Action potentials Flashcards
What is an action potential?
Rapid change in membrane potential across the membrane of a cell
What is the resting membrane potential of an axon?
-70mV
Where does an action potential start in an axon?
Axon hillock
What is required for an action potential to start at the axon hillock?
Must be depolarised to threshold value
What is meant by action potentials being all or nothing?
They only occur when the membrane is depolarised to threshold
otherwise they don’t occur at all
How is the axon hillock depolarised?
Inputs from pre-synaptic neurones to dendrite of neurone
What is the shape of an axon action potential?
Straight line at -70mV
upstroke to 40mV
then downstroke to below -70mV
then slow upstroke to -70mV
What is responsible for the upstroke of an axon action potential?
Membrane depolarisation to threshold causes voltage-gated sodium ion channels to open
get influx of sodium ions giving further depolarisation
What is responsible for the downstroke of an axon action potential?
Voltage-gated sodium ion channels inactivate and voltage-gated potassium ion channels are finally open
efflux of potassium ions gives repolarisation and then hyperpolarisation
What is meant by voltage-gated sodium ion channels being inactivated?
Not closed
but do not allow sodium ions to pass through them
How do voltage-gated sodium ion channels recover from inactivation?
By hyperpolarisation of the membrane
When do voltage-gated potassium ion channels close?
When the membrane has been repolarised to -70mV
Can action potentials occur immediately one after another? Why?
No
because of the refractory period
What are the two stages of the refractory period?
Absolute refractory period
Relative refractory period
What is responsible for the refractory period?
Voltage-gated sodium ion channels are inactivated
What is happening to voltage-sodium ion channels in the absolute refractory period?
All voltage-gated sodium ion channels are inactivated
What is happening to voltage-gated sodium ion channels in the relative refractory period?
Voltage-gated sodium ion channels are recovering from inactivation
How fast do voltage-gated potassium ion channels close? What is the importance of this?
Slowly
There is still some potassium ion efflux even when membrane potential has repolarised, giving hyperpolarisation
How is an action potential propogated along an axon?
Local current theory
What is the local current theory?
Influx of sodium ions repels other positively charged ions which spread out to nearby regions in the axon
this depolarises nearby regions of the axon to threshold
How are the local currents affected with increasing distance? Why?
As the local currents move further along the axon
they become dissipated as the positively charged ions have become more spread out
What is capacitance?
The ability of the membrane to store a charge
in other words, to prevent it from leaking out
What is resistance?
How impermeable the membrane is to a particular ion
How can resistance to an ion be increased? And decreased?
Increased - by decreasing the number of open ion chanels for that ion
Decreased - by increasing the number of open ion channels for that ion
Is it advantageous to an axon membrane to have high or low capacitance? Why?
Low capacitance - so more charge contributes to electrical excitability
Is it advantageous to axon membranes to have high resistance or low resistance? Why?
High resistance - to prevent ions leaking out of the axon and dissipating the current
How long is an axon action potential?
5ms
How is the action potential affected as it is propogated along the axon membrane?
Stays exactly the same
What type of feedback occurs in the upstroke of the action potential? How?
Positive feedback
Influx of sodium ions gives depolarisation which results in opening of more voltage-gated sodium ion channels which gives more sodium ion influx
What is the basic structure of a voltage-gated sodium ion channel?
Single polypeptide chain, called alpha subunit
Four parts
Each part is made up of six transmembrane spanning regions
What is the function of the fourth transmembrane spanning region in voltage-gated sodium ion channels?
Voltage sensor
What is the significance of the fifth and sixth transmembrane spanning region in voltage-gated sodium ion channels?
Between them is pore region
What is the significance of the third and fourth parts of the transmembrane spanning region in voltage-gated sodium ion channels?
Between them is the inactivation particle
What is the basic structure of a voltage-gated potassium ion channel?
Four polypeptide chains, called alpha subunits
Each one is made up of six transmembrane spanning regions
What is the function of the fourth transmembrane spanning region in voltage-gated potassium ion channels?
Voltage sensor
What is the significance of the fifth and sixth transmembrane spanning regions in voltage-gated potassium ion channels?
Between them is pore region
How do local anasthetics e.g. procaine affect voltage-gated sodium ion channels?
Block them
What are the two mechanisms by which local anasthetics block voltage-gated sodium ion channels?
Use-dependent
Use-independent
How do use-dependent local anasthetics work?
Block the ion channels only when they’re open
remain there when ion channel is inactivated or closed
How do use-independent local anasthetics work?
Block the ion channel even when they’re inactivated or closed because can diffuse across plasma membrane
remain there
What is conduction velocity?
Speed of action potential propogated along an axon
What is the exterior surface of some axons covered with?
Myelin sheath, has gaps in it
What is the myelin sheath?
Tightly packed membrane
What is responsible for forming the myelin sheath?
CNS - oligodendrocytes
PNS - Schwann cells
How do Schwann cells form the myelin sheath on axons in the PNS?
Continuously wrap around the axon
What are the gaps in the myelin sheath called?
Nodes of Ranvier
How are ion channels distributed along the axon in myelinated and unmyelinated axons?
Myelinated - lots of ion channels at nodes of Ranvier
Unmyelinated - ion channels distributed evenly along the axon
What is the function of the myelin sheath?
To act as in insulator
prevent the leakage of ions
How is an action potential propogated along a myelinated axon?
Saltatory conduction
What is saltatory conduction?
Refer to how action potentials seemingly jump from one node of Ranvier to the next
How does saltatory conduction work?
Action potential occurs at node of Ranvier
by local current theory, next node of Ranvier becomes depolarised to threshold
How does myelination affect the conduction velocity of an axon? Why?
Increases it
Less leakage of ions so more of them spread to next node of Ranvier
raising it to threshold quicker
How does myelination affect resistance? Why?
Increases it
Fewer ion channels for ions, since only occur at nodes of Ranvier although in large numbers
What is the most common demyelinating disease?
Multiple sclerosis
How does demyelination occur in multiple sclerosis?
Autoantibodies against proteins in myelin sheath
destroy myelin sheath
How does demyelination affect the propogation of an action potential along the axon?
Leakage of ions, current dissipates
next node of Ranvier may not be depolarised to threshold
action potential not generated
What is the solubility of use-dependent local anasthetics? And use-independent local anasthetics?
Use-dependent - hydrophilic, insoluble in lipids
Use-independent - hydrophobic, lipid-soluble
In how many directions is an action potential propogated along an axon? Why?
One direction only
Because region of axon behind action potential is in refractory period
so action potential can only be propogated forward
Which ion channels are located in the axon membrane?
Voltage-gated sodium ion channels
Voltage-gated potassium ion channels
Will a strong stimulus generate a stronger action potential than a normal-level stimulus?
No
action potentials are all the same size
How is the strength of a stimulus transmitted through action potentials?
By changing the frequency of action potentials
stronger stimulus means increased frequency of action potentials
How fast do voltage-gated potassium ion channels open? What is the significance of this?
Slowly
By the time they’ve opened, the voltage-gated sodium ion channels have already opened, given sodium ion influx and inacivated
so potassium ion efflux occurs after this
i.e. downstroke after upstroke
Do axons with a larger diameter of smaller diameter have a greater conduction velocity of action potentials? Why?
Greater diameter axons
Because they offer less resistance to ions spreading out through them
so ions can move through them faster
