><li>Axon terminal ( terminal bouton ) – the end of the axon </li></ul>
  16. 16. Visualizing the Neuron
  17. 17. Myelin <ul><li>Large axons have an insulating sheath - myelin </li></ul><ul><li>Supports, insulates & nourishes the axon and helps maintain chemical balance. </li></ul><ul><li>This sheath is made up of oligodendrocytes in the CNS & Schwann cells in the PNS. </li></ul><ul><li>Nodes of Ranvier = Gaps in the sheath which allow passage of the electrical signal through ion channels </li></ul><ul><ul><li>Saltatory conduction </li></ul></ul><ul><ul><li>Transmission very fast . </li></ul></ul>
  18. 18. Visualizing Myelinated Neurons
  19. 19. Presynaptic Terminals <ul><li>Neurons don't touch; separated by a space = synaptic cleft (synapse) </li></ul><ul><ul><li>Cell transmitting a signal = presynaptic </li></ul></ul><ul><ul><li>Cell receiving = postsynaptic; can be another neuron or an effector such as a muscle or gland </li></ul></ul><ul><li>Signal within cells is electrical, but between cells is chemical (b/c no direct contact) </li></ul><ul><li>Chemical messengers which transmit the signal = neurotransmitters </li></ul><ul><li>Neurotransmitters are stored in vesicles, released from terminals at the end of the axon into the synapse </li></ul><ul><ul><li>Terminal buttons </li></ul></ul><ul><ul><li>Motor endplates </li></ul></ul>
  20. 20. Anatomy of a Synapse
  21. 21. Diversity of Neurons <ul><li>Neurons can be classified in several ways: </li></ul><ul><li>Based on neurites </li></ul><ul><li>Based on dendrites </li></ul><ul><li>Based on connections/ function </li></ul><ul><li>Based on axon length </li></ul><ul><li>Based on neurotransmitter </li></ul>
  22. 22. Number of Neurite Processes <ul><li>Neurons classed in 3 (4) groups by number of processes: </li></ul><ul><li>Unipolar </li></ul><ul><ul><li>the simplest neuron </li></ul></ul><ul><ul><li>a single primary process with many branches </li></ul></ul><ul><ul><li>no dendrites from the cell body </li></ul></ul><ul><ul><li>common in invertebrates </li></ul></ul><ul><li>Bipolar </li></ul><ul><ul><li>oval shaped soma </li></ul></ul><ul><ul><li>2 processes: dendrite (info in) & axon (info out) </li></ul></ul><ul><ul><li>most sensory neurons are bipolar </li></ul></ul>
  23. 23. Number of Processes (Cont.) <ul><li>Pseudounipolar </li></ul><ul><ul><li>Sensory cells of touch, pressure, pain are special type of bipolar </li></ul></ul><ul><ul><li>1st develop as bipolar; the 2 processes fuse to form a single axon </li></ul></ul><ul><ul><li>axon splits at the cell body </li></ul></ul><ul><ul><li>one goes to spinal cord, other to periphery (skin, joints, muscles) </li></ul></ul><ul><li>Multipolar </li></ul><ul><ul><li>most common type in vertebrates </li></ul></ul><ul><ul><li>one axon and one or more dendrite </li></ul></ul><ul><ul><li>vary in size and shape </li></ul></ul>
  24. 24. Visualizing Cell Types
  25. 25. Classification Based on Dendrites <ul><li>Pyramidal cells/stellate cells </li></ul><ul><li>Spiny cells/aspinous cells </li></ul>
  26. 26. Classification Based on Function <ul><li>3 groups: </li></ul><ul><li>Sensory ( afferent ) neurons </li></ul><ul><ul><li>receive stimuli & transmit them to the central nervous system </li></ul></ul><ul><li>Motor neurons </li></ul><ul><ul><li>carry impulses away from the central nervous system to muscles or glands </li></ul></ul><ul><li>Interneurons </li></ul><ul><ul><li>link sensory to motor neurons </li></ul></ul>
  27. 27. Visualizing Cell Types - 2
  28. 28. Classification Based on Neurotransmitter <ul><li>Cholinergic neurons – </li></ul><ul><ul><li>use acetylcholine </li></ul></ul><ul><li>Gabanergic neurons – </li></ul><ul><ul><li>use gamma aminobutyric acid </li></ul></ul><ul><li>Dopaminergic neurons – </li></ul><ul><ul><li>use dopamine </li></ul></ul>
  29. 29. How Neurons Carry the Message <ul><li>Only neurons are involved in transmission of electrical signals. </li></ul><ul><li>Within a nerve cell, message is an electrical signal = action potential </li></ul><ul><ul><li>Cascading membrane depolarization creates the movement of the action potential as a nerve impulse </li></ul></ul><ul><ul><li>rapid, all or none impulses </li></ul></ul><ul><ul><li>in myelinated neurons, gaps at regular intervals allow regeneration of the action potential </li></ul></ul><ul><li>Between nerve cells the message is carried chemically </li></ul>
  30. 30. Principles of Impulse Transmission <ul><li>PRINCIPLE #1: </li></ul><ul><li>Information conveyed by an action potential is determined not by the form of the signal, but by the pathway the signal travels. </li></ul><ul><ul><li>action potential for sound looks like action potential for odor </li></ul></ul><ul><li>PRINCIPLE #2: </li></ul><ul><li>Principle of Dynamic Polarization : </li></ul><ul><li>electrical signals flow in a consistent direction </li></ul><ul><li>PRINCIPLE #3: </li></ul><ul><li>Principle of Connectional Specificity: </li></ul><ul><li>Contact is not random - cells communicate with certain targets and not others </li></ul>
  31. 31. Types of Signals <ul><li>Each sensory & motor nerve cell generates 4 types of signals: </li></ul><ul><ul><li>an input signal </li></ul></ul><ul><ul><li>an integration signal (trigger) </li></ul></ul><ul><ul><li>a conducting signal </li></ul></ul><ul><ul><li>an output signal </li></ul></ul>
  32. 32. Functional Regions <ul><li>Almost all neurons have 4 corresponding functional regions: </li></ul><ul><ul><li>receptive - local input </li></ul></ul><ul><ul><li>integrative - trigger </li></ul></ul><ul><ul><li>conductile - signaling </li></ul></ul><ul><ul><li>secretory - output </li></ul></ul>

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