M1 Peak Research Articles

Electrophysiological correlates of cognition improve with nap during sleep deprivation

The efficacy of a 30-min nap as a countermeasure in the reduction of cognitive decline following 24 h of sleep deprivation (SD) on subjective sleepiness scales, event-related potential (ERP) P300, and contingent negative variation (CNV) was evaluated. The experiment was performed in three sessions on different days between 7 and 8 a.m. on nine normal, healthy males, of age 25-30 years: Session 1. Baseline recordings; Session 2, after one night's total sleep deprivation, and; Session 3, after 1 week of Session 1, following one night's sleep deprivation along with a 30-min nap opportunity between 1.00 and 3.00 a.m. Subjective sleepiness scores increased after SD as compared to baseline, but reduced significantly after nap (P < 0.05). There was an increase in P3 peak latency of ERP following SD (16%, P < 0.01), which was reduced with nap (10.7%, P < 0.05).There was an increase in CNV M1 peak latency after SD (18%) which decreased with the use of nap (12.5%) (P < 0.01). The CNV reaction time increased following SD (39.3%) and decreased with the use of nap (24%) (P < 0.01). No significant effects on ERP N1, P1, N2 latencies, P2 and P3 amplitudes and CNV N1, P3, M2 peak latencies and M1, and M2 amplitudes were observed. It was concluded that a 30-min nap, between 1.00 and 3.00 a.m. during night SD, reduces the cognitive decline following 24 h of SD in terms of its electro-physiological correlates. The study is of applied value in optimization of cognitive performance in professions demanding night work schedules.

Small interfering RNA targeting decoy receptor 3 sensitizes colon cancer cells (SW480) to radiation treatment

AIM:To investigate whether small interfering RNA(siRNA)targeting decoy receptor 3(DcR3) can sensitize colon cancer cells(SW480)to radiation treatment and analyze the significance of DcR3 expression in the prediction of radiation resistance in colon cancer before radiotherapy. METHODS:After SW480 cells were irradiated with different doses of 137 Cs gamma rays for different durations,the expression levels of DcR3 were determined by enzyme-linked immunosorbent assay(ELISA).An expression vector expressing siRNA targeting DcR3 was then constructed.After the expression vector was trans-fected into irradiated and unirradiated SW480 cells,the colony formation ability of these cells was determined by colony formation assay,and cell apoptosis was analyzed by flow cytometry. RESULTS:Compared with unirradiated SW480 cells,the expression level of DcR3 increased in SW480 cells irradiated with different doses of 137 Cs gamma rays for different durations,especially remarkable in cells irradiated with 10 Gy 137 Cs gamma rays(P0.05).Compared with irradiated untransfected SW480 cells and unirradiated transfected SW480 cells,the colony formation was enhanced,the M1 peak in cell cycle was elevated and the number of apoptotic bodies increased in irradiated transfected SW480 cells. CONCLUSION:siRNA targeting DcR3 can sensitize colon cancer cells(SW480)to radiation treatment.High DcR3 expression may predict the development of radiation resistance in colon cancer.

Crustal response to continental collisions between the Tibet, Indian, South China and North China Blocks: geochronological constraints from the Songpan‐Garzê Orogenic Belt, western China

AbstractWe report SHRIMP U–Th–Pb monazite, conventional U–Pb titanite, Sm–Nd garnet and Rb–Sr muscovite and biotite ages for metamorphic rocks from the Danba Domal Metamorphic Terrane in the eastern Songpan‐Garzê Orogenic Belt (eastern Tibet Plateau). These ages are used to determine the timing of polyphase metamorphic events and the subsequent cooling history. The oldest U–Th–Pb monazite and Sm–Nd garnet ages constrain an early Barrovian metamorphism (M1) in the interval c. 204–190 Ma, coincident with extensive Indosinian granitic magmatism throughout the Songpan‐Garzê Orogenic Belt. A second, higher‐grade sillimanite‐grade metamorphic event (M2), recorded only in the northern part of the Danba terrane, was dated at c. 168–158 Ma by a combination of U–Th–Pb monazite and titanite and Sm–Nd garnet ages. It is suggested that M1 was a thermal event that affected the entire orogenic belt while M2 may represent a local thermal perturbation. Rb–Sr muscovite ages range from c. 138–100 Ma, whereas Rb–Sr biotite ages cluster at c. 34–24 Ma. These ages document regional cooling at rates of c. 2–3 °C Myr−1 following the M1 peak for most of the terrane. However, those parts of the terrane affected by the higher‐temperature M2 event (e.g. the migmatite zone) experienced initially more rapid (c. 8 °C Myr−1) cooling after peak M2 before joining the regional slow cooling path defined by the rest of the terrane at c. 138 Ma. Regional slow cooling between c. 138 and c. 30 Ma is thought to be the result of post‐tectonic isostatic uplift after extensive crustal thickening caused by collision of the South and North China Blocks. The clustering of biotite Rb–Sr ages marks the onset of rapid uplift across the entire terrane commencing at c. 30–20 Ma. This cooling history is shared with many other regions of the Tibet Plateau, suggesting that uplift of the Tibet Plateau (including the Songpan‐Garzê Orogenic Belt) occurred predominantly in the last c. 30 Myr as a response to the continuing northwards collision of India with Eurasia.

Magnetoencephalographic study of speed-dependent responses in apparent motion.

There have been only few studies of visually-evoked cortical responses to apparent motion as a function of stimulus speed. Most earlier findings on evoked peak magnitudes and latencies, utilizing various types of smooth and apparent motion stimuli, have demonstrated that greater spatial separation/speed resulted in enhanced peak magnitudes, decreasing onset latencies in individual extrastriate neurons and in shorter motor reaction times in subjects. However, some reports using partial-coverage magnetoencephalography stated that increasing the stimulus displacement actually triggered a substantial reduction of the evoked main peak latency while the magnitude showed no clear change. To resolve the issue of the dependency of evoked responses on stimulus speed in apparent motion, we presented moving bar stimuli to 6 subjects at velocities within a 100 fold range and investigated the ensuing evoked visual cortical activity using a whole-cortex magnetoencephalograph. The magnitude and the latency of the first major evoked peak M1 was measured and compared for 6 discrete bar-stimuli displacements in all subjects. Our results showed clearly that the M1 peak response magnitudes increased in a nonlinear way with higher apparent speeds (larger displacements), in compliance with the logarithmic Fechner law. We observed also that the fluctuations of the mean evoked M1 peak latency (140+/-10.6 ms) did not reach significance over the tested range of stimulus velocities. These findings probably reflect global motion processing mechanisms which rely on nonlinear speed-dependent feedback connectivity between striate and extrastriate visual cortex areas.

Oxygen isotope thermometry of quartz–Al2SiO5veins in high-grade metamorphic rocks on Naxos island (Greece)

Diffusion models predict that peak metamorphic temperatures are best recorded by the oxygen isotope fractionation between minerals in a bi-mineralic rock in which a refractory accessory mineral with slow oxygen diffusion rate is modally minor to a mineral with a faster diffusion rate. This premise is demonstrated for high-grade metamorphism on the island of Naxos, Greece, where quartz–kyanite oxygen isotope thermometry from veins in high-grade metamorphic pelites gives temperatures of 635–690 °C. These temperatures are in excellent agreement with independent thermometry for the regional M2 peak metamorphic conditions and show that the vein minerals isotopically equilibrated at the peak of metamorphism. Quartz–sillimanite fractionations in the same veins give similar temperatures (680±35 °C) and suggest that the veins grew near to the kyanite–sillimanite boundary, corresponding to pressures of 6.5 to 7.5 kbar for temperatures of 635–685 °C. By contrast, quartz–kyanite and quartz–biotite pairs in the host rocks yield lower temperature estimates than the veins (590–600 and 350–550 °C, respectively). These lower apparent temperatures are also predicted from calculations of diffusional resetting in the polyphase host-rock system. The data demonstrate that bimineralic vein assemblages can be used as accurate thermometers in high-temperature rocks whereas retrograde exchange remains a major problem in many polymineralic rocks.

Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)

On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock δ18OSMOW values of the metabasites show limited variations (10.4–12.0‰) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue- and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP–LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing AlIV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, AlIV and AlVI. Empirically calibrated thermobarometers allowed P–T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400–500°C/10–12 kbar was followed by a retrograde P–T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600°C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP–LT rocks escaped from an early erosion by a moderate second burial event and explains the long-lasting slow uplift with low average cooling rates.

Cerebral functional anatomy of voluntary contractions of ankle muscles in man.

1. Cerebral activation elicited by right-sided voluntary ankle muscle contraction was investigated by positron emission tomography measurements of regional cerebral blood flow. Two studies with eight subjects in each were carried out. Tonic isometric plantar and dorsiflexion and co-contraction of the antagonist muscles were investigated in study 1. Tonic contraction was compared with dynamic ramp-and-hold contractions in study 2. 2. All types of contraction elicited activation of the left primary motor cortex (M1). The distance between the M1 peak activation locations for tonic isometric dorsi- and plantar flexion was 17 mm. Co-contraction elicited activation of a larger area of M1 mainly located in between but partially overlapping the M1 areas activated during isolated dorsi-/plantar flexion. 3. A voxel-by-voxel correlation analysis corrected for subject covariance showed for dorsiflexion a significant correlation between tibialis anterior EMG level and cerebral blood flow activation in the cerebellum and the M1 of the medial frontal cortex. For plantar flexion a significant correlation was found between soleus EMG and cerebral activation in the left medial S1 and M1, left thalamus and right cerebellum. 4. The activation during dynamic isotonic and isometric dorsi- and plantar flexion was significantly more extensive than during tonic contractions. In addition to M1, activation was seen in the contralateral supplementary motor area and bilaterally in the premotor and parietal cortices. Isotonic and isometric contractions did not differ except in a small area in the primary somatosensory cortex. 5. One possible explanation of the different cerebral activation during co-contraction compared to that during plantar/dorsiflexion is that slightly different populations of cortical neurones are involved. The more extensive activation during dynamic compared with tonic contractions may reflect a larger cortical drive necessary to initiate and accelerate movements.

Soleus long-latency stretch reflexes during walking in healthy and spastic humans

The present study was carried out to investigate the long-latency soleus stretch reflexes M2 (peak latency of approximately 85 ms) and M3 (peak latency of approximately 115 ms) during walking in healthy and spastic multiple sclerosis (MS) patients. An 8° stretch was applied to the ankle extensors of the left leg in 8 healthy subjects during normal walking speed and 9 spastic MS patients and 10 age-matched healthy subjects during slow walking. When present in walking healthy subjects, M2 and M3 were modulated in a similar way and with the same amplitudes as previously described for the short latency soleus stretch reflex (M1). The spastic patients’ soleus M1 was significantly less modulated during walking. The patients’ M2 long-latency response was modulated in the same way as the age-matched healthy subjects. All patients’ M3 responses were absent or much suppressed during walking. The origin and functional importance of the short- and long-latency stretch reflexes in healthy and spastic persons are discussed in relation to the above findings and the behaviour of the stretch reflexes during matched isometric contractions. M3 is argued to be part of a transcortical reflex in healthy subjects.

The retrograde P–T–t path for low‐pressure granulites from the Reynolds Range, central Australia: petrological constraints and implications for low‐P/high‐T metamorphism

Several aspects of the petrogenesis of low‐pressure granulite facies rocks from the Reynolds Range (central Australia) are contentious, including: (a) the shape of the retrograde P–T –time path, and whether it is an artefact of repeated thermal events at different P–T conditions; (b) the type of regional metamorphism; and (c) the causes of metamorphism. Granulite facies rocks from the Reynolds Range Group experienced three major periods of mineralogical equilibration. Metapelitic rocks underwent dehydration‐melting reactions to form migmatites under peak M2 P–T conditions of c. 5.0–5.3 kbar and c. 750–800 °C. Metapsammitic rocks that did not melt during M2 show spectacular garnet–orthopyroxene intergrowths that developed at c. 3.5–3.7 kbar and c. 700–750 °C after penetrative regional deformation, but prior to amphibolite facies rehydration in discrete strike‐parallel zones. Rehydration occurred within the sillimanite stability field at P–T conditions close to the granite solidus (c. 3.2–3.4 kbar and 650–700 °C). Subsequently the terrane cooled into the andalusite stability field. Geochronological constraints suggest that: (a) peak‐M2 conditions were reached at c. 1594 Ma; (b) the garnet–orthopyroxene intergrowths in unmelted metapsammites probably developed between c. 1594 Ma and c. 1586 Ma; and (c) upper amphibolite facies rehydration occurred between c. 1586 Ma and 1568 Ma. The lack of petrological evidence for multiple dehydration and rehydration of the rocks suggests that the three episodes of mineralogical recrystallization can be linked to yield a single continuous retrograde P–T–t path of minor initial decompression (c. 1.5 kbar) from the M2 peak, followed by cooling (c. 100 °C) to the granite solidus over a period of c. 26 Ma. Late kyanite‐bearing shear zones that dissect the terrane are unrelated to this event and formed during the c. 300–400 Ma Alice Springs Orogeny. The shape of the P–T–t path and the duration of M2 metamorphism suggests that advective heating was not the major cause of high‐grade metamorphism, and that some other, longer lived heat source, such as the burial of anomalously radiogenic, pre‐tectonic granites, is required.

The Meatiq dome (Eastern Desert, Egypt) a Precambrian metamorphic core complex: petrological and geological evidence

The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn‐rich Spl–Qtz±Bt, Grt–Zn‐rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water‐rich H2O‐CO2±CH4/N2 inclusions) pass through peak M2 P–T conditions and flatter isochores (CO2‐rich H2O‐CO2±CH4/N2 inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to&lt;540 °C and the absence of crysotile to &gt;350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break‐off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low‐angle normal faults.

Structural and metamorphic geology of the Windmill Islands, east Antarctica: Field evidence for repeated tectonothermal activity

Structural and metamorphic relationships in the Windmill Islands, Wilkes Land, east Antarctica show evidence for repeated tectonothermal activity. An early metamorphic peak (M1), characterized by sillimanite‐biotite ± cordierite assemblages in pelitic rocks was coeval with and outlasted by D1 deformation which formed a pervasive horizontal fabric. A major shortening phase (D2) refolded the M1/D1 fabric. The syn‐ and post‐D2 thermal peak (M2) occurred at largely static conditions and is characterized by extensive in situ partial melting of quartzo‐feldspathic units, a high‐grade metamorphic overprint of M1 peak assemblages, and granite and charnockite emplacement. The M2 overprint increases in intensity from north to south and, in pelitic rocks, is characterized by the prograde breakdown of M1 sillimanite‐biotite to M2 garnet‐cordierite, resulting in M2 peak assemblages consisting of garnet‐cordierite‐biotite, garnet‐cordierite‐orthopyroxene and cordierite‐orthopyroxene. Pinnitization of early M1 cordierit...

Internal friction associated with dislocation relaxations in virgin martensite—II. Interpretation

Interpretations for the M2, M3 and M4 internal friction peaks observed (Part I) [ Acta metall. mater. 41, 3277 (1993)] during heating of virgin martensite from 82 to 350 K are presented. M4 and M3 peaks can be attributed to the generation of kink-pairs along non-screw and screw dislocations, respectively, while M2 peak is related to the interaction between non-screw dislocations and interstitial carbon atoms. Activation energies and pre-exponential factors of relaxation time τ 0 are determined for each peak. Peierls stress of martensite is estimated.

Role of boron in the multistable carbon-related defect in silicon

The conversion of the carbon-interstitial–phosphorous-substitutional (Ci-Ps) pairs from configuration M4 to M1 is shown to have a strong dependence on the method of sample fabrication. This conversion can be observed only in samples that have been contaminated by boron during sample fabrication. The temperature at which the conversion to M1 occurs is closely related to the level of boron contamination. In addition, it is shown that the M1 peak is not a single defect level. M1 consists of at least two overlapping levels that have similar properties but which are clearly related to different configurations. It is proposed that the conversion of M4 to M1 involves long-range migration of boron interstitials and that the M1 configurations involve a boron interstitial trapped at an isolated Ci-Ps pair.

Corticomotoneuronal cells contribute to long-latency stretch reflexes in the rhesus monkey.

To test the hypothesis that a transcortical reflex contributes to the stretch-evoked long-latency electromyographic (e.m.g.) response we documented the responses of identified corticomotoneuronal (c.m.) cells and their target muscles to perturbations of active wrist movements. Macaque monkeys performed ramp-and-hold wrist movements against elastic loads, alternating between flexion and extension zones; brief (25 ms) torque pulses were intermittently applied during the hold period. C.m. cells were identified by a clear post-spike facilitation in spike-triggered averages of forelimb muscle e.m.g. activity. Activity of c.m. cells and twelve wrist and digit flexor and extensor muscles was recorded during: (a) active ramp-and-hold wrist movements, (b) passive ramp-and-hold wrist movements, and (c) torque perturbations applied during the hold phase of active flexion and extension which either lengthened or shortened the c.m. cell's target muscles. Muscle-lengthening perturbations evoked a reproducible pattern of average e.m.g. activity in the stretched muscles, consisting of two peaks: the first response (M1) had an onset latency of 11.2 +/- 2.1 ms (mean +/- S.D.), and the second (M2) began at 27.9 +/- 5.1 ms. Torque perturbations which shortened the active muscles also evoked a characteristic e.m.g. response consisting of an initial cessation of activity at 13.5 +/- 3.4 ms followed by a peak beginning at 33.9 +/- 3.0 ms. The responses of twenty-one c.m. cells which facilitated wrist muscles were documented with torque pulse perturbations applied during active muscle contraction. Twenty of twenty-one c.m. cells responded at short latency (23.4 +/- 8.8 ms) to torque perturbations which stretched their target muscles. For each c.m. cell-target muscle pair, transcortical loop time was calculated as the sum of the onset latency of the c.m. cell's response to lengthening perturbations (afferent time) and the onset latency of post-spike facilitation (efferent time). The mean transcortical loop time was 30.4 +/- 10.2 ms, comparable to the mean onset latency of the M2 peak (27.9 +/- 5.1). The duration of a c.m. cell's response to torque perturbations provides a further measure of the extent of its potential contribution to the M2 muscle response. In all cases but two, the c.m. cell response, delayed by the latency of the post-spike facilitation, overlapped the M2 e.m.g. peak.

Electromyographic response to displacement of different forelimb joints in the squirrel monkey

The electromyographic (EMG) reflex response evoked in muscles stretched by imposed angular joint displacement has previously been studied in a variety of muscles and species. In most muscles studied, the EMG response consists of an initial burst of activity at latencies comparable to tendon tap responses followed by one or more bursts of EMG activity occurring at latencies less than somatosensory reaction time. The longer latency bursts or peaks of activity have often been assumed to be of similar origin in functionally diverse muscles. The present experiments were performed to examine the EMG response to imposed joint displacement in several different muscles of the squirrel monkey forelimb. The EMG was studied in muscles stretched by elbow, wrist, and metacarpophalangeal joint extension. Early (M1) and long (M2) latency peaks of activity could be observed in the EMG responses of short head of biceps (SHB), flexor carpi ulnaris (FCU), and flexor digitorum profundus (FDP), at latencies which are similar to those reported in other primate species allowing for differences in the size of the animals. The intervals between these peaks did not correspond to the period of oscillations in tension which occurred after the onset of the joint extension. The M2 peak consistently occurred later in SHB than in FCU and FDP. The M1 peak was smaller in distal than in proximal muscles and was frequently absent in FDP at resting levels of EMG activity. The ratio of M1 to M1 + M2 activity for different muscles revealed that proximal muscles had prominent early with less prominent long latency activity, whereas distal muscles had minimal early latency activity with prominent long latency activity. The onset of EMG activity approached tendon tap latencies in FCU at high base line EMG levels but in most cases occurred at approximately twice tendon tap latencies in FDP at high base line EMG levels. The results demonstrate the presence of significant differences between the EMG response to joint displacement in the proximal-distal series of forelimb flexors. Differences in the reflex response to joint displacement are discussed in view of twitch times and motor unit composition of the muscles studied.

‘Long-loop’ reflexes can be obtained in spinal monkeys

Extensor muscles of the fore- and hindlimb were stretched in the lightly anaesthetized monkey ( Macaca fascicularis) and cat. The resulting electromyogram contained a short latency peak consistent with a monosynaptic, segmental pathway; this peak was identified as the M1 peak of Tatton et al. [21]. A longer latency peak, identified as M2 and said to involve a pathway including supraspinal centres, was also present. After spinal section at a high cervical level, the electromyogram was reduced in amplitude, but both short (M1) and longer latency (M2) peaks were present. It is concluded that neither the cerebral cortex nor the cerebellum are necessary parts of the neural circuitry generating longer latency components of the electromyographic response to muscle stretch.

Motor unit responses in muscles stretched by imposed displacements of the monkey wrist.

Imposed angular displacements of monkey's wrist produce three major peaks of activity (terms M1, M2, and M3 peaks) in the averaged gross EMG activity ot the stretched muscles. The displacements were imposed on monkey's wrist by computer controlled step loads (range 60--540 g). Gross EMG was recorded simultaneously with the activity of single motor units (SMUs) in the stretched muscles. SMUs were identified and separated using a computer "shape-fitting" program. Average response histograms (ARHs) were constructed for each SMU's responses to randomly presented repetitions of up to five different step loads. Eighty-three percent of the SMUs showed a single excitatory response peak that was limited to a time interval corresponding to that of only one of the gross EMG peaks. Despite marked differences in the time courses of the imposed displacements, the time courses of the SMU excitatory responses were maintained. By increasing the background load and hence the tonic firing of the individual SMUs, the histograms show that the long latency of the M2 peak is not due to an inhibitory-excitatory sequence. The SMUs responding at longer latencies (M2 and M3 SMUs) show firing characteristics in keeping with those reported for fast twitch motor units while those of M1 SMUs correlate with those found for slow twitch units. These results establish that the M1, M2, and M3 peaks do not merely represent synchronized oscillatory activity of the motoneurons but largely result from the reflex excitation of separately responding motoneuron "subpopulations", each of which contributes to the generation of only one of the gross EMG peaks.