Membrane & action potentials

chemical forces moving ions

difference in ion concentration, causes diffusion from high to low concentration

electrical forces moving ions

cell interior is negatively charged - positive cations are retained and negative ions are expelled

2 types of ion channels

always open and allow free movement of ions - gated channels which require a stimulus to open (ligands/mechnical force/voltage) and are specific to particular ions

chemical gradient under resting conditions

Na+ concentration is 10x higher outside neuron than inside, K+ concentration is 15x higher inside neuron than outside, constant flow of K+ down gradient from inside to outside, neuron via open channels

how do ion pumps move Na+/K+

ion gradient maintained by continuous operation of Na/K ATPase pump, simultaneously moves 3Na+ from inside to outside, and 2K+ from outside to inside - each time the cell loses one + ion from intracellular environment

polarisation

difference in charge across membrane

resting membrane potential

difference in voltage across membrane when neuron is at rest - for most neurons = -70 mV

Na+ electrochemical gradient when channels open

chemical gradient drives ion movement into cell, electrical forces pulls + ions into cell, both act in same direction so Na+ will enter cell - eventually electrical and chemical gradient balance out - no more net flow

equilibrium potential

membrane potential required to exactly counteract the chemical forces acting to move one particular ion across membrane

K+ electrochemical gradient when channels open

chemical gradient drives ion movement out of cell, electrical force pulls + ions into cell, chemical force > electrical so K+ moves out

what happens when K+ moves out of neuron

cell charge becomes more negative, electrical gradient becomes stronger - eventually chemical force driving K out = electrical force pulling K in - no more net flow

nernst equation

used to calculate equilibrium potential = 61/z x log (outside conc/inside conc)

how are neurons stimulated

depolarisation from incoming signals - if membrane potential is depolarised beyond critical level of -55mV then action potential is triggered

sodium voltage gated ion channels

have both activation and inactivation gate. at rest - activation gate is closed. inactivation gate is open

potassium voltage gated ion channels

have one activation gate, opens to allow K+ to flow through channels, closes to stop flow

initial stimulation

when neuron receives excitatory stimulus, ligand-gated sodium channels open

small amounts of Na+ move down concentration gradient into neuron - resting potential becomes more positive

when membrane potential reaches -55mv …

voltage gated activation gates in Na+ channel open quickly, Na+ floods into neuron, neuron loses negative charge quickly and undergoes depolarisation

Na+ channel inactivation

when inside of neuron becomes highly positive, pore of voltage gated activation gates in Na+ channel is plugged by inactivation gate - Na+ flow into neuron stops

repolarisation

intracellular environment, becomes positive enough for voltage gated K+ channels to open slowly, K+ flows down concentration gradient out of cell, causes inside of neuron to quickly regain its negative charge

hyperpolarisation

increased negative charge inside neuron, voltage gated K+ channels close, slow process - some K+ continues to move out of cell, extra movement makes membrane potential more negative than resting potential

refractory period

neuron can't fire another action potential during hyperpolarisation - eventually Na/K ATPase pump restores resting membrane potential, neuron can fire another action potential