⚡ Lecture 10 – Nervous System: Membrane Potential (RMP, Graded, and Action Potentials)

graded potential

a small, variable change in the membrane potential casued by ion flow through gated channels; can vary in size, polarity, and duration depending on stimulus

action potential

a large, stereotyped change in membrane potential that occurs once the threshold is reached; propagates without decrement along the axon

resting membrane potential (RMP)

the steady-state membrane voltage of a cell at rest, created by unequal permeability and ion distribution across the membrane

hyperpolarization

a membrane potential change that makes the inside of a cell more negative than the resting potential

depoolarization

a change in membrane potential that makes the inside of the cell less negative (closer to zero or positive)

repolarization

the return of membrane potential back toward the resting value after depolarization

ligand-gated channel

an ion channel that opens or closes in response to a chemical signal (e.g. neurotransmitter binding)

voltage0gated channel

an ion channel that opens ot closes in response to changes in membrane voltage

mechanically-gated channel

an ion channel that opens or closes in response to physical distortion (e.g. stretch, pressue)

electrochemical gradient

the combines effect of an ion’s concentration (diffusive) gradient and the electrical gradient acing across the membrane

equilibrium potential (Eion)

the membrane potential where the electrical and diffusion forces on an ion are equal and opposite, producing no net ion movement

at Eion

ions still move in both directions, but there is no net movement of charge across the membrane

resting membrane potential (SO3)

arises from ions with different equilibrium potentials and unequal membrane permeabilities due to different leak channel abundance

typical ion distribution

high K+ and negatively charged proteins inside; high Na+ and Cl- outside

permeability hierarchy

K+ permeability is highest (many leak channels), Na+ is low (few leak channels), Cl- is moderate

each permeable ion

pushes the membrane potential toward its own equilibrium potential, weighted by its relative permeability

ion channels

have a primary role in creating membrane potential because they directly allow ions to move across the membrane

ion pumps (e.g. Na+/K+ - ATPase)

contribute indirectly by maintaining concentration gradients over minutes to hours

Na+/K+ - ATPase pump

moves 3 Na+ out and 2 K+ in, maintaining gradients but having small direct effect on RMP (<5mV)

leak (non-gated channels)

always open; responsible for maintaining the RMP

gated ion channels

open or close in response to stimuli (ligand, voltage, or mechanical); control graded and action potentials

exmaple locations of gated ion channels

ligand → dendrites and soma (synaptic potentials)

mechanical → sensory receptors

voltage → axons (action potentials)

graded potential amplitude

depends on the number of gated channels that open and the size of the resulting ion flow

duration

depends on how long the channels remain open and how far the signal spreads before decaying

spread of graded potential

decays with distance because current leaks across the membrane

four defining properties

1. threshold - an action potential is triggered only if membrane depolarization reaches a critical voltage

2. stereotypy - all action potentials have the same amplitude and duration once triggered

3. propagation without decrement - the signal travels the length of the axon without weakening

4. refractory period - a brief time after and action potential when another cannot (or is harder to) occur

graded vs. action potentials

• graded potentials vary in size and decay with distance

• action potentials are all-or-none and travel long distances unchanged

voltage-gated Na+ channels (VGNCs)

• open rapidly at threshold (activation gate)

• inactive shortly after opening (inactivation gate)

• responsible for depolarization phase of action potential

• exhibit positive feedback - depolarization causes more Na+ channels to open

voltage-gated K+ channel (VGKCs)

• open slowly after threshold is reached (activation gate only)

• responsible for depolarization hyper polarization

• exhibit negative feedback - K+ efflux restores resting potential and closes channels

refractory periods

• absolute refractory period: no new AP can be triggered (Na+ channels inactivated)

• relative refractory period: a stronger than normal stimulus that can trigger another AP (some Na+ channels reset, K+ still open)