Lecture 12 - The Action Potential Flashcards
Outline what the action potential is ?
summation of graded response in the dendrites tirgger a action potential at the trigger point if it reaches a threshold level
dependent on the relative permeability across the membrane
it is a change in voltage across a membrane
it is an all or nothing response
no loss of amplitude in signal
depolarisation is the signal - Na+ influx as Na is Electro and Chemically drawn into the membrane
after this we get hyper polarization - K+ outflux
during hyperpolarization - K+ is elecro drawn into the membrane at rest but chemically (conc) drawn out of the membrane - this gives a NET of K+ out of the memebrane - this is the main factor that contributes to the resting potential
however during an AP - the membrane interior goes from - to + , so K+ is now Electro and Chemicaly drawn out of the membrane - so K+ moves out alot
we have the absolute refactory period where , Na+ channels are Inactive - so cannot open to any singal - signal cannot fire - this prevents AP doubling back on itsself
then the relative refactory period where - Na+ channels are open but K+ (slow to close) have caused hyperpolarization still in effect , so to fire an AP the signal has to be LARGER to overcome the hyperpolarization and hit the threshold value
Na+ channel closes and opens instantly - no lag
K+ channel is slower the open and close - this causes the hyperpolarization to occur - as K+ channel stays open longer than just to balance the signal
use a VOLTAGE CLAMP to prove this
good phrase : Upstroke of the AP is due to an increase in the Permeability of the membrane to Na+ ions
explain the trigger point / axon hillock , and how graded repsonses occur
graded responses occur at points on the dendrite, they provide a small signal change
they will dissapate via diffusion over a certain distance - signal gets weaker
they will also reduce in signal strength over time due to diffusion of charges
so these signals are linked by Space and Time
if signals are close together in distance the additive (summation) effect is greater
if the signal is close occurrence Time wise , again the effect is additive
below threshold at hillock - no AP
above threshold at hillock - AP
size of the signal does not change the size of the AP
How is conductance linked to the action potential ?
conductance means permeability to ions
if we open ion channels then the membrane potential will move towards the equilibrium potential for that ion
so conductance is linked to how many channels are open for that ion
we need very few ions to move across the membrane to trigger a large change
what channels are involved in the action potential
at the neuronal junction - neroutransmitter release trasmitters from the pre synaptic membrane such as ACH - open channels on the post synaptic membrane
Na+ now can enter the dendrites - cause a graded signal - of great enough trigger action potential at hillock
at AP section we have Na+ Voltage gated channels for the Depolarization section and K+ voltage gated channels for the hyper polarisation section
we also have some Leak Channels - just always open all around the neuron for the resting and action potential
sodium pump is not involved
what is the Absolute refractory period ?
we have the absolute refactory period where , Na+ channels are Inactive - so cannot open to any singal - signal cannot fire - this prevents AP doubling back on its self
a channel can be Closed - Opened (fire AP if threshold signal met) - Inactive
what is the Relative refractory period ?
the relative refactory period where - Na+ channels are open - so can fire again
but K+ channels (slow to close) have caused hyperpolarization -still in effect , so to fire an AP the signal has to be LARGER to overcome the hyperpolarization and hit the threshold value
what is the structure of a voltage gated potassium channel ?
Made of 4 alpha sub units
each unit is 1-6 with a positive 4
difference is in Na+ it is one long protein not 4 separate proteins
forms a circular pore section
will undergo a confrontational change in response to a change in the membrane potential - causes it to open
what is the structure of a voltage gated sodium channel ?
a functional Na+ channel is one alpha sub unit
made of a unit of 6 intermembrane proteins that
the 4th of this unit is a positive region
this 6 unit repeats 4 times and forms a circular shape
it has a receptor section to inacitvate/ acitvate it
the 4*4 postive section will undergo a confirmational change in response to a change in membrane potential that will open the channel and allow Na+ to enter the cell
what is a consequence of too high a K+ concentration in the blood for a long time
indicated K+ is high in the body
if a pore stays open too long it is prone to being inactivated - slow sluggish response in person
how does lidocane and other related molecules act as a local anaesthetic
will exist in protonated and deprotonated form
the hydrophobic form will pass through the membrane
they can block Na+ channels - prevent neuronal firing
how does diameter affect the signals of action potentials ?
a thicker - wider diameter - axon will conduct an AP quicker than an thinner one
this is as the wider one has more volume for the Na+ molecules to travel along the axon in - the molecules are less impeded
the conductance of an axon - ie ability to conduct Rapidly is dependent on distance mathematically - so larger Distance - greater conductance and a quicker transmission
what do local currents do along the axon
Local currents will cause the propagation of the action potential - you get a wave of propagation along the axon
which is quicker, mylenated or unmylenated neurons ?
why?
Mylentaed neurons increase cinduction velocity - quicker
NOT VITAL - this is due to an increase in membrane resistance and decrease in membrane capacitance
mylein sheath that decreases conductance as they mean the neuron cant have Na+ channels where myelin is
then they have nodes of ranvier - where they have a high density of Na+ channels and can conduct
this gives and affect of saltatory conduction
myelin sheath is an insulator - the local curict is over 2 nodes so hence a greater distance
one node will depolarize the next and so on- this is far quicker than unmylenated due to the greater distance between voltage gated Na+ channels - the signal Jumps
an AP is only occuring at the nodes
what diseases affect the conduction of the AP ?
CNS - multiple sclerosis affects all CNS nerves
PNS - Landry-Guillian-Barre syndrome
these diseases are a result of the breakdown of damage to the myelin sheath - demyelination
why does demyelination affect the AP ?
damaged myelin - smaller local currents - cannot make the jump between nodes of ranveir - signal will not fire as threshold is not reached
it stops saltatory conduction
