Chapter 1 : The Cells of the CNS

Brain: Contents Page

Multiple Sclerosis

Multiple sclerosis (MS) affects conduction in axons within in the brain and spinal cord by damaging the myelin sheath provided by the oligodendrocytes. This results in patients having problems with muscle movement, balance and vision. Patients also feel fatigue. The myelin of the peripheral nervous system provided by Schwann cells is not affected by this disease.

Patients with MS may experience symptoms either intermittently or building up fairly continuously over time (relapsing and progressive forms). Between attacks, the symptoms may disappear completely, but more commonly the neurological problems become permanent, especially as the disease advances.

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MRI image of ther spinal cord from a patient with Multiple Sclerosis. The extent of the lesions can be seen in the MRI reconstruction of this patient's spinal cord.

Right: Carswell's 1838 image of the lesions in the brainstem and spinal cord of a patient with MS

Multiple sclerosis (MS) is an inflammatory disease in which the myelin sheaths around axons are damaged.

While the cause is not clear, the underlying mechanism is thought to be either destruction of CNS myelin by the inmmune system (autoimmunity), or a failure of production of myelin.  

The long-term outcome is not easily predictable. Good outcomes are more common in females, in people who develop the disorder in their youth, and those with a relapsing course particularly if they have only had a few attacks.

One hypothesis is that the blood brain barrier is breached because of infection, such as a viral infection. The entry of T-lymphocytes, activated by the virus, into the CNS is thought to cause an autoimmune attack on oligodendrocytes, resulting in a loss of myelin.

Remyelination starts but is incomplete, and repeated immune attacks cause relapses, that worsen the demyelination process.

Scars, sclerosis occurs in the white matter because of astrocytic hypertrophy.

These scars are often adjacent to the ventricular system, ventricles or central canal of the spinal cord.

Clinical Physiological Investigations

Evoked potential techniques have proved useful in screening for demyelination because of the sloing in axonal conduction associated with the loss of oligodendrocytes. One example is the use of the Visual Evoked Potential that shows slowing of conduction in patients with optic neuritis: these patients may have temporary loss or blurring of vision, double vision, or a lack of coordination between the two eyes.

Signs and Symptoms commonly include:

Visual problems including temporary loss or blurring of vision, double vision, or a lack of coordination between the two eyes. Vision may vary depending on the time of day or the circumstances - for example, it might get worse when you are stressed, tired or in unfamiliar surroundings. 

Balance problems and dizziness: Symptoms might mean patients are wobbly on their feet from time to time, or might need to move with more care than before to avoid losing balance.

Stiffness or Spasms: affect at least 20 per cent of people with MS at some time. Like all MS symptoms, spasms and stiffness affect people differently and can vary over time.

Speech difficulties of some kind affect between 40 and 50 per cent of people with MS. Not everyone is aware that MS can cause this problem, and can make assumptions about why their speech is affected.

More details of the pathways involved can be found in the specific sections on motor control in Chapters 4 and 5.

Fatigue in MS is not just an ordinary tiredness, as is experienced at the end of a hard day's work. It's as an overwhelming sense of tiredness that often occurs after very little activity. 

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Etiology

Environmental Factors

MS is more common in areas further away from the equator. It is virtually unheard of in places like Malaysia or Ecuador, but relatively common in Britain, North America, Canada, Scandinavia, southern Australia and New Zealand.

Viruses

It is not clear why people further away from the equator are more likely to get MS, but it is possible that something in the environment, perhaps bacteria or a virus, plays a role.

No single virus has been identified as definitely contributing to MS, but there is growing evidence that a common childhood virus, such as Epstein Barr virus (which can cause glandular fever), may act as a trigger.

This theory is still unproven and many people who do not have MS would have also been exposed to these viruses, so just like genes, they are unlikely to be the whole story.

Vitamin D

There is also a growing amount of research that suggests that a lack of vitamin D could be a factor in causing MS.

We get most of our vitamin D from exposure to sunlight. Low levels of vitamin D have been linked to higher numbers of people developing many different conditions, including MS. Vitamin D is also known to be of some important in neural development.

Smoking

A number of studies have looked at smoking in relation to MS, and have found that smoking appears to increase the risk of developing MS. It's not yet clear exactly why this is.

Additional Signs and Symptoms:

Bladder, Bowels, Sexual Function:

There are two main types of bladder problems in MS - storage and emptying - causing urinary retention or incontinence. Constipation is more common in MS than bowel incontinence. MS can affect sexual function for both men and women

Swallowing:

MS can cause swallowing difficulties if there is damage to any part of the brain that controls swallowing, or damage to the connections between the brain and the spinal cord (the ‘brainstem’).

Tremor:

Tremor may be experienced as small, shaking movements or as larger movements. If tremor develops, it tends to be some years after people’s first symptom of MS, typically between five and 15 years. 

Memory, Thought, Emotions

MS can affect memory and thinking, and also have an impact on emotions. Like all MS symptoms, you might experience this in varying degrees, or not at all.

It is clear that the pathology in MS can affect many aspects of CNS function - sensory, motor and autonomic.


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Chapter 1 : The Cells of the CNS

Brain: Contents Page

HumanPhysiology.Academy 2014-2015