Choose tutorial "Chattering Ion Channels" , and Both Na+ and K+ channels
Record single channel currents with the preset values :
Tstop = 16 ms
testing level duration = 12 ms
testing level voltage at 0 mV
Press Reset & Run
the channels open stochastically
Hint : You will likely need to repeat "Reset & Run" many times to get a good feel for things
Compare the activity of these Na+ and K+ channels
each channel's open current is fairly constant ( square step )
Na+ = inward
K+ = outward
Na+ channels :
channels open and inactivate relatively quickly
Typically open briefly and earliest in the depolarizing step
They inactivate ( closures soon after they open ) , so you often see short bursts of inward current
At 0 mV , Na+ current is usually inward ( below the baseline )
K+ channels :
K+ channels open more slowly but do not inactivate in the same way
they can remain open or flicker open/closed for longer
Tend to open a bit later and often show longer open times ( though still flickering )
At 0 mV ( assuming EK ~ -80 mV ) , the K+ current will be outward
Because these are single-channel recordings , you see individual "square" steps of current for each channel opening
Repeat with testing voltages of +30 mV and then −30 mV
@ +30 mV :
Na+ = inward current , small amplitude
K+ = outward current , large because +30 mV is far away from -80 , potassium's equilibrium potential
@ -30 mV :
Na+ = inward current , larger
K+ = outward , smaller
Discuss the amplitude and time course of the currents , and which type opens first
Na+ :
opens very quickly after depolarization and then inactivates
K+ :
open with a delay , and then stay open for longer
Construct an IV plot for this channel activity ( +80 mV to −80 mV , 40 mV increments ) , and confirm the ion selectivity
Increase the number of sodium channels to 3 and K+ channels to 3 when stepping to +30 mV , 0 mV , and then −30 mV
Discuss what happens , including how many channels are open at peak current
you see discrete “steps” in the current trace
each step corresponds to the opening of one additional channel
If multiple channels open at the same time, you’ll see a larger amplitude
Increase the number of sodium channels to 10 and K+ channels to 10 when stepping to +30 mV , 0 mV , and then −30 mV
Discuss what happens , including how many channels are open at peak current
Stochastic Variability : The exact number of channels open at any given time is random, governed by each channel’s probability of opening.
Faster vs. Slower Activation :
Peak vs. Sustained Current :
Single-Channel Currents :
Voltage Dependence : Inward
Activation Kinetics :
Increasing Channel Number : Increases the probability and magnitude of multiple-channel openings, thus raising peak currents and showing stepwise increments in the current traces.