Modulation of multisegmental monosynaptic responses in a variety of leg muscles during walking and running in humans

Abstract

Motor responses evoked by stimulating the spinal cord percutaneously between the T11 and T12 spinous processes were studied in eight human subjects during walking and running. Stimulation elicited responses bilaterally in the biceps femoris, vastus lateralis, rectus femoris, medial gastrocnemius, soleus, tibialis anterior, extensor digitorum brevis and flexor digitorum brevis. The evoked responses were consistent with activation of Ia afferent fibres through monosynaptic neural circuits since they were inhibited when a prior stimulus was given and during tendon vibration. Furthermore, the soleus motor responses were inhibited during the swing phase of walking as observed for the soleus H-reflex elicited by tibial nerve stimulation. Due to the anatomical site and the fibre composition of the peripheral nerves it is difficult to elicit H-reflex in leg muscles other than the soleus, especially during movement. In turn, the multisegmental monosynaptic responses (MMR) technique provides the opportunity to study modulation of monosynaptic reflexes for multiple muscles simultaneously. Phase-dependent modulation of the MMR amplitude throughout the duration of the gait cycle period was observed in all muscles studied. The MMR amplitude was large when the muscle was activated whereas it was generally reduced, or even suppressed, when the muscle was quiescent. However, during running, there was a systematic anticipatory increase in the amplitude of the MMR at the end of swing in all proximal and distal extensor muscles. The present findings therefore suggest that there is a general control scheme by which the transmission in the monosynaptic neural circuits is modulated in all leg muscles during stepping so as to meet the requirement of the motor task.

Training locomotor networks

“…8A, left traces), and the modulation of monosynaptic reflex responses. Courtine et al (2007b) observed similar modulation pattern of monosynaptic reflex responses from lower limb muscles during human locomotion (Fig. 8B).…”

“…In fact, state-dependent processing of sensory input has been demonstrated in humans as well. Courtine et al (2007b) showed that the gain in the monosynaptic neural pathway is modulated in virtually all lower limb muscles of normal subjects during walking in a manner similar to the patterns observed in the bipedally walking rat (Fig. 8B) .…”

Thoracolumbar multisegmental motor responses in the upper and lower limbs in healthy subjects

“…1 Recently, T11-12 stimulation has been reported to induce multisegmental monosynaptic responses in lower limb muscles in healthy subjects. [2][3][4] These responses have been recorded in both lower limbs. However, no studies have been published on the response to T11-12 electrical stimulation of the upper limbs in healthy subjects.…”

“…8 These multisegmental responses could also be useful in testing physiological function and circuitries between the upper and lower limbs, as related to the spinal cord. 2,3 The purpose of this electromyographic study is to compare the multisegmental muscular responses of upper and lower limb muscles with electrical stimulation of T11-12 spinal segment in healthy subjects.…”

“…Tendon vibration inhibited these lower limb responses similar to soleus H-reflex behavior. 2 However, tendon vibration might cause a presynaptic inhibition effect of the a-motoneurons that are activated directly by electrical stimulation.…”