Nervous System

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Nervous System
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Objectives

• Identify the principle parts of the nervous
  system
• Describe the cells that make up the nervous
  system
• Describe what starts and stops a nerve impulse
  (action potential)
• The role of neurotransmitters
• Compare the functions of the CNS PNS
• Identify the principle parts of the brain

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Support Cells

• 20 of cells in nervous system are neurons and
  the rest are neuroglial cells support cells
• Support protection
• Maintenance of surrounding chemical
  concentrations
• No impulse generation

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Types of neuroglial cells

• Astrocytes star shaped, largest most
  numerous, form tight sheaths around capillaries
  of the brain
• Microglia small, usually stationary but
  enlarge and mobilize in degenerating brain tissue
  for phagocytosis
• Oligodendroglia produce myelin sheath that
  envelopes nerve fibers in the brain and spinal
  cord
• Schwann cells found in nerves only (not in
  brain or spinal cord) help to form myelin sheath

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Schwann Cells

• In PNS these neuroglial cells produce myelin a
  fatty insulating material surrounding axon
  called the myelin sheath myelinated neurons
• Nodes of Ranvier are short uninsulated gaps where
  the surface of the axon is exposed

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Functions of the myelin sheath

1. Energy saving prevents slow leakage of Na in /
   K out (recall sodium-potassium pump 3 Na out
   2 K in)
2. Speeds transmission of impulses local
   depolarizing at Nodes of Ranvier is much faster
   than continuous propagation down an unmyelinated
   axon saltatory conduction action potentials
   appear to jump from node to node (5 mph vs. 250
   mph)
3. Helps damaged or severed axons of the PNS to
   regenerate severed end near soma can regrow in
   sheath channel (weeks-year )

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Myelinated Neuron
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Oligodendrocytes

• In CNS these protective sheaths of myelin do not
  regenerate when the cell dies this is why
  spinal cord injuries or MS causes permanent damage

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Objectives

• Identify the principle parts of the nervous
  system
• Describe the cells that make up the nervous
  system
• Describe what starts and stops a nerve impulse
  (action potential)
• The role of neurotransmitters
• Compare the functions of the CNS PNS
• Identify the principle parts of the brain

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Synapse/a chemical event

• Junctions that allow transfer of information from
  one neuron
• to another neuron
• to an effector cell (muscle)
• Pre synaptic neuron transmits signal to synapse
• Post synaptic neuron transmits signal away from
  synapse
• Synaptic cleft is the fluid filled space
  separating the pre/post synaptic neurons

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The Role of Neurotransmitters

• Convert an electrical signal (action potential)
  into a chemical signal (neurotransmitter)
• 4 step sequence
• Action potential arrives at axon bulb, Ca
  channels open and Ca diffuses into axon bulb
• Ca causes vesicles containing neurotransmitters
  to fuse with presynaptic membrane and release
  contents into synaptic cleft
• Neurotransmitter molecules bind to receptors on
  post synaptic membrane opening gated Na channels
• Na diffuses into postsynaptic membrane starts a
  new action potential (electrical signal)

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Neurotransmitters are excitatory or inhibitory

• gt 50 chemicals can function as neurotransmitters!
• Excitatory encourage the generation of new
  impulses in post synaptic neuron
• Inhibitory prevents generation of action
  potentials in post synaptic neuron
• Some neurotransmitters can do both depending on
  the receptor that they bind to
• Many small graded potentials are needed to fire
  the impulse across the cleft

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Somatic and Autonomic Divisions of the PNS
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Effects of neurotransmitters are short-lived

• Prompt removal of neurotransmitter causes signals
  to stop 3 ways to remove
• Taken back up by presynaptic neuron repackaged
• Destroyed by enzymes in synaptic cleft
• Diffuse away from synaptic cleft ? general
  circulation and destroyed
• Convergence one neuron receives information
  from several neurons
• Divergence action potential will go to several
  neurons

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Convergence effect

• Neurons may integrate and process thousands of
  simultaneous incoming stimulatory and inhibitory
  signals before generating and transmitting their
  own action potentials
• Bottom line individual neurons do not see,
  smell or hear yet combined actions allow us to
  experience these complex sensations
• Filter of information extraneous information
  expelled during REM sleep

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Motor neurons are single action cells

• Muscle cells are targets of presynaptic neurons
• Skeletal muscle does not process information
• Neuromuscular junction in large with many points
  of contact between neuron and muscle cell
• Threshold is reached in skeletal muscle every
  time a motor neuron sends a single action
  potential
• So the nervous system has absolute control of
  skeletal muscle!