Ion channels Flashcards
Briefly describe the channel activity underlying the action potential
Resting potential- leak Na and K channels
Rising phase- inward Na current via voltage-gated channels
Falling phase- outwards K current via delayed rectifier channels
Describe the voltage clamp methodology
Maintains voltage by neutralising influx of positive charge with negative ions
Used to measure current and therefore channel activity
Positive deflection implied outward movement
Negative deflection implies inward movement of cations
Describe I(Na) and I(K)
I(Na) negative deflection- inward current
Na channels open quickly but not instantaneously
Channels close despite maintained stimulus- inactivation
With increasing depolarisation- Inward current (increase then decrease) then outward current
Current reaches peak quickly (current/time)
I(K)- positive deflection - outwards current
Slow opening
Stay open
With increasing depolarisation- inreasing outward current
Total current plateaus quickly (current/time)
Describe current/voltage graphs
K- positive correlation- increased efflux with increased driving force
Na- initial increase influx with driving force then decrease influx as the Vm approaches ENa until the net movement is outward once Vm>ENa
What is Ohms law?
Explain its significance to I/V relationships.
Marvellous V=IR g= 1/R➡ V= I/g I=gV Proves that current also depends on driving force as I does not equal 0 when V=0 Therefore- I(Na)=g(Na)×(Vm-ENa)
Describe the current clamp methodology
Measures action potentials
Charge injected to provoke change in Vm and ion channel activity leads to further changes in Vm which detected by the electrode
What is ionic conductance?
Measure iof activity of ion channels
Calculated using voltage clamp
g(ion)=I(ion)/ (Vm-E(ion))
Conductance vs time-
Measures how channel activity varies with time
Calculated from current/time recordings
Conductance vs voltage-
Shows how channel activity varies with Vm
Calculated from current/voltage data
More depolarisation means more open channels
Gradient denote sensitivity not time
Describe the A-type K channel
Conducts K efflux- reduces excitability Voltage gated Inactivates- thus must be removed by hyperpolarization before they can respond again Fast acting- delays the action potential Nucleus tractus solitarius
Describe Na (Ca) channel
Alpha subunit- 4 domains (repeats) of 6 membrane-spanning segments
One alpha subunit forms channel around a central pore
Describe the K channel
Alpha subunit- single domain
Four alpha subunits form a channel
Same for delayed rectifier and A type
Describe how channels can be voltage gated
The part of the protein that senses Vm must contain charged amino acids and experience the change in Vm, ie. be within the membrane
S4 contains +ve charge AAs, entire S4 moved with change in Vm
How do channels inactivate?
Ball and chain model
Channel must open to inactivate
Na channel- cut between domains 3 and 4, no more inactivation
A-type K channel- NH3+ terminus of the alpha subunit- deletion of first 20 AAs, no more inactivation
What are different types of receptor?
Direct- ionotropic
Indirect- metabotropic
Excitatory- Na influx
Inhibitory- Cl influx
Describe direct transmission in the CNS
Small EPSPs and IPSPs Glutamate neurotransmitter Ionotropic Receptors- NMDA and non-NMDA Cation channels Also Can permeable Blocked by extracellular Mg and the drug APV
Describe the glutamate receptor activity in a patch clamp recording
When Mg is 0
Current is negative- inward- majority of current carried by Na
Current is positive- outeard - majority of current carried by K
At 0mV no channel activity because this is the reverse potential (Erev) halfway between ENa and EK- proves that channel is permeable to both
With Mg present
Unaffected at +60mV, more negative briefer channel openings- Mg block as it is attracted to the channel at -ve Vm
