6 Terms used in ionic electricity
- Charge: Ions have a charge given in multiples of one elementary electron charge. Cations (Na+, K+, Ca2+) are positively charged and anions (Cl-) are negatively charged.
- Two forces that move ions:
- Charges are moved by the force of electric fields: Opposites attract, so cations (positively charged) move towards a negative pole, and anions towards a positive pole.
- In addition, ions are moved by diffusion (thermal agitation) down their concentration gradients–even in the absence of electric fields.
- Current: A net movement of charge is an electric current measured in amperes (A) and symbolized by I. Thus, a net flow of ions is an ion current The direction of current is defined as the direction of positive charge movement, so in neurophysiology, a positive current means positive charges moving out of a cell, an outward current.
- Voltage (synonymous with potential): If cations are removed from a compartment (the cell), the compartment becomes more negative, a negative voltage will be set up inside the cell. A voltage is measured in units of volts (V) and symbolized by V. Positive charges want to move to regions of more negative voltages in the absence of other forces.
- Conductance: Membrane conductance describes how easily ions can cross the membrane. Conductance, symbolized G, is proportional to the number of ion channels open. Each open channel contributes a small conductance to the total. (A closely related inverse concept, resistance, expresses how hard it is for ions to move. Mathematically, resistance is the reciprocal of conductance (1/G). An insulator has high resistance and low conductance.)
- Capacitance: Electrical capacitors store charge. Two conductors separated by a thin insulator (membrane) form an electrical capacitor. The magnitude of a capacitance (symbolized C) is defined as the number of charges that you have to move to achieve a certain potential change. Therefore, the capacitance of cell membranes tells us how many ions have to be moved across the membrane to change the membrane potential during signaling.
- Equilibrium potential for an ion: The membrane potential at which the electrical force and the diffusion force (due to the concentration gradient) exactly cancel for that particular ion. At the equilibrium potential there will be no net force on that ion across the membrane.
- Reversal potential for an ion channel. The membrane potential at which no net charge movement occurs in that ion channel. The channel may not be perfectly ion selective so that at the reversal potential one kind of ion may move one way and another may move the opposite way, but the net effect is no charge movement. When only one kind of channel is open, the ion flow in that channel tends to move the membrane potential to the reversal potential for that channel. Equilibrium potential is a thermodynamic property of an ion, given its gradient. Reversal potential is a property of an ion channel, given its ion selectivity and the gradients of the permeant ions.