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Neuroscience
Lecture 2
L2
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Difference in charge across the two membranes gives
the membrane potential
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Theres more negative charge on inside cell, giving resting membrane potential of -70mV
-Have voltage/ligand gated receptors that can affect the
-a lipid bilayer is virtually impermeable to ions
-ions can only cross the membrane through ion channels (passive)
-at rest, the passive ion fluxes are balanced so the membrane potential is constant – the
balancing is done by the Na/K-ATPase pump (3 Na+ out and 2 K+ in)
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Therefore used for signalling
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Learn this definition!

Potential at which there’s no overall flow of the ions across membrane, when they're in
equilibrium
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Questions
What’s the Nernst potential? Define it
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Use Nernst potential end up with -75mV, when K+ is happy, distributed where it wants to be, its
membrane potential is -75mV
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(so
membrane potential of -70mV) Inject a known quantity of current into the cell (mimicking ions
through channel), and measuring the Vm response
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Depolarisation returning membrane potential towards 0
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Membrane potential= negative = polarised
But if inject enough to reach past the threshold potential, causes an action potential
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This only looks at the voltage- the membrane potential, not how the channels change
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The strategy of current clamp involves injecting a known quantity of current into the cell, and
measuring the Vm response
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So depolarisation = Vm moving towards 0
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How can we measure the membrane potential of a cell?
(ii) properties of voltage-gated ion channels
Measuring ion channel activity using voltage clamp

Voltage clamping a cell is done by a voltage clamp amplifier clamps the voltage at what you
want it to be
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Modifying current to get voltage to what you want it to be
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You set the voltage on the amplifier which takes a measurement of the voltage in real time,
measures the difference in the voltage and then injects the correct amount of current to get the
voltage to what you want to be
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This show the current flowing in
the cell
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The electrode doesn’t stick into the cell, they use a
glass electrode, a few microns in diameter, fine glass point
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This is called the
cell attached mode, any current flowing through can be measured, can study individual
channels and see them opening and closing
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It forms a tight seal
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Can use
this to work out the conductance
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This is whole cell
recordings
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This measures the whole cell behaviour, can measure the
populations of all the ion channels in the membrane
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If you change the membrane potential, change from -100 to -70 what happens? Nothing as the
threshold is -40/-55
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If you go past it
the current becomes less again, why? Because as you go more positive you head towards the
Nernst potential for sodium rather than potassium
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The more positive you go the more the sodium channels give up
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59
Conclusions from voltage clamp analysis

Closing at resting potential, open
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Questions
Whats the conclusion from voltage clamp analysis? Difference in K/Na?
What are these molecules actually doing?

Did an experiment where they changed the saltwater water potential the cells were in so see
how this affected the cells action potential’s kinetics
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Nowadays we can clone the channels themselves
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They all have 6 spanning transmembrane proteins-
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The K channel is a tetramer, four separate channel proteins forming
the overall channel, by four different genes
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The subunits are encoded by a
single gene
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Na channels are newer
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After a threshold membrane potential is reached the channel undergoes
an conformational change, helix S4 twisting itself
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As there's four subunits
there's 4 helices which twist
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The negative are
aspartate and glutamine
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There is
another gate
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The inactivation gate is the H gate
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This is due to the stability
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When the channel opens, the open form of the channel is unstable energetically
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Once threshold is reached the K channels shut, which is also not stable
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K channels has the ball and chain effect, but it happens very slowly, so isn’t counted
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K still leaving cell, when you repolarise the cell the H gate reopens, m closes and K
channels slowly start to close
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Ie the
TTX, can wash out drug after and works again
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Hereditary mutations in ion channels that cause disease alter ion channel properties
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Many disorders are possible
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WT see nothing, mutate the
channel and see a more active channel
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To do
experiment: take channel, close, put in diseases mutation and put into a cell without any
electronic activity
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Means you can do
whole channel recordings to study the behaviour of the channel
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- the action potential arises from the membrane’s selective permeability to Na+ and K+ ions via
the gating of voltage-dependent ion channels
- the sodium channel also has a voltage-independent gate that is responsible for
fast inactivation (linked to conformational stability)
- highly potent and selective toxins occurring in nature have been critical for
understanding how ion channels function
4
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- Hodgkin and colleagues took advantage of the squid giant axon to make early
electrophysiological recordings of the action potential and showed how it was
composed of Na+ and K+ conductances
- Neher and Sakmann’s patch clamp technique allowed scientists to study the
properties of individual ion channels
- expressing channels (and their mutants) in heterologous cells makes it easier to
study their properties

Questions
What are some key experiments in finding out this?



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