bluuebunny/crossref_metadata_2025 huggingface dataset:
On Broadening Failure Rate Distributions in PRA Uncertainty Analyses
Authors: Martz Harry F.
Published: May 2006

Several recent nuclear power plant probabilistic risk assessments (PRAs) have utilized broadened Reactor Safety Study (RSS) component failure rate population variability curves to compensate for such things as expert “overvaluation bias” in the estimates upon which the curves are based.

A simple two‐components of variation empirical Bayes model is proposed for use in estimating the between‐expert variability curve in the presence of such biases. Under certain conditions this curve is a population variability curve. Comparisons are made with the existing method.

The popular procedure appears to be generally much more conservative than the empirical Bayes method in removing such biases. In one case the broadened curve based on the popular method is more than two orders of magnitude broader than the empirical Bayes curve. In another case it is found that the maximum justifiable degree of broadening of the RSS curve is to increase α from 5% to 12%, which is significantly less than the 20% value recommended in the popular approach.
How rare is the bullet cluster?
Authors: Hayashi Eric , White Simon D. M.
Published: May 2006

The galaxy cluster 1E 0657-56 has a bullet-like subcluster that is moving away from the centre of the main cluster at high speed. Markevitch et al. (2004) recently estimated a relative velocity of Vbullet= 4500+1100-800 km s-1, based on observations of the bow shock in front of the subcluster. The weak lensing analysis of Clowe, Gonzalez & Markevitch (2004) indicates that a substantial secondary mass peak is associated with this subcluster. We estimate the likelihood of such a configuration by examining the distribution of subhalo velocities for clusters in the Millennium Run, a large Λ cold dark matter (ΛCDM) cosmological simulation. We find that the most massive subhalo has a velocity as high as that of the bullet subcluster in only about one out of every 100 cluster-sized haloes. This estimate is strongly dependent on the precise velocity adopted for the bullet. One of the 10 most massive subhaloes has such a high velocity about 40 per cent of the time. We conclude that the velocity of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured ΛCDM cosmogony.
Modeling of Heat Transfer in Low-Density EPS Foams
Authors: Coquard R. , Baillis D.
Published: May 2006

Expanded polystyrene (EPS) foams are one of the most widely used thermal insulators in the building industry. Owing to their very low density, both conductive and radiative heat transfers are significant. However, only few studies have already been conducted in the modeling of heat transfer in this kind of medium. This is due to their complex porous structure characterized by a double-scale porosity which has always been ignored by the previous works. In this study, we present a model of one-dimensional steady state heat transfer in these foams based on a numerical resolution of the radiation-conduction coupling. The modeling of the conductive and radiative properties of the foams takes into account their structural characteristics such as foam density or cell diameter and permits us to study the evolution of their equivalent thermal conductivity with these characteristics. The theoretical results have been compared to equivalent thermal conductivity measurements made on several EPS foams using a flux-meter apparatus and show a good agreement.
Brownian-Motion-Based Convective-Conductive Model for the Effective Thermal Conductivity of Nanofluids
Authors: Prasher Ravi , Bhattacharya Prajesh , Phelan Patrick E.
Published: May 2006

Here we show through an order-of-magnitude analysis that the enhancement in the effective thermal conductivity of nanofluids is due mainly to the localized convection caused by the Brownian movement of the nanoparticles. We also introduce a convective-conductive model which accurately captures the effects of particle size, choice of base liquid, thermal interfacial resistance between the particles and liquid, temperature, etc. This model is a combination of the Maxwell-Garnett (MG) conduction model and the convection caused by the Brownian movement of the nanoparticles, and reduces to the MG model for large particle sizes. The model is in good agreement with data on water, ethylene glycol, and oil-based nanofluids, and shows that the lighter the nanoparticles, the greater the convection effect in the liquid, regardless of the thermal conductivity of the nanoparticles.
Fractal Activity Generated Independently by Medullary Sympathetic Premotor and Preganglionic Sympathetic Neurons
Authors: Orer Hakan S. , Das Mahasweta , Barman Susan M. , Gebber Gerard L.
Published: May 2006

In anesthetized cats with cervical spinal cord transection, Fano factor analysis was used to test for time-scale invariant (fractal) fluctuations in spike counts of single preganglionic cervical sympathetic neurons (PSNs) and putative sympathetic premotor neurons located in the rostral ventrolateral medulla (RVLM) and caudal medullary raphe. The medullary neurons exhibited cardiac-related activity, and their axons projected to the spinal cord, as demonstrated by antidromic activation. The variance-to-mean spike count ratio (Fano factor) was plotted as a function of the window size used to count spikes. The Fano factor curves for seven PSNs, eight RVLM neurons, and eight raphe neurons contained a power law relationship extending over more than one time scale. In these cases, random shuffling of the interspike intervals in the original time series eliminated the power law relationship. Thus the power law relationships can be attributed to long-range correlations among interspike intervals rather than simply to the distribution of the intervals that is not changed by shuffling the data. It is concluded that PSNs and sympathetic premotor neurons in the medulla can independently generate fractal firing patterns.
Spontaneous Synaptic Activity Is Primarily GABAergic in Vestibular Nucleus Neurons of the Chick Embryo
Authors: Shao Mei , Hirsch June C. , Giaume Christian , Peusner Kenna D.
Published: May 2006

The principal cells of the chick tangential nucleus are vestibular nucleus neurons participating in the vestibular reflexes. In 16-day embryos, the application of glutamate receptor antagonists abolished the postsynaptic responses generated on vestibular-nerve stimulation, but spontaneous synaptic activity was largely unaffected. Here, spontaneous synaptic activity was characterized in principal cells from brain slices at E16 using whole cell voltage-clamp recordings. With KCl electrodes, the frequency of spontaneous inward currents was 3.1 Hz at –60 mV, and the reversal potential was +4 mV. Cs-gluconate pipette solution allowed the discrimination of glycine/GABAA versus glutamate receptor-mediated events according to their different reversal potentials. The ratio for spontaneous excitatory to inhibitory events was about 1:4. Seventy-four percent of the outward events were GABAA, whereas 26% were glycine receptor-mediated events. Both pre- and postsynaptic GABAB receptor effects were shown, with presynaptic GABAB receptors inhibiting 40% of spontaneous excitatory postsynaptic currents (sEPSCs) and 53% of spontaneous inhibitory postsynaptic currents (sIPSCs). With TTX, the frequency decreased ∼50% for EPSCs and 23% for IPSCs. These data indicate that the spontaneous synaptic activity recorded in the principal cells at E16 is primarily inhibitory, action potential-independent, and based on the activation of GABAA receptors that can be modulated by presynaptic GABAB receptors.
Rhythmic Properties of Neurons in the Rostral Ventrolateral Medulla of the Rat In Vitro: Effects of Clonidine
Authors: Granata Antonio R. , Cohen Morton I.
Published: May 2006

The rostral ventrolateral medulla (RVLM) is thought to be the main central site for generation of tonic sympathetic activity. In the rat in vitro slice preparation, we used intracellular recordings to identify different populations of neurons in the RVLM: 43 spontaneously active neurons with regular (R) or irregular (I) patterns of spike firing and 10 silent neurons. The degree of regularity was quantified by the coefficient of variation (CV = SD/mean) of interspike interval durations, as well as by the rhythmic properties of the spike autospectrum and autocorrelation. The distribution of CVs was clustered: R and I neurons were defined as those with CVs ≤12% ( n = 21) or >12% ( n = 22), respectively. The R-type and I-type neurons resemble the type II and type I neurons, respectively, which were previously characterized in the RVLM in vivo as barosensitive and bulbospinal. Both types may be important in generation of sympathetic tone. Clonidine (1–100 μM) was applied to 10 R-type neurons and 16 I-type neurons. The firing of 21/26 was depressed to the point of silence. However, 18/26 neurons were excited earlier in the perfusion. The later depression of firing occurred in both I and R neurons and in different cases was associated with either hyperpolarization or depolarization.
Upper Thoracic Respiratory Interneurons Integrate Noxious Somatic and Visceral Information in Rats
Authors: Qin Chao , Chandler Margaret J. , Foreman Robert D. , Farber Jay P.
Published: May 2006

The aim of this study was to determine if thoracic respiratory interneurons (TRINs) might receive peripheral noxious somatic and visceral inputs. Extracellular potentials of 78 respiration-related T3 neurons, whose activity was driven by central respiratory output, were recorded from the intermediate zone in pentobarbital anesthetized, paralyzed, and ventilated male rats. These neurons were identified as interneurons by their locations and by the absence of antidromic activation from the cervical sympathetic trunk and cerebellum. Thoracic esophageal distension (ED) was produced by water inflation of a latex balloon (0.1–0.5 ml, 20 s). A catheter was placed in the pericardial sac to administer 0.2 ml bradykinin (10−5 M) for noxious cardiac stimulation. Of 78 TRINs examined for ED, activity of 24 TRINs increased and activity of 8 TRINs decreased. Intrapericardial bradykinin increased activity in 26/65 TRINs tested and decreased activity in 5 TRINs. Seventy-four TRINs were tested for effects of brush, pressure, and pinch of the chest and upper back areas. No TRINs responded to brushing hair. Low-threshold responses to pressure were observed in 27 TRINs. Fourteen TRINs were wide dynamic range and 4 TRINs had high-threshold responses. Peripheral stimuli affected all types of TRINs, including inspiratory, expiratory, and biphasic neurons. Simultaneous phrenic recordings showed that effects of various somatic and visceral stimuli on TRINs were independent of central respiratory drive. Various somatovisceral and viscerovisceral patterns of input were observed in TRINs. The results suggested that TRINs participate in intraspinal processing and integration of nociceptive information from somatic fields and visceral organs.
Influence of Active Dendritic Currents on Input-Output Processing in Spinal Motoneurons In Vivo
Authors: Lee R. H. , Kuo J. J. , Jiang M. C. , Heckman C. J.
Published: May 2006

The extensive dendritic tree of the adult spinal motoneuron generates a powerful persistent inward current (PIC). We investigated how this dendritic PIC influenced conversion of synaptic input to rhythmic firing. A linearly increasing, predominantly excitatory synaptic input was generated in triceps ankle extensor motoneurons by slow stretch (duration: 2–10 s) of the Achilles tendon in the decerebrate cat preparation. The firing pattern evoked by stretch was measured by injecting a steady current to depolarize the cell to threshold for firing. The effective synaptic current ( I N, the net synaptic current reaching the soma of the cell) evoked by stretch was measured during voltage clamp. Hyperpolarized holding potentials were used to minimize the activation of the dendritic PIC and thus estimate stretch-evoked I N for a passive dendritic tree ( I N,PASS). Depolarized holding potentials that approximated the average membrane potential during rhythmic firing allowed strong activation of the dendritic PIC and thus resulted in marked enhancement of the total stretch-evoked I N( I N,TOT). The net effect of the dendritic PIC on the generation of rhythmic firing was assessed by plotting stretch-evoked firing (strong PIC activation) versus stretch-evoked I N,PASS (minimal PIC activation). The gain of this input-output function for the neuron (I-ON) was found to be ∼2.7 times as high as for the standard injected frequency current ( F-I) function in low-input conductance neurons. However, about halfway through the stretch, firing rate tended to become constant, resulting in a sharp saturation in I-ON that was not present in F-I. In addition, the gain of I-ONdecreased sharply with increasing input conductance, resulting in much lower stretch-evoked firing rates in high-input conductance cells. All three of these phenomena (high initial gain, saturation, and differences in low- and high-input conductance cells) were also readily apparent in the differences between stretch-evoked I N,TOT and I N, PASS and thus could be accounted for by the activation of the dendritic PIC. As a result, stretch-evoked I N,TOT and F-I provided an accurate prediction of the overall change in stretch-evoked firing. However, in about half of the low-input conductance cells, the rate of rise of firing in response to stretch was not smoothly graded but instead consisted of a rapid surge. Stretch-evoked I N,TOT was always smoothly graded. This suggests that although stretch-evoked I N,TOT can be used to predict the overall change in firing, prediction of the dynamics of firing may be less accurate.
Synaptically Activated Cl– Accumulation Responsible for Depolarizing GABAergic Responses in Mature Hippocampal Neurons
Authors: Isomura Y. , Sugimoto M. , Fujiwara-Tsukamoto Y. , Yamamoto-Muraki S. , Yamada J. , Fukuda A.
Published: May 2006

It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABAA receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABAA receptor-permeable anions, chloride (Cl–) and bicarbonate (HCO3–). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl– accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl– imaging technique with a Cl– indicator 6-methoxy- N-ethylquinolinium iodide (MEQ). The synaptically activated Cl– accumulation was mediated dominantly through GABAA receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl– accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABAA-mediated Cl– accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl– accumulation during synaptic activation.
Short-Latency Fixational Saccades Induced by Luminance Increments
Authors: Horwitz Gregory D. , Albright Thomas D.
Published: May 2006

We investigated the effect of peripheral visual stimulation on small-amplitude saccades that occur naturally during fixation. Two macaque monkeys were rewarded for fixating while a colorful stimulus flickered randomly in the periphery. Reverse correlation revealed a lawful relationship between the stimulus sequence and saccade occurrences: on average, a transient increase in stimulus intensity evoked saccades at a latency of ∼70 ms. The spectral tuning of this increase was roughly, but not exactly, consistent with a pure luminance increase. We conclude that peripheral luminance increases can evoke fixational saccades.
A Newly Identified Buccal Interneuron Initiates and Modulates Feeding Motor Programs inAplysia
Authors: Dembrow N. C. , Jing J. , Proekt A. , Romero A. , Vilim F. S. , Cropper E. C. , Weiss K. R.
Published: May 2006

Despite considerable progress in characterizing the feeding central pattern generator (CPG) in Aplysia, the full complement of neurons that generate feeding motor programs has not yet been identified. The distribution of neuropeptide-containing neurons in the buccal and cerebral ganglia can be used as a tool to identify additional elements of the feeding circuitry by providing distinctions between otherwise morphologically indistinct neurons. For example, our recent study revealed a unique and potentially interesting unpaired PRQFVamide (PRQFVa)-containing neuron in the buccal ganglion. In this study, we describe the morphological and electrophysiological characterization of this novel neuron, which we designate as B50. We found that activation of B50 is capable of producing organized rhythmic output of the feeding CPG. The motor programs elicited by B50 exhibit some similarities as well as differences to motor programs elicited by the command-like cerebral-to-buccal interneuron CBI-2. In addition to activating the feeding CPG, B50 may act as a program modulator.
Loud Sound-Induced Changes in Cochlear Mechanics
Authors: Fridberger Anders , Zheng Jiefu , Parthasarathi Anand , Ren Tianying , Nuttall Alfred
Published: May 2006

To investigate the inner ear response to intense sound and the mechanisms behind temporary threshold shifts, anesthetized guinea pigs were exposed to tones at 100–112 dB SPL. Basilar membrane vibration was measured using laser velocimetry, and the cochlear microphonic potential, compound action potential of the auditory nerve, and local electric AC potentials in the organ of Corti were used as additional indicators of cochlear function. After exposure to a 12-kHz intense tone, basilar membrane vibrations in response to probe tones at the characteristic frequency of the recording location (17 kHz) were transiently reduced. This reduction recovered over the course of 50 ms in most cases. Organ of Corti AC potentials were also reduced and recovered with a time course similar to the basilar membrane. When using a probe tone at either 1 or 4 kHz, organ of Corti AC potentials were unaffected by loud sound, indicating that transducer channels remained intact. In most experiments, both the basilar membrane and the cochlear microphonic response to the 12-kHz overstimulation was constant throughout the duration of the intense stimulus, despite a large loss of cochlear sensitivity. It is concluded that the reduction of basilar membrane velocity that followed loud sound was caused by changes in cochlear amplification and that the cochlear response to intense stimulation is determined by the passive mechanical properties of the inner ear structures.
Suppression of Corticospinal Excitability During Negative Motor Imagery
Authors: Sohn Young H. , Dang Nguyet , Hallett Mark
Published: May 2006

To investigate the effect of negative motor imagery on corticospinal excitability, we performed transcranial magnetic stimulation (TMS) studies in seven healthy subjects during imagination of suppressing movements. Subjects were asked to imagine suppression of TMS-induced twitching movement of their nondominant left hands by attempting to increase the amount of relaxation after receiving an auditory NoGo cue (negative motor imagery), but to imagine squeezing hands after a Go cue (positive motor imagery). Single- and paired-pulse TMS were triggered at 2 s after Go or NoGo cues. Motor-evoked potentials (MEPs) were recorded in the first dorsal interosseus (FDI), abductor pollicis brevis (APB), and abductor digiti minimi (ADM) muscles of the left hand. Paired-pulse TMS with subthreshold conditioning stimuli at interstimulus intervals of 2 (short intracortical inhibition) and 15 ms (intracortical facilitation) and that with suprathreshold conditioning stimuli at interstimulus interval of 80 ms (long intracortical inhibition) were performed in both negative motor imagery and control conditions. Compared with the control state (no imagination), MEP amplitudes of FDI (but not APB and ADM) were significantly suppressed in negative motor imagery, but those from all three muscles were unchanged during positive motor imagery. F-wave responses (amplitudes and persistence) were unchanged during both negative and positive motor imagery. During negative motor imagery, resting motor threshold was significantly increased, but short and long intracortical inhibition and intracortical facilitation were unchanged. The present results demonstrate that excitatory corticospinal drive is suppressed during imagination of suppressing movements.
Task Differences With the Same Load Torque Alter the Endurance Time of Submaximal Fatiguing Contractions in Humans
Authors: Hunter Sandra K. , Ryan Daphne L. , Ortega Justus D. , Enoka Roger M.
Published: May 2006

Endurance time, muscle activation, and mean arterial pressure were measured during two types of submaximal fatiguing contractions that required each subject to exert the same net muscle torque in the two tasks. Sixteen men and women performed isometric contractions at 15% of the maximum voluntary contraction (MVC) force with the elbow flexor muscles, either by maintaining a constant force while pushing against a force transducer (force task) or by supporting an equivalent inertial load while maintaining a constant elbow angle (position task). The endurance time for the force task (1402 ± 728 s) was twice as long as that for the position task (702 ± 582 s, P < 0.05), despite a similar reduction in the load torque at exhaustion for each contraction. The rate of increase in average electromyographic activity (EMG, % peak MVC value) for the elbow flexor muscles was similar for the two tasks. However, the average EMG was greater at exhaustion for the force task (22.4 ± 1.2%) compared with the position task (14.9 ± 1.0%, P < 0.05). In contrast, the rates of increase in the mean arterial pressure, the rating of perceived exertion, anterior deltoid EMG, and fluctuations in motor output (force or acceleration) were greater for the position task compared with the force task ( P < 0.05). Furthermore, the rate of bursts in EMG activity, which corresponded to the transient recruitment of motor units, was greater for the brachialis muscle during the position task. These results indicate that the briefer endurance time for the position task was associated with greater levels of excitatory and inhibitory input to the motor neurons compared with the force task.
Persistent Sodium and Calcium Currents in Rat Hypoglossal Motoneurons
Authors: Powers Randall K. , Binder Marc D.
Published: May 2006

Voltage-dependent persistent inward currents are thought to make an important contribution to the input–output properties of α−motoneurons, influencing both the transfer of synaptic current to the soma and the effects of that current on repetitive discharge. Recent studies have paid particular attention to the contribution of L-type calcium channels, which are thought to be widely distributed on both the somatic and the dendritic membrane. However, the relative contribution of different channel subtypes as well as their somatodendritic distribution may vary among motoneurons of different species, developmental stages, and motoneuron pools. In this study, we have characterized persistent inward currents in juvenile (10- to 24-day-old) rat hypoglossal (HG) motoneurons. Whole-cell, voltage-clamp recordings were made from the somata of visualized rat HG motoneurons in 300-μm brain stem slices. Slow (10 s), triangular voltage-clamp commands from a holding potential of −70 to 0 mV and back elicited whole-cell currents that were dominated by outward, potassium currents, but often showed a region of negative slope resistance on the rising phase of the command. In the presence of potassium channel blockers (internal cesium and external 4-aminopyridine and tetraethylammonium), net inward currents were present on both the rising and falling phases of the voltage-clamp command. A portion of the inward current present on the ascending phase of the command was mediated by TTX-sensitive sodium channels, whereas calcium channels mediated the remainder of the current. We found roughly the same relative contributions of P-, N-, and L-type channels to the calcium currents recorded at the soma that had previously been found in neonatal rat HG motoneurons. In most cells, the somatic voltage thresholds for calcium current onset and offset were similar and the peak current was largest on the ascending phase of the clamp command. However, about one-third of the cells exhibited a substantial clockwise current hysteresis, i.e., inward currents were present at lower voltages on the descending phase of the clamp command. In the same cells, 1-s depolarizing voltage-clamp commands were followed by prolonged tail currents, consistent with a prominent contribution from dendritic channels. In contrast to previous reports on turtle and mouse motoneurons, blocking L-type calcium channels did not eliminate these presumed dendritic currents.
Mechanisms of Neuronal Hyperexcitability Caused by Partial Inhibition of Na+-K+-ATPases in the Rat CA1 Hippocampal Region
Authors: Vaillend Cyrille , Mason Susanne E. , Cuttle Matthew F. , Alger Bradley E.
Published: May 2006

Extra- and intracellular records were made from rat acute hippocampal slices to examine the effects of partial inhibition of Na+-K+-ATPases (Na+-K+pumps) on neuronal hyperexcitability. Bath application of the low-affinity cardiac glycoside, dihydroouabain (DHO), reversibly induced interictal-like epileptiform bursting activity in the CA1 region. Burst-firing was correlated with inhibition of the pumps, which was assayed by changes in [K+]ouptake rates measured with K+-ion-sensitive microelectrodes. Large increases in resting [K+]odid not occur. DHO induced a transient depolarization (5–6 mV) followed by a long-lasting hyperpolarization (∼6 mV) in CA1 pyramidal neurons, which was accompanied by a 30% decrease in resting input resistance. Block of an electrogenic pump current could explain the depolarization but not the hyperpolarization of the membrane. Increasing [K+]ofrom 3 to 5.5 mM minimized these transient shifts in passive membrane properties without preventing DHO-induced hyperexcitability. DHO decreased synaptic transmission, but increased the coupling between excitatory postsynaptic potentials and spike firing (E-S coupling). Monosynaptic inhibitory postsynaptic potential (IPSP) amplitudes declined to ∼25% of control at the peak of bursting activity; however, miniature TTX-resistant inhibitory postsynaptic current amplitudes were unaffected. DHO also reduced the initial slope of the intracellular excitatory postsynaptic potential (EPSP) to ∼40% of control. The conductances of pharmacologically isolated IPSPs and EPSPs in high-Ca/high-Mg-containing saline were also reduced by DHO by ∼50%. The extracellular fiber volley amplitude was reduced by 15–20%, suggesting that the decrease in neurotransmission was partly due to a reduction in presynaptic fiber excitability. DHO enhanced a late depolarizing potential that was superimposed on the EPSP and could obscure it. This potential was not blocked by antagonists of NMDA receptors, and blockade of NMDA, mGlu, or GABAAreceptors did not affect burst firing. The late depolarizing component enabled the pyramidal cells to reach spike threshold without changing the actual voltage threshold for firing. We conclude that reduced GABAergic potentials and enhanced E-S coupling are the primary mechanisms underlying the hyperexcitability associated with impaired Na+-K+pump activity.
Importance of Cutaneous Feedback in Maintaining a Secure Grip During Manipulation of Hand-Held Objects
Authors: Augurelle Anne-Sophie , Smith Allan M. , Lejeune Thierry , Thonnard Jean-Louis
Published: May 2006

Previous research has shown that grip and load forces are modulated simultaneously during manipulation of a hand-held object. This close temporal coupling suggested that both forces are controlled by an internal model within the CNS that predicts the changes in tangential force on the fingers. The objective of the present study was to examine how the internal model would compensate for the loss of cutaneous sensation through local anesthesia of the index and thumb. Ten healthy adult subjects (5 men and 5 women aged 20–57 yr) were asked to grasp, lift, and hold stationary, a 250 g object for 20 s. Next, the subjects were asked to perform vertical oscillatory movements over a distance of 20 cm at a rate of 1.0 Hz for 30 s. Eleven trials were performed with intact sensation, and 11 trials after a local ring-block anesthesia of the index and thumb with bupivacain (5 mg/ml). During static holding, loss of cutaneous sensation produced a significant increase in the safety margin. However, the grip force declined significantly over the 20-s static hold period. During oscillatory arm movements, grip and load forces were continuously modulated together in a predictive manner as suggested by Flanagan and Wing. Again, the grip force declined over the 30-s movement, and 7/10 subjects dropped the object at least once. With intact sensation, the object was never dropped; but with the fingers anesthetized, it was dropped on 36% of the trials, and a significant slip occurred on a further 12%. The mean correlation between the grip and load forces for all subjects deteriorated from 0.71 with intact sensation to 0.48 after digital anesthesia. However, a cross-correlation calculated between the grip and load forces indicated that the phase lag was approximately zero both with and without digital anesthesia. Taken together, the data from the present study suggest that cutaneous afferents are required for setting and maintaining the background level of the grip force in addition to their phasic slip-detection function and their role in adapting the grip force/load force ratio to the friction on initial contact with an object. Finally, at a more theoretical level, they correct and maintain an internal model of the physical properties of hand-held objects.
Spectral Integration in A1 of Awake Primates: Neurons With Single- and Multipeaked Tuning Characteristics
Authors: Kadia Siddhartha C. , Wang Xiaoqin
Published: May 2006

We investigated modulations by stimulus components placed outside of the classical receptive field in the primary auditory cortex (A1) of awake marmosets. Two classes of neurons were identified using single tone stimuli: neurons with single-peaked frequency tuning characteristics (147/185, 80%) and neurons with multipeaked frequency tuning characteristics (38/185, 20%), referred to as single- and multipeaked units, respectively. Each class of neurons was further studied using two-tone paradigms in which the frequency, intensity, and timing of the second tone were systematically varied while a unit was driven by the first tone placed at a unit's characteristic frequency (CF) if it was single-peaked or at one of multiple spectral peaks if it was multipeaked. The main findings were: 1) excitatory spectral peaks in the frequency tuning of the multipeaked units were often harmonically related. 2) Multipeaked units showed facilitation in their responses to combinations of two harmonically related tones placed at the spectral peaks of their frequency tuning. The two-tone facilitation was strongest for the simultaneously presented tones. 3) In 76 of 113 single-peaked units studied using the two-tone paradigm, facilitatory and/or inhibitory modulations by distant off-CF tones were observed. This distant inhibition differed from flanking (or side-band) inhibitions near CF. 4) In single-peaked units, the distant off-CF inhibitions were dominated by tones at frequencies that were harmonically related to the CF of a unit, whereas the facilitation by off-CF tones was observed for a wide range of frequencies. And 5) in both single- and multipeaked units, sound levels of two interacting tones determined whether the two tones produced excitation or inhibition. The largest facilitation was achieved by using two tones at their corresponding preferred sound levels. Together, these findings suggest that extracting or rejecting harmonically related components embedded in complex sounds may represent fundamental signal processing properties in different classes of A1 neurons.
Role of Nucleus Tractus Solitarius 5-HT3Receptors in the Defense Reaction–Induced Inhibition of the Aortic Baroreflex in Rats
Authors: Sévoz-Couche C. , Comet M.-A. , Hamon M. , Laguzzi R.
Published: May 2006

Different stressful conditions elicit a typical behavior called the defense reaction. Our aim was to determine whether 5-HT3receptors in the nucleus tractus solitarius (NTS) are involved in 1) the inhibition of the baroreflex bradycardia and 2) the rise in blood pressure, which are known to occur during the defense reaction. In urethane-anesthetized rats, the defense reaction was elicited by electrical stimulation of the dorsomedial nucleus of the hypothalamus (DMH) or the dorsal part of the periaqueductal gray (dPAG). Direct electrical stimulation of the aortic depressor nerve was used to trigger the typical baroreflex responses. Aortic stimulation at high (100–150 μA) and low (50–90 μA) intensity produced a decrease in heart rate of –39 to –44% (relative to baseline, Group 1 responses, n = 113) and –19 to –24% ( Group 2 responses, n = 43), respectively. In spontaneously breathing rats, Group 1 and Group 2 bradycardiac responses were inhibited during DMH (–75 ± 4% and –96 ± 4%, n = 38 and n = 11, respectively), as well as dPAG (–81 ± 3% and –95 ± 4%, n = 36 and n = 10, respectively) stimulation. The aortic baroreflex bradycardia was hardly affected by DMH or dPAG stimulation when bicuculline (5 pmol), a specific GABAAreceptor antagonist, had previously been microinjected into the NTS. Likewise, NTS microinjections of granisetron, a specific 5-HT3receptor antagonist, prevented, in a dose-dependent manner, the baroreflex bradycardia inhibition. In addition, intra-NTS granisetron did not affect the rise in blood pressure induced by either site stimulation. These data show that 5-HT3receptors in the NTS are involved in the GABAergic inhibition of the aortic baroreflex bradycardia, but not in the rise in blood pressure, occurring during the defense reaction elicited by DMH or dPAG stimulation.
Global Contour Saliency and Local Colinear Interactions
Authors: Li Wu , Gilbert Charles D.
Published: May 2006

Our visual system can link components of contours and segregate contours from complex backgrounds based on geometric grouping rules. This is an important intermediate step in object recognition. The substrate for contour integration may be based on contextual interactions and intrinsic horizontal connections seen in primary visual cortex (V1). We examined the perceptual rules governing contour saliency to determine whether the spatial extents of contextual interactions and horizontal connections match those mediating saliency. To quantify these rules, we used stimuli composed of randomly oriented nonoverlapping line segments. Salient contours within this complex background were formed by colinear alignment of nearby segments. Contour detectability was measured using a 2-interval-forced-choice design. Contour detectability deteriorated with increasing spacing between contour elements and improved as the number of colinear line elements was increased. At short contour spacing, the detectability reached a plateau with alignment of a few line segments that together formed a contour subtending several visual degrees. At intermediate spacing, saliency built up progressively with a greater number of colinear lines, extending up to 30°. When contour spacing was beyond a critical range (about 2°), however, the detectability dropped to chance levels, regardless of the number of colinear lines. Contour detectability was found to be a function not only of the relative spacing of contour elements with respect to the noise elements but also of the average density of the overall pattern. Furthermore, training significantly improved contour detection, increasing the critical spacing of line elements beyond which contours were no longer detectable. Our data suggest that global contour integration is based on mechanisms of limited spatial extent, comparable to the interactions observed in V1. These interactions can cascade over larger distances provided the spacing of stimulus elements is kept within a limited range.
Dynamic Equilibrium of Neurotransmitter Transporters: Not Just for Reuptake Anymore
Authors: Richerson George B. , Wu Yuanming
Published: May 2006

Many electrophysiologists view neurotransmitter transporters as tiny vacuum cleaners, operating continuously to lower extracellular neurotransmitter concentration to zero. However, this is not consistent with their known behavior, instead only reducing extracellular neurotransmitter concentration to a finite, nonzero value at which an equilibrium is reached. In addition, transporters are equally able to go in either the forward or reverse direction, and when they reverse, they release their substrate in a calcium-independent manner. Transporter reversal has long been recognized to occur in response to pathological stimuli, but new data demonstrate that some transporters can also reverse in response to physiologically relevant stimuli. This is consistent with theoretical calculations that indicate that the reversal potentials of GABA and glycine transporters are close to the resting potential of neurons under normal conditions and that the extracellular concentration of GABA is sufficiently high when the GABA transporter is at equilibrium to tonically activate high-affinity extrasynaptic GABAA receptors. The equilibrium for the GABA transporter is not static but instead varies continuously as the driving force for the transporter changes. We propose that the GABA transporter plays a dynamic role in control of brain excitability by modulating the level of tonic inhibition in response to neuronal activity.
Motor Subcircuits Mediating the Control of Movement Extent and Speed
Authors: Turner Robert S. , Desmurget Michel , Grethe Jeff , Crutcher Michael D. , Grafton Scott T.
Published: May 2006

The functional correlates of movement extent, speed, and covariates were investigated using PET mapping of regional cerebral blood flow (rCBF) in 13 healthy right-handed adults. A whole-arm smooth pursuit tracking task was used to strictly control potential confounds such as movement duration, error, and feedback control. During each of four scans, images of relative rCBF were obtained while subjects matched the constant velocity movements of a target using a joystick-controlled cursor. Between scans, subjects were completely adapted to one of four joystick-to-cursor gains, thereby allowing constant visual stimulation and eye movements across arm movements that ranged in extent from 6 to 24 cm. Subjects were unaware of the changes in visuomotor gain. Analyses of arm and eye movements indicated that the only significant difference in behavior across the four gain conditions was the extent and velocity of arm movements, which were closely correlated with each other. Parametric statistical methods identified brain areas where rCBF covaried with the mean movement extent of individual subjects during individual scans. Increasing movement extent was associated with parallel increases of rCBF in bilateral basal ganglia (BG; putamen and globus pallidus) and ipsilateral cerebellum. Modest extent effects were detected also in the sensorimotor cortices bilaterally. No significant inverse relations were found. We conclude that a small subcircuit within the motor control system contributes to the control of movement extent and covariates and that the BG and cerebellum play central roles in the operation of that circuit.
Strategies for the Integration of Posture and Movement During Reaching in the Cat
Authors: Schepens Bénédicte , Drew Trevor
Published: May 2006

We have examined the relationship between the movement and the anticipatory postural adjustments (APAs) that precede that movement during a reaching task in the cat. We recorded ground reaction forces in all 3 planes from all 4 limbs as well as electromyographic (EMG) activity from limb and axial muscles. The reaching movement was always preceded by an APA that was characterized by a loading of the reaching forelimb and an unloading of the support forelimb. This loading of the reaching forelimb was preceded, and accompanied, by increased activity in shoulder and limb extensor muscles of the reaching limb; extensor muscle activity in the supporting limb was simultaneously decreased. An important finding from this study was that the onset of the APA and of the movement was temporally decoupled. Analyses of the onset of EMG activity showed that most of the muscles that we recorded could be classified as either related to the APA or related to the movement. These results support the idea of distributed, and perhaps independent, systems for the execution of the APA and of the prime movement. There was also postural activity in the supporting limb during the movement. Analysis of this activity, which is also anticipatory in nature, suggests that it was tightly linked to the movement. We suggest that this postural response is signaled as part of the command for movement. Some muscles, particularly the extensors of the reaching limb, received convergent input from the command signals for the APA and for the movement.
Progressive Adaptation of the Soleus H-Reflex With Daily Training at Walking Backward
Authors: Schneider Cyril , Capaday Charles
Published: May 2006

When untrained subjects walk backward on a treadmill the amplitude of the soleus H-reflex in midswing is equal to or exceeds the value in stance. This is a surprising result because during the swing phase of backward walking the soleus is inactive and its antagonist, the tibialis anterior, is active. We suggested that the high amplitude of the soleus H-reflex in late swing reflects task uncertainties, such as estimating the moment of foot contact with the ground and losing balance. In support of this idea we show that when untrained subjects held on to handrails the unexpected high-amplitude H-reflex during midswing was no longer present. We therefore asked whether daily training at this task without grasping the handrails would adaptively modify the H-reflex modulation pattern. In this event, within 10 days of training for 15 min daily, the anticipatory reflex activity at the beginning of training was gradually abated as the subjects reported gaining confidence at the task. However, when adapted subjects were made to walk backward with their eyes shut, the anticipatory reflex activity in midswing returned immediately. The reflex changes as a result of training were not due to changes in the motor activity or kinematics; they are likely part of the motor program controlling backward walking. This adaptive phenomenon may prove to be a useful model for studying the neural mechanisms of motor learning and adaptive plasticity in humans and may be relevant to rehabilitation programs for neurological patients.
Developmental Changes in Short-Term Synaptic Depression in the Neonatal Mouse Spinal Cord
Authors: Li Yan , Burke R. E.
Published: May 2006

We examined age-dependent changes in short-term synaptic depression of monosynaptic excitatory postsynaptic potentials (EPSPs) recorded in lumbar motoneurons in hemisected spinal cords of neonatal Swiss-Webster mice between postnatal day 2 (P2) and 12 (P12). We used four paradigms that sample the input-output dependence on stimulation history in different but complementary ways: 1) paired-pulse depression; 2) steady-state depression during constant frequency trains; 3) modulation during irregular stimulation sequences; and 4) recovery after high-frequency conditioning trains. Paired-pulse synaptic depression declined more than steady-state depression during 10-pulse trains at frequencies from 0.125 to 8 Hz in this age range. Depression during sequences of irregular stimulations that more closely mimic physiological activation also declined with postnatal age. On the other hand, the overall rate of synaptic recovery after a 4-Hz conditioning train exhibited surprisingly little change between P2 and P12. Control experiments indicated that these observations depend primarily, if not exclusively, on changes in presynaptic transmitter release. The data were examined using quantitative models that incorporate factors that have been suggested to exist at more specialized central synapses. The model that best predicted the observations included two presynaptic compartments that are depleted during activation, plus two superimposed processes that enhance transmitter release by different mechanisms. One of the latter produced rapidly-decaying enhancement of transmitter release fraction. The other mechanism indirectly enhanced the rate of renewal of one of the depleted presynaptic compartments. This model successfully predicted the constant frequency and irregular sequence data from all age groups, as well as the recovery curves following short, high-frequency tetani. The results suggest that a reduction in release fraction accounts for much of the decline in synaptic depression during early postnatal development, although changes in both enhancement processes also contribute. The time constants of resource renewal showed surprisingly little change through the first 12 days of postnatal life.
Graded Limb Targeting in an Insect Is Caused by the Shift of a Single Movement Pattern
Authors: Dürr Volker , Matheson Thomas
Published: May 2006

Grooming responses are movements of a multi-jointed limb that are targeted toward a stimulus site on the body. To be successful, they require a continuous transformation of stimulus location into a corresponding motor pattern or selection and blending of a subset of cardinal motor patterns. Tactile stimulation of one forewing of a locust elicits characteristic grooming movements of the ipsilateral hind leg. An initial targeted trajectory that moves the tarsus toward the site of stimulation is followed by a cyclic trajectory in the region of the stimulus. We have analyzed both components of this behavior to quantify the relative effects of somatotopic stimulus position and leg start posture on three parameters: initial movement direction, accuracy, and grooming distribution. Accuracy and grooming distribution were significantly affected by the stimulus location but were not influenced by the initial leg posture. Both cues systematically shifted the initial movement direction from the onset of the response. The subsequent cyclic component of grooming movements forms a behavioral continuum with no clustering in joint angle space. We therefore conclude that forewing grooming in locusts is generated by a single movement pattern that is continuously shifted by a sensory cue signaling position on the forewing surface. Both vertebrates and invertebrates can switch between distinct movement forms to groom different parts of their bodies. Our data provide the first evidence that invertebrates, like vertebrates, also have graded control of limb targeting within the somatosensory receptive field of a single form of motor response.