BIOLOGY 2C03 Lecture Notes - Exocytosis, Reversal Potential, Synaptotagmin 1

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28 Jan 2013
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Outline of Lecture 06 (02-19 B; Linden)
Synaptic Transmission
I. Ca++ couples action potential depolarization to neurotransmitter release
- Whereas Na and K help propagate an action potential, Na and K current are not necessary at the
presynaptic terminal for nxt release
- Evidence: Depolarization with TTX and TEA blockage of Na and K channels still results in nxt
release
- Voltage-gated Ca channels and Ca current are necessary for nxt release
- Evidence: numerous, e.g. removal of Ca blocks nxt release, Ca flood causes nxt release
- Ca promotes fusion of synaptic vesicles with presynaptic membrane
- Candidate interactions: synaptotagmin (particularly), calmodulin, PK-C
- Since nxt is stored in discrete vesicles of uniform size, nxt release is quantal and not graded
II. Neurotransmitters that affect ion channels cause predictable excitation or inhibition
- Nxt bind postsynaptic receptors and (1) open/close an ion channel or (2) modulate enzyme
activity (focus on the 1st here)
- ACh opens a channel equally permeable to Na and K whose reversal potential is ~ 0 mV (-11mV
by Nernst eqn), therefore ACh depolarizes the cell
- Nxt produce excitatory or inhibitory response if reversal potential of ion channel receptor (equil.
potential if single ion) is above or below threshold, respectively
- Consequence: opening of Na/Ca channels is excitatory, of K/Cl channels is inhibitory
- Excitation and inhibition aren’t synonymous with de- and hyperpolarization, e.g. if
depolarization is still inhibitory if reversal potential is below threshold
III. Time course of synaptic transmission
Presynaptic action potential Ca influx exocytosis of nxt vesicles nxt binds postsynaptic
receptors activation of postsynaptic ion channels postsynaptic action potential
Summary of major points
- Understand the role of Ca in nxt release
- Be able to predict response to depolarization and hyperpolarization: is it excitatory or inhibitory?
- Be able to describe the major events of synaptic transmission
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