Electrical properties of neuron Flashcards
(25 cards)
Bases of neuronal excitability
- Regulated Ion movement
across the membrane
(ion channels and pumps) - Existence of membrane
ionic gradients as quickly
releasable energy stores - Existence of an electric
resting potential
Resting Membrane Potential
- Controlled permeability of
the neuronal membrane - maintained by sodium potassium pump
- resting K+ channels
- -70mV
Ions involved in neuronal excitablity:
Sodium Na+
* Chloride Cl-
* Potassium K+
* Calcium Ca+2
* Magnesium Mg+2
* Bicarbonate HCO3-
Channel ion selectivity classification
Channels are selectively permeable to specific ion species
- Cationic (negative charge filter) - excitory
- Anionic (positive charge filter) - inhibitory
- K+ channels
- Na+ channels (smaller than K+)
- Ca+2 channels (synaptic transmission/modulation of AP)
- Cl- channels (
**Selectivity is not absolute (some other ions may transit as well)
**
- Ions are part of hydration spheres, so diff hsapes of water wrap around ion
- channels remove shells of water in diff way
Channels may also be classified according to the
stimulus that induces their opening (gating)….
- changes in transmemb voltage
- Voltage changes
- Ligands (small organic mol e.g GABA/glutamate/Ach) (desensitization) - made of 5/4 subunits
- Pressure
- pH
- light
Channel gating (opening and
closing) requires a specific event:
- Voltage change
- Ligand binding
- Phosphorylation
- Sensory stimulus
Channels agonists, antagonists, and allosteric modulators
- Complex molecular
interactions account for
competitive OR non
competitive antagonism - from endogenous or
exogenous ligands
Channel biophysics:
Closure vs. Inactivation
Different strategies bring about
the closure of a channel:
* Subunit movement
* Ball and chain
Ion Channel Inactivation
The stimulus for
opening the channel is
still present.
Nonetheless, after a
while the channel
tends to close:
Inactivation
(V-gated channels)
or
Desensitization
(ligand-gated channels
Function, Structure, and Pharmacology of the Na-K pump
- Stoichiometric transport of ion
(different from channels, non
stoichiometric) - Pumps use energy to move some
ions across the membrane - Different from channels, whose
ion movement is passive)
Energy source:
- ATP (ATPases)
- Gradient of (other) ions
(exchangers)
pumps vs. exchangers
- Exchange pumps differ from ATPases as they exploit a ion gradient to move a different ion against its gradient (no ATP involved)
- Pumps use ATP
for creating ion
gradients across
the membrane - move ions in a stochimetric function = specific number of ions going in and out
- Exchangers use
pre-existing ionic
gradients to
transport other
ions against their
gradient (most
often they use
the Na gradient)
Nernst potential
- equilibrium potential
- reversal potential
Nernst potential is the membrane voltage at which the net entropic force moving permeable ions along their conc gradient is equal + opposite to the electrostatic attraction/repulsion for the other side of the cell memb
-LOOK AT EQUATION
Ohm’s law regulates ion flow
In general, Ohm’s law
V = RI
or I = 1 V/R
or I = g x V (where g = 1/R)
**describes the flow of a current I through a conductance (1/r) g
- The opening of a selectively permeable channel induces a passive flow of current in the channels according to a modified Ohm’s law
I = gion (Vm – Eion)
g ion = conductance to that ion
Vm = actual transmembrane voltage
Eion = Nernst potential for that ion
Ionic current through a membrane channel
1)The transmembrane (resting) potential is the result of active ion movements (ATP-
driven) and of a high resting permeability to K+ ions
2) Channel opening is consequent to a specific stimulus or set of stimuli (voltage, ligand)
3) The flow of ions through a channel depends on the selective ion permeability of that
channel, and on the actual driving force at a particular time:
**I = gion (Vm – Eion)*
(Vm – Eion) = DRIVING FORCE for a particular permeant ion
g ion = conductance to that ion
Vm = actual transmembrane voltage
Eion = Nernst potential for that ion
Nernst potential for
biologically relevant ions
- Ionic gradients store energy
- The ionic gradient
- (Concentration Outside/Concentration Inside)
- Determine the strength of ionic currents
- (measure of energy stored)
Steps in the generation of the resting potential
- pumps create an ionic gradient using ATP to force out Na from the cell and to insert K into the cytosol
- K leakage channels allow K to freely cross the membrane
- The outflow of K+ creates a negatively charged cell interior
- This process originates the resting potential
Concentration difference
creates membrane potential
left = neuronal inside
right = cytosol
INITIAL CONDITION: MORE K INSIDE
* The movement of K ions creates accumulation of + electric charges to the right
* This is equivalent to an excess of negative charge in the inside
* THIS IS THE EXQUIVALENT OF THE MEMBRANE (NEGATIVE) POTENTIAL
Goldman Equation for the resting potential
- If K was the only permeable ion at resting, the resting
potential would be the Nernst potential for K. - Since we also have other ions (slightly) permeable at
resting, the following is the more complete expression for the resting potential
Summarize these cards
- The transmembrane (resting) potential is the result of active
ion movements (ATP-driven) and of a high resting permeability
to K+ ions - Channel opening is consequent to a specific stimulus or set of
stimuli (voltage, ligand, pressure, chemical factors) - The flow of ions through a channel depends on the selective
ion permeability of that channel, and on the actual driving
force at a particular time - Once opened, every membrane channel displays peculiar time dependent opening and closure properties (kinetic)
- Channels greatly differ in their closure kinetic (de-sensitization)
glycosylation
- attatchement of a sugar group attached to proteins/lipids
if excess sugar in blood..
- increased glycosolation
- increased inflammation
- nerve neuropathy
Alpha subunit is where..
- Ach binds
Ligand gated channels 2 examples
Glutumate exciatory
Gaba inhibitory
NMDA channel needed for..
- learning pathway
- high permeability to calcium ions and induces plasticity
- cell death
- NMDA associated neurotoxicity
- R1,R2A,R2B subunits –> A an B have developmeet expression w variability, in adults its R2A and in infant is R2B (difference = diff permebaility to calcium)
