Group I disynaptic excitation in flexor and bifunctional motoneurons during locomotion.

In the absence of locomotion, activation of extensor muscle spindle (Ia) and tendon organ (Ib) afferents evokes a widespread pattern of interneuronally mediated inhibition and excitation through the non-reciprocal reflex systems (1). During the extension phase of fictive locomotion, however, the same afferents evoke a disynaptic excitation of extensor motoneurons (2). Recently it has been reported that stimulation of flexor group I afferents also evokes a locomotor-dependent excitation of ankle flexor motoneurons (3). The present study further examined the distribution of locomotor-dependent group I excitation of flexor motoneurons and extended the analysis to motoneurons showing complex patterns of depolarization during locomotion, i.e. motoneurons innervating bifunctional muscles. Intracellular recordings of antidromically identified motoneurons were made (using glass microelectrodes filled with QX-314 to block action potentials) in decerebrate cats in which fictive locomotion was elicited by brainstem stimulation following neuromuscular blockade (see reference 2).