Common interneurones in reflex pathways from group 1a and 1b afferents of knee flexors and extensors in the cat.

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Abstract
Input from group I afferents of knee flexors and extensors to interneurons in Rexed''s laminae V-VI in the cat spinal cord was analyzed by use of intracellular recording and electrical stimulation of the nerves to differentiate between group Ia and Ib synaptic actions. The aim was to find out if these interneurons may mediate synaptic actions of both group Ia and Ib afferents. Of the 49 neurons analyzed 28% were excited by both group Ia and group Ib afferents; 32% were inhibited by both and 35% were excited by the 1 and inhibited by the other. Taking into account all of these actions, input from both subgroups of group I afferents was found in nearly 60% of neurons. Most were also excited and/or inhibited by group I afferents of ankle and toe extensors. Selective (excitatory and/or inhibitory) input from Ia afferents was found in 18% and from Ib afferents in 22% of the neurons. Excitation was evoked from Ia afferents of either knee flexors or extensors but not from both. In several of the neurons Ia IPSP [inhibitory postsynaptic potential] were evoked from both posterior biceps-semitendinosus and quadriceps. Intracellular staining with horseradish peroxidase revealed axonal projections of laminae V-VI interneurons to motor nuclei as well as to the intermediate zone, ipsilateral as well as contralateral. No correlation was found between patterns of input from group I afferents and axonal projections, and interneurons co-excited by Ia and Ib afferents were among those with different axonal projections.