neuroscience - chapter 4 Flashcards
Define Resting Membrane Potential.
The difference in electrical charge across the plasma membrane of a cell when the cell is not actively sending signals (i.e., when it is at rest). It is the electrical potential that exists due to the distribution of ions (like sodium, potassium, chloride, and others) on either side of the membrane.
How do neurons conduct information over a distance?
By using electrical signals. (sensory input and motor output)
How is neuronal communication achieved?
Via a chemical signal, the neurotransmitter is released at the axon terminals upon the arrival of an electrical signal travelling down the axon: the action potential.
Is every neuron post and pre synaptic at the same time?
Yes. (frame of reference)
What might we think axons are like.
Copper wires. This is not the case! The action potential does not involve electrons flowing down, but rather ions crossing the membrane. The crossing of ions travels down the axon like a wave.
What is a potential?
Potential: the inherent capacity for coming into being
Why do we say that the membrane is polarized?
Because there is a difference in electrical charge across the cell membrane. The inside of a cell is slightly more at rest than the outside.
What can we compare the potential to?
We can compare it to a bow and arrow. resting membrane potential: think bow is at rest, no its under tension (-65 or -70 is resting potential). if bow is at rest then no brain activity.
Does the neuronal membrane have an electrical potential?
Yes = voltage
What is an electrical potential?
difference in electrical charge
What is the resting membrane potential?
-65mV
How are ions distributed across the membrane when the neuron is “at rest” (no inputs)?
The ions are not evenly distributed across the membrane.
Tell me where Na+, Cl- and K+ are more and less concentrated across the membrane.
Na+ and Cl- are more highly concentrated outside the cell.
K+ is more highly concentrated inside the cell.
The membrane is semi-permeable. What is usually let through and what is not?
- Lipid-soluble molecules can cross
- Small, uncharged, water-soluble molecules can cross
- But ions cannot
Why can charged ions not cross the membrane?
-water is polar
-Ions attract clouds of water molecules
-the effective size of the ions becomes larger (“spheres of hydration”) - +pole of H2O molecule tend to go towards - shell forms. that is why harder for ions to squeeze through the membrane
-Charged water soluble molecules (ions), interact with the water, but would NOT mix with the non-polar lipid layers. (chat simpler reasoning: The membrane: Think of the membrane as having a layer that acts like oil (which repels water).
Charged ions: Ions are like tiny particles with an electric charge, and they are attracted to water. Since the membrane’s middle part is like oil, and oil and water don’t mix, ions can’t pass through it on their own.)
What CAN ions pass through?
channels!
Why can ions pass through channels?
-channels contain a selective pore (hole), don’t require energy (ATP)+non-directional, let specific ions through
-channels can open and close (gate)
-channels and pumps very different from one another!
Explain in greater detail how the selectivity works in channels.
-the selecting ions that are negatively or positively charged
-the positively charged region inside the pore repels K+, but allows Cl- to pass
-A negatively charged region inside the pore would allow positive ions to pass through
Explain in greater detail how the channels can also select based on size.
-Ions of the same charge have different diameters of hydration (K+ will pass, but not Na+)
-Na+ has a larger sphere of hydration, so a bigger volume
Explain the selectivity of the Na+ channel.
-A specialized site that binds to Na+ transiently
-Partially hydrated K+ is too big to fit through
SEE SLIDE 15 FOR VISUAL EXPLANATION
The resting membrane potential is determined by two forces, what are they?
-concentration (diffusional) force (concentration gradient)
-electrostatic (voltage) force (electrical gradient
-The balance of these two forces creates the resting membrane potential
Define diffusion
-adding salt to water
-channels in the membrane allow the ions to cross
-creates a balance between both sides
-it acts until there are equal concentrations on each side
-Equal concentrations are then maintained
-Gradients for each ion are independent.
Define electrostatic force.
A battery creates an electrostatic potential (e.g., 9 Volts) Ions have a charge, and are moved by the electrostatic force
● The Current also depends on Resistance
How do the two forces interact (electrical and diffusion)?
The inside of the cell is on the left
Lots of K+ and A- inside, but no channels to cross
With K+ channels, some K+ move out of the cell,
(down the K+ concentration gradient)
But this also establishes a voltage gradient (the inside
becomes relatively negative, which will eventually attract K+ from the outside)
At Equilibrium:
The forces have balanced out (“equilibrium potential”) Despite unequal concentrations, there’s no net movement of
K+
Show another example for Na+
Each ion has its own
Equilibrium Potential (Eion).
This is the electrical potential that would be reached across the membrane if only that ion were allowed to cross.
We can calculate Eion using Nernst equation.
What is the Nernst Equation used for?
It allows to determine the equilibrium potential of specific ions if allowed to move.
What is the voltage that perfectly balances the diffusion and electrostatic forces (no net movement of K+ at this potential)? And why?
The EK is about -80mV. We said RMP is ~ –65 mV, which is not far from ~ –80 mV. This is because K+ is the main contributor to RMP, since:
There is lots of K+ inside neurons
There are K+ channels that are always open
K+ diffuses out until the voltage gradient balances the concentration gradient.
What is the voltage that perfectly balances the diffusion and electrostatic forces (no net movement of K+ at this potential)? And why?
ENa is about +58V (from the Nernst equation).
Lots of Na+ outside:
inside = 15 mM, outside = 150 mM
Concentration gradient pushes Na+ in
The entry of the positive Na+ ion makes the cytosol more positive, so the electrical force now repels the Na+
How can we calculate the resting membrane potential?
-The Nernst equation would work if only K+ was involved. (only useful for one ion at a time)
-But, both K+ and Na+ affect the RMP
-The Goldman Equation:
Can account for multiple ions
Takes into account the permeabilities of the ions
-We therefore need the Goldman Equation to calculate the RMP.
*
The Goldman Equation
Is a generalization of the Nernst Equation
Nernst calculated an Equilibrium potential for one ion
The Goldman Equation uses information about both concentrations and permeabilities to determine the combined effect of multiple ions on the RMP:
(Note: If the permeability of Na+ was set to zero, the Goldman would reduce to the Nernst)
Detail how the resting membrane potential depends on the permeability (how easily ions can pass through) to K+ and Na+.
- If membrane were only permeable to K+, RMP would equal EK (-80 mV)
- If membrane were only permeable to Na+, RMP would equal ENa (+58 mV)
- If membrane is equally permeable to Na+ and K+, RMP would become the average of EK and ENa
However, many more K+ channels are open than Na+ channels (40:1) RMP comes very close to EK
RMP is pulled slightly to ENa
The permeability to K+ is X times greater than for Na+.
40 times greater
Describe the forces acting on K+ and Na+.
Forces on K+: Concentration: Outwards Electrostatic: Inwards
Forces on Na+: Concentration: Inwards Electrostatic: Inwards
Neither ion is at its Equilibrium Potential, so there’s leakage.
Leakage would tend to reduce the concentration gradients, if it weren’t for ion pumps.
How do ionic concentration gradients arise?
Ion pumps (maybe check out slide 32)
True or False? Very small changes in the concentration of an ion across the membrane can cause a large change in membrane potential (even if the overall ionic concentrations remain stable).
true!
True or false?
As far as the membrane potential is concerned, most of the “action” is taking place at the inside and outside surface of the membrane.
true!
What are the roles of astrocytes?
Astrocytes regulate the external K+ concentration
* Very delicate balance because of the high permeability
* A tenfold increase in extracellular concentration of K+, will wipe out the RPM (from -65 to -17 mV)
* Astrocytes can remove excess K+ from the extracellular space through pumps and channels.
summary of the resting membrane potential (RMP)
-A balance of diffusional and electrical forces acts on ions (e.g., Na+, K+)
-The action potential requires concentration gradients to exist
-The sodium-potassium pump maintains gradients by counteracting leak currents
-RMP is closer to EK than to ENa Permeability to K+ is higher
The ionic driving force (Vm - Eion) for
Na+ is much larger than the one for K+
