Transmission in a locomotor-related group Ib pathway from hindlimb extensor muscles in the cat

Abstract

It has been previously shown that phasic stimulation of group I afferents from ankle and knee extensor muscles may entrain and/or reset the intrinsic locomotor rhythm; these afferents are thus acting on motoneurones through the spinal rhythm generators. It was also concluded that the major part of these effects originates from Golgi tendon organ Ib afferents. Transmission in this pathway to lumbar motoneurones has now been investigated during fictive locomotion in spinal cats injected with nialamide and L-DOPA, and in decerebrate cats with stimulation of the mesencephalic locomotor region. In spinal cats injected with nialamide and L-DOPA, it was possible to evoke long-latency, long-lasting reflexes upon stimulation of high threshold afferents before spontaneous fictive locomotion commenced. During that period, stimulation of ankle and knee extensor group I afferents evoked oligosynaptic excitation of extensor motoneurones, rather than the "classical" Ib inhibition. Furthermore, a premotoneuronal convergence (spatial facilitation) between this group I excitation and the crossed extensor reflex was established. During fictive locomotion, in both preparations, the transmissions in these groups I pathway was phasically modulated within the step cycle. During the flexor phase, the group I input cut the depolarised (active) phase in flexor motoneurones and evoked EPSPs in extensor motoneurones; during the extensor phase the group I input evoked smaller EPSPs in extensor motoneurones and had virtually no effect on flexor motoneurones. The above results suggest that the group I input from extensor muscles is transmitted through the spinal rhythm generator and more particularly, through the extensor "half-centre". The locomotor-related group I excitation had a central latency of 3.5-4.0 ms. The excitation from ankle extensors to ankle extensors remained after a spinal transection at the caudal part of L6 segment; the interneurones must therefore be located in the L7 and S1 spinal segments. Candidate interneurones for mediating these actions were recorded extracellularly in lamina VII of the 7th lumbar segment. Responses to different peripheral nerve stimulation (high threshold afferents and group I afferents bilaterally) were in concordance with the convergence studies in motoneurones. The interneurones were rhythmically active in the appropriate phases of the fictive locomotor cycle, as predicted by their response patterns. The synaptic input to, and the projection of these candidate interneurones must be fully identified before their possible role as components of the spinal locomotor network can be evaluated.

Postural performance in decerebrated rabbit

“…Presence of weak EMG responses to tilts and pushes, with the temporal pattern similar to that in control, supports the second hypothesis. To test these hypotheses, in the present study we stimulated tonically two brainstem areas, which are known to affect the postural system, the MLR and VTF [10,25,26], and examined postural performance by applying lateral tilts and lateral pushes to the posterior part of the body. It was found that MLR stimulation, during the period preceding the onset of locomotion, caused an increase of the extensor muscle tone, as well as an augmentation of the EMG responses to tilts and pushes.…”

Neural control of locomotion; Part 1: The central pattern generator from cats to humans

“…It is thought that l-DOPA mimics the monoaminergic transmitters, which are normally released by descending pathways during periods of locomotion and facilitate the interneurones involved in the late discharges. The latter neurones were thought to be part of the 'spinal half-centers', which Brown had introduced to explain the generation of rhythmic loco motor patterns [30,31], see above.…”

Strategies for delineating spinal locomotor rhythm-generating networks and the possible role of Hb9 interneurones in rhythmogenesis

“…However, neither the precise connectivity of locomotor-related spinal interneurones nor their intrinsic properties have been defined. That is, despite the demonstration that many ventral interneurones are rhythmically active during locomotor activity in the cat (Angel et al, 2005;Baev et al, 1979;Gossard et al, 1994;Huang et al, 2000;Jankowska et al, 1967a;Matsuyama et al, 2004;Shefchyk et al, 1990) and rodent (Butt and Kiehn, 2003;Butt et al, 2005;Kiehn et al, 1996;Tresch and Kiehn, 1999;Zhong et al, 2006) little is known about the connectivity of these neurones…”