Oligosynaptic excitation of motoneurones by impulses in group Ia muscle spindle afferents in the cat

Abstract

Intracellular recording from hind-limb motoneurons was used to investigate whether di- and trisynaptic (oligosynaptic) excitatory post-synaptic potentials (EPSP) are evoked from group Ia muscle spindle afferents in those motoneurons in which such potentials are evoked from Ib tendon organ afferents or entire group I afferents. Ia afferents of triceps surae and plantaris were activated either selectively by single brief stretches of these muscles, or together with Ib afferents by electrical stimuli applied to the nerves. Muscle stretches below threshold for Ib afferents (10-35 .mu.m) evoked EPSP which appeared with latencies compatible with di- and trisynaptic coupling between the afferents and the motoneurons. The latencies of a majority of these EPSP were too short to allow their mediation by group II afferents, if any were activated by the applied stretches. They were also too short to be compatible with effects attributable to dorsal root reflexes. These EPSP are attributed to oligosynaptic actions of Ia afferents. Stretch-evoked di- and trisynaptic Ia EPSP were found in 83% of motoneurons in which EPSP were evoked by stimuli which activated both Ia and Ib afferents; in 5 motoneuron species they were found in > 90%. Group Ia muscle spindle afferents apparently evoke not only inhibitory but also excitatory actions in parallel with group Ib tendon organ afferents. The distribution of Ia oligosynaptic stretch-evoked excitation from ankle and toe extensor muscles was compared with the distribution of Ia non-reciprocal inhibition as previously described. Excitation predominated in posterior biceps-semitendinosus motoneurons and inhibition in other species of motoneurons investigated, except those of intrinsic foot muscles (tibial motoneurons); similar proportions of the latter showed excitation and inhibition. Occurrence of oligosynaptic EPSP and inhibitory post-synaptic potentials (IPSP) of Ia origin in some motoneuron species and in particular in individual motoneurons is indicative of a number of reflex pathways between group I avverents and these motoneurons. The disappearance of some of the EPSP evoked by near-threshold electrical stimulation following stronger stimuli indicates interactions between various functional groups of interneurons mediating group I actions.