ANKLE EXTENSOR GROUP-I AFFERENTS EXCITE EXTENSORS THROUGHOUT THE HINDLIMB DURING FICTIVE LOCOMOTION IN THE CAT

1. The effects of stimulating hindlimb extensor nerves (100-200 ms tra ins, 100 Hz, less than or equal to 2 times threshold) during the flexo r and extensor phases of the locomotor step cycle were analysed in the decerebrated, paralysed cat during fictive locomotion evoked by stimu lation of the mesencephalic locomotor region. 2. Stimulation during ex tension of either the medial gastrocnemius (MG), lateral gastrocnemius -soleus (LGS) or plantaris, (PL) nerves was equally effective in incre asing the duration and amplitude of electroneurogram (ENG) activity re corded in ipsilateral ankle, knee and hip extensor nerves. Enhancement of extensor ENG activity could be evoked with near threshold stimulat ion intensity and appeared within 10-40 ms of the onset of ankle exten sor nerve stimulation. Stimulation of anterior biceps during extension occasionally evoked a modest increase in the duration of activity of hip, knee and ankle extensors. Stimulation of quadriceps during extens ion enhanced the activity of proximal extensors and soleus, but inhibi ted other ankle extensors. 3. Selective activation of ankle extensor I a spindle afferents by muscle stretch also enhanced ipsilateral extens ion. It is argued that both muscle spindle and tendon organ afferents can contribute to the increase in extensor nerve activity evoked by gr oup I stimulation intensity during fictive locomotion. 4. During flexi on, stimulation of either the MG, Pl or LGS nerves at group I strength terminated on-going activity in ipsilateral flexors and initiated a b urst of activity in ipsilateral hip, knee and ankle extensors, i.e. re set the step cycle to extension. 5. Low strength stimulation of the mi xed muscle and cutaneous nerve innervation the plantar aspect of the f oot produced extension enhancement and resetting similar to that evoke d by group I muscle afferent stimulation. Stimulation of the cutaneous nerve supplying the dorsal aspect of the foot during extension enhanc ed extensor activity, and during flexion, enhanced the activity of fle xors. 6. The effects reported here during fictive locomotion may also occur during overground locomotion with natural activation of group I muscle spindle and tendon organ afferents. Extensor spindle and tendon organ afferents may thus serve as an excitatory reflex system helping to shape the amplitude, duration and timing of ipsilateral extensor a ctivity. Increased or unexpected activation of group I extensor affere nts or plantar foot afferents during locomotion could also compensate for increased loading of the limb.