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Results for “Recruitment, Neurophysiological”

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PubMed1998

Summation of elementary phonomyograms during isometric twitches in humans.

To study its summation principle, the phonomyogram (PMG) from the first interosseus dorsalis muscle was recorded in five subjects during single twitches evoked by electrical stimulation over the motor point. By increasing the current pulse from threshold to maximal intensity, PMG amplitude increased linearly with motor unit recruitment. The twitch amplitude-intensity relationship was also linear. The PMG amplitude was therefore linearly related to the external force. For all these relationships highly significant correlation coefficients were found. These relationships were interpreted as being a consequence of an orderly recruitment, although, contrary to what happens during voluntary contraction, the largest and strongest motor units were recruited before the smallest and weakest ones during axon electrical stimulation. The PMG onset always preceded twitch onsets as indicated by latency measurements [mean 3.2 (SD 1.3) ms versus 11.5 (SD 3.9) ms, respectively]. Moreover, PMG and twitch latencies may have been significantly reduced by recruitment, suggesting that orderly recruitment influenced both PMG amplitude and occurrence. These results were interpreted as being the result of the summation of elementary PMG from every contracting motor unit and the stiffness change of the muscle medium occurring with recruitment.

Acoustics↗
PubMed1998

Effects of repetitive dynamic contractions upon electromechanical delay.

The effect of repeated maximal effort isotonic contractions on electromechanical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow flexion trials. The kinematics and surface electromyographic (EMG) activity of the biceps brachii of these subjects were recorded. The period from the onset of the EMG until the beginning of movement was defined as the electromechanical delay. The period from the beginning of movement until the end of the EMG was defined as the second component of the contraction. Over the 4 day period there was an increase in the speed of limb movement. The mean power frequency and the duration of the EMG during the electromechanical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction remained stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.

Adult↗
PubMed2003

A model of the feline medial gastrocnemius motoneuron-muscle system subjected to recurrent inhibition.

Recurrent inhibition in the mammalian spinal cord is complex, and its functions are not yet well understood. Skeletomotoneurons (alpha-MNs) excite, via recurrent axon collaterals, inhibitory Renshaw cells (RCs), which in turn inhibit alpha-MNs and other neurons. The anatomical and functional structure of the recurrent inhibitory network is nonhomogeneous, and the gain and filtering characteristics of RCs are modulated by inputs circumventing alpha-MNs. This complex organization is likely to play important roles for the discharge and recruitment properties of alpha-MNs. Modeling this system is a way of investigating hypothesized roles for normal functioning including muscle fatigue and different forms of physiological pathological tremor. In this paper, a detailed model including alpha-MNs, RCs, and the muscle fibers innervated by the alpha-MNs is presented. Outlines of the experimental data underlying the model and the modeling philosophy and procedure are presented. Then the behavior of a RC model is compared with experimental data reported in the literature. Model and experimental data agree well for burst responses elicited by synchronous single-pulse activation of different numbers of motor axons. In addition, the static relation between motor-axon activation rate and RC firing rate agree fairly well in model and experiment, and the same applies to the dynamic responses to step changes in motor-axon rate. The ultimate objective is to use this model in probing the role of recurrent inhibition in the control and stability of (isometric) muscular force under normal and altered conditions occurring during fatigue and muscle pain.

Animals↗
PubMed2006

Electrode array for reversing the recruitment order of peripheral nerve stimulation: experimental studies.

One of the most challenging problems in peripheral nerve stimulation is the ability to activate selectively small axons without large ones. Electrical stimulation of peripheral nerve activates large diameter fibers before small ones. Currently available techniques for selective activation of small axons without large ones require long-duration stimulation pulses (>500 micros) and large stimulation amplitude, which shorten battery life of the implanted stimulator and could lead to electrode corrosion. In the current study, the hypothesis that small axons can be recruited before large ones with narrow pulse width (50 micros) using an electrode array was tested in both simulations simulation and experiments in the cat lateral gastrocnemius (LG) model. The LG nerve innervates both LG and soleus muscle groups with axons within 10-13 and 8-12 microm diameter ranges, respectively. A finite element model of LG nerve was constructed and simulations showed that, when activating 40% of LG, a conventional tripolar electrode activated only 9% of soleus whereas the electrode arrays of 5, 7, and 11 contacts activated 39, 46, and 60% of soleus respectively, suggesting that the arrays could activate small axons before fully recruiting large axons. In animal experiments, peak twitch force of LG and soleus were plotted as a function of stimulation amplitude to indicate the recruitment curve. At 40% activation of LG, a conventional tripolar electrode activated only 7% of soleus whereas the electrode arrays of 5, 7, and 11 contacts activated 43, 48, and 72% of soleus respectively. The electrode arrays also decreased significantly the recruitment curve slopes to only 10-20% of the value obtained for the tripolar electrode in both computer simulations and experiments. In conclusion, the 5-, 7-, and 11-contact arrays can be used to reverse the recruitment order of peripheral nerve stimulation with a narrow pulse.

Animals↗
PubMed1975

Relationship of neuronal discharges in the precentral gyrus of monkeys to the performance of arm movements.

Recordings have been made from 162 pyramidal tract neurones which discharged bursts of nerve impulses in characteristic temporal association with performances of a stereotyped motor task by monkeys. Clinical evaluation of the relationship between discharges of the neurones and free movement led to the view that each cell's firing was associated with a characteristic aspect of movement performance and the contraction of a particular group of muscles. Quantitative evaluation of these relationships led to the conclusion that coding of the recruitment of motor units to the movement task could have been conferred by the number of pyramidal tract neurones discharging to those motoneurone targets. A ramp of "recruitment" of pyramidal tract neurones preceded the development of a ramp of force by about 100 msec. This general conclusion was supported by the observations made in a single animal in which orderly discharge of precentral neurones in relation to a sterotyped movement performance was clearly evident.

Animals↗
PubMed1975

Recruitment and firing rate modulation of motor unit tension in a small muscle of the cat's foot.

Maintained contractions were elicited in the first deep lumbrical muscle of the cat's foot by electrical stimulation of the contralateral motor cortex or, reflexly, by pinching of the foot pad. The discharges of all significant motor units of the muscle were monitored by electromyography, and contractions of the various motor units were observed in isometric recordings of muscle tension. Over a wide range, muscle tension could be enhanced by an increased intensity of pad pinching or cortical stimulation. This increase in muscle tension was caused by a recruitment of new motor units as well as by an increase in the firing rate of already active motor units. The latter mechanism was clearly of great importance. Pad pinching or cortical stimulation could sometimes cause the muscle to produce a tension close to that of a maximum tetanic contraction. This was several times greater than the mean tension that would have been caused by motor unit recruitment alone (i.e. by the motor units firing at their minimum steady rate). Cortical stimulation as well as pad pinching commonly recruited weak units more easily than stronger ones of the same muscle. The recruitment order obtained in response to pad pinching often differed, however, in various details from the recruitment caused by cortical stimulation.

Animals↗
PubMed1976

Brbiturate spindle activity in the association cortex of the cat and its relation to spindle activity in the somatosensory system.

(1) Spontaneous barbiturate spindle activity was recorded simultaneously from nucleus ventralis poster-lateralis (VPL) of the thalamus, from the corresponding projection site in the primary somatosensory cortex (SI) and from the lateral (LAT) and middle suprasylvian (SSM) gyri. (2) Barbiturate spindles occurred more frequently in LAT and SSM than in the somatosensory cortex. Spindles in LAT and SSM were of longer duration than SI spindles while the intraspindle frequency was higher in spindles from the latter areas. The amplitude of the spindle waves was significantly higher in LAT and SSM than in SI. The largest spindle waves were recorded from the anterior part of LAT and SSM. (3) Barbiturate spindles recorded at various interelectrode distances were cross-correlated. In LAT and SSM the cross-correlation coefficients remained high up to an interelectrode distance of 4-5 mm while spindles recorded at this distance in SI gave low coefficients when cross-correlated. (4) Extensive removal of the somatosensory cortex had no effect on spindle formation in LAT or SSM. Furthermore, cutting of the cortico-cortical connections between two recording sites in either of these gyri produced no reduction in spindle wave synchrony between the recording sites. (5) Spindles in the VPL-SI only rarely started simultaneously with spindles in LAT and SSM. Those few coincident spindles which occurred were cross-correlated. The cross-correlation coefficients were invariably low suggesting that no wave synchrony existed between spindles in the VPL-SI system and those in the LAT and SSM.

Animals↗
PubMed1977

The mechanical properties of human motor units with special reference to their fatiguability and recruitment threshold.

The recruitment thresholds, twitch tension, twitch contraction time and fatiguability of human motor units have been studied in the first dorsal interosseous of the hand. Units recruited at contraction strengths less than 50 g had relatively low twitch tensions (0.18-1.9 g), long contraction times (59-146 msec) and were non-fatiguable. Units recruited at higher contraction strengths (less than 200 g) had higher twitch tensions (15-26g), faster contraction times (33-57 msec) and were highly fatiguable. It is concluded that during graded voluntary muscle contractions motor units are recruited in order of increasing contraction strength and diminishing fatigue resistance.

Adolescent↗
PubMed1978

Saturating summation of the afterhyperpolarization conductance in spinal motoneurones: a mechanism for 'secondary range' repetitive firing.

Summation of the potassium conductance (GK) changes underlying the spike afterhyperpolarization (AHP) has been studied in cat spinal motoneurones. Cells were directly activated by one to five short current pulses at constant rate, each evoking an action potential. The analysis was restricted to cells displaying an approximately exponential decay of the AHP conductance. In these neurones the AHP conductances given by successive spikes were found to summate in a non-linear manner. This nonlinear summation seemed well described by a neurone model based on modified Hodgkin-Huxley equations. From the model equations the total AHP conductance in motoneurones could be calculated from values of GK measured experimentally at different times during the summation process. Adaptation and steady-state firing in motoneurones are assumed to be governed by summation of AHP conductance. The same model was then utilized for simulating neuronal repetitive firing in response to current steps. Such simulations were performed after substitution of the model parameters with values measured in individual motoneurones which had also been fired repetitively by intracellular injection of long-lasting current steps. The amount of adaptation and the shape and slopes of the steady-state frequency-to-current relation were found to coincide in the model and in the corresponding motoneurones.

Action Potentials↗