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CSB332H1 Study Guide - Midterm Guide: Squid Giant Synapse, Stellate Ganglion, Synaptic Vesicle


Department
Cell and Systems Biology
Course Code
CSB332H1
Professor
Francis Bambico
Study Guide
Midterm

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Chapter 13 Release of Neurotransmitters
Release of neurotransmitter stimulus: depolarization of the nerve terminal
Release occurs as a result of Ca entry into the terminal through voltage-activated Ca channels
Invariably, a delay of about 0.5 milliseconds intervenes between presynaptic depolarization and transmitter
release because :
o The time taken for Ca channels to open
o Time required for Ca to cause transmitter release
Transmitter is secreted in multimolecular packets (quanta), each containing several thousand transmitter
molecules
1-300 quanta are released almost synchronously, depending on the type of synapse, in response to an AP
At rest, nerve terminals release quanta spontaneously at a slow rate, giving rise to spontaneous miniature
synaptic potentials (minis)
At rest, there is also a continuous, nonquantal leak of transmitter from nerve terminals
One quantum of transmitter corresponds to the contents of one synaptic vesicle and comprises several
thousand molecules of a low-MW transmitter
Release occurs by exocytosis, during which the synaptic vesicle membrane fuses with the presynaptic membrane
and the contents of the vesicle are released into the synaptic cleft
The components of the vesicle membrane are then retrieved by endocytosis, sorted in endosomes, and recycled
into new synaptic vesicles
The presynaptic terminals at vertebrate skeletal NMJ are typically too small for electrophysiological recording;
however, this can be done at a number of synapses, such as the giant fiber synapse in the stellate ganglion of a
squid
Characteristics of Transmitter Release
Axon Terminal Depolarization and Release
Stellate ganglion of the squid was used by Katz and Miledi to determine the precise relation between
presynaptic membrane depolarization and the amount of transmitter released
Simultaneous records were made of the AP in the presynaptic terminal and the response of the postsynaptic
fiber
Sketch of the stellate ganglion , illustrating the 2 large axons that
form a chemical synapse
o Pre and post synaptic axons are impaled with
microelectrodes to record membrane potential
o An additional microelectrode is used to pass depolarizing
current into the presynaptic terminal
To further explore the relation between the potential amplitude and
transmitter release, they placed a second electrode in the presynaptic terminal, through which they applied a
brief (1-2ms) depolarizing current pulses, thereby mimicking a presynaptic AP
The relationship between the amp of the artificial AP and that of the synaptic potential was the same as the
relation obtained with the failing AP during TTX poisoning
This result indicates that the normal fluxes of Na and K ions responsible for the AP are not necessary for
transmitter release; only depolarization is req
Synaptic Delay

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There is a lag time between the onset of the presynaptic AP and the beginning of the synaptic potential known
as the synaptic delay
Detailed measurements at the
frog NMJ show a synaptic
delay of .5ms too long to be
accounted for by diffusion of
Ach across the cleft (usually
50 microseconds)
A: the motor nerve is
stimulated while recording
with an extracellular
microelectrode at the frog
NMJ. With this recording
arrangement, current flowing
into the nerve terminal or the
muscle fiber is recorded as a
negative potential
B: (S) stimulus artifact; (AP)
the axon terminal AP; (EPC)
end plate current; the synaptic delay is the time between the AP in the terminal and the beginning of the EPC
When ACh is applied to the junction ionophoretically from a micropipette, delays as little as 150 microseconds
can be achieved, even though the pipette is much farther from the postsynaptic receptors than are the nerve
terminals
Furthermore, synaptic delay is much more sensitive to temperature than would be expected if it were due to
diffusion
Cooling the frog to 2.5 degrees increases the delay to as long as 7 ms, whereas the delay in the response to
ionophoretically applied Ach is not perceptibly altered -> the delay is largely in the transmitter release
mechanism
Evidence that Ca is Required for Release
Ca is the essential link in the process of synaptic transmission
When its conc in the ECF is decreased, release of Ach at the NMJ is reduced and eventually abolished
Evoked transmitter release is preceded by Ca entry into the terminal and is antagonized by ions that block Ca
entry
Transmitter release can be reduced either by removing Ca from the bathing soln or by adding a blocking ion
For transmitter release to occur, Ca must be present in the bathing soln at the time of a depolarization of the
presynaptic terminal
Measurement of Ca Entry into Presynaptic Nerve Terminals
Entry of Ca into the nerve terminal is through voltage-sensitive Ca channels that are activated upon
depolarization by the presynaptic AP
the presynaptic terminal is voltage clamped and treated with TTX and TEA to abolish voltage-activated Na and K
currents
A: records show potentials applied to the presynaptic fiber (upper trace), presynaptic Ca current (middle), and
EPSP in the postsynaptic fiber (lower)
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