Modulation, probably presynaptic in origin, of monosynaptic Ia excitation during human gait

Abstract

Modulation of presynaptic inhibition of Ia afferents projecting monosynaptically to soleus motoneurones was investigated during human gait. Changes in presynaptic inhibition of Ia afferents were deduced from alterations in the amount of heteronymous soleus H-reflex facilitation evoked by a constant femoral nerve stimulation. It has been shown that this facilitation is mediated through a monosynaptic Ia pathway and that during its first 0.5 ms it is still uncontaminated by any polysynaptic effect and can be used to assess ongoing presynaptic inhibition of Ia terminals to soleus motoneurones. During gait, heteronymous facilitation was reduced with respect to its control value (rest during sitting) and modulated during the step cycle: it reached its maximum at mid-stance and decreased to near zero by the end of stance. At the same time the H-reflex amplitude was to some extent similarly modulated. It is argued that this decrease in heteronymous Ia facilitation and in H-reflex amplitude reflects an increased, ongoing presynaptic inhibition of Ia terminals projecting onto soleus motoneurones, which could be from central and/or peripheral origin. D1 inhibition, i.e. the late and long-lasting inhibition of the soleus H-reflex evoked by a train of stimuli to the common peroneal nerve, was used as another method to assess presynaptic inhibition. This D1 inhibition was decreased during gait, and it is argued that this decrease might reflect an occlusion in presynaptic pathways or increased presynaptic inhibition of pathways mediating the conditioning volley.

New evidence of corticospinal network modulation induced by motor imagery

“…A conditioning test interval was determined for each participant by testing several intervals between 15 and 25 ms, with 1 ms steps. The interval used to obtain the greatest SOL H-reflex depression was 21 Ϯ 1 ms, confirming previous findings of Faist et al (1996). Conditioned and unconditioned H-reflexes were randomly elicited at rest and during imagined contractions of the TS muscles, with the ankle kept in a constant neutral position (angle: 90°).…”

The Interaction of a New Motor Skill and an Old One: H-Reflex Conditioning and Locomotion in Rats

“…As Figure 4 illustrates, the H-reflex changes were evident with higher and lower levels of background EMG and with larger and smaller M responses. This persistence contrasts with the H-reflex changes that occur with a switch from standing to walking or from walking to running (Capaday and Stein, 1987;Stein, 1995;Faist et al, 1996). Unlike these situations, in which the H-reflex changes markedly when the concurrent task changes, the effects of conditioning on the H-reflex did not disappear when the rat began to walk.…”

Modulation of Soleus H-Reflexes During Gait in Children With Cerebral Palsy

“…Soleus Hoffmann (H-) reflexes show a maximum in the stance phase and a minimum in the swing phase during walking and running Stein 1986, 1987). In addition to the rhythmic part of the modulation a general reflex depression also occurs during locomotion, compared with free standing or sitting (Brooke et al 1991;Capaday and Stein 1986;Faist et al 1996;Morin et al 1982). In a recent paper, we demonstrated that the rhythmic part of the modulation is already present in healthy children by the age of 6 yr, whereas the tonic depression of the reflexes during gait develops up to the age of 13 (Hodapp et al 2007).…”

“…Reflexes were obtained during 8 different phases of the step cycle. At least 12 reflexes were elicited in every phase in a randomized sequence (for details, see Faist et al 1996). The size of each H-reflex was measured as a peak-to-peak amplitude of the nonrectified EMG trace, and the mean H-reflex was calculated for each of the eight phases of the step cycle investigated.…”