Chapter 7 Synaptic Transmission

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Chapter 7Synaptic Transmission
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Synapse

• A functional connection between a neuron
  (presynaptic) another cell (postsynaptic)
• Chemical electrical synapses
• Synaptic transmission in chemicals is via
  neurotransmitters (NT)
• Electricals are rare in NS

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• Signaling between cells was originally thought to
  have occurred only by electrical signaling.
• In 1921, Otto Loewi published an experiment
  suggesting signaling by chemical transmission.

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Synaptic Transmission

• APs travel down axon to depolarize bouton
• Open VG Ca2 channels in bouton
• Ca2 driven in by electrochemical gradient
• Triggers exocytosis of vesicles release of NTs

Neurotransmitter Release

• Rapid because vesicles are already docked at
  release sites on bouton before APs arrive
• Docked vesicles are part of fusion complex
• Ca2 triggers exocytosis of vesicles

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Synaptic Transmission continued

• NT (ligand) diffuses across cleft
• Binds to receptor proteins on postsynaptic
  membrane
• Chemically-regulated ion channels open
• Depolarizing channels cause EPSPs (excitatory
  postsynaptic potentials)
• Hyperpolarizing channels cause IPSPs (inhibitory
  postsynaptic potentials)
• These affect VG channels in postsynaptic cell

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Acetylcholine (ACh)

• Most widely used NT
• NT at all neuromuscular junctions
• Used in brain
• Used in ANS
• Where can be excitatory or inhibitory
• Depending on receptor subtype
• Nicotinic or muscarinic

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Ligand operated channels
Ligand
Receptor
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Receptors Exhibit Two Types of Specificity
(1) Ligand Specificity-which ligand(s) are bound.
Similar to the specificity of an enzyme for its
substrate. (2) Effector Specificity-the response
to ligand binding

• Same ligand binding to a receptor can have
  different effects in different cell types ? same
  ligand specificty, different effector specificity

• Different ligands binding to different receptors
  on the same cell or different cells can have the
  same effect? different ligand specificity, same
  effector specificity

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G Protein-Operated Channels

• Receptor is not part of the ion channel
• Is a 1 subunit membrane polypeptide
• Activates channel indirectly through G-proteins

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Muscarinic ACh Channel

• Binding of 1 ACh activates G-protein cascade
• Opens some K channels, causing hyperpolarization
• Closes others, causing depolarization

Fig 7.25
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Acetylcholinesterase (AChE)

• Inactivates ACh, terminating its action located
  in cleft

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Neuromuscular Junction (NMJ)

• Cholinergic neurons use acetylcholine as NT
• Large synapses on skeletal muscle are termed end
  plates or neuromuscular junctions
• Produce large EPSPs called end-plate potentials
• Open VG channels beneath end plate
• Cause muscle contraction
• Curare blocks ACh action at NMJ

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Monoamine NTs

• Receptors activate G-protein cascade to affect
  ion channels

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Monoamine NTs

• Include serotonin, norepinephrine, dopamine,
  epinephrine
• Serotonin is derived from tryptophan
• Norepi dopamine are derived from tyrosine
• Called catecholamines

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Serotonin

• Involved in regulation of mood, behavior,
  appetite, cerebral circulation
• LSD is structurally similar
• SSRIs (serotonin-specific reuptake inhibitors)
  include antidepressants
• Prozac, Zolof, Paxil, Luvox
• Block reuptake of serotonin, prolonging its action

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Dopamine

• Involved in motor control emotional reward
• Degeneration of dopamine motor system neurons
  causes Parkinson's disease
• Reward system is involved in addiction
• Schizophrenia treated by anti-dopamine drugs

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Norepinephrine (NE)

• Used in PNS CNS
• In PNS is a sympathetic NT
• In CNS affects general level of arousal
• Amphetamines stimulate NE pathways

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Amino Acids NTs

• Glutamic acid aspartic acid are major CNS
  excitatory NTs
• Glycine is an inhibitory NT
• Opens Cl- channels which hyperpolarize
• Strychnine blocks glycine receptors
• GABA (gamma-aminobutyric acid) is most common NT
  in brain
• Inhibitory, opens Cl- channels

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EPSPs

• Graded in magnitude
• Have no threshold
• Cause depolarization
• Summate
• Have no refractory period

Fig 7.27
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Spatial Summation

• Cable properties cause EPSPs to fade quickly over
  time distance
• Spatial summation takes place when EPSPs from
  different synapses occur in postsynaptic cell at
  same time

Fig 7.31
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Temporal Summation

• Temporal summation occurs because EPSPs that
  occur closely in time can sum before they fade

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Synaptic Plasticity

• Repeated use of a synapse can increase or
  decrease its ease of transmission
• synaptic facilitation or synaptic depression
• High frequency stimulation often causes enhanced
  excitability
• Called long-term potentiation
• Believed to underlie learning

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Synaptic Inhibition

• Postsynaptic inhibition
• GABA glycine produce IPSPs
• IPSPs dampen EPSPs
• Making it harder to reach threshold
• Presynaptic inhibition
• Occurs when 1 neuron synapses onto axon or bouton
  of another neuron, inhibiting release of its NT

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