Patch Clamp / Ion Channel Calculator

Calculate single-channel conductance, open probability, and current from patch clamp data.

Single channel current i (pA) (pA)

Membrane potential Vm (mV) (mV)

Reversal potential Erev (mV) (mV)

0 for non-selective cation channels

Open probability Po

0-1

Number of channels N

Please check your inputs and try again.

Driving Force, Not Voltage

Single-channel conductance comes from Ohm's law, but the voltage term is the electrochemical driving force — the difference between membrane potential and the reversal potential — not the membrane potential itself.

γ = i / (V_m − E_rev)

At the reversal potential the driving force is zero, so no current flows regardless of how many channels are open. This is why E_rev identifies which ion a channel carries: it matches the Nernst potential of the permeant species.

E_rev	Ion	Typical channel
−90 mV	K⁺	Inward rectifier, delayed rectifier
−65 mV	Cl⁻	GABA_A, glycine receptor
0 mV	Non-selective cation	Nicotinic ACh, NMDA
+60 mV	Na⁺	Voltage-gated sodium

A reversal potential near 0 mV is the signature of a non-selective cation channel, since sodium and potassium currents balance there.

Whole-Cell Current

I = N × P_o × i

Macroscopic current depends on three separable quantities, and a change in whole-cell current cannot distinguish them. A drug halving the current might halve channel number, halve open probability, or halve single-channel conductance — completely different mechanisms with the same macroscopic result.

Non-stationary noise analysis separates them: plotting current variance against mean current across many trials yields both single-channel conductance and channel number without needing single-channel recording.

Channel	Conductance
BK potassium	100–250 pS — among the largest
Nicotinic ACh receptor	~30 pS
NMDA receptor	~50 pS
AMPA receptor	5–15 pS
CFTR chloride	~8 pS

Worked Examples

Example 1: K+ channel: i=2pA, Vm=-70mV, Erev=-90mV

γ=2pA/20mV=100pS

Result: Large conductance K+ channel

100 pS is characteristic of BK channels

Example 2: NMDA receptor: i=3pA, Vm=-70mV, Erev=0

γ=3/70=42.9pS

Result: Moderate conductance — non-selective cation

Po increases with glycine and glutamate co-agonists

Example 3: Identifying the permeant ion

Reversal potential measured at −88 mV

Result: Potassium-selective

The value matches the Nernst potential for K⁺ under physiological gradients. Shifting external K⁺ and re-measuring confirms it.

Example 4: Separating N, P_o and i

Whole-cell current halves after drug application

Result: Noise analysis required

Variance-mean analysis across trials yields single-channel conductance and channel number, distinguishing a change in i from a change in N or P_o.

Common Mistakes

⚠️

Using membrane potential instead of driving force

Conductance requires V_m minus E_rev. Using V_m alone gives a wrong answer whenever the reversal potential is not zero.

⚠️

Interpreting whole-cell current change as one mechanism

Current is N × P_o × i. Halving it could reflect fewer channels, lower open probability or reduced conductance.

⚠️

Assuming conductance is constant with voltage

Many channels rectify. Inward rectifiers conduct far better in one direction, so a single conductance value does not describe them fully.

⚠️

Forgetting that E_rev depends on ionic conditions

It shifts with the concentration gradient. Changing pipette or bath solutions moves the reversal potential predictably, which is how ion selectivity is tested.

Frequently Asked Questions

Whole-cell vs single-channel recording?

Cell-attached or excised patch: single channel (picoamperes). Whole-cell: sum of all channels in membrane (nanoamperes). I = N × Po × i (Hodgkin-Huxley framework). Gating: transitions between open/closed states detected as current steps.

Reversal potential identification?

At Vm=Erev: net current=0 (inward=outward). Erev tells you channel selectivity: near EK (-90mV)=K+ selective. Near ENa (+60mV)=Na+ selective. Near 0mV=non-selective cation. Nernst equations determine each ion's equilibrium potential.

Why use driving force rather than membrane potential?

Because current flows in proportion to how far the membrane potential is from the reversal potential. At E_rev no current flows regardless of voltage.

What does the reversal potential tell you?

Which ion the channel conducts. It matches the Nernst potential of the permeant species, so −90 mV indicates potassium and 0 mV a non-selective cation channel.

Why can't whole-cell current identify a mechanism?

Because it is the product of channel number, open probability and single-channel current. Any of the three could account for a given change.

What is non-stationary noise analysis?

Plotting current variance against mean current across trials, which yields single-channel conductance and channel number without single-channel recording.