The synapse: Where the impulse is passed from one cell to another

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• The synapse Where the impulse is passed from one
  cell to another
• Two basic kinds of synapses
• Electrical (gap junctions)
• Very fast
• Excitatory
• Does not require neurotransmiters
• Chemical
• Requires a neruotransmitter of some sort
• Fast (but slower than electrical)
• Can excite or inhibit
• Can modulate the permeability of a post synaptic
  element for an extended period of time

2

• The synaptic process Key events of a chemical
  synapse
• Action potential reaches the axon terminal where
  the presynaptic element resides.
• Causes the opening of CA channels.
• Ca forces the movement of microtubules onto
  synaptic vesicles pressing them to the
  presynaptic element.
• Vesicles bind to specific sites on the
  presynaptic element and open, spilling their
  contents (a neurotransmitter) into the synaptic
  cleft
• Neurotransmitters (the ligand) bind to receptors
  at specific binding sites on the post synaptic
  cell membrane causing either
• Deformation of the receptor protein which opens a
  ion channel
• Deformation of the receptor protein which
  activates a second messenger (G-protein coupled
  receptors).
• Ultimately both mechanisms can either cause
• depolarization of the post synaptic element
  (EPSP)
• hyperpolarizing of the post synaptic element
  (IPSP)

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Typical excitatory vs inhibitory synaptic events
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The neuromuscular junction synapse from neuron
to muscle

• Synapses can occur between
• Sensory cells and neurons
• Neurons and muscles
• Neurons and glands

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G-protein coupled receptors can amplify
transmitter effects and modulate cell sensitivity
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Synaptic complexity Neurons have a variety of
receptive (dendritic) fields
Adendritic
Spindle radiation
Spherical radiation
Laminar radiation Planar
Laminar radiation Offset
Laminar radiation Multi
Conical radiation
Biconical radiation
Fan radiation
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• Synaptic complexity Types of synapses.
• Axo-dendritic
• Dendro-dendritic
• Dendro-axonic
• Axo-axonic
• Dendro-somatic
• Axo-somatic

Computational complexity of synaptic regions
Electron micrograph image of a reciprocal
dendro-dendritic synapse (D1, D2) and an axon
from a third cell (A) makes an asymmetric synapse
on D2. Arrows point to synaptic vesicles and
presumed polarity of chemical transmission. From
Cat Thalamus (x 33 000)
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Synaptic complexity There not that simple inside
either
The take home message here is that a synapse is
like a tiny computational compartment!
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Synaptic complexity Spines represent pockets of
synaptic computation
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Synapses change synaptic plasticity

• Plasticity occurs for a number of reasons
• Development aging
• Experience (learning, exhaustion)
• The net result of plastic nervous systems is that
  they can adapt!

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Electrophysiology Direct method(s) for
monitoring neurons

• Intracellular (glass electrode)
• Patch electrode
• Sharp electrode
• Extracellular (wires/metals)
• Hook electrodes
• Beveled wire
• Silicon electrodes

• Examples of Indirect methods
• FMRI
• CT
• Optical imaging
• Calcium imaging

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Standards of evidence to establish a causal
neural basis of behavior
Take home message To say a neuron causes a
behavior, you need to establish that a neuron (or
group of neurons) is both necessary and
sufficient! -a correlation just wont do!