NESC 2570 Study Guide - Final Guide: Nernst Equation, Inhibitory Postsynaptic Potential, Resting Potential
27 Jun 2018
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What signals can be relayed by electrical synapses? What functional roles
do electrical synapses have? Is transmission at electrical synapses regulated?
Electrical synapses are gap junctions that are channels with very large
pores and transmit all ions and allow for passage of second messengers
(cAMP and cGMP). We expect them to transmit messages very quickly
because it only needs an ionic current through the gap junctions. They have
roles in escape responses and should synchronize neurons activity of like-
minded neurons. And yes the electrical synapses is regulated,
phosphorylation does this, connexons get phosphorylated that modulate the
opening of gap junction channels.
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1)
Describe neurotransmission at a chemical synapse using the terms "voltage-
gated potassium channel", "voltage gated calcium channel", "voltage gated
sodium channel", "metabotropic receptor" and "ionotropic receptor". How
is neurotransmission at chemical synapses terminated?
Action potential arrives at the pre synaptic specialization which opens
voltage gated sodium channels open which lead to the depolarization of the
membrane. Membrane depolarization leads ot the opening of the calcium
channels which binds to a calcium sensor protein on synaptic vesicles
which in turn allows the vesicles to fuse with the plasma membrane and
release neurotransmitters. Neurotransmitter can bind to two types of
neurotransmitter receptors.
Ionotropic - ligand gated ion channels
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Metobotropic - g protein coupled receptors that activate a trimeric G
protein which can activate an enzyme that produces a second
messenger or bind and modulate the opening ion channels.
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They yield the same result which is a post synaptic potential
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Voltage gated sodium channels cause the opening of potassium channels
which allows for the repolarization of the membrane. Calcium channels
close at the repolarization of the membrane and neurotransmitter is taken
up or can diffuse out of the synaptic cleft. This results in the inability for
threshold to be met at the post synaptic neuron
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2)
What is the reversal potential (Erev) of a synapse? How is the post synaptic
current related to membrane potential (Vm) and Erev?
Reverse potential is defined as the post synaptic membrane potential at
which there is no net flux of charges through the ligand gated ion channels
or the channels regulated by the metobotropic receptors. This tells us
whether or not we have a synapse when the membrane depolarizes or
hyperpolarizes. When the synapse open, the membrane potential moves to
the reverse potential of the channels and you can deduce how the current
should look. Sometimes, ion channels can close before reversal potential is
met. Synapse will open and the membrane potential will move towards the
reversal potential so you can deduce what the current looks like. For
example, if the membrane depolarizes, making the membrane potential
more positive than the reversal potential, positive charges either need to
come into the cell or negative charges need to leave the cell. This works
vice versa as well, if the reversal potential is more negative than the
membrane potential, then the post synaptic neuron needs to hyperpolarize
meaning that positive charges need to leave the cell or negative charges
need to enter the cell.
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3)
A researcher is characterizing synaptic connections in a squid. She
hypothesizes that one of the synapses she is studying may utilize ionotropic
NT receptors that are equally permeable to sodium and potassium.
Extracellular and intracellular ion concentrations are show in the table
What reversal potential should she find at this synapse if her hypothesis is
correct?
4)
Use Goldman's equation
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Reversal potential is 0mV - this means that when the post synaptic
membrane will move towards 0 mV and the course of that, it might reach
threshold to release an action potential. This makes this an excitatory
potential
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A synapse contains chloride selective ionotropic GABA receptors. Assuming
the intra and extracellular ion concentrations given in the table to the right,
the reversal potential of postsynaptic currents at this synapse is _____ mV.
If the resting membrane potential is -70mV and action potential threshold
is -35mV, the synaptic current will (hyperpolarize/depolarize the membrane.
The synapse is (excitatory/inhibitory)
5)
The reversal potential is -58mV
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At Vm = -7mV, and AP threshold of -35mV, the synaptic current will
depolarize the membrane but will be inhibitory because threshold for
activation is -35mV and this is much larger than the resting potential
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Use Nernst equation
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Shunting inhibitory synapse
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Review Questions
October 20, 2015
9:52 PM
