Lecture 5: Graded Potentials and Passive Membrane Properties

Question 1

What is conductance?

Answer 1

inverse of a resistance – something that makes a membrane able to pass current (ie. ion channel)

Question 2

What is equilibrium potential (Eion)?

Answer 2

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

Question 3

What is reversal potentials (Erev)?

Answer 3

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)

Question 4

What are the two different approaches to testing any Vm signal?

Answer 4

• alter [ion]o bathing the neuron (ie. alter Eion)

- find the reversal potential – alter Vm (artificially) until net ion flux stops

Question 5

Even when an ion is permeable, what determines whether or not it will make a net flux (current)?

Answer 5

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)

Question 6

What is the reversal potential for an EPSP channel?

Answer 6

0 mV

Question 7

Under what situation can an Erev be the same as an Eion?

Answer 7

if there is an ion channel where only one ion can go through

Question 8

In most neurons, what is the threshold to trigger an AP?

Answer 8

Vm ≈ -45 to -55 mV

Question 9

What do PSPs summate according to?

Answer 9

according to the currents that create them, which depend on driving force

Question 10

When currents are generated by the cell membrane itself, in which direction does inward current flow?

Answer 10

downward

BUT this is opposite to how we draw the resulting voltage changes

Question 11

When currents are generated by the cell membrane itself, in which direction does outward current flow?

Answer 11

upward

BUT this is opposite to how we draw the resulting voltage changes

Question 12

What is the reversal potential of typical EPSP?

Answer 12

0 mV – complicating, because this value does not match any known value of Eion for typical ions

Question 13

Why is the reversal potential of typical EPSP = 0 mV?

Answer 13

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

Question 14

What is the reversal potential of typical IPSP?

Answer 14

ECl

Question 15

What are IPSPs driven by?

Answer 15

increase in permeability to Cl- ions

Question 16

What is the reversal potential of non-polarizing/depolarizing IPSPs?

Answer 16

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

Question 17

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?

Answer 17

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

Question 18

AP vs. graded potential

• graded
• signal type
• size
• summation
• degradation

Answer 18

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)

Question 19

Why do graded potentials decay with distance?

Answer 19

neurites are leaky and semi-conductive

Question 20

How are neurites semi-conductive?

Answer 20

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

Question 21

Is electrotonic conduction perfect?

Answer 21

NO – it’s efficient and rapid, but ionic fluid still has some resistance to electrical current flow, in addition to the membrane leaking ions

Question 22

What does transmembrane current arise from?

Answer 22

ions moving through ion channels

Question 23

Where does transmembrane current flow?

Answer 23

flows through conductances (local ion channels in a synapse, or sensory receptors in a dendrite) that opened when the signal was generated

Question 24

How does eletrotonic current spread?

Answer 24

by charge displacement through the cytoplasm

Question 25

Where does electrotonic current flow?

Answer 25

flows within neurite, through cytoplasm

Question 26

What does leak current do?

Answer 26

moves charge back to ECF (also via ion channels)

Question 27

Where does leak current flow?

Answer 27

flows through any conductances (ie. leak ion channels) that happen to be in the membrane and open when electrotonic current flows by

Question 28

What are the 4 components of an equivalent circuit?

Answer 28

• membrane resistance (Rm)
• internal resistance (Ri)
• external resistance (Re/Ro)
• membrane capacitance (Cm)

Question 29

What is membrane resistance (Rm)?

Answer 29

reflects both resistance of plasma membrane, and the small amount of current flow that leaks across open ion channels (even ‘at rest’)

Question 30

What is internal resistance (Ri)?

Answer 30

some resistance to flow of electrotonic current provided by cytoplasm

Question 31

What is external resistance (Re/Ro)?

Answer 31

small resistance to flow of electrotonic current provided by ECF

Question 32

What is membrane capacitance (Cm)?

Answer 32

electrical feature arising from properties of lipid bilayer

Question 33

As electrotonic current travels down dendrite, what is it effected by?

Answer 33

membrane resistance (Rm) and internal resistance (Ri)

Question 34

To keep the maximum amount of current inside the neurite, what should Rm and Ri be?

Answer 34

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

Question 35

How quickly does membrane potential decline with distance?

Answer 35

exponentially, depending on resistances

Question 36

What is the length constant (λ) (space constant)?

Answer 36

constant value for a given neurite that describes how much a passive/graded potential decays with distance

Question 37

What are the different ways that the length constant can be defined

Answer 37

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

Question 38

What does the length constant describe?

Answer 38

amount of current that is preserved (not lost) as a membrane potential change spreads through a neural compartment

Question 39

More current will remain within the neurite if… (2)

Answer 39

• 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

Question 40

What are the 2 physical properties that affect ‘Rm and Ri’ properties in real neurons?

Answer 40

membrane permeability

size (diameter)

Question 41

How does membrane permeability affect Rm and Ri?

Answer 41

more open conductances (ion channels) in neurite means more holes in membrane – Rm decreases without an effect on Ri

Question 42

How does size (diameter) affect Rm and Ri?

Answer 42

affects resistivity (total resistance per unit of volume), where larger volumes have lower resistivity – Rm and Ri both decrease as size increases

Question 43

What is hyperpolarizing inhibition?

Answer 43

inhibition caused by hyperpolarizing beyond RMP

Question 44

What is non-hyperpolarizing or shunting inhibition?

Answer 44

inhibition caused by not hyperpolarizing beyond RMP – effect of inhibitory synapses with non-hyperpolarizing reversal potentials

Question 45

What can the effect of non-polarizing/depolarizing IPSPs be understood through?

Answer 45

length constant (λ)

Question 46

How can shunting inhibition be explained by λ?

Answer 46

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)