Membrane potentials and APs Flashcards
Diffusion
Spontaneous with no energy input
Flux
Number of molecules that cross a unit area per unit time
How is the membrane potential of a cell measured?
→ A reference electrode is placed outside the cell (zero-volt level)
→ Another electrode placed inside the cell
→ Measures the voltage that is negative compared with the outside (i.e. reference)
What is the membrane potential?
Difference between voltage inside and outside of the membrane
Key components of lipid cell membrane
- Hydrophobic
- Barrier to ion movement and separates ionic environments
- Can selectively change its permeability to specific ions
- Contains ion channels which open and close depending on transmembrane voltage, presence of activating ligands or mechanical forces
When will there be movement of ions across ion channels?
- When there is an imbalance across the membrane
- Until an equilbrium is reached
[Direction of flux is determined by direction of concentration grad]
The Nerst Eqn
Used to calculate equilibrium potential E, using concentrations inside and outside membrane
What does the Nerst Eqn assume?
Perfectly unique selectivity for ions
Goldman-Hodgkin-Katz GHK equation
Describes membrane potential Em more accurately
→ Membranes have mixed and variable permeability to all ions (e.g. at rest permeability to K+ is greater than Na+)
→ Each ion’s contribution to membrane potential is proportional to how permeable the membrane is to the ion
Depolarisation
Membrane potential becomes more positive than resting potential
Repolarisation
Membrane potential goes back to resting potential following depolarisation
Overshoot
Membrane potential becomes more positive thant 0mV
Hyperpolarisation
Decreased beyond RMP
What is the concept of a graded potential?
Changes in MP varying in response to type/strength/duration of stimualtions
How do graded potentials spread?
Decrementally
→ Charge build up caused by initial depolarisation of graded potential can leak from axon
→ Size of potential charge decreases along the axon
What kind of event is an action potential?
All or nothing
→ Graded potential has to reach a threshold for activation i.e. the opening of many Na+ channels
→ Same response regardless of size, intensity and duration of stimuli
Where do APs occur?
In excitable cells e.g. neurons, muscle cells and some endocrine tissues
What are APs called in neurones? What do they allow for?
Nerve impulses
- Reliable and quick transmission of information over long distances
What intracellular processes can APs activate?
- Muscle cells → AP first in a series of events leading to contraction
- Beta cells of pancreas → AP stimulates insulin release
What triggers each phase of an AP?
Change in membrane permeability for Na⁺ (Pₙ) and K⁺ (Pₖ)
What does membrane permeability depend on?
Conformational state of ion channels
- Opened by membrane depolarisation
- Inactivated by sustained depolarisation
- Closed by membrane hyperpolarisation/repolarisation
Phases of the AP
- RMP
- Depolarising stimulus
- Depolarisation/Upstroke
- Repolarisation
- Hyperpolarisation
Depolarising stimulus
→ Graded potential
→ Depolarisation below threshold potential of -55mV
→ All or nothing AP
Depolarisation
→ Starts at threshold potential
→ VGSCs open v. quickly (Increase in Pₙ)
→ Influx of Na+ → depolarisation
→ VGKCs open slowly (Slight increase in Pₖ)
→ Efflux of K+
→ Less K+ leaving than Na+ entering
→ Membrane potential becomes more positive
Repolarisation
→ Peak depolarisation → VGSCs close and become inactivated (absolute refractory period)
→ Increase in Pk as VGKCs are fully open
→ Efflux of K+
→ Membrane potential becomes more negative again
Hyperpolarisation
→ VGKCs still open i.e. continued K+ efflux
→ Eventually VGKCs close as MP reaches Ek
→ Both Na+ and K+ channels are at resting state
→ RMP restored by Na⁺-K⁺ ATPase
VGSC is active but closed - relative refractory period
What is the refractory period?
Time when neuronal membrane is unresponsive to further threshold depolarisations
When is the absolute refractory period?
Happens in repolarisation
1. Early on → inactivation gate of VGSC closes
2. Later → activation gate of VGSCs closes
What is the absolute refractory period?
Neuronal membrane is unable to respond to further stimulation, and cannot initiate another action potential
When is the relative refractory period?
Happens in hyperpolarisation
→ Inactivation gate is open but activation gate remains closed
What is the relative refractory period?
When another AP may be initiated by a stimualtion that is larger than normal
How is unidirectional flow of the AP achieved?
→ Absolute refractory periods ensure that neurons that have already been fired recently can’t be stimulated again so soon.
What are the ion movements during AP mainly carried out by?
Voltage gated ion channels not ion pumps
How big are the changes in ion concentration that cause changes in MP?
Very small
What can affect the rate of decay of a depolarisation?
- Insulation → slower decay
- Internal or axial membrane resistance
- Diameter of axon → slower decay
Where are VG channels mostly located?
At Nodes of Ranvier, to allow for saltatory conduction
RMP is re-established by.. (in an AP)
Repolarisation with more K+ channels opening
How does myelination affect conduction velocity?
Presence of myelin increases speed of conduction
What could cause a reduction of myelination?
Diphtheria
MS
How does axon diameter affect conduction velocity?
Increased axon diameter increased conduction velocity
How does temperature affect conduction velocity?
Increased temp increased speed of conduction
Diseases or conditions that can affect conduction velocity
→ Regrowth of axon after injury is smaller and thinner
→ Reduced myelination with multiple sclerosis and diphtheria
→ Cold
→ Anoxia
→ Compression
→ Some drug inc. anaesthetics
