Aim: 

Stimulation of group I extensor muscle afferents resets the locomotor rhythm by either prolongation of the extensor phase, or by termination of the flexor phase during fictive locomotion in the cat. The same stimuli also give rise to polysynaptic group I EPSPs in extensor motoneurones while the classical "autogenic Ib inhibition" of extensor motoneurones is suppressed during fictive locomotion. We aimed to identify the interneurones causing the resetting and mediating the polysynaptic group I excitation and to describe the activity of Ib inhibitory interneurones during fictive motor activity.

Methods: 

In decerebrate feline preparations, extracellular interneurone recordings were collected in the L6-S1 segments prior to and during fictive (i.e. animal under paralysis) locomotion or scratch. In the absence of motor activity we examined axonal projections within the DLF and to motoneurones innervating the ipsilateral gastrocnemius and hamstring muscles and sensory-evoked activity by electrical stimulation (2-5 stimuli at 200 Hz) of hindlimb muscle afferents. Sensory-evoked activity was compared before, during and after motor activity and emerging spontaneous firing was related to the motor rhythm.

Results: 

Not all of the putative resetting interneurones (n = 14) showed rhythmically modulated sensory-evoked firing and spontaneous activity as expected if they were to distribute locomotor/scratch drive potentials to motoneurones. The sensory-evoked activity of the classical "Ib interneurones" was reduced during motor activity as expected; however some of these neurones showed rhythmic spontaneous firing during fictive scratch.

Conclusions: 

Candidate resetting and Ib inhibitory interneurones include two subtypes: those with and those without spontaneous firing during fictive motor activity.