Chapter 5 Nerve Cells

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Chapter 5 Nerve Cells

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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Nerve cells

• Around 100 billion neurons in the brain initially
• Adult stage 15 billion
• Means of communication in the nervous system
• Excitatory and inhibitory in nature

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Nerve cells

• Neurons
• Sensorimotor activities
• Higher mental functions
• Neuroglia cells
• Support and protect neurons

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Neurons

• Three parts
• Soma body with nucleus
• Dendrite receives inputs
• Axon sends information
• Signal is transmitted electrochemically down
  shaft of axon.
• Connects to target cells (neurons, muscles,
  glands) at synapse.
• Can either inhibit or excite target cells.
• Can be up to x10,000 length of soma

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Soma

• Protoplasm
• Nucleus
• Cytoplasm
• Protein filled liquid
• Microscopic Organelles
• neurofibril
• mitochondria powerhouse of the cell
• ribosomes
• lysosomes
• Nissl bodies
• Golgi com

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Metabolic activity

• Firing requires oxygen and glucose
• Constant supply is required (via capillaries of
  vascular system).
• If supply is disrupted
• Meaningful neural activity will cease immediately
• Neurons will die within hours if supply is not
  restored

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Intracellular transport

• Proteins generated and degraded in the soma.
• Proteins must by moved to axons/synapse
• Proteins transported via Microtubules in Axon
• Anterograde Transport toward synapse (new
  proteins, transported by protein kinesin)
• Retrograde Transport toward cell body
  (recycling, transported by protein dynein)
• Suicide Transport (apoptosis, form of retrograde
  transport)
• Microtubules act as tracks for moving proteins

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Nucleus

• Control Center
• Contains DNA
• Contains Nucleolus
• Engaged in Protein Synthesis
• Contains RNA - Code for Enzyme for Synthesis of
  Specific Enzymes Needed

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Nuclear structures and functions

• Neurofibrils Channels for Communicationin Cell.
  These filaments repulse each other, so their
  development enlarges the diameter of the axon and
  dendrite
• Mitochondria Contain Enzymes for Metabolism
  creates ATP, used for chemical energy.
• Free Ribosomes Synthesize Proteins
• Lysosomes Intracellular Digestion (break down
  waste)
• Nissl Bodies Amino Acids to Proteins
• Golgi Complex Protein Secretion and its
  Transportation (post office of the cell)

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Neural communication

• Dendrites
• Afferent influenced by other neurons/receptors.
• Branching and short
• Axons
• Efferent influence other neurons/receptors.
• Dependent on electrical signal at axon hillock
• End in axon terminals with synaptic knobs that
  release neurotransmitters.

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Myelin Sheath

• Myelin Sheath
• Multilayer Lipid (fatty) Material
• Insulates Nerve Fiber
• Analogy plastic insulation on wire in house
• Regulation
• Speed Signals can jump between nodes
• Efficiency Escape of Electrical Energy
• Segmented nodes of Ranvier with internodes

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Saltatory conduction

• Normally, chemical signal is relatively slow down
  an axon.
• However, in mylinated segments the signal can
  jump between nodes.
• Facilitates Speed of Fiber (120 msec)

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Glial cells for myelin

• In CNS
• Oligodendroglia single cell can support Myelin
  sheaths for up to 50 axons
• Multiple Sclerosis
• In PNS
• Schwann cells each covers only about 1mm, so
  many cells per axon.
• Can Regenerate
• Injury to Motor Pathways
• Amputation

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Synapse

• Connection Between Neurons
• Three Parts
• Knob (Axon presynaptic)
• Contain Vesicles
• Filled with Neurotransmitters Released when
  Necessary
• Chemically Stimulate Receiving Nerve Cell Body
• Synaptic Cleft gap between one neuron and the
  next.
• 2/100,000mm Wide
• Receptive Sites (Receiving Cell Axon or Dendrite
  postsynaptic)

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Action Potentials

• When a neuron fires, it releases
  neurotransmitters from terminals into synaptic
  cleft.
• Chemical in Neurotransmitter Stimulates or
  inhibits Postsynaptic Cell (Dendrite, Axon or
  Cell Body)
• If the postsynaptic cell receives enough
  stimulation, this neuron will fire.
• Firing is an all-or-none response
• After firing, neuron must wait a while before it
  can fire again.

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Types of neuron

• Classified By Number of Processes Multipolar
  many dendrites, one axon (brain)
• Bipolar retina, smell, facial nerve
• Unipolar dorsal spine

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Axonal Length Types

• Golgi Type I
• Axons range from feet to inches.
• E.G. Motor cortex
• Golgi Type II
• Axons are very short
• E.G. interneurons