action potentials + membranes Flashcards
How can you achieve intracellular membrane potential recording?
glass capillaries fine tip <1 micrometer
electrode filled with salt solution, wire inserted into salt solution and connected to amplifier, which measures p.d. between the tip and a ground
How does the fluid mosiac model describe the structure of the plasma membrane?
a mosai of components, with cholesterol, proteins and carbohydrates floating in the sea of phospholipids.
PPLs and proteins re able to diffuse rapidly and laterally in te membrane
What are the 3 functions of the mosaic membrane?
- bilayer is impermeable to ions and organic molecules so allows to cell to maintain intraceullar envirnment
- proteins in the membrane allow cell to cumminicate with its environment and allow selective movement of molecules across the membrane
- bilayer separates ionic chargein the intraceullaand extracellular salt fluids, therefore acts as insulaor, enabling charge to be stored on the neuronal membane
why is the asymmetric distribution of ions not ufficient to geneate a membrane potential?
the negative andposiitve charges are balanced across the membrane by different ions
In which direction do K+ ions flow through the membrane?
from inside to outside down the conc grad
through leak channels
Describe how a membrane system reaches K+ electrochemical equilibrium.
K+ flow through leak channels to the outside of the cell
as positive K+ leave there is a net accumulation of -ve charges on the inner membrane leaflet: becomes hyperpolarised relative to extracellular fluid
generates an electrical force that tends to attract K+ ions back into the cell
as membrane becomes more hyperpolarised, electrical gradient = diffusional gradient = equilibrium
What does the Nernst equation refer to?
the membrane potential at which electrochemical equilibrium is reached
What does the Nernst equation decree?
membrane potential should be proportional to log[K+]out
Why do experimental measurements of the membrane potential deviate from the Nernst potential for K+ ions at low values of [K+]out?
leak channels are weakly permeable to other ions (Na+)
have to use GHK equation (average of Nernst potential or each permeable ions weighted by relative permeabilities)
How can an ion channel select for K+ over Na+ ?
to dissolve, cations need to be surrounded by the -ve dipoles of the water molecules
to pass through pore, cations need to shed hydration shells
water molecules are replaced by interactions with oxygens from -COOH or -OH groups in the proteins of the selectivity filter
the spatial configuration of these replacement atoms makes interactions 100x more favourable for K+ over Na+
How do you maintain ion gradients across the membrane?
active transport against electrochemical gradients
pumps: K+/Na+ pump (3 Na+ out and 2 K+ in) : directly caused small hyperpolarisation of the membrane
Difference between a cotransporter and an ion exchanger?
ion exchanger: driving ion and driven ion move in opposite directions
cotransporters: direction of transport the same for both
what is secondary active transport?
when the energy stored in electrochemical gradients is used to drive active transport
e.g. flow of Na+ down its ECG is used to expel Ca2+
Why are higher demand placed on transporters in active neuronal networks?
neurons are surrounded by glia which also use AT to maintain ion gradients and uptake neurotransmitters
much higher demand to maintain intra and extracellular environments
if channels blocked, membrane potentials collapse rapidly
In which direction is membrane potential defined?
relative to the exterior of the cell
