DESIGN FEATURES OF THE SEGMENTAL MOTOR CONTROL SYSTEM: THE EFFICACY OF MONOSYNAPTIC SPINDLE IA CONNECTIONS ONTO THEIR HOMONYMOUS MOTONEURONS (EPSP, SPINAL CORD, COMPOSITE, NEUROPHYSIOLOGY).
AuthorVanden Noven, Sharyn
AdvisorStuart, Douglas G.
MetadataShow full item record
PublisherThe University of Arizona.
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AbstractIn the field of spinal-cord neurophysiology, the nature of and the rules which govern the strength of functional connections between muscle afferents and motoneurons supplying the same muscle are important to delineate. This study addressed a facet of this issue by testing the possibility that the strength of the spindle Ia-motoneuronal connections is stronger (as demonstrated by the differing amplitudes of the mean maximum composite Ia EPSPs) if both neurons supply the same sub-volume of the muscle, providing the various sub-volumes of the muscle are capable of independent action. Intracellular recordings were made of the Ia EPSP responses of semimembranosus (SM) and lateral gastrocnemius (LG) motoneurons in anesthetized low-spinal cats to electrical stimulation (Group I range) of nerve branches supplying different parts of the homonymous muscle, as well as different heteronymous muscles. For study of SM motoneurons, stimulated nerve branches included those supplying the anterior (SMa) and posterior (SMp) heads of the SM muscle and three providing heteronymous input from the anterior (BFa) and posterior (BFp) parts of biceps femoris and the distal part of the semitendinosus (STd) muscle. Ia EPSPs were partitioned such that stimulation of the SMa nerve branch produced significantly larger EPSPs in SMa motoneurons than in SMp cells; likewise, stimulation of the SMp nerve branch produced larger EPSPs in SMp motoneurons than in SMa cells. Study of the differences in the strength of heteronymous Ia input (i.e., from BFa, BFp and STd) between the SMa and SMp cell groups correlates with the different actions reported previously for the two heads of the SM muscle. For study of LG motoneurons, the stimulated nerve branches were those supplying the four neuromuscular compartments of the LG muscle (LG1, LG2, LG3 and LGm) and the nerve to a heteronymous muscle, soleus (SOL). In all five instances, partitioned Ia effects were evident. An association is suggested between the present results and previous electromyographic studies. The previous studies have shown that the muscle heads (SM) or neuromuscular compartments (LG) under consideration in this study are capable of somewhat separate actions. The present study also included assessment of the relative extent to which the partitioned Ia effects could be attributed, in part, to one or two developmental factors, topographic specificity and species specificity. The analysis suggested that both factors were potentially implicated, with species specificity somewhat predominant over topographic specificity.