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Summary The Adaptive Brain lectures
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Summary of the lectures of The Adaptive Brain, which is a part of the minor Biomedical Sciences and Neuroscience: track Neuroscience.
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SMV HC THE ADAPTIVE BRAINCh. 2 Electrical Signals of Nerve Cells
Neurons consist mainly of lipids, which are super good conductors and do not carry electrical
current. Electrical signaling is fundamental neuronal process that underlies all aspects of
brain functioning. However, as neurons are poor generators and conductors of electricity.
They have developed a mechanism to help overcome this limitation which is based upon the
flow of ions across the membrane.
Electrical signals that neurons convey can be measured with micro-electrodes:
- Receptor potentials in touch receptors in your skin. Membrane potential changes
because of flow of ions.
- Synaptic potential if you activate one single synapse
- Action potential when threshold potential is reached. This amplitude is way bigger
than the synaptic potential.
Experiment: culture individual nerve cells and build electrodes in the neurons. One electrode
to stimulate and the other one to record. Once you activate record electrode they measure a
-65 mV membrane potential, this is the unstimulated resting membrane potential. If you
activate the stimulation electrode there is an inject of negative current to -80 mV
(repolarization) first and then an inject of positive current (depolarization). If you increase the
current that you inject into neuron, they reach certain plateau (threshold) and an action
potential happens. The more current we inject, more action potentials occur (the frequency
increases, not amplitude). All or none event, once it reaches the threshold you get an action
potential.
Action potentials are very important. They allow for long range transport and encode
information of stimulus intensity in their frequency. The membrane potential depletes very
fast if you don’t get action potential because of the lipid bilayer (passive conduction). Action
potentials can carry on a long distance, they don’t deplete.
Neurons have -65 mV resting membrane potential. The plasma membrane is impermeable
to ions. Inside the cell a there is a lot of K, and outside a lot of Na and Cl. Movement of ions
is producing a negative membrane potential. Proteins in membranes that allow movement of
ions:
- ATPase (Na/K pump): active transporter, consumes a lot of energy. Pumps 2 K in
the cell and 3 Na out the cell against their gradient.
- Ion channels: selective for certain ions. Will only allow passage of one certain ion.
Can only open and close, ions move with the gradient. Opening and closure of
these channels is controlled by the membrane potential, the ion channels are
voltage gated channels. 2 most important are the K and Na channel. When
neuron is at rest Na channels are all closed. K channels are a little bit open; K is
leaking a bit to the outside of the cell.
So the K channels are a little bit leaky and K travels outside the cell. This will make the
membrane potential a bit negative, because of the positive ions that are flowing out of the
,cell. Electrical force wants K back inside the cell, membrane gets depolarized, no netto flux
of K. Now there is a membrane rest potential of -65 mV.
Nernst equation is the equation for equilibrium, the membrane potential. Goldman equation
also included membrane permeability. The rest membrane potential is close to the K
equilibrium. During action potentials Na channels start to open, membrane potential goes to
Na equilibrium, then Na channels are closing again, K are opening again and will go down to
K equilibrium (lower than rest membrane potentials).
, Ch 2-4. Action Potentials are Generated by Ion Currents
Electric current: flow of electrical charge (ampere=coulomb/second)
Membrane resistance: a measure of how much the membrane opposes the passage of
electrical charge
Membrane conductance: the permeability of the cell membrane to those ions
Voltage: difference in charge between two points. Creates energy that will ‘push’ charges to
move (potential energy). Voltage difference influences the current of ions.
Membrane potential: the voltage difference between the inner and outer surface of the cell
membrane
Capacitator: can store charges. Membranes are capacitators because the lipid bilayer is
impermeable to ions.
Ions can flow through specific channels. When channels open ions flow to reach
electrochemical equilibrium.
Experiment: test hypothesis that AP occurs due to increased permeability to Na. They
studied this with squid giant axons. Bigger diameter of axon, faster ions flow through axon,
very fast information conducting. They isolated these axons and measured the membrane
potential with a voltage clamp. Penetrate them with two electrons: recording electrode and
current-passing electrode. They saw that squid axons resting MP -70 mV. Force current into
electrode thereby changing the voltage of MP, jumped to +30 mV. Measure the current that
is required to change the membrane in such voltage.
- Put in negative current to -130 mV. They saw fast capacitive current go to -130
mV and after that nothing happened.
- Put in positive current to 0/+30, also saw capacitive current but something else
was triggered, transient inward current and slow outward current. This was an
action potential.
Which ions are involved?
1. Determine at what membrane potential current reverses. Compare to
electrochemical equilibrium potential of ions in solution.
2. Change external concentration of certain ions, to test if the currents are altered or
reversed.
3. Specifically inhibit certain channels to test if this blocks the currents. Tetrodotoxin
(TTX) used which blocks sodium channels, powerful neurotoxin.
When they removed sodium, outward current is still there but inward current is completely
gone. So the inward current is generated by the flow of sodium in the cell. Removed
potassium, outward current gone and inward current still there. When you block the sodium
channels neurons can’t generate action potentials.
Patch clamp is the refinement of the voltage clamp method. Put a micropipette on the
membrane of a neuron. In the pipette there will be some ion channels, on which you can
apply gentle suction. You suck in a little bit of membrane with ion channels. Now you
measure current flow that is flowing through those channels. You can change the voltage
and measure the current that is required to change the voltage. You can also patch the
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