CMT 1A: Fast responses to stepping on an unexpected surface height depend on int

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J Neurophysiol. 2009 Sep;102(3):1684-98.

Fast responses to stepping on an unexpected surface height depend on intact

large-diameter nerve fibers: a study on Charcot-Marie-Tooth type 1A disease.

van der Linden MH, de Kam D, van Engelen BG, Hendricks HT, Duysens J.

Department of Rehabilitation, 898, Radboud University Nijmegen Medical Centre,

P.O. Box 9101, 6500 HB, Nijmegen, The Netherlands.

The contribution of reflexes from the large myelinated afferents in the control

of normal and perturbed gait in humans is a highly debated issue. One way to

investigate this topic is by studying normal and perturbed gait in patients

lacking large myelinated fibers in the distal limb (Charcot-Marie-Tooth [CMT]

type 1A disease).

Such patients should have delayed and decreased reflexes if the latter depend on

these large myelinated fibers.

To elicit the reflexes, both patients and controls had to step on a platform

that was either at the same level or lowered by 5 cm. In control subjects,

landing on a level surface induced short-latency responses in the biceps femoris

and tibialis anterior muscles, whereas such responses were largely absent in the

patients.

Similarly, stepping down unexpectedly induced a very fast muscle synergy,

leading to a brake of the forward propulsion in the controls, which was

significantly reduced and delayed (on average 32 ms) in the patients. The

observed changes correlated with both sensory and motor deficits.

Nevertheless, it is concluded that the results are primarily related to the

sensory deficits, since the delayed or absent responses appeared in both upper

and lower leg muscles, whereas only the latter showed motor deficits.

The data are taken as evidence that large-diameter afferents from the distal leg

are essential for fast reflex activations induced by stepping on a level or

lowered surface unexpectedly.