Nerve Tissue

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Nerve Tissue The Nervous System
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Nervous Tissue

• Ectodermal in origin Neurulation
• Formation of neural tube and neural crest.
• Cell Types
• a) Neurons - CNS and PNS
• b) Ependyma - CNS
• c) Neuroglia - CNS
• d) Schwann Cells - PNS

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Neurulation
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Nerve Cell Origins

• A) Neural Tube
• 1) Matrix Layer - matrix cells become ependymal
  -gt neural tube lining cells
• 2) Mantle Layer migrate from matrix
• a) Glioblasts
• Astroblasts -gt Astrocytes
• Oligodendroblasts -gt Oligodendrocytes
• b) Neuroblasts -gt neurons
• 3) Marginal Layer - mantle neuron cell bodies
  form gray matter while their axons migrate out
  into an outer Marginal layer forming white matter
  of CNS.

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Nerve Cell Origins

• B) CNS CT forms Microglia cells.
• Neural Crest - PNS and other structures
• 1) Chromaffin Cells
• 2) Schwann Cells
• 3) Melanocytes
• 4) Odontoblasts

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Neurons

• Nerve cells are capable of depolarization
• (Excitable)
• Function in impulse formation, transfer,
  interpretation.
• Neurons form functional links (circuits)
• a) afferent sensory neuron that respond to
  changes in environment (stimuli)
• b) interneurons connector neurons
• c) efferent motor neuron (response)

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Neuron

• Cell Body with nucleus Perikaryon
• Cell Body - cytoplasm has prominent numbers of
  basophilic granules (ribosomes and ER) Nissl's
  Bodies
• Usually two kinds of fibrous processes - dendrite
  and axon.
• Dendrite
• a) non-myelinated,
• b) brings impulse to NCB,
• c) contains microtubules.
• Axon
• a) myelinated,
• b) takes impulse from NCB,
• c) contains microfilaments.

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Neuron Morphology

• Several levels of neuron organization
• a) apolar - modified neurons with no fibrous
  process, unique to Pineal gland and Adrenal
  medulla.
• b) unipolar (pseudounipolar) - one axon, no
  dendrite, PNS afferent.
• c) bipolar - one axon and one dendrite special
  neuron types retina of eye olfactory neuron.
• d) multipolar one axon and many dendrites
• CNS efferent and afferent
• PNS Efferents.

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Fibrous Processes

• Axons
• Cylinder of cytoplasm axoplasm
• Up to 1 meter (40 inches)
• Plasma membrane axolemma
• Axons are encased in a sheath of cells
• Schwann in the PNS,
• Oligodendrocytes in CNS.

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Myelin

• Encasing cells have myelin (lipid) in membranes -
  produce a white appearance in living state.
• When the cell wraps many times myelinated or
  medullated neuron with white outer cover
• Serves to speed up impulse transfer
• Ends of Schwann Cells produce segmented nodes
  Nodes of Ranvier

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RF reticular fibers P perineurium (nucleus) S
Schwann Cell (nucleus)
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Unmyelinated Neurons

• Present in CNS and PNS
• In PNS, axon in simple cleft in Schwann Cell, no
  wrapping, no Nodes of Ranvier.
• In CNS, totally unsheathed, many in number (3-4
  x as many as myelinated.)

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• Ultrastructural features of myelinated (A) and
  unmyelinated (B) nerve fibers.
• Nucleus and cytoplasm of a Schwann cell
• axon
• microtubule
• neurofilament
• myelin sheath
• mesaxon
• node of Ranvier
• interdigitating processes of Schwann cells at the
  node of Ranvier
• side view of an unmyelinated axon
• basal lamina.

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Synaptic Communication

• Gap between adjacent neuron ends synapse.
• Chemical Synapse
• In the synapse, the axon terminal secretes
  chemical neurotransmitter (e.g. acetylcholine)
  which transfers impulse from one neuron to the
  next via a specific receptor on the post-synaptic
  cell.
• Common neurotransmitters include
• Acetylcholine, Norepinephrine, Dopamine,
  Serotonin and Glutamate, and GABA
• Electrical Synapse
• Gap junctions - cytoplasmic connections between
  adjacent cells.

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