Biology 202
2004 Second Web Paper
On Serendip

Many patients suffering from Multiple Sclerosis have difficulty maintaining balance and walking and many suffer falls. These falls and other motor skill impairments result not only from the deterioration of motor neurons, but also from consequent decline in proprioceptive capacity. The inability of a Multiple Sclerosis patient to effectively process reafferent feedback amplifies the neuronal impairment found elsewhere in the patient's Central Nervous System.

Multiple Sclerosis (MS) is a demyelinating syndrome affecting roughly half a million Americans. MS degenerates myelin into plaques (known as sclerosis) which impair electrical conductivity along axons. The underlying mechanisms this process are not well understood at the present time. However it is known that as MS eats away at a patient's myelin sheathes, muscles may gain in average tension or become weak and difficult to mobilize. One of the basic diagnostic tests for MS involves the electrical measurement of evoked potentials, which are an estimate of the time it takes for the CNS to transmit action potentials. Demyelination slows the speed of evoked potentials. (1)

While MS erodes the body's ability to transmit sensory information and send motor instructions, the links which enable the body to marshal muscles for movement resultant from a stimulus decay as well. This decreases the precision and subsequently the utility of concerted motor functions. For example, while driving an automobile a difficulty in properly transmitting the sight of a car veering dangerously in traffic results in an increased braking time. If instead of an oncoming car, one were to see his foot about to step onto a loose rock on a slope, a lag time may prove equally dangerous. This affirmation of body position and subsequent integration is known as reafferent feedback.

While carrying out complex motor functions such as walking, a person must perceive of his position and movement in order to maintain balance, plot a course of action (or inaction) relative to this information and vary muscle movements appropriately. This complex phenomenon is known as proprioception. The sensory apparatus which are responsible for detecting changes in muscle movement and orientation are known as proprioceptors. These proprioceptors, containing myelinated neurons known as gamma motor neurons, are susceptible to sclerosis. (2)

Two common symptoms associated with all types and severities of MS are optic neuritis, which is a term describing plaque interference with optic nerve function, and vertigo, which is a dizziness which is often associated with impartial or irregular sensory input, inner ear problems or brainstem damage. (3) Motor impairment or ataxia is also symptomatic of MS.

Ataxia related to MS comes in several forms and is well documented. MS patients suffering from upper limb ataxia have greater difficulty than healthy individuals in pointing at objects in varying states of motion. (4) Patients with vestibular ataxia have difficulty maintaining a normal gait when they are not deliberately visually monitoring their movements.

While both upper limb and vestibular ataxia are manifestations of direct effects upon topographically specific regions of the CNS and brain pertaining to balance or coordination, proprioceptive ataxia results from a malfunction or deterioration of the gamma motor neurons themselves. (5)

No comprehensive studies have been able to quantify the deleterious effect of sclerosis upon gamma motor neurons and coordination nor have any studies been able to figure out why and how MS makes people lose their balance. But we do know that MS can destroy gamma motor neurons and optic nerves and cause people to fall and injure themselves.

Falling down (especially repeatedly) is a symptom that there are major nervous or muscular system malfunctions. Standing upright is perhaps the most significant evolutionarily derived trait that humans possess, on par with the opposable thumb. Thus we can likely assume that sclerosis-induced damage to the nervous system is rather extensive once a patient begins to fall down with regularity. A fall represents a localized failure of a portion of a balance-related system or a broad failure of an entire system. Either way, a fall represents a general failure of the CNS and other systems to properly maintain an erect state.

In the event of a localized failure within a system, an apt analogy for the resultant general malfunction (a fall) is the propagation of error through a system of equations or a computational model. Assume an MS patient with optic neuritis. Distorted visual imagery of a set of stairs represents an error in perception that is passed along when several other parts of the system attempt to act on the imprecise information. There may or may not be a direct relation between the degree of optic distortion and the degree to which the subject's foot deviates from its normal path down the stairs. Now assume a subject experiencing minor distortion of a proprioceptive signal from the foot combined with a minor distortion of visual input due to optic neuritis. Each individual distortion may be slight enough that alone it could be compensated for, however, the two distortions may be enough to send the subject tumbling down the steps- his foot being unable to feel its way onto the step.

Assessment of the cause of a fall can be difficult. The root problem may be ataxia or vertigo. One way to potentially rule out the malfunction of reafferent feedback and the optic nerves is through the Romberg Test for balance. This test simply consists of nudging a free-standing subject whose eyes are closed. (6) Perhaps in the future, cheap and effective scans for sclerosis may be able to reveal the site-specificity of plaque buildup. With this knowledge we could determine the correlations between site-specific buildup and proprioceptive malfunctions.

So what does it mean to fall? What does it mean to lose track of your own body? Falling is not just the result of a loss of proprioception or a malfunction of the reafferent feedback system. Falling down is a fundamental failure of the body to either locomote properly or protect itself. When one falls, one regresses into a more childish state, where walking cannot be taken for granted. Falling represents a physical (and mental) devolution, an unlearning of one of the most fundamental motor skills. Falling can be discouraging in the same way physical deterioration can be anguishing. MS patients can be depressive due to their inability to properly control their bodies. Scientists at the British MS Research Centre in Bristol have developed a machine to measure gait irregularities and inform patients if they are in danger of falling. (7) This machine will also serve as a collector of vital data on MS patients' falls and balance problems, enabling development of more comprehensive preventative means.

References

1) Canadian MS society diagnosis ,
Evoked potentials- time it takes CNS to send and receive signals. Demyelination slows time.

2) Proprioception literature review by Darryn Sargant from Australasian Journal of Podiatric Medicine.

3) National MS Society, MS and vertigo

4) MS patients have more difficulty pointing at objects in varying states of motion than non patients. Influence of visual and proprioceptive afferences on upper limb ataxia in patients with multiple sclerosis.
J. Neurol. Sci. Feb. 1, 1999. 163(1):61-9.
Quintern J, Immisch I, Albrecht H, Pollmann W, Glasauer S, Straube A.
Department of Neurology, Ludwig-Maximilians University, Klinikum Grosshadern, Munich, Germany

5) MS encyclopedia, vestibular ataxia- gait disorder. Result of improper visual proprioception.

6) MS encyclopedia, Romberg test

7) BBC: Bristol study in brief, "people are unaware of how bad their balance is..."


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