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At least 235 records · Page 13Linked to original sources
PubMed1991

Discharge variability and physiological properties of human masseter motor units.

Intramuscular electrodes were used to study discharge variability in motor units of human masseter whose physiological properties were determined using spike-triggered averaging. Subjects voluntarily controlled the mean firing rate of a selected motor unit at 10 Hz for 15 min of continuous activation. Discharge variability was assessed at the beginning and end of this period. In 81% of units, the discharge variability at a mean interspike interval (ISI) of 100 ms increased after 15 min of continuous activity. There was a wide range of discharge variability within the population of masseter units studied, but no significant correlations were found between initial discharge variability and recruitment threshold, twitch tension or time-to-peak tension (TTP). There was, however, a significant correlation between motor unit fatigability and its initial discharge variability. This represents a link between the motoneuron and the functional properties of the muscle fibers in innervates.

Adolescent↗
PubMed1992

Extraocular motor units: type classification and motoneuron stimulation frequency-muscle unit force relationships.

Extracellular and intracellular techniques were used to study single motor units of the abducens nucleus and lateral rectus muscle in the cat. Using a combination of two motor unit properties, the fusion frequency and an index of fatigability, the population of twitch motor units could be separated into 4 subgroups: fast fatigable (FF), fast fatigue resistant (FR), slow fatigable (SF) and slow fatigue resistant (S). Nontwitch motor units, a fifth subgroup (NT), formed 10% of the total studied population. The twitch tension and the maximum tetanic tension of the FF motor unit type were significantly stronger than all other motor unit types. The use of frequency varying stimulation patterns did not further differentiate the motor unit types. The relation between a series of single motoneuron stimulation frequencies and the resultant single muscle unit forces generated a slope defined as a motor unit's kt value. Motor units with low kt values had higher twitch tensions, higher maximum tetanic tensions, higher fusion frequencies and lower fatigue indices than motor units with high kt values. Motoneuron recruitment was tested by electrical stimulation of the medial rectus subdivision of the contralateral oculomotor nucleus. No correlations were seen between recruitment order and the mechanical parameters of the single abducens motor units.

Animals↗
PubMed1993

Variation in firing order of human soleus motoneurons during voluntary and reflex activation.

The activation of motoneurons in a muscle pool is said to proceed as an ordered array in response to both dorsal root stimulation and voluntary activation, with small motoneurons being recruited before larger ones. We have examined 19 voluntarily recruited soleus motor units in 5 normal subjects and found that in 18 cases, the lowest threshold motor unit recruited by slowly increasing 'tonic' voluntary activity was different from the lowest threshold unit recruited by electrical stimulation of the tibial nerve in the popliteal fossa (phasic Hoffman reflex). The initially recruited voluntary units were, however, part of the pool influenced by the stimulated afferents because, during tonic activation, the timing of their discharge could be shown to be altered by electrical stimulation at a lower intensity than that required for H recruitment at rest. These findings suggest that the pool of soleus motoneurons responding to the voluntary command "tonically plantar flex your ankle" differs somewhat, in order of activation, from the pool responding to phasic stimulation of the largest diameter fibers in the tibial nerve, perhaps because of inhomogeneities in the distribution of descending or segmental inputs to the soleus motoneuron pool. Whether this partitioning is functional or a reflection of minor, random variations in synaptic density remains to be determined.

Action Potentials↗
PubMed1993

Recruitment of inhibition by enhanced activation of synaptic NMDA responses in the rat cerebral cortex.

Intracellular recordings of layer V neurons from rat neocortical slices were obtained to examine the effects of reducing extracellular magnesium on inhibition. Magnesium-free solutions induced interictal and ictal-like events in cortical neurons. Changes in synaptic events underlying epileptogenesis were studied when extracellular calcium was raised (from 2 to 3-7 mM) since this delayed seizure activity. With increasing time of exposure of cells to magnesium-free solutions, there was a significant increase in the size and duration of both the depolarizing and slow synaptic hyperpolarizing responses, but the fast synaptic hyperpolarization significantly declined in amplitude. When cells were recorded with cesium acetate-filled microelectrodes slow hyperpolarizing responses were blocked, but depolarization of cells to 0 mV allowed an isolated fast hyperpolarizing response to be recorded following synaptic stimulation. The amplitude of this response was unchanged after exposure to magnesium-free solutions. Synaptic responses of cells initially bathed in an N-methyl-D-aspartate (NMDA) antagonist (CPP) were unchanged by subsequent exposure to magnesium-free solutions. CPP exposure by itself caused a decrease in depolarization duration, increase in fast hyperpolarizing amplitude, and decrease in slow hyperpolarization amplitude and duration. When the fast hyperpolarization was viewed in isolation (cesium recording electrodes) at 0 mV, the amplitude of this event was unchanged by exposure to CPP. Given these results stimulus-response characteristics of neocortical neurons were reassessed under control conditions. With higher intensity stimuli larger depolarizing and slow hyperpolarizing responses were evoked, but the fast hyperpolarization showed a decremental response. These effects were reversed when CPP was added. When NMDA activity was enhanced by exposure to magnesium-free solutions or electrical stimulation, the amplitude of excitatory events and slow hyperpolarizations increased, but fast inhibitory responses showed limited capacity for incremental recruitment. This suggests fast inhibition is saturated (maximal) at submaximal levels of excitation, and can be overcome by increasing levels of excitation. Such a process is active under physiological conditions, altering the efficacy of inhibition.

Animals↗
PubMed1993

Recruitment of motor units in human flexor carpi ulnaris.

In 1949, Denny-Brown reported that the motoneuron pool of the human flexor carpi ulnaris (FCU) muscle was fractionated into subpopulations, each subpopulation being activated during a different voluntary motor task. The following report presents data on motor unit recruitment in the human FCU muscle for the tasks of isometric flexion and ulnar deviation, cocontraction of the forearm muscles and non-isometric flexion of the wrist. These observations show that every FCU motor unit tested reliably, contributed to all four tasks, that is, no separate subpopulations were observed for any of the contractions tested. Furthermore, the order of recruitment was the same for all four tasks.

Action Potentials↗
PubMed1996

Human motor unit activity during post-vibratory and imitative voluntary muscle contractions.

Applying mechanical vibration for short periods to a muscle tendon induces long-lasting involuntary contractions which develop soon after the vibration offset in the previously vibrated muscle. In the present study, the question was raised as to whether these post-vibratory motor responses are mediated by the activity of supraspinal neural population or whether they may involve in addition some peripheral facilitatory influences operating at the motoneuronal level. To investigate this question, we analysed the unitary activity of 48 motor units belonging to the wrist extensor radialis muscles of the human hand, after attempting to classify them as slow or fast, during both post-vibratory and voluntary contractions having almost the same amplitudes and time-courses. The motor units were found to be activated in much the same way with both types of contraction. Similarities were observed as regards: the nature of the motor units activated, the order of recruitment of the motor unit population, the motor units' force recruitment thresholds, the mean interspike interval and the standard deviation. These analogies suggest that post-vibratory contraction may mainly involve a supraspinal tonic drive, but the possibility that these involuntary contractions may have a spinal origin cannot be completely ruled out.

Action Potentials↗
PubMed1975

Regulation of unit activity in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex.

Recruiting responses and related synchronous activities appear to be mediated by thalamic inhibition originating in nucleus reticularis thalami, a structure jointly regulated by an ascending projection from mesencephalic reticular formation and a descending influence from the frontal cortex. Extracellular unit activity was recorded in the anterior nucleus reticularis thalami (RVA) during recruiting responses, augmenting responses, stimulation of the mesencephalic reticular formation (MRF), and cryogenic blockade of the inferior thalamic peduncle (ITP). During recruiting responses, RVA units responded to medial thalamic (MT) stimulation with prolonged high frequency bursts. Analysis of the post-stimulus time histograms of these responses showed tham to have the same latency, duration, incrementing character, and envelope shape as the phasic thalamic inhibitory postsynaptic potentials (IPSPs) which appear to mediate recruiting responses. Brief stimulation of the MRF, which abolishes recruiting responses and thalamic IPSPs, prevented the response of RVA units to MT stimuli, and inhibited the spontaneous discharge of these units for 20 sec or more. We propose that the desynchronizing effect of MRF activation results from the abolition of thalamic inhibition originating in RVA. Cryogenic blockade of the ITP, which abolishes recruiting responses in the thalamus and cortex, also prevented R units from responding to MT stimuli. This result suggests that the MT activates R units via a thalamo-frontocortico-R pathway and explains the long latency of R bursts and thalamic IPSPs following MT stimulation. R cells that fired prolonged bursts during recruiting responses did not respond during augmenting responses. This result suggests that separate thalamic inhibitory mechanisms are involved in these two types of synchronization.

Animals↗
PubMed1975

Post-activation potentiation and the kindling phenomenon.

Potentials evoked in the hippocampus and preoptic region of rats by single biphasic pulses applied to the amygdala were compared during recruiting, after post-tetanic potentiation (PTP) and after amygdaloid kindling. The same components were enhanced temporarily by recruiting and PTP as were enhanced permanently by kindling. Trains of tetanic stimulation with parameters which partially mimicked the cellular discharge parameters during an amygdaloid afterdischarge (AD) were applied to the amygdala at a frequency of 1 per 5 sec continuously for 2.5 h or for 15 min a day for 10 days to produce a total of 1800 trains. Amygdaloid kindling rates were then measured and compared with control groups. Animals pretreated with tetanic stimulation required significantly fewer ADs to develop maximal seizures. Further experiments showed that tetanic stimulation, but not recruiting stimulation (10 c/sec), low frequency stimulation (1 c/sec), or handling, would produce a permanent change in potentials evoked in secondary sites by single pulses applied to the amygdala. This change in evoked potential amplitude was significant but smaller than that produced by kindling. Also tetanic stimulation, but not recruiting or single pulses, facilitated subsequent kindling.

Amygdala↗
PubMed1975

Effects of procaine hydrochloride, diazepam, and diphenylhydantoin on seizure development in cortical and subcortical structures in rats.

Procaine HCl and diphenylhydantoin (DPH) increased the duration and propagation of epileptiform afterdischarges (ADs) produced by electrical stimulation of the amygdala in rats. Procaine and DPH also increased the rate of seizure development (kindling) produced by repeated stimulation of the amygdala. Procaine and to a limited extentDPH would themselves act as convulsants in well kindled subjects. Diazepam, on the other hand, retarded or blocked amygdaloid kindling. Diazepam trigered a high frequency (20-30 c/sec) rhtthm in the amygdala, hippocampus and preoptic area. None of these drugs had any significant effect on potentials evoked in secondary limbic sites by single electrical pulses applied to the amygdala. Also, none of these drugs had any effect on recruiting or post-tetanic potentiation (PTP) in secondary sites produced by amygdala stimulation and none of the drugs had any effect on amygdaloid AD thresholds. The effects of these drugs on the responses evoked by anterior neocortex stimulation were quite different. Diazepam had no effect on any of the characteristics of the discharge or convulsion even at twice the dose levels used for the amygdala group. Procaine and DPH, however, blocked not only the eonvulsion but the AD as well. Eighty percent of the procaine- and DPH-treated rats failed to respond with neocortical AD even at current levels as high as 2000 muA. The few cortically stimulated subjects that did respond with an AD showed a subcortical rather than a neocortical seizure response. DPH had no effect on recruiting or PTP of the transcallosal response. Both procaine and DPH produced a weak but significant increase in the amplitude of the transcallosal evoked potential, while diazepam produce a weak decrement in that response.

Amygdala↗
PubMed1976

Regulation of slow potential shifts in nucleus reticularis thalami by the mesencephalic reticular formation and the frontal granular cortex.

Novel stimuli or electric stimulation of the mesencephalic reticular formation (MRF) produced large positive slow potentials (SPs) in rostral nucleus reticularis thalami (RVA) that accompanied the negative SPs known to occur in frontal cortex. SP durations (20-30 sec) were similar to the periods of unit inhibition that occur in RVA following MRF stimulation. Trains of 8 c/sec medial thalamic stimuli produced phasic negative SPs in RVA similar in duration to the intervals of unit excitation that follow each stimulus pulse. These results suggest that the polarity and duration of the SPs in RVA reflect changes in excitation of the underlying neurons. Direct activation of a specific region of RVA produced complete inhibition of visual cortex responses evoked by optic tract stimuli, a finding which suggests that RVA has an inhibitory action on the thalamus. A tone reinforced by electric shock also elicited SPs in frontal cortex (negative) and RVA (positive). In contrast to the long duration of the MRF- or novelty-elicited SPs, the durations of the conditioned SPs were phasic and were regulated by the tone--shock interval. Bilateral cryogenic blockade of the interconnections between the frontal cortex and medial thalamus abolished SPs of all origins in the frontal cortex. The blockade also abolished conditioned SPs in RVA, but did not affect the MRF-elicited ones. Thus, the subcortical SPs that accompany orienting to novel stimuli are distinct from those which occur during the higher cognitive process of conditioned expectancy and require the integrity of the mediothalamic-frontocortical system.

Animals↗
PubMed1978

Digital memory recorder in electromyography and nerve conduction studies.

Specifications are given: (i) for a 3-channel digital memory that, when connected to an inkjet writer, allows action potentials from muscle and nerve to be recorded without distortion (upper limiting frequency 10,000 Hz); (ii) for a 14-channel digital memory connected to a 14-channel inkjet writer for the measurement of the territory of the motor units by recording simultaneously from 14 leads of a multi-electrode spaced over the cross-section of the muscle; (iii) for a digital circuit that rejects signals that exceed a given amplitude and duration and interfere with electronic averaging of potentials from sensory nerve less than 1 muV in amplitude.

Electromyography↗
PubMed1979

Hyperbaric hyperreflexia: tendon-jerk and Hoffmann reflexes in man at 43 bars.

Tendon jerk (TJR) and Hoffmann (H) reflexes of the soleus muscle were studied in two men during a 26-day simulated oxygenhelium dive to a maximum pressure of 43 bars. The amplitude of the TJR response was observed to increase markedly after compression and also at the end of decompression. This biphasic pattern of enhanced reflectivity was reproduced by synchronised but much smaller variations in H reflex response. The positive facilitatory effects of applying Jenkdrassik's manoeuvre were reduced and the negative effects increased during hyperbaric exposure. Excitability cycles (twin-pulses methods) revealed modifications in one subject similar to those observed in clinical hyperreflexia, namely a shortening of Phase III and an enhancement of Phase IV. The other subject exhibited a notable depression of Phase IV. Abnormal EMG recordings are described including randomly-triggered slow wave potentials with no mechanical effects, and fixed, long latency late responses of a reflex nature with a definite mechanical effect. The contention that alterations in vestibulo-spinal relations may result in the release from tonic presynaptic inhibitory control of myotatic reflex arc excitability is discussed as a partial explanation for these and related findings.

Adult↗
PubMed1980

Intracortical origin of recruiting responses in the cat cortex.

Field potentials associated with recruiting responses produced by low-frequency stimulation of non-specific thalamic sites were simultaneously recorded from an intracortical tri-probe and the pial surface. Separation of the tri-probe tips along the probe axis (typically 100 mum) permitted discrimination of spatial field potentials. Analyses were performed on the data to provide estimates of the intracortical locations of the ion sources and sinks giving rise to the surface negative recruiting response. At the peak of a recruiting response event, the results suggest a sink in Brodmann layer II simultaneous with a source in layers III, IV and V. A hypothesis of a simultaneous 'push-pull' spatial modulation of pyramidal cell activity in layer II with the activity of pyramidal cells in layers IV, V and VI is proposed.

Animals↗