NEUS 609 - Electrophysiology

ion channels

Rapid changes in membrane potential are mediated by ___ (integral membrane proteins)

___ = large integral-membrane proteins with a core transmembrane domain that contains a central aqueous pore spanning the entire width of the membrane. 

separation of charge

main function of cell membrane is the ___

impermeable

cell membrane is made of a lipid bilayer that is ___ to large and/or polar molecules due to its hydrophobic inside

Concentration gradients

___  are what dictate the movement of ions across the cell membrane

size, electrostatic interactions

Ion channels’ selectivity is based on the ion __ and its ___ inside the channel

• Molecules that make up the channel create a selectivity filter with binding sites inside the channel that allow for ___.

hydration shell

unless passing through membrane, ions in aqueous solvent are surrounded by a ____

chemical gradient

if there is more of ion X on the outside than the inside, then there is a force pushing ion X to inside (which is only effective when the channel for ion X is open)

electrical gradient

if the ion X has a positive charge (X⁺) then in the typical situation of a ~-60 mV intracellular potential, that ion will be driven towards the inside of the cell (attracted to the negative on the inside and repelled from the positive on the outside)

equilibrium potential

the membrane potential at which there is no longer any driving force on the ion to move, so that the NET flow of that ion is 0.

conductance

the inverse of resistance 

1/R

depends on type of channel and how many of them are open

Resistance

How easily the current flow (increases when channels are closed, inactive, blocked, & when the neuron diameter is smaller)

Ohm’s law

V = I * R

Describe the behavior of an ion channel acting as a simple resistor

slope/conductance

For channels that don’t have a LINEAR change in diving force with change in potential (NOT following Ohm’s law) that means that there is a change in ___ at some specific potential

• Ex: NMDA receptor blocked by Mg2+ until potential high enough to force it out of cell and thus out of channel

reversal potential

also called equilibrium potential

get 0 NET current flow

driving force equation

DV = Vm - Eref

membrane potential equation

Vm = Vin-Vout

positive

direction of current is the direction of the ___ ions

• outward current has a __ value

negative

inward current has a ___ value

electrotonic potentials

passive changes in membrane potential that do not lead  to opening of gated ion channels

Nerst equation

Ex = (58mV /z) * ln([Xo]/[Xi])

• finds equilibrium potential

Goldman equation

Vm = (RT/F) * ln((Pk[Ko] + PNa[Nao]+PCl[Clo])/(Pk[Ki]+PNa[NAi]+PCl[Cli])

• valid at rest

• no longer useful when permeability changes

driving force

– (specified per ion or per channel) – the difference between the current membrane potential and the equilibrium potential for that ion/channel

Ik = gammak *DV = gamma k * (Vm-Eref)

intracellular axial resistance

Ra

how much resistance is created by axon thickness (thicker = more space = less resistance)

capacitance

• greater surface area aka bigger cells have a greater ___

• the thicker the membrane, the lower the ___ which is partially why myelin reduces capacitance and thus makes conduction faster

time constant

___ = time to charge the membrane to 63% of its steady-state value = Rm * Cm

length constant

__ = distance to decay to 37% of initial value = √((r_m/r_a))

• Resistance of a unit length of membrane r_m depends inversely on the total number of channels in a unit length of the neuronal process.

• the larger the axonal diameter, the lower the r_a

measure of the efficiency of electrotonic conduction

strategies to speed up AP conduction

• increase axonal size (lower r_a)

• use myeline (decrease C_m aka capacitance so prevent bundle of charge and allows it to propagate

desensitization

Ligand-gated channels can enter a refractory state with prolonged exposure to agonist either via direct chemical interactions or via phosphorylation.

• most ion channels

ligand gated ion channels

receptors for neurotransmitters ACh, GABA, glycine, serotonin: 5 subunits, each subunit with 4 transmembrane a-helixes, conserved loop of 13 amino acids flanked by cysteine residues that form a disulfide bond.  Called: Cys-loop receptor families.

gap junction channels

bridge the cytoplasm of two cells at electrical synapses. Channel is formed from a pair of hemichannels. Each hemichannel is made of 6 identical subunits, each containing 4 transmembrane a-helixes.

voltage-gated ion channels

Usually activated (opened) by membrane depolarization and selective for Ca²⁺, Na⁺, or K⁺. All have a core motif of 6 transmembrane segments (S1-S6).  S5 and S6 connected by conserved P-region which loops into and out of the extracellular face of the membrane and forms the selectivity filter.

how to determine ion channel structure/function

• antibodies

• genetic engineering'

• site-directed mutagenesis

• naturally occurring mutations

ion pumps

maintain proper concentrations of Na⁺, K⁺, Ca²⁺, Cl^- and other ions. Undergoes conformational change in the protein that makes up the pump and therefore rate of ion flow is 100 to 100,000 slower than through channels.

also require ATP as this is active transport

rectification

When a channel passes more current at certain potentials than at others