When diseased, this huge, complex system has simple signs: weakness, slowness, and clumsiness of purposeful movement; changes in muscle tone, stretch, and superficial reflexes; and the addition of new reflexes.
There is a quantitative relationship between only two of these signs. If muscle tone is increased, the tendon reflexes will be increased. This is always true, unless the muscle is so stretched that striking its tendon can stretch it no further. The opposite relationship is not true: tendon reflexes may be pathologically brisk, while muscle tone is normal.
A minimal corticospinal system syndrome might consist of unilateral absent abdominal reflexes or an extensor plantar response. A larger constellation might consist of flaccid weakness of the arm and leg, pathologically brisk tendon reflexes, normal superficial reflexes, and a flexor plantar response.
The signs vary and are unrelated to each other in a quantitative way, except as mentioned above. The variations are dependent on site, size, and rate of progress of the lesion. A proper examination includes verification of all the possible manifestations of a corticospinal system lesion.
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