4 & 5. Resting and Action potentials Flashcards
Flux
the rate of transfer of molecules
= number of molecules that cross a unit area per unit time e.g. m-2. s-2
Is there net flux in dynamic equilibrium?
No
Equilibrium potential
The potential at which electrochemical equilibrium has been reached.
Concentration gradient balanced by electrical gradient
When is electrochemical equilibrium achieved?
when electrical force prevents further diffusion across the membrane
What does the Nernst equation show?
It allows you to calculate the equilibrium potential of a cell (dependent on temperature and charge)
What are the intracellular and extracellular concentrations of the 4 main ions involved in action potentials?
Na+: inside: 10 outside: 150
K+: inside: 150 outside: 5
Cl-: inside: 5 outside: 120
Ca2+: inside: 0.1 outside: 2
Explain why the resting membrane potential of most cells is around -70 mV
Equilibrium potential of K+ is -90 mV and Na+ is +72
Membranes have mixed K+ and Na+ permeability
But more permeable to K+ so resting membrane potential is closer to equilibrium potential of K+.
What does the Goldman-Hodkin-Katz equation show?
Accounts for relative permeability of the membrane at any one time to different ions so you can figure out the resting membrane potential.
Resting potential
-70mV
Depolarising
-70 to 0 (becoming more +)
Hyperpolarising
-70 to -90 (becoming more -)
Overshoot
0 to +
Repolarising
+ to -
What is the difference between graded potentials and action potentials?
APs are an all-or-nothing event (have the same amplitude every time)
GPs can vary in amplitude and the amplitude is affected by the strength of the stimulus. GPs can be positive or negative and they decrease in altitude as they travel away from the point of origin.
How can nerve cells use graded depolarisations and hyperpolarisations for signalling?
Contribute to initiating or preventing action potentials
May summate or cancel each other out, create a more integrated signal
Where do graded potentials occur?
Synapses
Sensory receptors
Where do action potentials occur?
In excitable cells
Mainly neurones and muscle cells e.g. in muscle cells lead to contraction
Also in some endocrine tissues e.g. In B cells of pancreas provoke release of insulin
What does permeability to ions depend on?
Conformational state of ion channel
Most channels in neurology are voltage operated
What are the 5 phases of an action potential?
- Resting membrane potential
- Depolarising stimulus
- Upstroke
- Repolarisation
- After hyperpolarisation
Describe phase 1 of an action potential
Caused by K+ moving out of cells
Finite permeability to Na+
Membrane potential nearer equilibrium potential for K+ than for Na+
Describe phase 2 of an action potential
Stimulus depolarises membrane potential
(Graded response- small and slow depolarisation)
Moves it in +ve direction towards threshold
Describe phase 3 of an action potential
Starts at threshold potential
At threshold potential lots of Na+ channels respond
PNa increases because voltage-gated Na+ channels open quickly
Na+ ions enter the cell down their electrochemical gradient
Depolarisation
Membrane potential moves toward Na+ equilibrium potential
PK increases as the voltage-gated K+ channels start to open slowly
K+ ions leave the cell down their electrochemical gradient
(Slowly- Less than Na+ entering)
Describe phase 4 of an action potential
PNa decreases because voltage-gated Na+ channels inactivate (Channels blocked)
Na+ entry stops
PK increases as more voltage-gated K+ channels open and remain open
K+ leaves the cell down its electrochemical gradient
Membrane potential moves toward the K+ equilibrium potential
Describe phase 5 of an action potential
(At rest) voltage-gated K+ channels are still open
K+ continues to leave the cell down the electrochemical gradient
Membrane potential moves closer to the K+ equilibrium
Additional K+ channels are still open, so slight hyperpolarisation (Close slowly)
Some voltage-gated K+ channels then close
Membrane potential returns to the resting potential
What occurs at the start of repolarisation?
Part of channel protein reacts to change in voltage
and blocks the channel so Na+ cant get through
Na+ channel gate is open, but blocked by inactivation protein
Na+ entry to cell stops
Depolarisation recognised by K+ channels and more open
What is the underlying mechanism for the absolute refractory period? What is this?
Na+ channel inactivation is the underlying mechanism for
the Absolute refractory period
Inactivation gate is closed
New AP cannot be triggered even with very strong stimulus
What happens later in repolarisation?
Absolute refractory period continues
Na+ Channel activation gate closes
Inactivation protein still present
What occurs during post-hyperpolarisation?
Relative refractory period
Inactivation gate is open
Stronger than normal stimulus required to trigger an action potential
How long does a normal action potential last?
2 ms
Threshold potential
Once this potential is reached an AP is triggered
Refractory state
Unresponsive to threshold depolarization
What is the regenerative relationship between PNa+ and membrane potential?
Once threshold is reached the cycle continues
Positive feedback behaviour
Cycle continues until the voltage-gated Na+ channels inactivate- closed and voltage-insensitive
Membrane remains in a refractory (unresponsive) state until the voltage-gated Na+ channels recover from inactivation
Ion channels vs Ion pumps in an action potential
Ion channels: Main pathway for ion movement, rapid
Ion pumps: NOT directly involved in ion movement during an AP, slow, need ATP
Why can the AP only move in 1 direction?
Originating area is refractory
Nodes of ranvier
Unmyelinated gaps along the axon
Allow rapid impulse propagation as impulse can jump to areas of high concentration of Na+ channels
Allows movement via saltatory conduct
What influences conduction velocity?
Axon diameter
Axon myelination
Large diameter myelinated axons speed
120 m/s
Low internal resistance
Small diameter non-myelinated axons speed
1 m/s
Can’t travel by saltatory conduct
High internal resistance
How does conduction velocity change with increased axon diameter?
Increases
Less resistance to current flow inside large diameter axons
How does conduction velocity change with myelination?
Increases
AP’s only occur at nodes of Ranvier
What slows conduction velocity?
Reduced axon diameter (i.e. re-growth after injury)
Reduced myelination (i.e. MS and diphtheria)
Cold, anoxia, compression and drugs (some anaesthetics)
