Electrical Excitability Flashcards
What is an action potential?
A change in voltage across a membrane
What does the action potential depend on?
Ionic gradients
The relative permeability of the membrane
What is meant by action potentials being ‘all or nothing’?
There is no half, or double, action potentials etc
What happens once an action potential has been generated?
It is propagated without loss of amplitude
What happens once the membrane has been depolarised to threshold voltage?
Voltage gated Na channels open
What is the effect of the opening of voltage gated Na channels?
It allows Na influx as Na ions attempt to move to their equilibrium potential
What is the equilibrium potential of Na ions?
+61mV
What does the influx of Na ions cause?
It depolarises the membrane further, causing more voltage gated Na channels to open and even more depolarisation
By what process does depolarisation cause more Na ion channels to open?
Positive feedback
What is the positive feedback causing further Na channels to open to basis of?
The all or nothing characteristic of the action potential
What happens in maintained depolarisation?
Na channels close by a mechanisms called inactivation
Voltage gated K channels are opened
What happens when voltage gated K channels are opened?
There is K efflux, as K attempts to move towards its own equilibrium
What is the equilibrium potential of K?
-88mV
What does the combination of K efflux and Na channel inactivation cause?
Repolarisation of the membrane
What is happening in the upstoke of the action potenital?
Na channels open
What is the result of the Na channels that open to cause depolarisation being voltage gated?
As the moment potential becomes more positive, positive feedback means that more channels will open until they all are
Why can depolarisation not stop half way?
At any voltage above threshold, it will be at a voltage where more channels open, thus causing more depolarisation
What is true of Na channels in the absolute refractory period?
Nearly all N channels are in the inactive state
What level is excitability at in the absolute refractory period?
0
What is true of the Na channels in the relative refractory period?
They are recovering from inactivation
What happens to excitability in the relative refractory period?
It returns to normal as the number of channels in the inactivated state decreases
What happens as a stimulus gets longer?
A larger depolarisation is necessary to initiate an action potential
Why does a longer stimulus mean a larger depolarisation is required to initiate an action potential?
Because Na channels become inactivated during the stimulus
How are Na and Ca voltage gated channels similar?
In structure
Describe the structure of a Na/Ca voltage gated channel?
They main pore forming subunit is one peptide consisting of four homologous repeats.
Each repeat consists of 4 transmembrane domains
What gives the Na/Ca channel its voltage sensitive properties?
One of the transmembrane domains is able to sense voltage across the membrane
What does a functional Na/Ca channel require?
One subunit
How do voltage gated K channels compare to Na/Ca channels?
They are similar in structure, except each repeat is actually a separate subunit.
Describe the structure of voltage gated K channels
Each subunit has 6 transmembrane domains, one of which is voltage sensitive.
What does a functional K channel require?
4 subunits
Give an example of a local anaesthetic?
Procaine
How do local anaesthetics act?
By binding to and blocking Na channels, thereby stopping action potential generation
In what order to local anaesthetics block conduction in nerve fibres?
Small myelinated axons
Non-myelinated axons
Large myelinated axons
What is the result of local anaesthetics blocking conduction in nerve fibres in the order that they do?
They tend to effect sensory before motor neurons
What kind of molecules are local anaesthetics?
Weak bases
How do localised anaesthetics cross the membrane?
In their unionised form
When do local anaesthetics block the Na channels?
When the channel is open
Do local anaesthetics have a high affinity to the activated or inactivated state of the Na channel?
Inactivated
What are electrodes used for in extracellular recording of action potentials?
To raise the membrane to threshold to generate an action potential
How can conduction velocity be calculated?
By recording changes in potential between the stimulating (cathode, -ve), and recording (anode, +ve) electrodes along an axon, and using the equation conduction velocity = distance / time
What does the depolarisation of a small region of membrane produce?
Transmembrane currents in neighbouring regions
What is the result of transmembrane currents being produced in neighbouring regions?
As Na channels are voltage gated, it opens more channels, causing propagation of the action potenital
What happens if the local current spreads further?
The conduction velocity of the axon is faster
What properties of an axon lead to a high conduction velocity?
A high membrane resistance
A high axon diameter
A low membrane capacitance
Why does a high axon diameter lead to a high conduction velocity?
Has a low cytoplasmic resistance
What is Ohm’s Law?
V=IR
What does Ohm’s Law state?
The higher the resistance of the membrane, the higher the potential difference across it
What does more voltage across the membrane result in?
More voltage gated Na channels being open
What is the result of more voltage gated Na channels being opened?
It is easier to reach the threshold to fire an AP
What effect does an increase in number of Na channels have on conduction velocity?
It increases it
Why does large axon diameter increase conduction velocity?
Ohm’s Law states that the lower the resistance, the larger the current, and therefore the action potential will travel further, increasing action potential
What is capacitance?
The ability to store charge
What will a membrane with high capacitance do?
Take more current to charge, or a longer time for a given current
What happens to a the time taken to charge in membrane with a low capacitance, for a given current?
It is shorter, thus increasing conduction velocity
What happens to conduction velocity with myelination of axons?
It increases considerably
What kind of axons are myelinated?
Large diameter axons, such as motor neurones
What kind of axons are not myelinated?
Smaller ones, such as sensory neurones
Why does myelination increase conduction velocity?
It reduces capacitance and increases membrane resistance of the axon
What does myelination allow for?
Saltatory conduction
What is saltatory conduction?
Where the AP ‘jumps’ between Nodes of Ranvier
Why does saltatory conduction occur?
Because the myelin sheath acts as a good insulator, causing the local circuit current to depolarise the next node above threshold and generate an AP
What do Nodes of Ranvier have?
High density of voltage gated Na channels
How does Na channel density differ from Nodes of Ranvier in unmyelinated axons?
They have an even distribution of channels
Which cells myelinate peripheral axons?
Schwann cells
Which cells myelinate axons in the CNS?
Oligodendrocytes
How do cells myelinate axons?
They envelop axons in their plasmalemma
Give an example of a disease that strips areas of some axons of their myelin sheaths
Multiple Sclerosis
What type of disease is MS?
Autoimmune
What happens in MS?
Myeline is destroyed in certain areas of the CNS
What effects can demyelination have?
Can have dramatic effects on the ability of previously myelinated axons to conduct action potentials properly, leading to decreased conduction velocity, complete block or cases where only some action potentials are transmitted
