What is Vrest?
Vrest originates in the ability of the membrane to allow different ions in and out of the cell in a selective manner
What is the Nernst potential?
It is used to calculated the exact V that is generated by a specific ion, for a known concentration difference across a membrane permeable to that ion - the balance between electrical chemical forces (electrochemical gradient) acting on a given ion
Therefore when Vm is exactly at Vnernst for an ion, there will be no net flow of that ion across the membrane (equilibrium)
Each ion will try to pull Vm to its Vnernst. The ion with the highest permeability (more open channels) will win
How does ionic permeability affect membrane potential?
The membrane has to be permeable to ions to let them through, ions can only cross the membrane through pores of channels
No ion flow and membrane potential can be generated unless there are specific channels present in the membrane allowing these ions to get through
What influences the resting membrane potential?
There are many leak K+ channels (always open) in neuronal membranes, therefore thr relative permeability of K+ (Pk) is predominant factor in determining Vrest - closer to Ek.
However, Vrest is not identical to Ek because there are also some elaskage of Na+ and Cl- ions through channels. Therfore, Pna and Pcl can influence Vrest as well, but to a much lesser extent than Pk
What ions are inside and outside the cell at rest?
Inside: K+ and A- (negatively charge anions e.g. AA and proteins)
Outside: Na+ Ca2+ Cl-
How does signal propagation in nerve cell happens?
- Passive propagation: due to static membrane properties and is determined by measuring the voltage change resulting from a current pulse passed through the axonal membrane that may not be large enough to generate action potential - here there is diminishing of signal as distance increase. Axon diameter also affects signal propagation where the large the diameter, the further it can travel. Therefore passive propagation usually only happens over short distances. (myelin also influences signal propagation - behaves like insulation)
- Active propagation (action potential): Electrical properties, triggered by changes in Vm. These properties enable the conduction of electrical signals without decrement over large distances
Are electrotonic responses graded or summated?
Passive propagation do both
Graded = bigger stimulus leads to bigger response
Summated = multiple stimuli = summated response
Describe the all-or-non signals of action potentials:
- if the stimulus is too low there is no AP
- if the signal is above a threshold, the AP is always the same size, it does not get larger for stronger stimuli
- As the AP travels along, it triggers the next section of axon to fire
What are the 2 types of forces driving ions across the membrane?
- Chemical driving force
- Electrical driving force
Once the ion e.g. K+ diffusion has proceeded to a certain point,the electrical driving force on K+ balances the chemical driving force. This potential is called the equilibrium potential
Describe the properties of voltage-gated Na+ and K+ channels during resting, activated and inactivated state
Na+ ion channels have both internal(h) and external(m) activation gates whilst K+ channel only have one gate, (n)
Resting state: h - open, m - close, n - close: therefore Na+ and K+ channel is closed
Activation state: h - open, m - open: Na+ channels open (delayed opening of K+ n gates)
Inactivated state: h- close, m - open: Na+ channels closed (as Na+ gates gradually close, K+ channels open too)
The delayed opening (rectifier) n gates open after 1-2 mace of threshold depolarisation. K+ flows out of the cell and speeds up the depolarisation process
Why does the action potential have an undershoot?
The open K+ channels increases Pk higher than at rest and therefore voltage moves towards Ek
Hyperpolarisation membrane causes K+ channels to close, however the K+ channels don’t respond instantly to changes in the membrane potential therefore a significant amount of channels are still open even after resting potential is reached.
At the same time, the internal Na+ h gates re-open and the membrane is ready to generate another action potential - refractory period
What is the action potential refractory period?
Absolute refractory period (when the potential is depolarised and hyperpolarised): A second AP cannot be produced, regardless of the stimulus strength
Relative refractory period: APs can be generated but with
- Increased threshold because they have to overcome hyperpolarisation
- Reduces amplitude because fewer Na+ channels are available to open (many are still in the inactive state) and so less Na+ can flow into the cell
How do we ensure that action potential always propagates forward?
The refractory period sets the direction of an action potential
- Depolarising current from AP can spread passively in either direction
- Ahead if the AP, Na+ channels are in a closed state, but ready to be opened (h gates open), therefore the spreading current can trigger an AP in this neighbouring region
- Behind the AP, Na+ channels are in an inactive state and cannot open. Therefore the spreading current has no effect on these channels and action potentials cannot be triggered
Describe the structure of the myelin sheath
Axons are wrapped in myelin sheath which consist of several layers of a specialised membrane (70-80% lipids and 20-30% proteins)
Myelin is uniform and impermeable to movement of ions
Short gaps - nodes of Ranvier - exist between the myelin sheaths, exposing the axon
How does signal propagation in myelinated axon happens?
Membrane areas covered by myelin do not become depolarised and therefore cannot generate action potentials.
This forces the current to travel down the axon to the node of Ranvier where there is no myelin and the concentration of V-gated Na+ channels is high.
Thus AP jump from one node to another. Between the nodes, there is passive spread of potential
This process is called Saltatory conduction
What is the function of Saltatory conduction?
Saltatory conduction increases the conduction velocity
Small non-myelinated axons conduct at about 0.25m/sec whereas large myelinated axons conduct at about 120m/sec
