membrane potentials and excitability

cell plasma membrane

all cells are surrounded by a fluid, lipid-protein bi-layer called the cell membrane

functions: provides cellular structure, fluidity of membrane/cell, physical barrier that prevents free passage of substances

semi permeable membrane

passive transport

seeks to establish equilibrium between intracellular and extracellular compartments

solute will flow from high concentration → low concentration

positively charged particles will flow into negatively charged compartments (opposite charges attract)

simple free diffusion - O_2, CO₂ lipid soluble molecules freely diffuse across membrane

protein mediated (facilitated) diffusion - channels in membrane allow specific ions to flow through = ion channels

active transport

consumes energy (ATP) to transport molecules against their concentration gradients

leak channel (ion channel)

they are always open

each channel is selective for specific ions

differences in the number and type of leak channels in the membrane determines the plasma membrane’s selective permeability

at rest the typical plasma membrane has higher permeability for K⁺ than for Na⁺ or Cl⁺

gated channels (ion channel)

ligand dated ion channels

• ligand receptor embedded in the channel

• ligand binding to receptor induces conformational change to open or close the channel

voltage gated ion channels

• voltage sensor in the ion channel protein

• changes in membrane potential induce a conformational change to open or close the channel

active transport: Na⁺ -K⁺ ATPase

maintains Na⁺ and K⁺ gradients across plasma membrane

uses energy (ATP)

vesicular transport

endocytosis (including phagocytosis)

exocytosis

resting membrane potential (RMP)

difference in electrical charge across the plasma membrane in the cell at rest

approx. -70mV (-60 to -100 mV)

inside the membrane is more negative than the outside

used to transport substance across the membrane - ions will move across membrane is they are given the chance

equilibrium potential

charge gradient + concentration gradient = electrochemical gradient

at equilibrium, the charge gradient balances the concentration gradient, and there is no net flow of ions across the membrane

for each ion at a given concentration gradient, the charge gradient that leads to equilibrium potential. calculated using the Nernst equation

Nernst equation

electrical potential (E) of an ion is calculated using this - it is base on concentrations outside and inside the cell

RMP implications for ion flux

if ion channels open in response to specific stimuli, the ions will move towards their equilibrium potential. the membrane potential (MP) will change as a result

given RMP= -70mV and E k+ = -90 mV, if we open K⁺ channels in the membrane3, an efflux of K⁺ ions will occur

changes in membrane potential

excitable cells (neurons, cardiac cells) actively induce changes in their membrane potential

depolarisation influx of positively charged ions, known as excitatory post synaptic potential (ePSP)

hyperpolarisation: efflux of positively charged ions and an influx of negatively charged ions, known as inhibitory post synaptic potential (iPSP)

action potentials

if depolarisation at a certain point reaches a threshold voltage of -55mV (action potential threshold), an action potential in generated

brief, rapid, large (100mV) change in membrane potential during which potential reverses

serves as long distance signals (propagates along axon and from neuron to neuron)

involves voltage gates Na⁺ and K⁺ channels

action potential text

depolarisation pushes membrane potential across the action potential threshold (-55mV)

reduced voltage open voltage gated sodium channels in that portion of the plasma membrane

at the peak of the AP, voltage gated potassium channels open and sodium channels close

voltage gated potassium channels then close leaving plasma membrane hyperpolarized. resting membrane potential and concentration gradients are restored

refractory period

period when a further stimulus will not trigger another action potential

refractory period lasts 1-2ms. neurons can still transmit up to 500-1000 impulse per second

summation

postsynaptic response is the combined effect of thousands of synaptic events (iPSPs and EPSPs) from many neurons

neuron is brought to action potential threshold at the axon hillock via summation - temporal summation, spatial summation

prpagation

APs reproduce along the axon from the axon hillock to the synaptic terminals

depolarisation is one portion of the membrane opens more voltage gated sodium channels in the adjacent portion of the membrane

depolarisation travels along the cell

myelin

functions as an electrical insulator

high density of voltage gates sodium and potassium channels at gaps between myelin (nodes of ravenier)

saltatory conduction (action potential jumps from one node to next) = faster propagation

myelination increases the conduction velocity of action potentials