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Graduate Seminar, NJIT • June 14, 2012
Analytical & Computational Neuroscience: Lecture 2
Action Potentials and Chemical Synapses
Math 430 * Math 635 * Fall 2015 * Victor Matveev
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Graduate Seminar, NJIT • June 14, 2012
Neuron is charged: VinVout ≈ 70mV
Outside: high Na+, low K+
Inside: low Na+, high K+
Neurons and synapses
Human brain:
~1011 neurons
~1015 synapses
Node of Ranvier
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Graduate Seminar, NJIT • June 14, 2012
Synapses are connections between two neurons
Structure of neuron in more detail
Neuron is charged: VinVout ≈ 70mV
Outside: high Na+, low K+
Inside: low Na+, high K+
Human brain:
~1011 neurons
~1015 synapses
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Graduate Seminar, NJIT • June 14, 2012
Myelin Sheath and Nodes of Ranvier
• Schwann cells (peripheral NS) similar to Oligodendrocytes (CNS)
• Myelinated axonal segments:
Electric potential propagates passively and fast
• Nodes of Ranvier
Action potential initiated at first node of Ranvier, and is
amplified at other nodes by voltage-dependent ion channels
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Graduate Seminar, NJIT • June 14, 2012
gNa gK gL
VNa VK VL
Cm
Vm
Action potential (spike)Excitable cells: neurons, endocrine cells, muscle cells
Low K+
Moderate Ca2+ High Na+
Equivalent RC circuit
Vm(t )
Vrest=-75 mV
Applied current
gCa gNa
gK
(A) Vm increases
(B) Na+ channels open
(C) K+ channel open
AP time course
Vm
High K+
Very low Ca2+
Low Na+
Cell Blood
Molec Cell Biol (2000) 4th Ed. Lodish, Berk, Zipursky et al
Units: mM
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Graduate Seminar, NJIT • June 14, 2012
gNa gK gL
VNa VK VL
Cm
Vm
Equivalent RC circuit
Vm(t )
Vrest=-75 mV
Applied current
gCa
gNa
gK
(A) Vm increases
(B) Na+ channels open
(C) K+ channel open
(D) Na+ channels close
(E) K+ channels close
AP time course
A
B
C D
E
Action potential (spike)Excitable cells: neurons, endocrine cells, muscle cells
Vm
High K+
Very low Ca2+
Low Na+
Low K+
Moderate Ca2+ High Na+
Cell Blood
Molec Cell Biol (2000) 4th Ed. Lodish, Berk, Zipursky et al
Units: mM
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Neurotransmitter-filled vesicles
Presynapticcell (axon)
Ca2+ ions
~40nm
Postsynaptic cell (neuron or
muscle)
Neurotransmitters: glutamate, GABA, acetylcholine, serotonin, dopamine…
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Action potential (depolarizing pulse)
Ca2+ ionsCa2+ channelsopen
Neurotransmitter-filled vesicles
Presynapticcell (axon)
Postsynaptic cell (neuron or
muscle)
Neurotransmitters: glutamate, GABA, acetylcholine, serotonin, dopamine…
~40nm
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Ca2+ channelsopen
Several Ca2+ ions bound to release site
Presynapticcell (axon)
Action potential (depolarizing pulse)
Postsynaptic cell (neuron or
muscle)
~40nm
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Exocytosis (fusion)
Neurotransmitter receptors/channels
Neurotransmitter diffuses into synaptic cleft
Presynapticcell (axon)
Postsynaptic cell (neuron or
muscle)
~40nm
Vesicle fusion initiated by SNARE protein complex interacting with Ca2+-sensor synaptotagmin
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Neurotranmitter binds to postsynaptic receptors
Neurotransmitter receptors/channels
Presynapticcell (axon)
Postsynaptic cell (neuron or
muscle)
~40nm
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Postsynaptic ion channels open
Neurotransmitter receptors/channels
Presynapticcell (axon)
Postsynaptic cell (neuron or
muscle)
~40nm
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Graduate Seminar, NJIT • June 14, 2012
Chemical Synaptic Transmission
Postsynaptic ion channels open
Influx of Na+ & Ca2+ (excitation)
K+ efflux, Cl- influx (inhibition)
Presynapticcell (axon)
Postsynaptic cell (neuron or
muscle)
~40nm
Total transmission
delay ~ 200 μs
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Graduate Seminar, NJIT • June 14, 2012
Synapses and Neurotransmitters (NT)
• In mammalian brain: roughly 104 synapses per neuron: 1015 total
• A given neuron secretes a single NT (single type on NT load), e.g.:
Dopaminergic neuron: synapses release dopamine
Cholinergic neuron: synapses release acetylcholine
…Etc.
• A given neuron may be activated by many NTs
Afferents (inputs): multiple NTs
Efferent (output): single NT (…well, for the most part)
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Graduate Seminar, NJIT • June 14, 2012
Synapses and Neurotransmitters (NTs)
• Gap junctions: direct “pore”; no NT needed!
• Chemical:
○ Ionotropic (fast): receptors activate ion channels
○ Metabotropic (slow): receptors activate second messengers
• Depending on NT, chemical synapses may be
○ Excitatory
- Examples (ionotropic): glutamatergic, cholinergic
- Postsynaptic channels permeable to Na+ and Ca++
○ Inhibitory
- Examples (ionotropic): GABAergic
- Postsynaptic channels permeable to K+ and Cl-
Note: chemical synapses allow inhibitory interaction between neurons!
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Graduate Seminar, NJIT • June 14, 2012
“Excitable” Cells
Neurons are not the only cells that change Vm
• Endocrine cells
Hormone similar to neurotransmitter
• Muscle Cells
Ca2+ important for AP and muscle contraction