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

Abstract

Input from group I afferents of ankle and toe extensors, other muscles, skin nerves and descending tracts to interneurons of Rexed''s laminae V-VI in the cat spinal cord was analyzed using intracellular recording from these interneurons. Adequate stimuli (muscle stretches) were used to activate selectively group Ia muscle spindle afferents of triceps surae and plantaris while other fiber systems were excited electrically. Ia and Ib afferents of ankle and toe extensors co-excited, co-inhibited or exerted opposite synaptic actions in 41, 33 and 50% of the analyzed interneurons, respectively. Taking into account both excitatory and inhibitory input from these 2 groups of afferents, 64% of the interneurons appeared to be used in common in reflex pathways from muscle spindles and tendon organs of ankle and toe extensors. Selective input from Ib afferents of triceps surae and plantaris (excitation and/or inhibition) was found in 36% of the interneurons; there was no evidence for a similarly selective input from Ia afferents. A great majority (> 90%) of the interneurons excited by group I afferents were also inhibited by group I afferents, from either the same or other muscles. Both monosynaptic and disynaptic epsp [excitatory post-synaptic potential] from Ia and/or Ib afferents from other muscles and from fibers in the ipsilateral funiculi were found in a great proportion of the same interneurons, together with disynaptic epsp from low threshold cutaneous afferents. Intracellular staining with horseradish peroxidase revealed 4 different patterns of axonal projections of the analyzed interneurons: projections to motor nuclei and the intermediate region, projections only to the intermediate region, locally or combined with projections to different rostro-caudal levels, and projections to the opposite side of the spinal cord. A large proportion of interneurons projecting to motor nuclei displayed input from both Ia and Ib afferents although such an input was a feature of interneurons with other projections as well. No systematic differences in the input from group I afferents were found for interneurons with different axonal projections. In contrast disynaptic epsp of cutaneous origin and monosynaptic epsp upon stimulation of ipsilateral spinal tracts appeared predominantly in interneurons projecting to motor nuclei.