Passive and Active Properties of Cell Membrane Flashcards
What is a membrane potential?
- the plasma membrane of all cells has one
- (Vm)
the potential is due to separation of opposite charges (+/-) across the membrane - the membrane potential can be measured as the difference in potential between the tip of a microelectrode inserted into the cell and a reference electrode outside of the cell by connecting both to a potentiometer or voltmeter
it is recorded in units of mV ( millivolts) - which is 1/1000s of a volt.
Vm varies considerably between different cell types
(e.g for nerve cells -65mV)
how is the membrane potential determined?
- the selective permeability of the membrane to certian ionic species
- the concentration gradients of the permanent ions across the membrane
- the diffusion of ions down their concentration gradients generates an electrical potential across the membrane which is the resting membrane potential !!!!
What is the nernst equation?
The Nernst Equation enables the determination of cell potential under non-standard conditions.
large changes in membrane potential involve only small changes in ion concentrations in the ECF and ICF
the net difference in electrical charge occurs at the inside and outside surfaces of the membrane - the membrane acts as a capacitor ! - an electrical device that stores charge !!
ions are driven across the membrane at a rate proportional to the difference between the membrane potential (vM) and the equilibrium potential (Eion)
driving force = Vm-Eion( same as Ek)
if the concentration difference across the membrane for an ion is known - an equilibrium potential (Eion) for that ion can be calculated from the nenst equation !!!
The Nernst Equation is derived from the Gibbs free energy
Vm= (RT/zF ) ln (K)0/ (K)i
^ need to learn off by heart !!!
(REFER TO PAGE 4 OF LECTURE URGENTLY FOR EQUATION AND WORKED EXAMPLES !!)
Vm= membrane potential R = gas constant T = absolute temperature z= ionic valence F = faraday constant
at resting potential?
- at resting potentia (-65mV) the passive movement of ions down their electrochemical gradient is :
Na+ inward ( causing depolarisation)
Ca2+ inward ( causing depolarisation)
K+ outward ( causing hyperpolarization)
Cl- inward ( usually) ( causing hyperpolarisaiton ( usually)
The ionic basis of the resting membrane potential?
- the neuronal membrane , at rest is permeable to ions other than K+ , anmely Na+ and Cl-
thus the membrane potential (Vm) is dependent upon their concentration gradients and their relative permeabilites.
what is the Goldman- Hodgkin-Katz equation?
GHK is useful to finding out Vm !!!!
( dont need to know equation just what it has in it .. and how to do it !.)
R= gas constant T= absolute temperature F= faraday constant
What is the effect of the opening of Na+ selective or K+ selective channels upon the membrane potential ?
- Na Channel opening = the membrane potential is driven towards ENa
- K+ channel opening= the membrane potential is driven towards Ek!!!
what are ion channels?
- protein complexes that span the lipid bilayer to form a central pathway that allows rapid flow of selected ions!
- most ion channels are regulated- they exist in open(o) , closed(c) and additional conformations
there are many types of open ion channels ( e.g.gated)
what are 3 types of gated ion channels?
- membrane voltage - voltage -gated ion channels
- chemical substances(ligand gated ion channels)
- physical stimuli - e.g. mechanical , thermal
What are the ion channels responsible for the action potential in neurones?
- Voltage - activated Na+ channels (Nav ) - depolarising
- Voltage - activated K+ channels (Kv) - hyperpolarising
( refer to page 8 of notes for proper diagram)
What is an action potential?
- reliable when it reaches threshold, there is a rapid depolarisation which activated more sodium channels to open.
- sodium channels open for 1 msec!!!
action potentials are brief electrical signals in which the polarity of the nerve cell membrane is momentarily ( about 2 msec) reversed.
Action potentials propagate along nerve cell axons with constant magnitude and velocity ( for a given axon)allowing signalling over long distances.
- action potentials are generated when threshold is reached - they are all or none !!
(N.B. Hodgkin and Katz used an alarmed squid to measure action potential!)
What do you know about Voltage - activated Na+ and K+ channels - IN THE CASE OF POSITIVE FEEDBACK?
- Both highly selective for their respective cations
- both activated by membrane depolarization -Na+ channels rapidlu; K+ channels with a slight delay.
- The activation of Na+ Channels is self-reinforcing - the opening of a few channels causes further depolarization etc. ( THIS IS POSITIVE FEEDBACK !)
INCREASE IN Na+ CONDUCTANCE > INWARD Na CURRNET > DEPOLARISATION > …. ( back to beginning of cycle )
What do you know about Voltage - activated Na+ and K+ channels - IN THE CASE OF NEGATIVE FEEDBACK?
- The activation of K+ channels is self -limiting - outward movement of K+ causes repolarisation which turns off the stimulus for opening - this is NEGATIVE FEEDBACK !
DEPOLARIZATION >INCREASE IN K+ CONDUCTANCE > OUTWARD K+ CURRENT > REPOLARIZATION
What is the refractory Period and Na+ channel Inactivation?
- although voltage- activated Na+ Channels Inintially open in response to depolarization, they enter a non conducting , inactivated state during maintained depolarization!
- repolarization is required for the channel to enter the closed state in readiness for opening and the generation of a further action potential.
- inactivation contributed to the repolarizing phase of the action potential and is responsible for the refractory period.
What is the difference between Absolute and Relative Refractory Period?
( refer to page 11 of notes )
Absolute refractory period - no stimulus , however strong, can elicit a second action potential ( all Na+ channels inactivated.)
relative refractory period: - a stronger than normal stimulus may elcit a second action potential ( mixed population of inactivated and closed channels)
