Chapter 48: Nervous System

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Chapter 48 Nervous System
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Neurons and Supporting cells

• Organization of Nervous system
• Diffuse nerve net-hydra
• Brain in front
• Central nervous system (CNS)- planarian
• Ganglia (cluster of neurons)- in annelids
• Peripheral nervous system (PNS)-connects CNS with
  rest of body

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Information Processing

1. Sensory input- sensory neurons transmit external
   stimuli or internal conditions to CNS
2. Integration- interneurons integrate (analyze and
   interpret). Greatest complexity in neural
   circuits cells.
3. Motor output- leaves CNS via motor neurons that
   communicate with effector cells (muscle or
   endocrine cells)

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Question 1

• a. Arrange the following neurons in the correct
  sequence for information flow during the
  kneejerk reflex interneuron, sensory neuron,
  motor neuron.
• b. Which of the neuron types is located entirely
  within the CNS?

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Neuron Structure
cell body
sensory neuron
cell body
axon
interneuron
associative
dendrites
dendrites
cell body
motor neuron
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Neuron Structure

• Dendrites- receive signals
• Synapses- site of communication from presynaptic
  cell to postsynaptic cell by neurotransmitters
• Axon- transmits the signal

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Question 2

• Would severing a neuron's axon stop the neuron
  from receiving or from transmitting information?
  Explain.

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Supporting cells

• Astrocytes- structural support
• Radial glia- form tracks
• Oligodendrocytes (in CNS) and Schwann cells (in
  PNS) form myelin sheaths around axon
• Similar to plastic protecting electric covers

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Question 3

• What would be the most obvious structural
  abnormality in the nervous system of a mouse
  lacking oligodendrocytes?

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Action potentials are the signals that conducted
by axons

• If gated channels, membrane potential will change
  when open or closed
• Hyperpolarization- increase in membrane potential
  (inside is more neg)
• Ex. Opening of K channels
• Depolarization- decrease in membrane potential
  (inside is less neg)
• Ex. Opening of Na channels

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Action potentials contd

• Threshold- depolarization up to a certain
  membrane voltage
• Action potential-once reached amount of stimulus
  has been reached signal that carry information
  along axons very brief

Animation Link
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Question 4

• Draw the sketch below and label the following
  outside cell, inside cell. Show where the
  concentrations of Na and K are highest.

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Conduction of Action Potentials

• For an action potential to function long
  distances, it regenerates itself as it moves
  along the axon, increasing action potential as it
  moves down axons
• What effects speed of transmission?
• Diameter of axon
• Myelin sheath

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Question 5

• Arrange the following from lowest to highest
  conduction speed (a) myelinated, smalldiameter
  axon (b) myelinated, largediameter axon (c)
  unmyelinated, smalldiameter axon.

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Neurons communicate with other cells at synapses

• Two types of communication
• Electrical synapses- gap junctions that allow
  electric current to flow from one cell to another
• Chemical synapses-most release neurotransmitter
  in a synaptic vesicle, it depolarizes the
  terminal membrane and vesicles fuse with terminal
  membrane, releasing neurotransmitters by
  exocytosis. It can
• then have a direct
• or indirect effect on
• postsynaptic cell.

Online Link
Mouse Party Link
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Direct synaptic transmission

• Neurotransmitter binds to channel (the receptor),
  it opens and allows ions to diffuse across the
  membrane
• Result- postsynaptic potential (change in
  membrane potential)
• Excitatory postsynaptic potentials (EPSPs)
  membrane potential brought down to threshold
• Inhibitiory postsynaptic potentials (IPSPs)
  increase past threshold

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Indirect transmission

• Neurotransmitter binds to a receptor that is not
  part of the channel, activates a signal
  transduction pathway
• Slower onset but longer lasting than direct
  transmission

Indirect Link
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Types of neurotransmitters

• Acetylcholine (Ach)- released at neuromuscular
  junctions, exciting motor cells other times, can
  be an inhibitor
• Biogenic amines- neurotransmitters from aa
• One group, catecholamines from aa tyrosine
  include epinephrine and nonepinephrine
  (hormones) dopamine (brain-affect sleep, mood,
  attention and learning)

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Types of neurotransmitters

• Amino acids and peptides
• Aa-most in brain
• Peptides- short chains called neuropeptides
• Gases- some neurons release gases especially NO
  (nitric oxide) and CO (carbon monoxide)
• Aa-most in brain
• Peptides- short chains called neuropeptides

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Question 6

• A single postsynaptic neuron can be affected by
  neurotransmitter molecules released by many other
  neurons, some releasing excitatory and some
  releasing inhibitory neurotransmitters. What will
  determine whether an action potential is
  generated in the postsynaptic neuron?

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Question 7

• There are many different types of
  neurotransmitters. Each neuron secretes only one
  type of neurotransmitter. Some neurotransmitters
  hyperpolarize the postsynaptic membrane. Are
  these excitatory or inhibitory neurotransmitters?