How Nerves Work 4 Flashcards
Describe the shape of the action potential graph ( Membrane potential against time)
The graph starts at -70mV (resting potential) then slightly increases as it reaches the threshold and then it rapidly spikes up to +40 before rapidly falling to below the resting potential (hyperpolarising)
Action potential graph
What is happening as the graph increases from the resting potential up to around -55mV?
This is the cell reaching the threshold to start an action potential from the Graded potential.
Action Potential graph
What is happening when the graph spikes from -70mV to +40mV?
This is a rapid depolarisation of the cell. This is due to the opening of sodium voltage gated channels when the threshold reaches around -55mV and sodium enters the cell in large volumes due to its concentration and electrical gradient favouring this movement. This causes the inside of the cell to be positive so it is depolarised.
Action Potential Graph
What is happening when the graph rapidly goes from its peak to below the resting potential?
The sodium voltage gated channels don’t stay open for long and when they shut potassium voltage gated channels are triggered by their own threshold and these open to let potassium out of the cell. Potassium flows along its concentration gradient and this hyperpolarises the cell until they close and the cell returns to its resting potential.
Why does the cell never go over +40mV when it’s depolarising on its action potential?
It never goes above this value due to it being sodiums equilibrium potential.
Define Absolute Refractory Period
This is when the cell cannot fire any action Potentials as the sodium voltage gated channels close whilst they recover.
Define Relative Refractory Period
This is when the cell can again fire action Potentials due to the sodium voltage gated channels having recovered.
How do action Potentials convey the stimulus properties?
Action Potentials are always the same size so their amplitude cannot be used as an indication of the stimulus (as Graded is). Instead their frequency is used (firing frequency) so a bigger stimulus will make a Graded potential stay past it’s threshold for longer therefore increasing the frequency of the action potential.
Describe how the action potential self propagates
The depolarisation spreads along the membrane, decaying as it goes. Each time the depolarisation is large enough to reach the threshold for the next sodium voltage gated channels to open. Therefore the signal will be the same size at the end as it is at the start.
Why doesn’t depolarisation keep going back and forth along the membrane?
The sodium voltage gated channels need to time to recover so the depolarisation does spread backwards slightly but doesn’t affect these channels.
How do larger axons increase the speed of action Potentials?
Larger axons conduct a lot quicker as they reduce the axial resistance so less current leaks out. They also allow the sodium voltage gated channels to be further spread out and as the opening of these is what slows down depolarisation this increases the speed.
What is myelination and how does this speed up depolarisation of the membrane? What is the term of the propagation of the action potential along myelinated axons?
Myelin is folds of membrane from: Schwann cells in the PNS and oligodendrocytes in the CNS can form myelin sheaths around the axon and this makes the axons smaller so they can conduct quicker. Sodium voltage gated channels are only located at the nodes of Ranvier so it cannot propagate as normal but the myelin increases the resistance of the membrane so it allows the action potential to spread to the next segment with little decrement. This not only speeds up conduction but it also is metabolically good due to ions only crossing at the nodes and it saves space as an unmyelinated neurone would have to be large to conduct at the same speed.
The term is saltatory conduction.
What conditions can demyelination cause?
It can cause Multiple Sclerosis and Guillain-Barré syndrome. The depolarisation decays quicker then it normally would do it can’t reach the threshold and fire the next action potential.
What is Compound Action Potential?
Nerve trunks that contain a lot of conduction varieties will get lots of readings on a graph. Fastest conducting ones will arrive first at the point of recording. Their readings are related to their conduction velocity which in turn is related to their anatomy.
