Neuronal Biophysics Flashcards
Ohm’s Law:
1) I = ?
2) G = ?
3) I = ?
I=GV
G= 1/R
I= V/R
What describes the relationships between current (I), voltage (V), conductance (G) and resistance (R)?
Ohms law
According to ohm’s law if voltage increases what happens to current?
What if resistance increases?
Current increases
Current decreases
What is the Nernst equation?
What is the simplified version at 37*C?
Ex = RT/zF ln [x]o/[x]i
Ex = 61.5 log [x]o/[x]i
What are the extracellular and intracellular concentrations of Cl-?
Ex 123mM
In 4.2 mM
Remember that valency and charge affect the Nernst eq. What are the equations for Ca2+ and Cl-?
Eca = RT/2F ln [X]o/[X]i
Ecl = RT/F ln [X]i/[X]o
What happens to the ratio of extracellular to intracellular ion conc in the Nernst eq if the ion is negatively charged?
Inverted
What eq shows contribution of different ions to resting potential?
Goldman-Hodgkin-Katz
If membrane permeable to Na and K what’s is the GHK eq at 37*C?
Vm = 61.5log (Pk[k]o + Pna[na]o)/(Pk[k]i + Pna[na]i)
What creates an asymmetric distribution of ions across the membrane?
Integral membrane proteins: Ion channels, pumps and transporters
What is an important feature of an ion channel?
Selective permeability
What’s different between the movement of ions through ions channels and transporters compared to pumps?
Channels/transporters move ions down electrochemical grad
Pumps move ions AGAINST electrochemical grad
What part of a VG ion channel contains the selectivity filter?
Pore
Structure of pore determines which ions pass
VG ion channels have a gate which controls the pore but what controls the gate?
Voltage sensor
Movement of sensor causes conformational changes in channel protein cause gate to open
Describe the structure of a VG ion channel
4 identical subunits/domains
Each domain contains 6 transmembrane alpha helices (s1-s6. S4 is voltage sensing)
How does the K+ selectivity filter work?
Interactions between K+ and waters if hydration replaced by interactions with O2 atoms lining pore that bind K+
Why can’t Na+ pass K+ selectivity filter?
Too small
Can’t contact O2 atoms on both side of filter, not thermodynamically efficient to remove waters of hydration
How do Na channel gates work? (Activation and inactivation gates)
Activation gate (m) opens upon depolarization Inactivation gate (h) blocks pore 5ms after depolarization
How do Na channel gates contribute to refractory period?
Inactivation gate (h) remains blocking pore for a short time after depolarization Further Na influx not possible
What is mainly responsible for setting resting Vm?
What else contributes?
Background K+ activity
Na+ and K+ ATPase
Activation of which channels depolarise the membrane and is therefore excitatory?
Na+
Ca2+
Non selective cation channels
Activation of which channels hyperpolarize membrane and are inhibitory?
K+
Cl-
What are the Nernst potential for: K, Na, Ca, non-selective cation and Cl-?
Ek= -90mv Ena= +65 mv Eca = +110 mv Ecat = 0 mv Ecl= varies, in adult CNS usually -90mv
What is the equation for driving force of an ion?
Driving force = Vm - Eion
What direction of flow has a positive ionic driving force?
Outward
What direction of flow has a negative ionic driving force?
Inward
What is a reversal potential?
Vm at which direction of current flow reverses
If only K+ channels open what is the reversal potential?
Why in reality is Erev never exactly Eion?
= Ek = -90mv (zero current at -90)
Membrane never 100% selective for one ion, always leaks
What is voltage-clamp recording?
What’s it used for?
Voltage controlled, current measures
Used to test properties of ion channels, esp VG channels
What’s current-clamp recording?
What’s it use for?
Current controlled, voltage response recorded
Mimics APs and synaptic input
What deflection is an outward current?
Upward
What deflection in an inward current?
Downward
Calcium-activated K+ channels which phases of AP and why?
Repolarization. Activated by actuation of VG Ca channels
Afterpolarization - open longer than VG k+ channels, slow repolarization
Why is AP unidirectional?
Refractory period
Na channels inactivated behind depolarization
Why is there a high conc of VG channels at nodes of Ranvier?
Saltatory conduction in myelinated axons
What determines firing properties of neurones?
Ion channel expression profile
What technique allows the characterisation of ion channels and their contribution to firing patterns?
Patch-clamp
Describe the general principle of patch-clamp
Pipette (containing electrolyte solution) placed on cell membrane. Suction applied to create high-resistance gigaohm seal.
Ions fluxing through channels in membrane flow into pipette
What does on-cell/cell-attached patch clamp allow?
Recording trough single ion channel without disrupting cell interior
Explain whole-cell recording
Cell membrane ruptured and cytoplasm continuous with pipette
Use to record current from entire cell (voltage clamp a whole cell)
Explain inside-out patch clamp
Pipette retracted in on-cell configuration to excuse patch of membrane
Cytosolic membrane surface exposed
Used to study single channel activity
Explain outside-out patch clamp
Pipette retracted from whole cell configuration to break off patch of membrane
Extracellular surface exposed
Used to study effects if extracellular cues like NTs
What type of recording shows gating properties of channels?
Whole cell
What is the threshold for VG Na and Kv activation?
VG Na - -50mv
Kv - -20mv
When are outwardly rectifying k+ channels open?
Permeable to K+ when Vm > Ek (more positive than Ek)
Open at depolarization
When are inwardly rectifying k+ channels open?
Hyper polarization
At Vm < Ek (more negative than Ek)
When is the pore of inwardly rectifying K+ channels blocked? And what is it blocked by?
Positive potentials (>Ek)
Intracellular substance like Mg2+ and polyamines
Which current is inactivating?
A current (Ia)
Which current is non-inactivating?
M current
H current is conducted through what type of channels?
Hyperpolarisation activated, cyclic nucleotide gated (non-selective) cation channels (HCN channels)
GIRK channels are a type of what kind of K+ channel?
Inward rectifier
What channels are large-conductance also called BK channels?
Ca-dependent k channels
What effect does M current have on AP firing?
So what is the effect of inhibiting M current?
Acts as brake on AP firing, inhibits firing
Increased firing, excitation
What inhibits M current?
Muscarinic Ach receptor (M1)
- activates Gq -> PLC
- PKC and Ca inhibit M channel
(This excites sympathetic neurones)
How does Muscarinic Ach receptor inhibit cardiac muscle?
M2 receptor activates GIRK channel via Gi activation
What current contributes to background/leak K+ conductance?
M current
Other than VG Na and K channels what 4 other channels contribute to rhythmic activity of neurone?
VG Ca channels Ca-dependent K channels HCN channels (hyperpolarisation activated cation channels - Ih) T-type Ca channels
What is the role of VG Ca channels?
Increase [Ca]i
What is the role of Ca-dependent k channels?
Long lasting afterhyperpolarisation
What is the role of HCN channels?
Depolarization (and stop repolarization )
What is the role of T-type Ca channels?
Cause burst of AP firing
Does a mature neurone have low or high intracellular Cl-?
What type of channels does it express more?
Low
Outwardly directed channels - KCC2
Does an immature neurone have low or high intracellular Cl-?
What type of channels does it express more?
High
Inwardly directed channels - NKCC1
