Society and the Brain Lessons 3-4

Resting membrane potential

The charge difference across a neuron's membrane when it is not actively firing (typically -65 to -75 mV, with the inside more negative than the outside.)

Ion channel

A protein embedded in the cell membrane that allows specific ions to cross

Electrochemical driving force

Net push on an ion determining which way it moves resulting from a combination of the chemical gradient and electrical gradient

Chemical (concentration) gradient

The force that drives ions from an area of high concentration to an area of low concentration (diffusion).

Electrical gradient

The force that drives positively charged ions toward negatively charged areas, and vice versa

Equilibrium potential

Membrane voltage at which an ions chemical and electrical gradient cancels each other out (NO NET MOVEMENT)

Potassium (K⁺) leak channels

Channels that are open at rest and allow K⁺ to continuously cross the membrane (their abundance is why K⁺ has the largest influence on resting membrane potential.)

Na⁺/K⁺ pump

A membrane protein that actively transports Na⁺ out of the cell and K⁺ into the cell (against their concentration gradients) (requires ATP) (active transport).

Active transport

Movement of molecules across a membrane that requires energy (ATP), typically moving substances against their gradient

Selective permeability

The property of a membrane that allows some ions/molecules to cross more easily than others, depending on which channels are present and open.

Action Potential

A change in membrane potential that neurons use to send signals down its axon

Depolarization

Membrane potential becomes less negative (moves toward 0 or is positive)

Repolarization

Membrane potential moves back toward its negative resting value after having been depolarized

Hyper polarization

Membrane potential becomes more negative than resting potential as a result of potassium channel staying open for too long

Excitatory

An influence that pushes the membrane towards firing

Inhibitory

Influence that pushes the membrane potential away from firing

Threshold

The membrane potential that must be reached (via depolarizing current) for voltage-gated sodium channels to open and trigger an action potential.

Rising phase

The phase where the membrane becomes extremely permeable to Na⁺, causing rapid depolarization.

Overshoot

The peak of the action potential, where membrane potential becomes positive, right before the falling phase begins.

Falling phase

The phase where voltage-gated K⁺ channels open (slower than Na⁺ channels) and Na⁺ channels inactivate, causing repolarization.

Undershoot

A brief dip below resting potential, approaching K⁺'s equilibrium potential (-85 mV), after the falling phase.

Absolute refractory period

The period right after an AP fires when no new AP can occur, because Na⁺ channels are inactivated and cannot reopen regardless of stimulus strength.

Relative refractory period

The period after the absolute refractory period when an AP can occur, but only with a stronger-than-normal stimulus.

Inactivated (channel state)

A state of a voltage-gated Na⁺ channel in which it cannot reopen regardless of voltage, distinct from simply being closed.

Unidirectionality (of action potentials)

The property that APs only travel one direction down an axon, because the region just behind is in its absolute refractory period.

Tetrodotoxin (TTX)

A pufferfish toxin that blocks voltage-gated sodium channels, preventing action potentials entirely.

TEA (tetraethylammonium)

A toxin that blocks voltage-gated potassium channels; doesn't block the AP but alters its properties (e.g. falling phase)

Nernst Equation

Calculates the equilibrium potential for a single ion, based on its concentration inside vs. outside the cell.

Goldman Equation

Calculates the overall membrane potential by accounting for multiple ions at once, weighted by their relative permeabilities.