Lecture 4: Action Potentials Flashcards
Describe the process of an action potential, based on the diagram
Something happens to bring the membrane potential to threshold. This triggers the all or nothing action potential. The membrane potential becomes more positive (eventually passing above 0). Then the membrane potential falls towards zero and then there is hyperpolarisation
When is no action potential possible?
the absolute refractory period
When is action potential possible but it is more difficult to initiate?
the relative refractory period
Action potentials are regenerative. What does this mean?
They are propagated without decreasing the amplitude
When there is a depolarisation and the membrane potential becomes more positive, does it approach the equilibrium potential for Na+ or K+?
Na+
What is the conductance of an ion and what two things does it depend on?
the amount of a particular charged ion that can flow across a membrane and it depends on
- permeability due to ion channels
- equilibrium potential (driving force)
Describe the conductance of Na+ throughout an action potential
the conductance of Na+ rises rapidly at the beginning of an action, and falls rapidly back to zero
Describe the conductance of K+ throughout an action potential
the conductance rises much more slowly and doesn’t get as high and then very slowly decreases
Describe voltage gated Na+ channels (4)
- there is an activation gate which is opened when the membrane potential reaches a certain voltage
- selectively filter (specific for Na+)
- there is an inactivation gate
- blocked by chemicals such as TTX
Describe the makeup of voltage gated Na+ channels in terms of the subunits
- 4 α subunits
- 2 β subunits
What are the 4 parts of the voltage gated Na+ channel
- voltage sensing activation gate
- a pore with a selectivity filter
- an inactivation gate inside the cell
- a modulation function
Describe the voltage gated Na+ channel when it is not activated
There are positive charges in the way which closes the pore and prevents Na+ from entering because like charges repel
What are the three phases that the voltage gated Na+ channel can be in?
- not activated
- activated
- inactivated
Describe the voltage gated Na+ channel when it is activated
there has been a depolarisation to bring the membrane potential to threshold which moves the positive charge out of the way, allowing Na+ to flow down its electrochemical gradient into the cell
Describe the voltage gated Na+ channel when it is inactivated
the ball and chain part of the gate inside the cell closes to that the channel is open but blocked from the inside
Which responds faster, the ball and chain or the movement of the charge out of the pore?
they both respond to change in the voltage but the charge movement responds faster
Describe the voltage gated K+ channels (4)
- has an activation gate
- selectively filters (specific for K+)
- has an inactivation gate
- blocked by toxins
Which is faster, the voltage gated Na+ channel or the voltage gated K+ channel?
voltage gated Na+ channel
What is happening to the activation gates (both Na+ and K+) at resting membrane potential?
they are both closed so there is no conduction through the channels
When the resting membrane potential is brought to the threshold, what voltage gated channels open?
the Na+ ion channels
When the resting membrane potential is brought to the threshold, what voltage gated channels remain shut? What does this mean for the conductance of K+ ions?
K+ ion channels which means that the conductance is 0
Just after the resting membrane potential occurs, the K+ voltage gated channels open. What happens to the K+ ions? What does this cause?
they flow out of the cell which causes repolarisation
During repolarisation, which voltage gated ion channels close first?
the inactivation gate of the Na+ voltage gated channels close first
What is the absolute refractory period? Including when it occurs and why it occurs
This is when no action potential can occur and it happens when the Na+ channel inactivation gate it closed but the K+ channel is still open. This means that K+ ions still flow out which pushes the membrane potential further from threshold (hyperpolarisation)
Which gate opens after the absolute refractory period?
the inactivation gate for Na+ but the ions don’t flow in because the activation gate is still closed
What is the relative refractory period?
This occurs when the Na+ channel inactivation gate is open (the activation gate is still closed) but the K+ channel is still open. K+ ions still flow out and so the membrane potential becomes further from threshold so the neuron is less excitable
When there is a repolarisation and the membrane potential becomes more negative, does it approach the equilibrium potential for Na+ or K+?
K+
The inactivation gate of the voltage gated Na+ channel blocks the channel pore BECAUSE the inactivation gate is part of the pore
the first statement is true and the second statement is false because the inactivation gate is not part of the pore, it is part of the intracellular loop (ball and chain)
When considering the speed of conduction of electricity (ions), what does Rm stand for?
membrane resistance
When considering the speed of conduction of electricity (ions), what does Rin stand for?
axial (internal) resistance
What are two things in the axon which affect the speed of conduction?
- internal (axial) resistance
- membrane resistance
What is the internal (axial) resistance regulated by?
the cross-sectional area of the axon
What are the two ways to increase the speed of conduction?
decreasing internal (axial) resistance increasing membrane resistance
Explain how decreasing the internal resistance can increase the speed of conduction along the axon
decreasing the resistance increases the speed of conduction by allowing ions to flow faster which increases AP conduction
Explain how increasing the membrane resistance increases the speed of conduction along the axon
increasing the membrane resistance stops the axon from being “leaky” which means that less ions are able to cross the membrane which forces more ions to flow and this increases the speed of conduction
What modifications are made to the axon to increase the speed of conduction?
the Schwann cells in the PNS and the oligodendrocytes in the CNS make myelin which provides great electrical insulation to the axon and increases the speed of conduction.
Explain what saltatory conduction is and why this increases the speed of conduction
- saltatory conduction is the propagation of an action potential from one node of ranvier to another
- this increases the speed of conduction because if everything is insulated the whole way along, the AP fails because the ions won’t travel far enough from the point of origin as they end up along the whole volume of the axon
Where is there a high density of Na+ channels and where is there no channels?
- high density at the nodes of ranvier
- no channels at the internode
Describe the voltage-gated K+ channels (4)
- activation gate
- selectively filter (specific for K+)
- inactivation (slower than for the voltage-gated Na+ channel
- blocked by toxins
