03-18 Introd; Resting Membrane Potentials Flashcards
What is the RMP for an SKM cell?
-90mV
What is ohm’s law?
V = IR (voltage = current X resistance)
or
I = GV (current = conductance X voltage)
units of current
amperes (A) = 1 coulomb of charge/sec
units if potential different (E)
volts
units of work
joules (1 joule is amt of work req’d to move 1 coulomb of charge across a 1 volt difference
units of resistance (R)
measured in ohms (also 1/G)
units of capacitance (C)
farads
What is the capacitor in neurons?
the lipid bilayer of the plasma membrane
What is the conductance (G) pathway in neurons?
ion channels
What are the resistors (R) in neurons?
ion channels are also resistors b/c flow through them is limited by the size of the pore (re-call the R = 1/G)
What are the major classes of ion channels?
- leak channels
- voltage-gated
- ligand-gated
- mechanosensory
How much energy is req’d by ion channels to allow flux through them?
none, flux through ion channels is PASSIVE; they conduct orders of magnitude more ions through per second than pumps
Give examples of where mechanosensory channels are located.
Pacinian corpuscles
For a vertebral spinal cord neuron, compare and contrast the intracellular vs. extracellular concentration of Na+
Out = high (117mM) In = lower (30mM)
For a vertebral spinal cord neuron, compare and contrast the intracellular vs. extracellular concentration of K+
Out = low (3mM) In = high (90mM)
For a vertebral spinal cord neuron, compare and contrast the intracellular vs. extracellular concentration of Cl-.
Out = low (4mM)
In = high (117mM)
(think opposite of Na+)
For a vertebral spinal cord neuron, compare and contrast the intracellular vs. extracellular concentration of “Anions-.”
Out = high (116mM) In = none (0mM)
Which channels are predominantly responsible for the maintenance of the negative RMP?
leaky K+ channels (not voltage- or ligand-gated; constitutively open at rest)
What class of channels do leaky K+ channels belong to? What are some examples?
tandem-pore domain channels; they have names like TREK, TRAAK, TWIK, TASK, etc.
When measuring current through a single pore (with patch-clamp technique) or across a whole membrane, what is the graphing convention for tracing (i.e. how does a tracing looking showing a flux of positive ions)?
flow of positively charged ions is downward deflection
What are some examples of the intracellular anions that contribute to negative charge balance?
impermeable groups: phosphates on nucleic acids
The intracellular concentration of K is much higher than the extracellular conc., and the intracellular anions that balance its positive charge in the cell are two big to follow it out the leaky channels. Since you have these constitutively-open, leaky K+ channels, why doesn’t the cell just waste all of its K+?
The resulting (-) charge inside the cell due to anions that are two big to follow K+ outside the cell pulls the K back into the cell.
What is the point at which equilibrium is reached between the chemical gradient push of K+ out of the cell equals the electrical gradient push of K+ back into the cell?
The Equilibrium Potential (Eion)—there’s NO net flow of ions at this point. That is in SUMMATION of all the different ions, however.
What is the equation to calculate the Eion?
The Nernst Equation!
Eion = (RT/zF)*ln([ion_in]/[ion_out])
What is the Nernst equation for monovalent cations (like K+) at 37°C (body temp)?
E_k+ = 61*ln([ion_in]/[ion_out])
Compare and contrast ion channels and pumps
Channels-always open, passive diffusion through them, large # ions can flow through fast (high flux)
Pumps-no pore so not always open, usually requires energy (e.g. ATPase, or conc grad of another ion), slower (low flux)
What is the most important ion pump in neurons? How much energy does it use?
Na+/K+ ATPase; uses 20-40% of total brain’s energy requirement; 70% electrically active neuron’s energy requirement!
What is the ratio of the Na+/K+ ATPase? What does the mean in terms of electroneutrality?
3Na+out/2K+in therefore ELECTROGENIC
What is ouabain? Mechanism?
What another example in this drug class still used?
a cardiac glycoside poison that blocks Na+/K+ ATPase activity—>incr [Na+ in]—>upreg Na+/Ca2+ exchanger—>incr intracellular [Ca2+]—>ionotropically increases contraction
**in this situation the Na+/Ca2+ exchanger is operating in the opposite direction
**Digoxin is still used
How sensitive are neurons to changes in membrane potential?
Very, even a 1-2mV ∆ can have big consequences
What is RDP?
rapid-onset dystonia parkinsonism
- hereditary mutation in Na+/K+ ATPase
- leads to dystonic spasms in limbs, speech, swallow balance
- occurs after stressful event
- can cause seizures
Myotonia Congenita
SKM Cl- channel mutation
- examples include Thomsen’s and Becker’s
- delayed relaxation after voluntary mov’t—>stiffness and/or weakness
- myotonic goats
