Where does conductance collide?

In circuit theory, conductance is how readily a component lets electric current pass for a given voltage across it: it is the reciprocal of resistance, written G = 1/R, and measured in siemens. Kennelly's 1893 Impedance set conductance within the algebra of alternating-current circuits, as the real part of admittance alongside susceptance, giving engineers a single quantity for the ease of current flow. Here the referent is a fixed electrical property of a wire or component, a number that describes the part itself, not the changing behaviour of a living ion channel, an index of bodily arousal, or the rate at which a leaf trades gas with the air.[1]

Unit

A component's ease of carrying current, in siemens (one over ohms)

Recovery means

Not applicable

Measured by

Found by dividing current by voltage, or as the reciprocal of a measured resistance, in siemens

In neurophysiology, conductance is how permeable a patch of nerve membrane is to a flow of ions, set by how many ion channels of a given type are open at a given moment: it governs how much current crosses the membrane for a given voltage. Hodgkin and Huxley's 1952 A quantitative description of membrane current modelled the nerve impulse as separate sodium and potassium conductances, the g terms, that rise and fall with voltage and time, while a single open channel is measured in picosiemens. Though it shares the electrical unit, the referent is a biophysical property of ion channels in living tissue, not a fixed number stamped on a circuit part, a sweat-driven measure of arousal, or a leaf's gas-exchange rate.[2]

Unit

The membrane's permeability to ionic current, in siemens (picosiemens per channel)

Recovery means

After a spike the voltage-gated conductances decay back toward their resting values as channels close or inactivate

Measured by

Estimated by voltage clamp from the current that flows at a controlled voltage, and at the single-channel level by patch clamp

In psychophysiology, skin conductance is how easily a tiny applied current passes between two points on the skin, which rises when the sweat glands are activated by the sympathetic nervous system: because that activation tracks emotional and mental arousal, the measure is used as an index of how aroused or engaged a person is. Boucsein's 2012 Electrodermal Activity is the standard reference for how the signal is recorded and scored, in microsiemens, as a slowly drifting level and brief responses to events. The unit is again electrical, but the referent is a marker of inner arousal read off the body's sweating, not a property of a circuit, a nerve channel, or a leaf pore.[3]

Unit

The skin's momentary conductance between two electrodes, in microsiemens

Recovery means

Each brief skin conductance response has a recovery time, the interval over which it falls part-way back toward baseline

Measured by

Recorded by holding a small constant voltage across two skin electrodes and reading the resulting current as level and responses

In plant physiology, stomatal conductance is the rate at which water vapour and carbon dioxide pass through the stomata, the adjustable pores on a leaf, for a given difference in concentration across them: it sets how fast a leaf loses water and takes up the carbon dioxide it needs for photosynthesis. Jarvis's 1976 The interpretation of the variations in leaf water potential and stomatal conductance found in canopies in the field treated it as a response to light, humidity, temperature, and carbon dioxide, and it is reported as a flux of gas per unit leaf area, in units such as moles per square metre per second. Borrowed from the electrical idea by analogy, this referent is not an electrical quantity at all but a rate of gas diffusion through a leaf, unrelated to a circuit part, a nerve channel, or a measure of arousal.[4]

Unit

The flux of gas through leaf pores per unit driving gradient, in mol per square metre per second

Recovery means

Not applicable

Measured by

Measured with a leaf gas-exchange system or porometer from the transpiration flux and the vapour gradient at the leaf surface