On premotoneuronal integration in cat and man

Abstract

The first part of this thesis is concerned with reflex pathways from Ib afferents from Golgi tendon organs. Convergence at an interneuronal level was analyzed with intracellular recordings from lumbar motoneurones in acute spinal cats, and in human experiments using the H-reflex of flexor carpi radialis (FCR) as test. In both cases facilitation of Ib transmission by electrical stimulation of low threshold cutaneous afferents was obtained. On the basis of the results obtained in animal experiments, it is hypothesized that the cutaneous feed-back could supply a gain control in Ib pathways. Activation of a skin field in the course of an exploratory movement could then lead to a purposeful decrease of muscular force. The finding that cutaneous facilitation of Ib transmission in the human forearm could only be obtained from restricted receptive fields is taken to support this hypothesis. Other afferent systems converging onto the Ib inhibitory interneurones are discussed with emphasis on the Ia muscle spindle afferents. It is proposed that the latter provide dynamic sensitivity to the Ib inhibition and play a special role in the Ib control of muscle force which is exerted at the motoneuronal level. The second part is concerned with an interneuronal pathway from Ia afferents to motoneurones of forearm muscles, disclosed by a late (3 ms central latency) facilitation of the FCR H-reflex by conditioning stimulation of the median and ulnar nerves and by weak tendon taps. For the analysis of this interneuronal pathway the post stimulus time histogram (PSTH) method was utilized, activating individual wrist flexor motor units. Electrical stimulation of low threshold afferents in the median and ulnar nerves evoked an excitation with a latency at least 3 ms longer (mean 4.2 ms, range 3-6 ms) than the monosynaptic latency. Increasing the afferent input to voluntarily activated wrist flexor motor units resulted in a decrease of the non-monosynaptic excitation. This depression of the excitation could not be explained only by occlusion, but must involve an inhibitory mechanism. Findings are reported suggesting that this inhibition is not exerted directly onto the motoneurones, but acts on the interneurones mediating the excitation. Several characteristics of the non-monosynaptic excitation and its depression bear resemblance to those of a cervical propriospinal neuronal system in the cat, used as a model for the discussion.(ABSTRACT TRUNCATED AT 400 WORDS)