Psychology 304: Brain and Behaviour Lecture 13

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Psychology 304 Brain and BehaviourLecture 13
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Announcement In preparation for the midterm exam,
I will hold additional office hoursWednesday,
October 14 300-400 PMFriday, October 16
300-430 PMTuesday, October 20 1200-200
PM Naghmeh, your TA, will hold one additional
office hour Friday, October 16 1000-1100
AMMonday, October 19 1000-1100 AM
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Conduction and Transmission of Electrochemical
Neural Signals
1. How is an action potential conducted along
the axon of a neuron?
2. What is the structure of the synapse between
two neurons?
3. How are neurotransmitters released from
presynaptic neurons?
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How is an action potential conducted along the
axon of a neuron?

• Once an action potential has occurred near the
  axon hillock, it travels along the axonal
  membrane to adjacent voltage-activated Na
  channels. The electrical signal opens the
  channels, generating an action potential on this
  portion of the membrane.

• These events are repeated along the length of
  the axon, through to the terminal buttons.

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• In myelinated axons, ions can pass through the
  membrane only at nodes of Ranvier. The
  electrical signal from the previous action
  potential is conducted passively along the
  myelin segment, until it reaches the node of
  Ranvier, at which point a full action potential
  is generated.

• Thus, myelination increases the speed of axonal
  conduction, via saltatory conduction.

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Saltatory Conduction
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What is the structure of the synapse between two
neurons?

• Action potentials arriving at terminal buttons
  trigger the release of neurotransmitters. The
  neurotransmitters are released into the synapses
  (i.e., synaptic clefts) between neurons,
  allowing neurons to communicate.

• Most communication between neurons occurs
  across axodendritic synapses.

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Synapsticcleft
Synaptic cleft
Structure of an Axodendritic Synapse
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Synaptic cleft
Vesicles
Presynaptic membrane
Postsynaptic membrane
Electron Micrograph of an Axodendritic Synapse
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• Several other synaptic arrangements, however,
  have been identified

Axosomatic synapses Synapses of axon terminal
buttons on cell bodies (i.e., somas).
Dendrodendritic synapses Synapses of dendrites
on dendrites.
Axoaxonic synapses Synapses of axon terminal
buttons on axons.
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An Axoaxonic Synapse
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Directed synapses Synapses at which the site of
neurotransmitter release and the site of
neurotransmitter reception are relatively close.
Nondirected synapses Synapses at which the site
of neurotransmitter release and the site of
neurotransmitter reception are relatively
distant.
Gap junctions Electrical synapses. Narrow
spaces between adjacent neurons are bridged by
fine tubular channels that contain cytoplasm.
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Gap Junctions
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How are neurotransmitters released from
presynaptic neurons?

• When stimulated by an action potential,
  voltage-activated calcium channels open and Ca2
  ions enter the terminal button. This causes
  vesicles in the button to fuse with the
  presynaptic membrane and release their contents
  into the synaptic cleft.

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Release of Neurotransmitters into the Synaptic
Cleft
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Neurotrans-mitter
Synaptic cleft
Presynaptic membrane
Release of Neurotransmitters into the Synaptic
Cleft
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Conduction and Transmission of Electrochemical
Neural Signals
1. How is an action potential conducted along
the axon of a neuron?
2. What is the structure of the synapse between
two neurons?
3. How are neurotransmitters released from
presynaptic neurons?