PSY236 Lecture Notes - Lecture 9: Second Messenger System, Pyrophosphate, Positive K
2 Jun 2018
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PSY236 Week 9 Lectures:
Neurophysiology
• Neurophysiology looks at how neurons fire – how they produce action
potentials
• Resting neuron membrane potential
• The action potential
• The propogation of the action potential
Resting membrane potential
• Remember the cell membrane (phospholipid bilayer)
• Resting membrane potential is an electrochemical gradient (i.e. intracellular
ion concentration is different from extracellular ion concentration). Since
these are different – it has the POTENTIAL to change
• Extracellular vs. intracellular fluid ion concentration at rest
• Ions important for neurophysiology
o Cations = NA+, K+, CA2+
o Anion = Cl-
• Phospholipid layer = will let uncharged molecules through but not ions
o Ions require channels in the form of large protein molecules
• Electrochemical gradient occurs between ICF and ECF
• Phospholipid bilayer membrane = continuous around all of neuron, membrane
helps to maintain resting potential
Taking a closer look at a neuron at rest
• Extracellular fluid → High NA+/Low K+
• Intracellular fluid → High K+/Low NA+
• Na+ = sodium
• K+ = potassium
There are many proteins in the cell: negative
• Intracellular fluid → proteins and chloride ions
• The difference in electric charge between ICF and ECF gives a potential
• The many proteins and chloride ions inside the cell make the overall resting
potential of the inside of the cell NEGATIVE
Usual resting membrane potential = -70mV
• The difference in electric charge between ICF and ECF means that some ions
will easily move across the plasma membrane if their channels are open
Regulation of Na+ and K+
• Electrical gradient from ECF to ICF
• Concentration gradient from ICF to ECF
• Ions cross the plasma membrane via proteins called ion channels
• At rest, K+ channels are open, K+ flows in AND out
Resting membrane potential
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• Electrical gradient = the difference in electrical charge between two adjacent
areas. In an area is negative, positive ions will flow to it
• Positive K+ ions will flow from the outside of the cell to the inside
• Concentration gradient = the difference in concentration of a particular ion
between two adjacent areas. If an area has many K+ ions, the K+ ions will
flow to an area with less K+ ions
• The inside of the cell has more K+ ions than the outside. K+ ions will then
cross from the inside to the outside of the cell
So why aren’t the levels of K+ the same between inside and outside at rest?
• K+ ions move into the cell due to electrical gradient and move out of the cell
due to the concentration gradient – so why is there more K+ inside than out?
• There is a sodium/potassium pump in the cell membrane. This pump brings in
2 K+ and removes 3 Na+ ions. More K+ ions are then inside and more Na+
ions are outside of the cell
• The pump requires energy (Adenosine Triphosphate – ATP) to work and gets
the ion concentrations in the neuron ready for action potentials (ready to fire).
ATP is required as sodium is being transported out of the cell and K+ is being
transported into the cell – both travelling against their concentration gradient
• Na+ is pumped out and K+ is pumped into the cell via the Na+/K+ pump – 3
Na+ get taken out and 2 K+ ions taken in: this requires energy
• There are also Na+ channels = these are closed in a resting neuron
o On the axon Na+ channels are voltage-gated
o They will only open with certain membrane potentials (e.g. -50 to +30
mV)
• There are also voltage-gated K+ channels = these are also closed in a resting
neuron
o Voltage-gated K+ channels only open when the cell becomes positive
• These channels and pumps are along the axon (much smaller)
o The resting membrane potential is always negative
Measuring the action potential
• The intracellular and extracellular electrical potentials can be measured by
recording electrodes and an oscilloscope
• Action potentials produce a rapid reversal in potential
Action potentials begin at the Axon Hillock
• Communication with dendrites from other neurons brings positive or negative
ions into the cell
• Enough positive charge at the hillock will fire the neuron
Changes of charge in the axon alters how ion channels work
• Less negative charge in axon opens sodium (Na+) channels
• There is a threshold of excitation (potential) required to open these channels (-
50mV)
• Massive influx of positive charge into the cell
• Polarity of the membrane switches for milliseconds
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Changes in charge occur in a wave: along the axon
• After the change in charge has occurred, the membrane returns to its original
resting membrane potential
• A neuron at rest (-70mV)
o At rest, K+ channels open and at equilibrium, voltage-gated K+
channels closed
o Na+ channels are closed, Na+/K+ pump is setting up potential
The axon hillock receives positive charge which reaches cell threshold (-50mV)
• The Na+ channels closest to the change in potential change their molecular
conformation: they OPEN, massive influx of Na+ ions – depolarization
• Some K+ ions leave as now more negative outside! Na+/K+ pump is closed
When the potential becomes positive the Na+ channels close: K+ GOES OUT
• The less negative (depolarized) part of the cell continues to open sodium
channels along the length of the axon: the first part of the axon is now in its
refractory period: this part of the neuron is still too positive to fire:
o Voltage-gated K+ ions channels open – big efflux of K+ ions
The middle section of the axon is now in its refractory period, the first part of
recovering
• The third part of the axon has now become depolarized – the first is
hyperpolarized
• The actual concentration of ions does not change that much, a small
movement of ions causes a large change in POLARITY = more positive inside
and more negative outside
Refractory periods
• Absolute refractory period = 1ms after action potential (third)
• Relative refractory period = 2-4ms after action potential (first and middle)
• The neuron cannot fire again until the resting potential is restored
Na+/K+ pump and open K+ channels restore the RMP to -70mV
• The membrane of the neuron is ready to fire again approx. 4ms after the action
potential has occurred in its section of the membrane
The action potential → rapid reverse of membrane potential
• Influx = into the neuron
• Efflux = out of the neuron
• Polarize = make negative
• Voltage-gated Na+ & K+ channels open
• Hyperpolarization = undershoot that occurs due to continued k+ efflux before
neuron stabilizes)
Summary: resting membrane potential
• The resting membrane potential is produced by the electrochemical gradient
between the intracellular and extracellular fluid bordering the phospholipid
bilayer membrane of the neuron
• Cells can have different RMPs, commonly neurons have an RMP of -70mV
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