why can't you use a current clamp to figure out an action potential's conductances?
b/c current-clamp provides poor control over the membrane potential or the currents that flow through the membrane -
when inject current, it bypasses the membrane, triggers conductance changes that cause an action potential
both conductance and resistance are changing in action potential, so cannot solve Ohm's law and know what happens to currents
how does a voltage-clamp work?
voltage-clamp keeps Vm constant, measures Im (command potential)
you set the command voltage, and it compares changes in voltage inside the cell to that command voltage, and converts the voltage difference to a proportional current thats delivered to the cell through an electrode
thus it detects different btwn set command potential and the actual membrane potential; and produces a proportional current to maintain your set command potential in the cell
what is the sign of an inward/outward current in a voltage clamp?
inward current on voltage-gated Na+ channels: voltage clamp is negative (injects negative charged ions)
outward current on voltage-gated Na+ channels: voltage clamp is positive (injects positive charged ions)
action potential of squid giant axon
what can an I-V plot show?
study ion currents through membranes - which ions mediate the action potential
use I-V plot to detemrine reversal potential
how do you know the equilibrium potential, VNa, from this I-V graph?
VNa = +50 mV
what is TTX?
a neurotoxin that blocks voltage Na+ channels
eliminates transient inward current
makes your tongue numb when you eat fugu
what is TEA?
blocks K+ channels
blocks K+, prevents persistent outward current
what is the I-V plot for a membrane containing only non-rectifying K+ Channels?
what is the slope equal to?
slope = all values of Vm b/c conductance doesn't change w/ voltage
what does the patch clamp method allow study of?
single-channel level study
how does the patch clamp work?
pipette is applied to outside of cell membrane; creates high resistance seal between glass and membrane; pipette now records the channels that're present in that patch of membrane
may be 1, or several, channels
thus allows single channel microscopic recording or many channel macroscopic recording
what is the cell-attached mode for the patch clamp?
initial patch configuration
intracellular environment remains intact
what is the inside-out patch?
pull pipette away from cell; patch remains sealed to pipette, former cytoplasmic face of memrbane is exposed to bathing sol'n, where ion concentratiosn and other conditions can be manipulated
what is the whole cell configuration of path clamp configuration?
patch is ruptured while pipette is still attach to the membrane; as result, the pipette, containing much larger volume of solution thatn cell, dialyzes interior of cell; diffusible molecules are lost; cytosol replaced w/ pipette sol'n
here, macroscopic combined activity of channels in intact membrane is recorded