Whilst trying to explain how white blood cells enter the brain, using an analogy with a carpark, you asked "Whether cars (white blood cells) were central to the problem of progressive MS?". Others asked "What is the problem in PPMS?", "Why don't Current MS drugs work in PPMS?"
No doubt you may already have the answer (However, it is obvious from your comments that some/many of you do not) and for those MSers who don't like the simplistic approach, I will cut to the chase so that you can then "Switch Off" and not view/comment on the Mini series of the same title that will address the same issue in a more digestable manner.
Relapses are triggered by white blood cells entering the brain and damaging the oligodendrocyte (0) and nerves (N). This coupled with swelling of the nerve, serves to stop the nerve impulses firing properly. This leads to symptoms.
The best treatments of (relapsing) MS aim at preventing white blood cells (WBC) reaching the brain and if this happens it will stop the cascade of damaging events (loss of myelin and nerve damage) from occuring.
Current effective treatments work by stopping the white blood cells entering the brain. In my current opinion these will not work to stop the underlying cause of progressive MS. We know this because of the many failures of trying to treat progressive MS.
Some Doctors however continue to believe that progessive MS is a problem of white blood cells and carry on regardless. Indeed some Progressive MSers will probably respond to these drugs, particularly those with gadolinium enhancing lesions. However, we need a new approach to treat progressive MS, because the main problem appears to not be with the white blood cells (although I think that these need to be dealt with also) but appears to be a problem with the nerves, that has been triggered by the damage caused by white blood cells.
Nerves transmit nerve impulses (see videos in British or American English to Explain) from one nerve to another across a synapse(see video). Transmission of the nerve impulse uses energy to transmit the impulse, which results from movement of ions largely sodium (Na) and potassium. (K) Sodium enters the cell by sodium channels (pores that allow sodium to flow through them) in the nerve. Once the impulse passes the nerve resets itself so that it can transmit the next signal. Within the nerve there is a exchanger (imagine a revolving door). The system pumps calcium (Ca) out of the cell in exchange for sodium. This all helps maintains the health of the cell and te balance of sodium within the cell The myelin sheath made by oligodendrocytes allows the nerve impulse to travel faster. This is because the nerve impulse jumps acrossto the gaps (Nodes of Ranvier) between individual myelin sheaths. (There are lots of videos/tutorials on youtube those by khanacademy.org are informative. (If you watch these- a schwannn celll myelinates peripheral nerves and those in the brain are myelinated by oligodendrocytes but otherwise the message applies to the brain)
In progressive MS (both primary and secondary), loss of myelin is a feature of the disease. Initially this damage may be repaired. But as we get older our repair mechanisms become less effective. Furthermore the disease may kill too many of the repairing-cells and the scarring from the damage may stop repair. This makes the nerves vulnerable to further damage.Why one person gets relapsing MS verses Progressive MS is not clear at the moment. It may depend on age, sex, where the attack occurs within the nervous system and how well an individual can repair and deal with the consequences of damage. As the compensation mechanisms become exhausted or are not there in the first place (from PP MSers) damage from progressive MS takes over from damage caused during relapsing MS.
Once nerves are demyelinated, to keep the nerve impulses moving the nerve must use alot more energy. Low level inflammation which is present in all MS types such as that produced by microglial cells produces mediators such as nitric oxide that stop cells from making enough energy. This blocks the action of the energy-dependent sodium-potassium pump. This results in excess sodium entering the nerve, via the sodium channels. This excess sodium in the cell can cause the sodium-calcium exchanger to go into reverse and it pushes sodium out of the cell whilst pulling calcium into the cell. This can be bad news as too much calcium in cells triggers them to commit suicide. Therefore simple electrical activity of the nerves can be a problem for demyelinated nerves. As nerve circuits are damaged and lost the brain compensates by using different circuits to perform the task. As nerves are lost it puts more of a strain on the remaining nerves, leading to further nerve loss and the accumulation of progressive disability.
With this scenario progressive disease can occur in the absence of white blood cells and explains why current MS drugs are not useful for progressive MSers. Importantly it gives us clues on how to treat MS. One obvious route is to repair the myelin, but others such as blocking the accumulation of toxic concentrations of ions in the nerve are even more feasible, because these types of drugs are already availble to treat other diseases such as epilepsy. The guess work is which ones or combination are more likely to succeed.This however is just theory and will become more concrete when treatments based on this theory are shown to be active. Until then we keep an open mind to alterative ideas.
Too technical? See Part I and the rest of the series.