Cell Membrane Potentials and Ionic Gradients

Study Notes

Most living cell membranes are electrically polarized, meaning there's a difference in electrical charge between the inside and outside of the cell.
The transmembrane voltage range is +30 to -100mV.
Membrane potential is necessary for signaling, especially in neuromuscular systems, and allows cells to communicate and respond to stimuli.

Na-K ATPase and Ca2+ ATPase are active transport proteins that create large concentration differences of sodium and potassium across the membrane, allowing for depolarization or repolarization of the cell.

The body maintains a constant ratio of extracellular to intracellular potassium by moving potassium into or out of the cell.
The inside of the membrane is relatively more negative because K+ tends to move out of the cell.
The equilibration for potassium is -94 mV (no net movement of the ion), with [K+]i = 155 mM and [K+]o = 2.5-4.5 mM.

Sodium is very high outside and low inside, making the outside more positive, which makes it want to enter the cell.
The equilibration for sodium is +61mV (no net movement of the ion), with [Na+]i = 12 mM and [Na+]o = 145 mM.

The Nernst equation describes the relationship of voltage to ion concentration differences and is given by E = potassium outside/potassium inside.
The greater the concentration, the greater the tendency of an ion to move in one direction.

Most of the conductance is due to potassium, which causes the cell membrane to become negative when a positive ion moves out of the cell.
The sign of the membrane potential is decided by whether a positive ion leaves the cell or a negative ion enters the cell.

The Goldman equation is used when the membrane is permeable to multiple ions and takes into account ionic conductance and permeability.
The equilibrium potential is dependent on the polarity (direction), concentration difference for each ion (gradient), and permeability of the membrane to each ion (net movement).

Ionic current is the driving force (how much the ions want to move, expressed in voltage) multiplied by the conductance for that ion.
Membrane potential can be measured using an electrode filled with an electrolyte solution impaled into the interior of a fiber and another in the ECF (indifferent electrode).

The resting membrane potential for sodium is -86 mV because the conductance of potassium is 100 times greater than sodium.
The two important factors for the origin of normal resting potential are potassium diffusion potential and sodium diffusion through nerve membrane.
Minor factors include the Na-K ATPase pump and chloride.

Depolarization is the process of making the membrane less negative (Vm), which can overshoot and become positive.
Repolarization/Hyperpolarization makes the membrane more negative (Vm), which can undershoot.
Inward current is the flow of positive charge into the cell, depolarizing, while outward current is the flow of positive charge out of the cell, repolarizing.
Threshold potential is the membrane potential at which the action potential commences, with depolarization becoming self-sustaining and inward current greater than outward.
Overshoot is when the membrane potential is positive, and undershoot is when it is more negative than at rest.

Action potential is caused by a wave of depolarization that spreads down motor neurons and excitable tissues, triggered at -55 mV, which is called the threshold.
The difference between subthreshold response and action potential is that action potential has a much larger amplitude and is propagated down the length of tissue without decreasing in strength, with all or none response.
Stages of action potential include resting, depolarization, and repolarization, with changes in sodium and potassium conductance.

Refractory periods are periods during which a second action potential cannot be triggered.
There are absolute and relative refractory periods, depending on the state of sodium channel inactivation gates.

Propagation of action potential occurs through depolarization of local segments, spread of local current to adjacent regions, and becomes self-sustaining.
Conduction velocity can be increased by larger neuron diameter, which has lower internal resistance to electrical movement, and by myelination, which acts as insulation, prevents signal loss, and allows signals to jump from node to node.

AP stages include resting, depolarization, and repolarization.
Sodium activation gate steps include closed but available, open, and inactivated states.