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Chapter 5: Synaptic Transmission (Neurotransmitters (Neurotransmitter…
Chapter 5: Synaptic Transmission
Electrical Synapses: Occur at Gap Junctions
Gap junction channels formed by 2 connexons- each of which has 6 connexins.
Very fast transmission
Chemical Synapses
Three types
Axosomatic: Axon to cell body
Axoaxonic: Axon to axon
Axodendritic: axon to dendrite
Axospinous: Axon to dendritic spine
Categories of CNS Synapses
Gray's Type I: Asymmetrical, usually excitatory
Gray's Type II: Symmetrical, Usually Inhibitory
Neuromuscular Junction (NMJ): Chemical, one of largest synapses in body
Neurotransmitters
Nonclassical neurotransmitters
Lipids (Anandamide)
Gases (Nitric Oxide)
Neuropeptides (Endorphins, enkephalins, Corticotropin-releasing factor [CRF], Brain-derived neurotrophic factor [BDNF], Others
Classical Neurotransmitters
Amino Acids (Glutamate, GABA, Glycine)
Monoamines (Dopamine, Norepinephrine, serotonin)
Acetylcholine
Three Criteria to Determine Neurotransmitter:
Substance released in response to presynaptic depolarization & release must be calcium-dependent
Specific receptors for substance must be present on postsynaptic tail
Substance present in presynaptic terminal
Neurotransmitter Synthesis
Neuropeptide synthesis/storage
Transporters in vesicle membrane concentrate finished NTs into synaptic vesicles
Synthesized in ER, split in Golgi, bud off into secretory granule
Precursor proteins broken down by enzymes during shipment to axon terminals
Neurotransmitter Release through Ca mediated exocytosis
Voltage-gated Ca channels open upon depolarization, Ca enters cell
Ca2+ channels are concentrated in active
zones
Ca2+ sensitive proteins cause the vesicles to fuse with the cell membrane
SNARE Proteins
Syntaxin
and
SNAP-25
in plasma membrane (t-SNARES)
Synaptotagmin
in vesicle membrane binds to Ca2+
Synaptobrevin
in synaptic vesicle membrane (v-SNARE)
Botulism blocks ACh release at neuromuscular junctions
Neurotransmitter Receptors
Transmitter-gated ion channels (aka: ligand-gated or
ionotropic)
NT binds to the receptor and causes a conformational change
EPSP and IPSP
G-protein coupled receptors (metabotropic)
Act in 2 ways
Inhibit or Activate Ion channels
Stimulate or Inhibit Effector Enzymes in cell membrane that synthesize or break down second messenger molecules
Neurotransmitter is first messenger
Second messengers activate protein kinases to phosphorylate substrate proteins
cAMP for example
Reversal potential (Erev): Vm that net current reverses
Occurs when ligand-gated channels are permeable to more than one ion
If Vm < 0, net current flow would be inward (Vm becomes more positive)
Autoreceptors: Presynaptic receptors sensitive to the neurotransmitter released by the presynaptic terminal (Negative feedback)
Neurotransmitter Recovery/Degradation
Reuptake via transporters
Diffusion
Enzymatic destruction
Mutation in glutamate transporter causes seizures, other bad stuff
Nerve gases inhibit AChE from breaking down ACh
Neuropharmacology
Agonist: Mimics actions of neurotransmitters
Antagonist: inhibits Neurotransmitter receptor
EPSP summation
Temporal Summation: same synapse, rapid summation
Spatial Summation: Simultaneously, different sites
Dendritic length constant (λ)— indicates how far depolarization can spread
At λ, depolarization is 37% of original
Shunting inhibition: Synapse inhibits current flow to axon hillock