Comparison of voluntary and reflex activation of motor units. Functional organization of motor neurones.

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Abstract
Motor unit potentials of voluntary polysyn-aptic spinal reflex activity were recorded. A bipolar needle electrode was used and the recorded potentials were classified as to shape, size, amplitude and polarity. Constancy of the voluntary motor unit pattern was stable enough to serve as a basis for comparison. When comparing the flexion reflex and voluntary contraction 24 out of 35 units were i-dentical and the second activated unit followed the same scheme. When comparing tendon jerk and voluntary activation essentially the same results were noted. Voluntary and reflex activation interact in that during subliminal voluntary effort, reflex activity was enhanced. In 2 patients with spastic paraplegia and no voluntary power the same unit appeared first in both monosynaptic and polysynaptic reflex activation. A trained subject under auditory feedback could bring about reversal of recruitment order of nearby units almost constantly. Even though only units of nearby threshold were altered in recruitment order, input specificity cannot be ignored. Apparently inherent properties of motor neurons are the main factor in recruitment. A population of motor neurons have a low threshold for any stimulus and will be used all day. Training and disuse, however, seem to be concerned primarily with high threshold units. Close interaction between voluntary and reflex activity exists at the motor unit level. This forms a logical basis for various types of physical therapy in which reflex activity is used to reinforce voluntary motor activity.