Thresholds of cortical activation of muscle spindles and α motoneurones of the baboon's hand

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Abstract
Much current thinking about voluntary movement assumes that the segmental Y loops can function as a servomechanism operated by the brain. However, the a motoneurons of the baboon''s hand receive a powerful monosynaptic (CM) projection from the precentral gyms. If servo-driving from the same cortical area is to be possible, it must project independently to the fusimotor neurons and have sufficient power to increase the afferent signalling from the muscle splndles. The cortical thresholds for contraction of m. extensor digitorum communis and for acceleration of the discharges of its muscle splndles have therefore been compared. Significant results in this cortext require that the splndles studied be coupledin parallel with the responding extrafusal muscle fibers. Many splndles were not unloaded by the submaximal contractions evoked by cortical stimulation, although all so tested were unloaded by maximal motor nerve twitches. Reasons are given for thinking that such apparent lack of parallel coupling is an artifact of complex intramuscular anatomy and limitation of shortening by "isometric'' myography. A brief burst of corticosplnal volleys at 5500/sec, which is specially effective in exciting a motoneurons over the CM projection, failed to excite splndle afferents at or below the threshold for a cortical "twitch". In a few eplleptiform discharges, bursts of splndle acceleration occurred independently of the clonic contractions. A relatively direct and independent cortico-fusimotor (CF) projection may therefore exist. Prolonged near-threshold stimulation at 50-100/ sec, which allows time for temporal summation in the less direct projections (e.g. cortico-interneuronal, cortico-rubro-splnal) and does not cause frequency-potentiation at CM synapses, gives abundant evidence of independent a and fusimotor projections, whose actions hardly outlast the stimulation period. Although independent CF projections would permit servo-driving in natural movements of the hand (given adequate loop gain), there has been no evidence of servo-driving by cortical stimulation or in thespontaneous contractions of light anesthesia. Independent projections would provide for controlled ay co-excitation in the servo-governing of natural movements. The CM projection itself may be part of an important control loop for voluntary movement in primates. A corollary would be a diminished importance of CF projections for segmental loops and an increased importance for maintaining the splndle input to cortical loops.