lecture 4: membrane potentials Flashcards
understand how differences in ECF and ICF composition and the relatively high resting membrane permeability to K result in the establishment of RMP (detailed)
- the energy consuming action of the NA/K ATPase moves Na and K across the membrane, to establish and maintain the normal ECF and ICF concentrations of those ions
- this results in a chemical gradient for Na to move into the cell and for K to move out
- nerve cells have a large number of K leak channels
- this means the resting cell is relatively permeable to K
- there is a strong chemical gradient driving K out of the cell, so K exits the cell via these channels, carrying positive charge with it
- as K leaves the cell, the loss of positive charge leaves a slight deficit of positive charge inside the cell, making the cell slightly negative inside (and outside positive)
- a second gradient for K movement has now been established
- since K carries a positive charge, and the cell is inside negative, there is now an electrostatic attraction bringing k back into the cell
- eventually the cell will reach a point at which the outward movement of K down its chemical gradient is exactly matched by the inwards movement of K down its electrical gradient
- transmembrane voltage at this point = equilibrium potential
the membrane potential depends on
- concentration gradients of ions
- relative permeability of membranes for ions
when is the cell in electrochemical equilibrium
when its at a transmembrane voltage where the electrical charge across the membrane exactly balances the outward diffusion of potassium, there is no net gain or loss of potassium
how does K diffuse out of the cell
through a leak channel
what is the RMP largely a function of
the difference in K concentration across the cell membrane and resting the membranes permeability to K
What happens when the cell is at rest - for K and Na
- electrostatic and chemical forces for potassium movement are equal and opposite, so there is no net gain or loss of K
- the cell is relatively impermeable to Na, so no net gain or loss of Na
–> steady state
what would happen if we suddenly made the membrane permeable to Na?
Na would enter the cell down its electrochemical gradient, bringing positive charge and making the inside of the cell positive (or less negative)
what might happen to change the permeability of the membrane?
- chemical
- voltage
- mechanical
ion channel gating = chemical
- a chemical signaling molecule binds to a receptive site on the channel, typically resulting in a change in the shape of the channel
- ions can then cross the membrane driven by their electrochemical gradient
- the signaling molecule unbinds, and the ion channel changes shape again to close and membrane permeability is reduced
ion channel gating - voltage
- voltage gated channels are common in areas of excitable membrane, crucial in generation and conduction of action potentials and synaptic transmission
- channels are gated by the voltage across them
- may be then opened by a change in voltage, then pass to an inactivated state so they cannot open again until the membrane has been repolarized (accounting for refractory periods)
ion channel gating - mechanical
- channels are gated in response to physical/mechanical forces
- crucial in triggering signals in response to touch and pressure
- typically generate a receptor potential
what happens to membrane potential when membrane permeability changes?
- a change in permeability means that the membrane typically becomes more or less permeable to specific ions
- ions may move (or stop moving) across the membrane
- ions carry electrical charge
- the change in charge distributions across the membrane will result in a change in the membrane voltage
- that is, the membrane potential changes
- these changes in membrane potential may be local potentials or action potentials
membrane potential for potassium
-94mV
membrane potential for sodium
+65mV
what does RT stand for in the membrane potential equation
energy per mol at this temp
what does F stand for in the membrane potential equation
Convert chemical energy units to electrical
what does ln stand for in the membrane potential equation
non linear relationship
what does Ko/Ki stand for in the membrane potential equation
concentration gradient
ko= concentration of potassium outside
ki = concentration of potassium inside
what does Pna stand for in the membrane potential equation
permeability
