An EMG study of functional cortico-motoneuronal connections in humans.

We set out to decompose the EMG signal into its constituent motor unit action potential components to track motor unit firing rates with a high degree of accuracy and extract their average firing rate. We were able to show that this average firing rate tracks the subject's force trajectory from beginning to end. We propose that this average firing rate is a volitional control signal pointing to the existence of a 'volitional unit'. This volitional unit has to do with the projection of a group of functionally related cortico-motoneurons on a group of spinal motoneurons in the motoneuronal pool of a muscle. Our study of motor unit firing patterns during their steady state showed that spinal motoneurons respond to a descending central input in a Gaussian manner. We have further shown that the central drive itself, as represented by the average firing rate of the active motor units, also displays a Gaussian firing behavior. We have also described the existence of a 'translation factor', highly correlated to the motor unit size, which is unique to each spinal motoneuron and determines the motoneuronal response, and its resulting firing rate, to the descending inputs. As for force generation, we have shown that expressing the twitch force of a motor unit in a dynamic fashion using the 'electrotwitch' concept of firing rate x macro area, approximates motor unit force output better and accounts for firing rate related force changes more effectively than force estimates based on the mechanical twitch.

The volitional unit: a functional concept in cortico-motoneuronal connections in humans.

We used the EMG precision decomposition technique to resolve complex EMG signals and derive information about the firing times of a family of motor units (MUs) and the force they produce. The active units shared a common behavior among their firing rates, a concept described by DeLuca et al. and termed the 'common drive'. This 'common' behavior was extracted as the average of the firing rates of MUs and found to track the subject's force trajectory. In this paper, we propose the existence of functional cortico-motoneuronal connections which provide for a large number of combinations between affector cortical motoneurons (CMNs) and effector spinal motoneurons (SMNs) for the generation of a purposeful movement. We argue that these connections provide the essential link between volition and movement and function as a 'volitional unit' which consists of the CMNs, the SMNs and the anatomical and interneuronal connections between them.

Correlates of motor unit size, recruitment threshold, and H-reflex jitter.

The aim of this research was to study the neurophysiology of the anterior horn cell (AHC) using single-fiber EMG (SFEMG) study of the flexor carpi radialis (FCR) H-reflex. Twelve men and 7 women, ages 20-80 years, were studied. The mean H-jitter was 138 +/- 59 microseconds. H-jitter increased with age (while the M-jitter did not) and was greater in men than in women. There was a direct correlation between the H-jitter and H-latency which was used as an indirect measure of the AHC's size. Given that small AHCs have a higher input resistance than large ones, the H-jitter can be used as an indirect indicator of the AHC's input resistance and therefore its size. When subjects fell asleep, the H-jitter increased over tenfold the baseline value, raising the possibility of an alternative, oligosynaptic pathway. H-reflex jitter studies provide a useful clinical neurophysiological tool for the study of AHC physiology.

The use of multiple Tinel's sign in the identification of patients with peripheral neuropathy.

We have found that the presence of a Tinel's sign is often pervasive in subjects with peripheral neuropathy and/or risk factors for developing it. There is a paucity of research dealing with the usefulness of Tinel's sign as it relates to peripheral neuropathy (PN). The following investigation was carried out to determine if multiple Tinel's sign or MTS can be used to identify patients with an underlying PN or with risk factors for developing a PN. A total of 102 subjects were enrolled in the study prospectively. We found MTS to have a sensitivity of 79% with a specificity of 42% in detecting peripheral neuropathy and a sensitivity of 76% with a specificity of 44% in determining the presence of risk factors for developing a peripheral neuropathy. We conclude that MTS can be a fairly sensitive indicator of PN or risk factors.