Lecture 20: From neurons to brains

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Lecture 20 From neurons to brains

• The computational brain
• Reflex arcs
• Homeostasis
• Learning
• Personality
• Diversity of the vertebrate brain
• Evolutionary diversity
• Hippocampus
• Vasopressin
• Environmental diversity
• Hippocampus
• Ecstasy

• Action potential review
• Translating electrical signals into chemical
  signals (and back again)
• Excitatory vs inhibitory neurotransmitters
• Integrating signals
• EPSPs
• IPSPs
• Summation

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Action potential

• Definition an all-or-none change in voltage
  that propagates itself down the axon

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Action potential

• Definition an all-or-none change in voltage
  that propagates itself down the axon
• Naturally occurring action potentials begin at
  the axon hillock

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Action potential

• Definition an all-or-none change in voltage
  that propagates itself down the axon
• Naturally occurring action potentials begin at
  the axon hillock
• Action potentials do not occur anywhere else in a
  neuron not in dendrites, not in cell bodies

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Figure 48.9 The role of voltage-gated ion
channels in the action potential (Layer 5)
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Figure 48.11 Saltatory conduction
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How do you get from electrical signals to
chemical signals and back again?
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Translating signals

• The action potential moves down the axon until it
  reaches the terminal (synapse)

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Translating signals

• The action potential moves down the axon until it
  reaches the terminal (synapse)
• Its wave of depolarization opens
  voltage-activated Ca2 channels

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Translating signals

• The action potential moves down the axon until it
  reaches the terminal (synapse)
• Its wave of depolarization opens
  voltage-activated Ca2 channels
• Influx of Ca2 causes vesicles to fuse with
  presynaptic cell membrane

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Translating signals

• The action potential moves down the axon until it
  reaches the terminal (synapse)
• Its wave of depolarization opens
  voltage-activated Ca2 channels
• Influx of Ca2 causes vesicles to fuse with
  presynaptic cell membrane
• Transmitter diffuses across synaptic cleft and
  binds to receptors on post-synaptic cell

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Excitatory and inhibitory neurotransmitters

• If a transmitter depolarizes the post-synaptic
  neuron, it is said to be excitatory

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Excitatory and inhibitory neurotransmitters

• If a transmitter depolarizes the post-synaptic
  neuron, it is said to be excitatory
• If a transmitter hyperpolarizes the post-synaptic
  neuron, it is said to be inhibitory

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Excitatory and inhibitory neurotransmitters

• If a transmitter depolarizes the post-synaptic
  neuron, it is said to be excitatory
• If a transmitter hyperpolarizes the post-synaptic
  neuron, it is said to be inhibitory
• Whether a transmitter is excitatory or inhibitory
  depends on its receptor

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Figure 48.12 A chemical synapse
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Excitatory and inhibitory neurotransmitters

• Acetylcholine is excitatory because its receptor
  is a ligand-gated Na channel

Fig 48.7
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Excitatory and inhibitory neurotransmitters

• Acetylcholine is excitatory because its receptor
  is a ligand-gated Na channel
• GABA is inhibitory because its receptor is a
  ligand-gated Cl- channel

Fig 48.7
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Excitatory and inhibitory neurotransmitters

• Acetylcholine is excitatory because its receptor
  is a ligand-gated Na channel
• GABA is inhibitory because its receptor is a
  ligand-gated Cl- channel
• Other transmitters (e.g. vasopressin) have
  G-protein-linked receptors

Fig 48.7
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Excitatory and inhibitory neurotransmitters

• Acetylcholine is excitatory because its receptor
  is a ligand-gated Na channel
• GABA is inhibitory because its receptor is a
  ligand-gated Cl- channel
• Other transmitters (e.g. vasopressin, dopamine)
  have G-protein-linked receptors
• Effects depend on the signal transduction pathway
  and cell type

Fig 48.7
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• Some synapses form on the dendrites, cell body,
  or the axon hillock.

Fig 48.13
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How do post-synaptic neurons integrate
information from more than one pre-synaptic cell?
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Summation of postsynaptic potentials

• The opening of a ligand-gated channel produces a
  post-synaptic potential either excitatory
  (EPSP) or inhibitory (IPSP)

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Summation of postsynaptic potentials

• The opening of a ligand-gated channel produces a
  post-synaptic potential either excitatory
  (EPSP) or inhibitory (IPSP)
• If two post-synaptic potentials occur at the same
  time in different places, or at the same place in
  rapid succession, their effects add up.

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Summation of postsynaptic potentials

• The opening of a ligand-gated channel produces a
  post-synaptic potential either excitatory
  (EPSP) or inhibitory (IPSP)
• If two post-synaptic potentials occur at the same
  time in different places, or at the same place in
  rapid succession, their effects add up.
• This adding up is called spatial or temporal
  summation

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• Because voltage spreads along the dendrites and
  cell body without an action potential, the
  strength of PSPs decay with distance

Fig 48.13
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• Because voltage spreads along the dendrites and
  cell body without an action potential, the
  strength of PSPs decay with distance
• The closer a synapse is to the axon hillock, the
  stronger its influence on post-synaptic firing.

Fig 48.13
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The way in which a neurons EPSPs and IPSPs sum
to cause (or prevent) an action potential
represents a computation.
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Figure 48.19 Embryonic development of the brain
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Computational brain

• Some of its many computations include
• Reflex arcs
• Homeostasis
• Learning
• Personality

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Neural activity is thought itself.
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Isnt it?
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Diversity of the vertebrate brain

• Evolutionary diversity

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Diversity of the vertebrate brain

• Evolutionary diversity
• Acquired diversity

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Diversity of the vertebrate brain

• Evolutionary diversity
• Hippocampus of food-storing birds
• Acquired diversity

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Prairie vole Montane vole

• Highly social
• Biparental
• Pair-bonds

• Solitary
• Minimally parental
• Promiscuous

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Diversity of the vertebrate brain

• Evolutionary diversity
• Hippocampus of food-storing birds
• Vasopressin and monogamy
• Acquired diversity

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Transgenic mouse expressing V1a receptor under
control of prairie vole promoter
pvV1a transgene
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1623
pA
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Diversity of the vertebrate brain

• Evolutionary diversity
• Hippocampus of food-storing birds
• Vasopressin and monogamy
• Acquired diversity
• Hippocampus in taxi drivers

Time as London taxi driver
Volume of hippocampus
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Diversity of the vertebrate brain

• Evolutionary diversity
• Hippocampus of food-storing birds
• Vasopressin and monogamy
• Acquired diversity
• Hippocampus in taxi drivers
• Brains of ecstasy users

Serotonin in the brains of ecstasy users (top)
and non-users (bottom)
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Is the brain an evolutionary hypothesis?