Experimental data of the motor unit interspike intervals variability of the Soleus muscle

Leonardo Abdala Elias
Escola de Engenharia El?trica e de Computa?

Marcus Fraga Vieira
Laborat?rio de Biomec?nica - FEF/UFG

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     Last modified: February 28, 2007

Abstract
When the motoneuron discharges with a steady mean frequency, the information is probably transmitted in the interspike intervals. However, there is an intrinsic variability on the nervous system, which is also verified in interspike intervals. This variability is an important physiological mechanism, because it is capable of establishing an accurate relationship between the receptor and its stimulus. Besides, a variable and noisily output contributes to a softer mechanical response.
In vertebrates, it can be observed that the main sources of variability are the synaptic input variations in motoneurons (synaptic noise), since this element should integrate a great quantity of postsynaptic potentials in the time and space in your synaptic sites (Calvin and Stevens, 1968).
In this study, we collected experimental data concerning the motor unit interspike intervals variability of Soleus muscle in humans. Therefor, an experimental protocol was developed: data was acquired in three individuals (men only), without neuromuscular disorder and who aged between 21 and 45 years, after they have signed the consent term. The subjects have remained sat down on a rigid bank with adjustable height, with his hip and knee bent 90?, his feet completely on the ground, and his ankle in neutral position. The electrodes were disposed as it is described on the previous article (Elias and Vieira, 2007). Volunteers had been instructed to keep in isometric plantar flexion, in order to produce clear discharges from a single motor unit. After verbal command, acquisitions were made for 2 minutes, and the electromyographic signals were supplied as feedback to the volunteers.
The experimental data were processed in Matlab? and the histograms and statistics of the interspike intervals were calculated (mean, standard deviation, coefficient of variation, kurtosis and skewness).
In all cases, 20 an 40 seconds acquisitions were made, which is enough time to discharge a single motor unit. However, in these periods, there were never less than 120 samples, which is enough to infer on the intended mechanism.
The motor units presented firing rate of 6.45?0.868 imp/s (mean?sd), presenting slow motor units (type S). The histograms already presented a leptocurtic characteristic (kurtosis ? 19.165?21.47) and positive deviation (skewness ? 2.693?2.079), similar to those reported in the experimental literature related to muscles whose motoneurons pool is located at the lumbar area. (Poliakov et al., 1995; Vieira, 2002).
The coefficients of variation were located in a range between 0.086 and 0.255, but the highest value was between 0.130 e 0.160.