When Neurons Meet: The Synapse

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When Neurons Meet The Synapse
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Synapse

• The term synapse was first introduced by
  Charles Sherrington to describe the zone of
  contact between neurons

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Synapse Characteristics

• Synaptic Cleft 20-40 nm distance
• Presynaptic Active Zones Presynaptic Vesicles
• Postsynaptic receptors
• NT flow (always Unidirectional)
• Transmission delay of .3 to 5 msec

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Exemplar - Multipolar Synapse

• From thousands of pre synaptic terminals
• From inhibitory and excitatory cells

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Membrane

• Stimulation
• From thousands of pre synaptic terminals
• From inhibitory and excitatory cells
• By NT binding result in inhibitory and excitatory
  membrane response (graded potentials)
• Membrane excitatory postsynaptic potentials
  (EPSPs)
• Membrane inhibitory postsynaptic potentials
  (IPSPs)

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Anatomy of the Synapse
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Pre Synaptic Membrane

• The terminal button at the synapse
• Synaptic vesicles loaded with NT sit
• Where action potentials cause synaptic vesicles
  to fuse with presynaptic membrane and release
  massive NT
• The greater the stimulation the greater NT
  release
• Terminal buttons occasionally release small
  amounts of NT even w/out cell stimulation

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Synaptic Vesicles

• Synaptic Vesicles
• Formed in Cell nucleus
• Transported via microfilament tracks to terminal
  button
• At synaptic terminal vesicles are filled with
  several thousand molecules of NT

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Pre-synaptic Membrane Active Zones

• Vesicles line up and move into active zone in
  revolver-like fashion
• Note Fusion of synaptic vesicles and
  presynaptic membrane

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Synaptic Cleft

• Region separating pre/post synaptic cell
  (Cytoplasmic discontinuity)
• Width 20-40 nm distance
• NT are released into synaptic cleft

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Post-Synaptic Membrane

• Contain NT docking sites called receptors
• - NT released into synaptic cleft bind to
  specific postsynaptic sites
• - NT do not produce EPSP or IPSP
• - NT binding produce inhibitory and excitatory
  response (graded potentials)
• NT do not produce EPSP or IPSP

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Post Synaptic Membrane

• Stimulation
• From thousands of pre synaptic terminals
• From inhibitory and excitatory cells
• By NT binding result in inhibitory and excitatory
  membrane response (graded potentials)
• Membrane excitatory postsynaptic potentials
  (EPSPs)
• Membrane inhibitory postsynaptic potentials
  (IPSPs)

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NT Binding

• NT bind to specialized - receptors
• NT-receptor Binding opens specific ion channels
• Opening of specific ion channels initiate EPSP
  and IPSP

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NT Receptor Binding

• Two Types of Post synaptic Receptors
• Ionotropic receptors NT binding results in
  direct opening of specific ion channels
• Metabotropic receptors binding of NT initiates
  a sequence of internal molecular events which in
  turn open specific ion channels

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NT binding -gt Membrane Potential Response
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Mechanism of Neurotransmission
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Anatomy of the Synapse
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Types of Synapses

• Axo-dendritic synapse
• Axo-axo synapse
• Axo-soma synapse

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What happens after NT release?

• Presynaptic
• Synaptic vesicles are recycled
• Repackaged
• In Synaptic Cleft
• Transmission termination
• Postsynaptic
• Membrane excitatory postsynaptic potentials
  (EPSPs)
• Membrane inhibitory postsynaptic potentials
  (IPSPs)

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At the Synaptic Cleft

• NT Termination
• Three methods
• Diffusion
• Reuptake
• Enzymatic degradation
• E.g., AchE hydrolizes Ach into acetyl and choline

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And,

• Neurotransmitters flow from one neuron to another
  neuron across the synapse continue the neural
  message

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When a Neuron Meets a Muscle The Neuromuscular
Junction

• Site where motor neuron meets the muscle fiber
• Separated by gap called the neuromuscular cleft
• Motor end plate
• A pocket of the muscle that surrounds the
  neurons den
• Acetylcholine is released from the motor neuron
• Causes an end-plate potential (EPP)
• Depolarization of muscle fiber

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Illustration of the Neuromuscular Junction