Unit 1 Flashcards
Action Potential
Transient depolarization triggered by a depolarization beyond threshold
Simple Diffusion
Movement from one location to another as a result of random thermal motion
-passive
Driving force
Determined by electrochemical gradient acting on the solute between two compartments
Electrochemical potential energy difference
Contribution from concentration gradient and from any difference in voltage that exists between two compartments
Equilibrium
No net driving force for X
Facilitated diffusion
Proteins enable ions to cross membranes by moving them downhill
-passive
Pores
Channels that are always open
-leak channels
Channels
Can be opened/closed by the action of specific mechanisms
-gated
Carriers
Conformational change needed to open gate
-“steps”
Active transport
Proteins enable ions to cross membranes by moving them across as energy-dependent fashion
Secondary active transport
Movement of solute using the gradient created by a pump
Cotransporters (symporters)
Use existing gradient to move an ion across the membrane down the gradient
-both in same direction
Exchangers (antiporters)
Use an existing gradient to move one ion to the side of membrane of lower concentration in exchange for another ion that is moving to opposite side of membrane where it is present in higher concentration
Membrane potential
Separation of positive and negative charges across the cell membrane
- depolarization = more positive
- hyperpolarization = more negative
Nernst Equation
Equilibrium potential for any ion
Ionic gradient
Net diffusion of ions towards compartment of lower concentration
Steady state condition
Neither Na or K is in equilibrium but the net flux of charge is null
Resting potential
Determined by the relative proportion of different types of ion channels that are open together with the value of their equilibrium potentials
Goldman equation
Shows that the resting membrane potential of a cell could be changed by either changing the gradient for a given ion (change Nernst potential) or by changing the relative permeability for an ion
Cations
Positive charge
Anion
Negative charge
Current (I)
Net flow of charge from one point to the other
-amperes (A)
Resistance (R)
Resistance to movement of current
-Ohms
Capacitance
Ability of a system to store an electric charge
Current clamp
Measurement of cell voltage while controlling the applied current
Voltage clamp
Measurement of a cell current while controlling cell voltage
Capacitative current
Only flows while Vm is changing
Time constant
Time required for voltage to fall to 37% of its initial value
Feedback amplifier
Injects opposing current if there is a difference from intended voltage to maintain a constant Vm
-equal but opposite in sign
Patch clamp technique
Resolves unitary currents through single-channel molecules
Cell-attached patch
Only measure single channel that is inside pipette. Cytoplasm remains intact
-Pipette solution = extracellular
Whole cell patch
Recording of all currents on cel membrane. Direct access to cytoplasm
-Pipette solution =intracellular
Outside out patch
Extracellular side of channel facing out into bath solution. Broken ends of membrane join.
-Pipette solution = intercellular
Inside out patch
Cytoplasmic side of channel is facing into bath solution.
-Pipette solution = extracellular
Probability of channel opening
Fraction of total time that the channel is in the open state
-applied voltage favours open state
Rising phase
Depolarization phase (positive going)
- rapid
- from -Vm to max + value
Depolarization phase
Negative going
Afterhyperpolarization
Repolarization undershorts to a voltage min., more negative than Vrest
Overshoot
Part of AP that lies above 0 mV
Absolute Refractory Period
Impossible to fire another AP
- initiation of spike to when repolarization almost complete
- due to Na channel inactivation.
Relative refractory period
Minimal stimulus necessary for activation is stronger or longer than predicted by the first AP
-due to high K conductance and min Na conductance
S4 Segment
Positively charged residue every 3rd amino acid
-voltage sensing
P region
Dips into membrane but doesn’t cross it
- ion selectivity of channel
- lines the pore of the channel
Selectivity filter
Narrow regions that act as molecular sieves
-must shed waters of hydration to traverse channel
Waters of hydration
Ions in solution are surrounded by a cloud of water molecules that are attracted by the next charge of the ion
-smaller ions = more waters of hydration
Channelopathies
Neurological diseases caused by altered function of ion channel subunits or the proteins that regulate them
Delay Rectifiers (Kvs)
Delay in activation.
Outward rectifiers - carry current preferentially in an outward direction
Voltage gated
A-type
Currents that are low threshold, rapidly activating and inactivating K currents
BK channels
Calcium activated
- large conductance
- sensitive to voltage and Ca
SK channels
Calcium activated
- small conductance
- only sensitive to Ca (not voltage)
KIR Channels
Inward rectifying K channels
- not voltage gated
- open at rest (contribute to resting potential)
GIRKS
G protein coupled KIR channels
- activated by G protein
- Mg blocks channels at + values
K2P channels
Mediate “leak” k currents
-2 subunits only
HVA Ca Channels
High Voltage Activated
- more + Vm
- L type and PQNR type
LVA Ca Channel
Low Voltage Activated
- more - Vm
- T type
T-Type Ca Channel
Tiny Conductance
Transient current
LVA
-fast acting
L Type Ca Channel
Large Conductance
Long lasting current
HVA
-slow acting
N type Ca Channel
Found in Neurons
Neurotransmitter release
HVA
-fast acting
P/Q type Ca Channel
Located in Cerebellun Very closely related Neurotransmitter release HVA -fast acting
Potential difference (V or E)
Potential to do work
- moving charge from one place to the other
- measured in volts (v)
