Maturation of lower extremity EMG responses to postural perturbations: relationship of response-latencies to development of fastest central and peripheral efferents

Abstract

EMG responses to toe-up tilt perturbations on a movable platform system were analysed in 86 children between the age of 12 months and 13 years. To assess the relative contribution of peripheral and central nerve conduction properties, a concomitant recording of the fastest efferent pathways in the central and peripheral motor system was made using non-invasive transcranial magnetic stimulation of motor cortex and peripheral nerve roots. This allowed the determination of the fastest downstream efferent connection times from motor cortex to lumbar motor neuron pools and to measure the fastest efferent conduction from these motor neuron pools to effector muscles in the lower leg. The sequence observed for stance stabilizing EMG responses was similar to that obtained in earlier studies with short latency (SL) and middle latency (ML) companents occurring in the stretched triceps surae muscle and long latency (LL) responses occurring in the non-stretched tibialis anterior muscle. Homologous responses were also obtained in upper leg muscles, being recruited consistently later than those in lower leg muscles across all age groups. In the short latency range two different SL1- and SL2-responses were obtained in children of all age groups as well as in adult controls. Both the SL1- and the SL2-responses showed a flat developmental profile, reaching adult values between 20 and 30 months of age which correlated with that of the fastest efferents from lumbar motor neuron pools to leg muscles, i.e. the final motor path. ML-responses showed a steeper developmental profile.(ABSTRACT TRUNCATED AT 250 WORDS)

The significance of somatosensory stimulations to the human foot in the control of postural reflexes

“…Even though the ankle immobilizer was able to restrict, to a large degree, the joint motion, there was still a little movement possible (e.g., about 0.33° at 50 ms), which might not be sufficient to trigger the joint receptors (Simoneau 1992) but adequate to trigger the spindle afferents (Matthews and Stein 1969). In fact, this is in agreement with Diener et al (1984a), who believed that the SL reflexes are the results of monosegmental reflexes possibly mediated by fast spindle afferents, and with Müller et al (1991), who stated that the joint afferents are not related to the first phase response of the SL reflexes.…”

“…With respect to the ML responses, although they are believed to be the reflexes relayed through polysynaptic spinal pathways (Matthews 1984;Müller et al 1991), it is still not clear yet which particular set of receptors, afferents, or a segmental or supersegmental loop is responsible for the mediation of these reflexes (Müller et al 1991). In the present study, it was found, first, that the ML responses in the GAS muscle were significantly delayed when the plantar pressure became smaller while standing on softer surfaces.…”

A neurophysiological study of mirror movements in adults and children

“…Attractive as this argument may appear, our conclusion should be tempered by the fact that our inability to demonstrate interhemispheric inhibition by TMS is not necessarily evidence for lack of function. It is known that the threshold for excitation (and therefore accessibility) of the corticospinal projection by using TMS becomes lower as age increases 19,44 ; thus, it is possible that in a similar way the immature callosal system may simply not be easily accessible to external activation by TMS yet be operational functionally. Nevertheless, the inability to demonstrate significant interhemispheric inhibition in the children suggests that bilateral activation of both motor cortices might occur during unilateral tasks in children and thus remains an attractive hypothesis to explain their mirror movements.…”

Effect of a Single Dose of Diazepam on Balance Measures in Older People

“…Muscle responses to unexpected perturbations result in EMG responses at different latencies, including the short latency response (monosynaptic stretch reflex), medium latency response, and long latency response 28 . The medium latency responses are mediated via an automatic oligosynaptic spinal reflex 28–30 with supraspinal modulation but without conscious control. This automatic oligosynaptic reflex loop is faster than a voluntary, conscious (transcortical or long latency response) response but slower than a monosynaptic stretch reflex (short latency response).…”