Electrical Signalling Flashcards
What is needed for ion movements?
Ion channels and transporters
What does ion flow do?
Establishes concentration gradients
Generates membrane potential
Used for electrical signalling
Causes action potentials
What are action potentials?
Transient alterations in membrane potential that propagate along axons
What are the consequences of an excitable cell being activated?
Synpatic vesicle release (Neurons)
Hormone release (endocrine cells)
Contraction (muscle cells)
What is the resting membrane potential?
A difference in charge across the membrane
What happens with an impermeable membrane?
e.g. Concentrated potassium chloride concentration in the left chamber and a dilute potassium chloride concentration in the right hand chamber and an impermeable membrane between them
Voltmeter will have a zero read at equilibrium
There isn’t actually an electrical circuit here as this is impermeable so there’s no current flowing
No movement of ions and balanced on both sides
What happens with a fully permeable membrane?
e.g. K/Cl flow, where the membrane allows full, free passage between ions with a concentrated solution on the left and a concentrated solution on the right and a fully permeable membrane between them
At equillibrium the voltmeter will read 0
Both the potassium and chloride ions will both flow from left to right until they reach equilibrium and the same concentrations
What happens with a selectively permeable membrane?
e.g. (K/Cl) membrane only allows potassium ions through it- will not allow chlorine ions through it
There is a concentration gradient where potassium ions can flow down it but chlorine ions cant because this membrane isn’t permeable to them.
We end up with an excess of negative charge on the left and an excess of positive change on the right
What is electrochemical equillibrium?
When two opposing forces or two competing forces balance each other out, then the system is said to be at an electrochemical equilibrium
What is the effect of increasing extracellular K+?
As extracellular potassium concentration is raised, the membrane depolarises i.e. become less negative, tending towards zero
But
When similar experiments done with sodium, it had hardly any effect –> under equilibrium conditions the membrane is selectively permeable to potassium but not to sodium.
What is the rising phase/depolarisation in an action potential?
Where the membrane potential is increasing by going towards and then above zero
What is the overshoot phase in an action potential?
Where the potential goes above zero
What is repolarisation in an action potential?
This is where the membrane becomes more negative after depolarisation
What is the undershoot in an action potential and what does it cause?
Where the membrane potential becomes more negative that the original resting membrane potential
It causes hyperpolarisation
Why is calcium important in neural transmission?
We need a calcium influx to get the fusion of synaptic vesicles and their release
Calcium is also often used as an on/off switch in biology
What drives depolarisation?
An influx of sodium
With resting membrane potential, the membrane is more permeable to potassium than sodium
In the depolarising rising phase the permeability to sodium increases, and its an influx of sodium which is driving depolarisation
What causes the action potential to return to RMP?
An efflux of potassium
So the permeability to sodium decreases and permeability to potassium increases so permeability to potassium is higher than sodium and this causes a fall in membrane potential. We get an efflux of potassium rather than the influx of sodium so everything normalises then we get back to the resting potential
What do voltage-gated Na+ and K+ channels do in relation to action potentials?
Opening of voltage-gated Na+ channels causes depolarisation
Closure of Na+ and opening of K+ voltage-gated channels then causes repolarisation
The voltage-gated K+ channels remaining open after the action potential reaches resting level causes hyperpolarisation
What do voltage-gated Na+ and K+ channels do in relation to action potentials?
Opening of voltage-gated Na+ channels causes depolarisation
Closure of Na+ and opening of K+ voltage-gated channels then causes repolarisation
The voltage-gated K+ channels remaining open after the action potential reaches resting level causes hyperpolarisation
What does Tetrodotoxin (TTX) from a puffer fish do?
It blocks voltage-gated sodium channels
If depolarisation done in the presence of TTX you don’t get the inward flow of sodium voltage gates channels, you just get the out flow through the voltage gated potassium channels
What does Tetraethylammonium (TEA) do?
It blocks voltage-gated potassium channels
If depolarisation is done in the presence of TEA you don’t get the outward flow because its been blocked, you only get the inward flow through the voltage gated sodium channels
What are excitatory postsynaptic potentials?
They are depolarisations heading up towards zero that are not strong enough alone to cause an action potential
However
As soon as there is enough of these to raise the membrane potential above the threshold they will fire an action potential
What is the most common cause of an excitatory post-synaptic potential?
Movement of sodium is the most common cause of EPSP
Sodium is higher on the outside than on the inside so if we get an influx its going to raise the membrane potential
What is the most common cause of an inhibitory postsynaptic potential?
Movement of chloride is the most common cause of IPSP
Membrane is negative on the inside, you’ve got more chloride outside than inside, opening up a chloride channel will allow chloride to flow in, making it even more negative inside – occurs through GABA or glycine receptors
Are action potentials always the same size?
Yes action potentials are always the same size regardless of the size of a stiumulus
BUT
A bigger stimulus can increase the frequency of action potentials, called spiking, where bigger potentials give rise to multiple action potentials referred to as a train of spikes
What is the absolute refractory period?
The various confirmational changes involved in the open and resetting of the voltage gated channels takes some time so we have an absolute refractory period.
This means that if you get a second stimulus you wont fire a second action potential during this period
The fact we have an absolute refractory period does confer some advantage- it confers directionality of an action potential, it cannot bounce back
What is a sub-threshold stimulus?
One not big enough to cause an action potential
What is passive electrical flow in axons?
Passive current flow depolarizes the membrane potential in the adjacent region of the axon, once the current begins dissipating the membrane potential gets pregressively smaller
It demonstrates that axons are poor electrical conductors
There is also current leakage during passive flow which contributes to the diminishing effect
What is active conduction in axons?
When stimulated with a supra-threshold stimulus, it has an action potential which is the same size all along, it doesn’t dissipate.
Active flow is far more efficient than passive flow is
Is passive flow important?
Yes- its important during action potential propagation
Where the channels are open we get an influx of sodium and you get passive flow further along the axon which actually depolarises the membrane in this region, allowing voltage gated channels to open in the next region and hence this passive flow is important in propagating the action potential
Where are channels and transporters concentrated?
At the nodes of Ranvier
What is saltatory propagation?
It is the propagation of action potentials along myelinated axons from one node of Ranvier to the next
