Excitability of spinal motor neurones in normal subjects and patients with spasticity, Parkinsonian rigidity, and cerebellar hypotonia.

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Abstract
The recovery of the H reflex conditioned by a preceding stimulus was used to determine the excitability of spinal motor neurons in normal subjects and patients with severe spasticity, Parkinsonian rigidity and cerebellar hypotonia. In normals the recovery lasted more than 4 sec. The second return of the test reflex occurred 80 [plus or minus] 8 msec, after the conditioning stimulus with stimuli just above threshold for the reflex and at half the time with maximal reflexes. In spasticity with stimuli just above threshold, the test reflexes were twice as high and the 2nd return occurred earlier. \Vith maximal reflexes the early facilitation was the same as in normals and during recovery from the 2nd return the test reflexes were 30% higher. The increase of excitability was not related to the degree of spasticity. In Parkinsonian rigidity excitability was increased in the period of 2nd facilitation and the following depression. With stimuli just above threshold, however, the 2nd return of the reflex occurred at the same time as in normals and the early facilitation was not increased. Electrocoagulation of the ventro-lateral nucleus of the thalamus reduced the rigidity. Excitability was also diminished but remained above normal. In cerebellar hypotonia excitability was depressed 800 msec, to 2 sec. after the conditioning stimulus and early facilitation was increased.