Membrane biophysics Flashcards
What is current?
Rate of flow of charge
What is a capacitor?
A device that stores charge, made of two conductors separated by an insulator
One plate stores negative charge while the other stores positive charge
How is a membrane thickened?
Through myelination
What is the relationship between capacitance, surface area and insulator thickness?
Capacitance is proportional to surface area and inversely proportional to insulator thickness
What factors determine the membrane resistance and capacitance?
Membrane resistance depends on density of open ion channel
Capacitance depends on surface area and membrane thickness
What is the time constant and what is it equal to?
The time it takes for a membrane potential to change by 63% of its maximum
TC = Membrane resistance x capacitance
What is the length constant?
The distance from injection point where a voltage signal has decreased by 63%
The longer the length constant, the better the conductor
What are the three components of resistance?
Membrane resistance
Axial resistance
Extracellular fluid resistance
What is excitability equal to?
1/threshold current to initiate an AP
What is the local circuit theory?
The depolarisation of a region of an axon membrane (active zone) causes induces depolarisation further down down the axon (resting zone). The current can’t move backwards because the preceding region is in its refractory period.
What two factors make a good length constant?
A high membrane resistance coupled with a low axial resistance
What is a myelin sheath and what does it do?
A Schwaan cell wapped around a neuron
Increases membrane resistance and decreases capacitance
Also increases the length constant
Explain Saltatory conduction
Myelinated sheaths are separated by Nodes of Ranvier which have a high density of Na+ and K+ ion channels. These are put in place to regenerate action potentials and allow for rapid depolarisation
APs are passed between them by electrotonic (passive) conduction
What is refractoriness?
A decrease in membrane excitability following an AP
What does the Nernst equation measure
Equilibrium potential
Give two examples of how sodium current is blocked
Tetrodotoxin
Replacement of Na+ with cations that are too large to pass through the channel
Why does the resting membrane potential never reach Ek as expected?
It isn’t only the potassium channels that are open
What is the advantage of the voltage clamp technique over radioactive ion flux measurements?
It has a greater time resolution, measuring current events on a microsecond timescale
How does the voltage clamp technique work?
Membrane potential is controlled and membrane current is measured
A feedback amplifier measures membrane potential and if it isn’t equal to the voltage command (what you choose) it injects a current to return it to that voltage
If the current is enough to activate voltage gated channels, an ionic current is generated and the amplifier injects an equal and opposite charge which is how the membrane current is measured
What determines the resting membrane potential?
The selective permeability of the membrane to K+ coupled with different electrochemical gradients
How do voltage gated channels know when to open or close?
They have voltage sensing domains
What is conductance?
A measure of ability to pass ionic current
How does hyperpolarisation occur?
Permeability to K+ is higher than at rest and conduction is higher for K+ than Na+
What does the patch clamp technique enable you to do?
Measure the current across a patch of membrane rather than the whole cell using a fine micropipette
What is the mechanism behind the absolute refractory period?
The inactivation gate of the channel is closed which makes the channel non-conducting. The channels need time at negative potentials before they can become active again
Why is the open probability of potassium channels never zero?
There are two populations, one that opens at the resting membrane potential and another (VG) that respond to changes in membrane potential
What does axial resistance depend on?
Axon diameter, the bigger the axon the lower the resistance
