Lecture 5: Graded Potentials and Passive Membrane Properties Flashcards
What is conductance?
inverse of a resistance – something that makes a membrane able to pass current (ie. ion channel)
What is equilibrium potential (Eion)?
property of a specific ion in a specific system
represent the point where the forces exerted on that ion species are balanced (equal and opposite), so there is no net movement of that ion
What is reversal potentials (Erev)?
property of a conductance that generates a particular current (by allowing a specific ion or group of ions to move)
represent the point where there is no net current moving through that open conductance (ie. the number of charges moving one way is the same as the number moving the other)
What are the two different approaches to testing any Vm signal?
- alter [ion]o bathing the neuron (ie. alter Eion)
- find the reversal potential – alter Vm (artificially) until net ion flux stops
Even when an ion is permeable, what determines whether or not it will make a net flux (current)?
depends on driving force, or voltage difference (ΔV) between its Eion and the current Vm
I = ΔVxG
at Eion : I = 0 x G = 0 (no driving force = equal ion movement back and fort = no current)
What is the reversal potential for an EPSP channel?
0 mV
Under what situation can an Erev be the same as an Eion?
if there is an ion channel where only one ion can go through
In most neurons, what is the threshold to trigger an AP?
Vm ≈ -45 to -55 mV
What do PSPs summate according to?
according to the currents that create them, which depend on driving force
When currents are generated by the cell membrane itself, in which direction does inward current flow?
downward
BUT this is opposite to how we draw the resulting voltage changes
When currents are generated by the cell membrane itself, in which direction does outward current flow?
upward
BUT this is opposite to how we draw the resulting voltage changes
What is the reversal potential of typical EPSP?
0 mV – complicating, because this value does not match any known value of Eion for typical ions
Why is the reversal potential of typical EPSP = 0 mV?
both PNa and PK increase during an EPSP
at 0 mV, amount of Na+ going in is the same as amount of K+ going out – Na+ and K+ have the same amount of charges, therefore there is no net current
What is the reversal potential of typical IPSP?
ECl
What are IPSPs driven by?
increase in permeability to Cl- ions
What is the reversal potential of non-polarizing/depolarizing IPSPs?
less negative than RMP
- sometimes IPSPs even produce slight depolarizations if the neuron is actually ‘at rest’
- these inhibitory synapses trigger an increase in membrane permeability whose reversal potential can be near or above RMP, but is still below
Difference between integrated PSP (solid orange) and sum of E1 and E2 (dotted orange) is lower than expected if you just subtract the IPSP amplitude (dotted red). Why?
although IPSP is near its Erev when neuron is otherwise at rest, any simultaneous depolarization of membrane (from EPSPs) moves Vm away from IPSP Erev, increasing the IPSP driving force, which allows more current to flow through the open IPSP conductance
more driving force = more current = more change in membrane
AP vs. graded potential
- graded
- signal type
- size
- summation
- degradation
AP:
- graded: no
- signal type: digital or binary signal (on/off)
- size: generally comes in one size in each neuron
- summation: cannot be summed together – no such thing as half an AP or double AP
- degradation: spreads (within an axon) without degrading
graded potentials:
- graded: yes
- signal type: analog signal
- size: different sizes
- summation: can be summated/integrated with other graded potentials when they spread into the same patch of membrane
- degradation: degrades as it spreads in neurite (down a neurite)
Why do graded potentials decay with distance?
neurites are leaky and semi-conductive
How are neurites semi-conductive?
contain (semi-)conductive fluid (cytoplasm) surrounded by insulator (membrane) – allows current to flow or spread within the neurite, if a membrane potential changes at one location, via electrotonic conduction
Is electrotonic conduction perfect?
NO – it’s efficient and rapid, but ionic fluid still has some resistance to electrical current flow, in addition to the membrane leaking ions
What does transmembrane current arise from?
ions moving through ion channels
Where does transmembrane current flow?
flows through conductances (local ion channels in a synapse, or sensory receptors in a dendrite) that opened when the signal was generated
How does eletrotonic current spread?
by charge displacement through the cytoplasm
Where does electrotonic current flow?
flows within neurite, through cytoplasm
What does leak current do?
moves charge back to ECF (also via ion channels)
Where does leak current flow?
flows through any conductances (ie. leak ion channels) that happen to be in the membrane and open when electrotonic current flows by
What are the 4 components of an equivalent circuit?
- membrane resistance (Rm)
- internal resistance (Ri)
- external resistance (Re/Ro)
- membrane capacitance (Cm)
What is membrane resistance (Rm)?
reflects both resistance of plasma membrane, and the small amount of current flow that leaks across open ion channels (even ‘at rest’)
What is internal resistance (Ri)?
some resistance to flow of electrotonic current provided by cytoplasm
What is external resistance (Re/Ro)?
small resistance to flow of electrotonic current provided by ECF
What is membrane capacitance (Cm)?
electrical feature arising from properties of lipid bilayer
As electrotonic current travels down dendrite, what is it effected by?
membrane resistance (Rm) and internal resistance (Ri)
To keep the maximum amount of current inside the neurite, what should Rm and Ri be?
Rm should be high and Ri should be low, so that internal pathway is the path of least resistance
- if you had high Rm and Ri, you would still lose current
How quickly does membrane potential decline with distance?
exponentially, depending on resistances
What is the length constant (λ) (space constant)?
constant value for a given neurite that describes how much a passive/graded potential decays with distance
What are the different ways that the length constant can be defined
if directly measuring potentials along a neurite: λ is the distance over which change in membrane potential will decrease to 37% (1/e) of its original value
if only have one set of electrodes, or want to know why two neurites could have different λ: λ is defined mathematically, using the relationship of different resistances in the neurite circuit to the flow of electrotonic current
What does the length constant describe?
amount of current that is preserved (not lost) as a membrane potential change spreads through a neural compartment
More current will remain within the neurite if… (2)
- it is easy to move charge within that neurite’s cytoplasm
- that charge isn’t all leaking back out of the neurite through the membrane
What are the 2 physical properties that affect ‘Rm and Ri’ properties in real neurons?
membrane permeability
size (diameter)
How does membrane permeability affect Rm and Ri?
more open conductances (ion channels) in neurite means more holes in membrane – Rm decreases without an effect on Ri
How does size (diameter) affect Rm and Ri?
affects resistivity (total resistance per unit of volume), where larger volumes have lower resistivity – Rm and Ri both decrease as size increases
What is hyperpolarizing inhibition?
inhibition caused by hyperpolarizing beyond RMP
What is non-hyperpolarizing or shunting inhibition?
inhibition caused by not hyperpolarizing beyond RMP – effect of inhibitory synapses with non-hyperpolarizing reversal potentials
What can the effect of non-polarizing/depolarizing IPSPs be understood through?
length constant (λ)
How can shunting inhibition be explained by λ?
shunting inhibition decreases Rm, and therefore affects neuron’s length constant
even if Vm is relatively unchanged by an IPSP (because conductance is near its Erev so very little current is flowing), ion channels are opened, and membrane permeability at location of the synapse is still increased
- this reduces Rm around the area of the synapse, lowering λ in that region
- PSPs passing through that part of the dendrite will experience more degradation
Na+ ions entering at excitatory synapse are not diffusing in dendrite all the way to the inhibitory synapse (electrotonic current is carried by charge displacement, not bulk ion flow)
