Voltage-gated channels Flashcards
Discuss the structure of a ligand-gated channel
5 subunits
4 transmembrane domains (pentameric organistaion)
M2 lines pore
Discuss the structure of the cys-loop superfamily of ligand-gated receptors
Cysteine amino acids in N-terminal
Inward currents are …
Outward currents are …
DOWN
UP
Discuss features of Nav channels
- open rapidly in response to depolarisation
- (+) f.b. loop (AP)
- an INACTIVATION MECHANISM limits duration of excitatory response
- ROLE = excitatory responses
Discuss features of Kv channels
-open slowly in response to depolarisation
-Kir leak channels active at rest
(molecular basis of mem. permeability, enabling semipermeable barrier)
-ROLE= restore and maintain resting state
-2 subclasses:
fast-inactivating (A type)
non-inactivating (delayed rectifier)
Discuss features of Cav channels
- transient increases in intracellular Ca
- Ca removed by transporter & binding
- Ca important 2nd messengers, coupling stim. to responses
What voltage-g channel type was the first to be cloned?
Nav
- protein isolated by high-affinity binding of blocker TTX
- defined as S4 voltage superfamily (along w. Cav & Kv)
What are the major classes of Cav channels?
HIGH VOLTAGE ACTIVATED (HVA)
- require large depolarisations to open
- plateau responses
- non-inactivating
LOW VOLTAGE ACTIVATED (LVA)
- open below threshold for Nav channels
- transient responses
- inactivating
Name some electrophysiological recording methods?
Intracellular
Voltage-clamp
Patch-clamp
Discuss INTRACELLULAR elctrophys. recording method
Voltage and current change with time
-illustrates neuronal firing patterns
Firing patterns are shaped by Kv channels:
–Fast activating Kv channels (purkinje cells)
BLOCK (by TEA) slows firing rate & increases threshold of activation
–Intermediate activating Kv channels (CA1 cells)
–Slow activating Kv channels (Dopamine)
Discuss VOLTAGE-CLAMP elctrophys. recording method
Voltage set
Thus measure CURRENT
-illustrates ionic mechanisms of function
Kinetics (rising & decaying) reflect gating process (open/closed, inactivating/desensitised)
Discuss PATCH-CLAMP elctrophys. recording method
- Recording pipette filled with conductive saline attached to mem.
- Slight suction seals mem. to pipette tip
- Channel activity measured as step changes in current
- Retracting pipette pulls of mem. patch, often with channels attached
WHOLE CELL: total cell current (ensemble activity of all channels)
OUTSIDE IN: access extracellular side of channel
(e.g. ligand-gated Rs)
INSIDE OUT: access to intracellular side of channel
(e.g. Ca-dependent, cyclic nucleotide-gated channels)
What is channel behaviour determined by?
Probability
What does probability determine?
- threshold voltage for activation
- rates of channel opening & closing
- rates inactivation & recovery
What did Hodgkin & Huxley predict?
A multistep activation process (multi-exponential fit)
Probability describes the
- curved time course
- of rise in current
- after an abrupt step change in voltage
The multi-exponential fit indicates 4 stages of channel opening!
(as there are 4 subunits in a voltage-g channel)
