High-threshold Group Research Articles

Treatment thresholds for intervention in posthaemorrhagic ventricular dilation: a randomised controlled trial

ObjectiveTo compare a low versus a higher threshold for intervention in preterm infants with posthaemorrhagic ventricular dilatation.DesignMulticentre randomised controlled trial (ISRCTN43171322).Setting14 neonatal intensive care units in six countries.Patients126 preterm infants...

Long-term outcomes in patients with septic shock transfused at a lower versus a higher haemoglobin threshold: the TRISS randomised, multicentre clinical trial.

We assessed the predefined long-term outcomes in patients randomised in the Transfusion Requirements in Septic Shock (TRISS) trial. In 32 Scandinavian ICUs, we randomised 1005 patients with septic shock and haemoglobin of 9g/dl or less to receive single units of leuko-reduced red cells when haemoglobin level was 7g/dl or less (lower threshold) or 9g/dl or less (higher threshold) during ICU stay. We assessed mortality rates 1year after randomisation and again in all patients at time of longest follow-up in the intention-to-treat population (n=998) and health-related quality of life (HRQoL) 1year after randomisation in the Danish patients only (n=777). Mortality rates in the lower- versus higher-threshold group at 1year were 53.5% (268/501 patients) versus 54.6% (271/496) [relative risk 0.97; 95% confidence interval (CI) 0.85-1.09; P=0.62]; at longest follow-up (median 21months), they were 56.7% (284/501) versus 61.0% (302/495) (hazard ratio 0.88; 95% CI 0.75-1.03; P=0.12). We obtained HRQoL data at 1year in 629 of the 777 (81%) Danish patients, and mean differences between the lower- and higher-threshold group in scores of physical HRQoL were 0.4 (95% CI -2.4 to 3.1; P=0.79) and in mental HRQoL 0.5 (95% CI -3.1 to 4.0; P=0.79). Long-term mortality rates and HRQoL did not differ in patients with septic shock and anaemia who were transfused at a haemoglobin threshold of 7g/dl versus a threshold of 9g/dl. We may reject a more than 3% increased hazard of death in the lower- versus higher-threshold group at the time of longest follow-up.

Lower versus Higher Hemoglobin Threshold for Transfusion in Septic Shock

BackgroundBlood transfusions are frequently given to patients with septic shock. However, the benefits and harms of different hemoglobin thresholds for transfusion have not been established.MethodsIn this multicenter, parallel-group trial, we randomly assigned patients in the intensive care unit (ICU) who had septic shock and a hemoglobin concentration of 9 g per deciliter or less to receive 1 unit of leukoreduced red cells when the hemoglobin level was 7 g per deciliter or less (lower threshold) or when the level was 9 g per deciliter or less (higher threshold) during the ICU stay. The primary outcome measure was death by 90 days after randomization.ResultsWe analyzed data from 998 of 1005 patients (99.3%) who underwent randomization. The two intervention groups had similar baseline characteristics. In the ICU, the lower-threshold group received a median of 1 unit of blood (interquartile range, 0 to 3) and the higher-threshold group received a median of 4 units (interquartile range, 2 to 7). At 90 days after randomization, 216 of 502 patients (43.0%) assigned to the lower-threshold group, as compared with 223 of 496 (45.0%) assigned to the higher-threshold group, had died (relative risk, 0.94; 95% confidence interval, 0.78 to 1.09; P=0.44). The results were similar in analyses adjusted for risk factors at baseline and in analyses of the per-protocol populations. The numbers of patients who had ischemic events, who had severe adverse reactions, and who required life support were similar in the two intervention groups.ConclusionsAmong patients with septic shock, mortality at 90 days and rates of ischemic events and use of life support were similar among those assigned to blood transfusion at a higher hemoglobin threshold and those assigned to blood transfusion at a lower threshold; the latter group received fewer transfusions. (Funded by the Danish Strategic Research Council and others; TRISS ClinicalTrials.gov number, NCT01485315.)

Choosing a Community: Providing Local Public Goods When Opting Out is an Option

Different community organizations require varying levels of contribution and effort from, and compete for the time, effort, and resources of a heterogeneous population of potential volunteers. Many are not able to sanction free-riders in any meaningful way, and must account for people's ability to opt out of joining groups altogether. I conduct threshold local public goods experiments with two groups, one with a low threshold and one high, and subjects with randomly distributed opportunity costs (wages''). I allow subjects to opt out of joining a group in some treatments. Overall, I find that subjects self-select -- high-wage subjects choose the low-threshold group, and vice versa. An fee'' -- a small required contribution -- for the high-threshold group increases both provision rates and low-wage subjects' earnings. When subjects are allowed to opt out of joining a group, provision rates fall, but the low-cost group continues to thrive as long as the high-cost group does not have an entry fee. When it does, it once again has a higher provision rate than the low-cost group. Allowing opting out worsens earnings overall for the low-wage individuals, but increases them for high-wage individuals.

Early surfactant therapy and nasal continuous positive airways pressure for mild respiratory distress syndrome - a pilot study

Objectives To determine if the administration of Curosurf® 100 mg/kg within 24 hours after birth, to infants with respiratory distress syndrome (RDS) receiving nasal continuous positive airways pressure (NCPAP) and inspired oxygen (FiO2) 0.3–0.4, decreased the need for mechanical ventilation (MV) during the first week of life when compared to infants in whom the required FiO2 was allowed to rise above 0.4 before Curosurf® was administered. Design and subjects Twenty-seven preterm infants were randomised into either a low- or a high-threshold group. The low-threshold group received Curosurf® immediately and the high-threshold group received Curosurf® if their FiO2 rose above 0.4. Infants who received surfactant were returned to NCPAP if respiratory effort was adequate. Setting The Neonatal Intensive Care Unit, Groote Schuur Hospital, Cape Town. Results The mean gestational age for the entire cohort was 31  2 weeks. There were no significant differences between the groups with respect to the need for MV in the first week of life. However, the duration of any form of assisted ventilation (NCPAP and MV) was significantly less in the low-threshold group (p= 0.042). The low-threshold group had a lower mean PaCO2 at 24 hours (p = 0.015). Conclusions The administration of 100 mg/kg Curosurf® to preterm infants with RDS requiring NCAP at a threshold FiO2 0.3–0.4, improved alveolar ventilation and reduced the duration of any form of assisted ventilation, compared to waiting until the FiO2 was above 0.4. There was no significant reduction in the need for MV.

Guidance of Block Needle Insertion by Electrical Nerve Stimulation

Little is known regarding the final needle tip location when various intensities of nerve stimulation are used to guide block needle insertion. Therefore, in control and hyperglycemic dogs, the authors examined whether lower-intensity stimulation results in injection closer to the sciatic nerve than higher-threshold stimulation. During anesthesia, the sciatic nerve was approached with an insulated nerve block needle emitting either 1 mA (high-current group, n = 9) or 0.5 mA (low-current group, n = 9 in control dogs and n = 6 in hyperglycemic dogs). After positioning to obtain a distal motor response, the lowest current producing a response was identified, and ink (0.5 ml) was injected. Frozen sections of the tissue revealed whether the ink was in contact with the epineurium of the nerve, distant to it, or within it. In control dogs, the patterns of distribution using high-threshold (final current 0.99 +/- 0.03 mA, mean +/- SD) and low-threshold (final current 0.33 +/- 0.08 mA) stimulation equally showed ink that was in contact with the epineurium or distant to it. One needle placement in the high-threshold group resulted in intraneural injection. In hyperglycemic dogs, all needle insertions used a low-threshold technique (n = 6, final threshold 0.35 +/- 0.08 mA), and all resulted in intraneural injections. In normal dogs, current stimulation levels in the range of 0.33-1.0 mA result in needle placement comparably close to the sciatic nerve but do not correlate with distance from the target nerve. In this experimental design, low-threshold electrical stimulation does not offer satisfactory protection against intraneural injection in the presence of hyperglycemia.

Blue light regulation of host pigment in reef-building corals

Reef-building corals harbor an astounding diversity of colorful GFP (green fluorescent protein)-like proteins. These pigments can easily be detected and thus may serve as intrinsic optical markers of physiological condition, provided that the determinants that control their expression are well understood. Here we have analyzed the effect of light on the regulation of major classes of GFP- like pigments in corals of the taxa Acroporidae, Merulinidae and Pocilloporidae. Pigment levels in the tissues of all studied species were observed to be tightly controlled by light. Two groups could be dis- tinguished by their distinctly different light-dependent regulation. The low-threshold group contains mainly cyan fluorescent proteins; they are expressed in considerable amounts at very low light inten- sities, and their tissue content increases with light to a maximum at a photon flux of 400 µmol m -2 s -1 . The high-threshold group includes green and red fluorescent proteins as well as non-fluorescent chromoproteins. These pigments are essentially absent in corals grown under very low light, but their tissue content increases in proportion to photon flux densities >400 µmol m -2 s -1 . The enhancement of coral pigmentation is primarily dependent on the blue component of the spectrum and regulated at the transcriptional level. The specific regulation patterns suggest complex functions of GFP-like proteins related to the photobiology of reef corals. Moreover, the distinct response of coral coloration to light climate promises that the pigment complement can also be predicted in natural habitats. Our results stress the potential of GFP-like proteins as intrinsic markers of physiological processes, as well as overall health, in corals.

Mechanism of Reflex Regulation of the Gastroduodenal Function by Acupuncture

Many clinical studies focus on the effects of acupuncture on digestive disorders. However, few studies describe the mechanism by which these effects are produced. We present some recent experimental work on the mechanism of acupuncture for reflex regulation of gastroduodenal function in anesthetized rats. In anesthetized rats, it has been proven that acupuncture to the abdomen excites sympathetic nerves via spinal reflexes causing inhibition of motilities while acupuncture of limbs excites vagus nerves via supraspinal reflexes causing an increase in the motilities. It has also been shown that in order to inhibit gastric motilities, acupuncture stimulation of the abdomen must be strong enough to excite group VI fibers of the afferent intercostal nerves. To increase gastric motilities, acupuncture stimulation to hind limbs must be strong enough to excite the high-threshold group III fibers of tibial nerves. It has also been shown that the neural mechanism of duodenal motility stimulation by acupuncture involves the same body regions and intensity of stimulation as that of gastric motilities. Theories regarding the underlying mechanism have proposed somato-autonomic reflexes and responses via endogenous opioids, etc., but without definitive conclusions.

Afterhyperpolarization?firing rate relation of turtle spinal neurons

This study addressed the afterhyperploarization-firing rate relationship of unanesthetized turtle spinal motoneurons and interneurons. The afterhyperploarization of their solitary action potential at rheobase was compared to that during the cells' minimum and maximum firing rates. Like previous mammalian findings, afterhyperpolarization duration and area at rheobase were 32 and 19% less for high- versus low-threshold motoneurons. Contrariwise, maximum firing rate was two times less for the high-threshold group. Other new findings were that for high- versus low-threshold interneurons, afterhyperpolarization duration and area were 25 and 95% less, and maximum firing rate 21% higher for the high-threshold group. For combined motoneurons versus interneurons, there were no differences in afterhyperpolarization duration and area at rheobase, whereas maximum firing rate was 265% higher for the interneurons. For high-threshold motoneurons alone, there were significant associations between minimum firing rate and afterhyperpolarization duration and area measured at rheobase. In summary, this study showed that (1) the afterhyperploarization values of both turtle spinal motoneurons and interneurons at rheobase provided little indication of their corresponding values at the cells' minimum and maximum firing states, and (2) the evolution of afterhyperploarization from rheobase to maximum firing state differed both qualitatively and quantitatively for motoneurons versus interneurons.

270 A Randomized Controlled Trial of Low vs High Hemoglobin Thresholds for Transfusion in Extremely Low-Birth-Weight Infants

Background: Infants of extremely low-birth-weight (ELBW, or <1000g) are frequently exposed to multiple transfusions of packed red blood cells. Our objective was to evaluate the effects of using low vs. high levels (thresholds) of blood hemoglobin concentration for transfusion of ELBW infants on mortality or severe morbidity until discharge.Methods: We selected infants <1000 g birth-weight and <48 hr of age, excluding infants with established blood disease, shock or sepsis. We randomized 451 (223 low, 228 high) infants to transfusion threshold algorithms of low vs. high hemoglobin. The algorithms differed by 10–20 g/l and were adjusted for age and need for respiratory support (low from 115 to 75 g/l, high from 135 to 85 g/l). These were maintained until discharge. Additional transfusions were permitted for shock, sepsis or surgery.Results: Maternal and infant prognostic factors were similar between the groups. Median (IQR) birth-weight was 770 (670–887) g and gestational age 26 (25–27) wk. Transfusion and Hematologic Outcomes: Comparing infants in the low vs. high threshold group, the time to first transfusion was delayed by a median of two days, fewer infants were exposed to any transfusion (194 (87%) vs. 214 (94%), p<0.01), and fewer transfusions were given (means 4.8 vs. 5.5, p=0.09). Hemoglobin levels fell faster from admission to discharge (from 164 to 101g/l vs. from 165 to 111g/l; mean difference after first week was 11 g/l, p<0.0001). Clinical Outcomes: There were no significant differences between low and high threshold groups respectively in mortality (22% vs. 18%, OR 1.3 95% c.i. 0.8–2.2), bronchopulmonary dysplasia as oxygen supplementation at 36 wk (59% vs. 56%, OR 1.2 c.i. 0.7–1.8), retinopathy of prematurity of stage 3 or worse(18% vs. 17%, OR 1.2 c.i.0.7–2.0) or periventricular leukomalacia/ventriculomegaly by ultrasound (19% vs. 21% OR 0.8 c.i. 0.5–1.4). Rates of combined death or severe morbidity were not significantly different (74% vs. 70% (OR 1.3 c.i. 0.8–2.0). Overall, 28% of all infants (or 35% of survivors) survived with no severe morbidity to discharge.Conclusion: Using a regimen of low rather than high hemoglobin transfusion thresholds results in a reduced exposure to blood transfusion for infants of extremely low birth weight with postnatal hemoglobin levels approximately 11 g/l lower. There is no difference in mortality or severe morbidity in these very high-risk infants managed with low vs. high hemoglobin thresholds.

An electrophysiologic study of mechanoreceptors in the sacroiliac joint and adjacent tissues.

The somatosensory afferent units in the sacroiliac joint of an animal model were investigated using an electrophysiologic technique. To identify the mechanosensitive receptive fields in the sacroiliac joint, and to determine their distribution and characteristics. The sacroiliac joint is considered to be a source of lower back pain. Although there have been clinical studies on the diagnosis of sacroiliac joint pain, no satisfactory diagnostic method other than joint blocks has been reported. It still is not clear whether the sacroiliac joints actually transmit pain to the central nervous system. The sensory innervation of the sacroiliac joint has not been fully characterized neurophysiologically. Experiments were performed on 10 adult cats weighing 2.6 to 4 kg. The animals were anesthetized with intravenous sodium pentobarbital. An L4-L7 laminectomy was performed. The L4-L6 dorsal roots were cut at their proximal ends, split, and draped over a bipolar recording electrode. Glass probes were used to search the sacroiliac joint and adjacent tissues for mechanosensitive units. When units were identified, they were stimulated electrically to obtain conduction velocities and by Semmes-Weinstein monofilaments to determine mechanical thresholds. In the sacroiliac joint and adjacent muscles, 29 discrete mechanosensitive units were identified. Of these 29 units, 26 were found in the posterior sacroiliac ligament and the remaining 3 in the adjacent muscles. Also, 16 units (55%) were identified in the proximal third of the sacroiliac joint. Conduction velocities of the units ranged from 3.1 to 22 m/second (average, 9.2 m/second), and 26 units were group III. Mechanical thresholds of the units ranged from 4.6 to 164.3 g (average, 69.7 g). Whereas 28 units (96.6%) had thresholds higher than 7 g, one unit (3.4%) had a threshold lower than 7 g. Group III units with mechanical thresholds higher than 7 g may serve as nociceptors, and units with thresholds of lower than 7 g may serve as proprioceptors. The current study showed that most of the units in the sacroiliac joint were high-threshold group III units that perhaps had a nociceptive function. This result suggests that the sacroiliac joint may be a source of lower back pain in humans. This study also showed that the sacroiliac joint has little proprioceptive function.

The effect of lumbar sympathectomy on increased tactile sensitivity in spinal nerve ligated rats

The aim of this study was to investigate the reason for the contradictory results following surgical sympathectomy on increased tactile sensitivity in spinal nerve ligated rats. For this purpose, firstly the results of L5 spinal nerve ligation alone and both L5 and L6 (L5/6) spinal nerve ligation were compared in Sprague–Dawley rats. Secondly, the difference in tactile sensitivity between the plantar surface (the middle glabrous area on the foot pads of the hind paw) and on the toe (the proximal half of the third and fourth toe of the hind paw) after the spinal nerve injury was studied. Third, we divided the L5 spinal nerve ligated rats into two groups, (i.e. low and high threshold groups) based on the degree of tactile sensitivity and investigated the effect of surgical lumbar sympathectomy (L2–L5) on tactile sensitivity in both the plantar and toe areas. The results show that the tactile sensitivities of L5 spinal nerve ligated rats and L5/6 spinal nerve ligated rats were not different. However, tactile sensitivities of the plantar surface were less than those of toe area suggesting that the response from toe is a better indicator of neuropathic pain. Surgical sympathectomy reduced the response from only the toe area and only in the low threshold group. These results suggest that the reason for the contradictory results of surgical sympathectomy in spinal nerve ligation models is, at least in part, the difference in the degree of mechanical allodynia in each study.

Mechanosensitive afferent units in the shoulder and adjacent tissues.

Electrophysiologic experiments were done to identify the somatosensory afferent units in the shoulder and adjacent tissues of the rabbit. Conduction velocity and mechanical threshold of the units were examined to determine whether the units have a nociceptive function or proprioceptive function. Eighty mechanosensitive units were identified in the shoulder and adjacent tissues. Forty-six of these units were found in the rotator cuff area, 26 in the supraspinatus muscle, and eight in the infraspinatus muscle. These tissues contained low to high threshold Group II and Group III units. This study showed there are nociceptors that receive pain sensation and proprioceptors that receive position sense with motion of the shoulder in the rotator cuff and the supraspinatus and infraspinatus muscles. The high density of nociceptors found in the rotator cuff and infraspinatus muscles near their insertion to the shoulder suggests these tissues are more sensitive to noxious stimuli.

Effect of vasopressin on neuronal responses of theNucl. Caudalis of the spinal trigeminal tract in rats

Effect of vasopressin on responses of individual neurons of thenucl. caudalis of the spinal trigeminal tract was studied on rats under urethan anesthesia; the responses were evoked by nociceptive (stimulation of the tooth pulp) or non-nociceptive (stimulation of Aa fibers of the infraorbital nerve) afferent activation. After injection of 10 nM vasopressin into the recording zone, responses evoked by stimulation of the tooth pulp were suppressed in all studied neurons of the high-threshold group; the same was true as to responses induced by stimulation of the tooth pulp and infraorbital nerve in most neurons of the convergent group. At the same time, vasopressin did not change the responses evoked by stimulation of Aa fibers of the infraorbital nerve in neurons of the low-threshold group. Possible involvement of vasopressin in the process of pain suppression is discussed.

Transmission in a locomotor-related group Ib pathway from hindlimb extensor muscles in the cat.

It has been previously shown that phasic stimulation of group I afferents from ankle and knee extensor muscles may entrain and/or reset the intrinsic locomotor rhythm; these afferents are thus acting on motoneurones through the spinal rhythm generators. It was also concluded that the major part of these effects originates from Golgi tendon organ Ib afferents. Transmission in this pathway to lumbar motoneurones has now been investigated during fictive locomotion in spinal cats injected with nialamide and L-DOPA, and in decerebrate cats with stimulation of the mesencephalic locomotor region. In spinal cats injected with nialamide and L-DOPA, it was possible to evoke long-latency, long-lasting reflexes upon stimulation of high threshold afferents before spontaneous fictive locomotion commenced. During that period, stimulation of ankle and knee extensor group I afferents evoked oligosynaptic excitation of extensor motoneurones, rather than the "classical" Ib inhibition. Furthermore, a premotoneuronal convergence (spatial facilitation) between this group I excitation and the crossed extensor reflex was established. During fictive locomotion, in both preparations, the transmissions in these groups I pathway was phasically modulated within the step cycle. During the flexor phase, the group I input cut the depolarised (active) phase in flexor motoneurones and evoked EPSPs in extensor motoneurones; during the extensor phase the group I input evoked smaller EPSPs in extensor motoneurones and had virtually no effect on flexor motoneurones. The above results suggest that the group I input from extensor muscles is transmitted through the spinal rhythm generator and more particularly, through the extensor "half-centre". The locomotor-related group I excitation had a central latency of 3.5-4.0 ms. The excitation from ankle extensors to ankle extensors remained after a spinal transection at the caudal part of L6 segment; the interneurones must therefore be located in the L7 and S1 spinal segments. Candidate interneurones for mediating these actions were recorded extracellularly in lamina VII of the 7th lumbar segment. Responses to different peripheral nerve stimulation (high threshold afferents and group I afferents bilaterally) were in concordance with the convergence studies in motoneurones. The interneurones were rhythmically active in the appropriate phases of the fictive locomotor cycle, as predicted by their response patterns. The synaptic input to, and the projection of these candidate interneurones must be fully identified before their possible role as components of the spinal locomotor network can be evaluated.

Recovery characteristics of auditory nerve fibres in the normal and noise-damaged guinea pig cochlea

Spontaneous activity was analysed in auditory-nerve fibres innervating normal and noise-damaged cochleas. Spike occurrences were conceived as point processes. Joint interval distributions and serial correlation coefficients reveal a weak history effect for succeeding intervals. The point process is regarded as a renewal and the recovery function, being proportional to the hazard function, is determined from the interval probability density function. In 29 out of 60 fibres the latter shows peculiarities which result in a deviation from a monotonically increasing recovery function. For three fibres of low characteristic frequency the interval probability function shows an oscillatory pattern and for 26 fibres this function exhibits an early, sharp peak around 1.1 ms irrespective of characteristic frequency, spontaneous rate, or cochlear damage. The recovery function is not different between fibres with normal and those with abnormally high thresholds and exhibits an exponential recovery with one time constant of average value 1.6 ms. Bursting activity is found in only one fibre from the abnormally high threshold group.

Influences of central gray matter stimulation on thalamic neuron responses to high- and low-threshold stimulation of trigeminal nerve structures

Responses of the thalamic non-specific medial nuclei and relay ventral posteromedial nucleus neurons evoked by stimulation of the teeth pulp of A α and A α fibers of infraorbital nerve and caudal nucleus of the spinal trigeminal tract were studied in cats under thiopental-chloralose anesthesia. Three groups of neurons were revealed: low-threshold, high-threshold and convergent. In medial nuclei, 29% of neurons formed the low-threshold group, 12% made up the high-threshold group and 59% were convergent. In the ventral posteromedial nucleus 47% of neurons formed the low-threshold group, 4% made up the high-threshold group and 49% were convergent. Ninety per cent of the medial nuclei neurons and 79% of the ventral posteromedial nucleus neurons responded to stimulation of the caudal nucleus spinal trigeminal tract. The conditioning central gray matter stimulation could suppress responses of low-threshold, high-threshold and convergent neuron groups. In medial nuclei 100% of responses induced by stimulation of teeth pulp and A α fibers of the infraorbital nerve were completely suppressed and 86% of responses induced by stimulation of the A α fibers of the infraorbital nerve stimulation were also completely suppressed. In the ventral posteromedial nucleus 40% of responses induced by stimulation of teeth pulp and A αfibers of the infraorbital nerve were completely suppressed and 26.4% of responses induced by stimulation of the A α fibers of the infraorbital nerve were completely suppressed. Conditioning stimulation of the central gray matter suppressed responses evoked by stimulation of the caudal nucleus spinal trigeminal tract and in the most part neurons of the medial nuclei and neuron responses of the ventral posteromedial nucleus. The inhibitory influence of central gray matter stimulation on neuronal responses evoked by stimulation of the caudal nucleus spinal trigeminal tract shows that the central gray matter had a direct influence on the activity of thalamic neurons.

Neurophysiological study of bulbar projections of different groups of superior laryngeal afferents

The projections of superior laryngeal afferents to bulbar neuronal structures were investigated using ortho- and antidromic testing during acute experiments on cats anesthetized with Nembutal. It emerged that endings of the most excitable (group Aβ) afferent fibers were mostly distributed ipsilaterally within the solitary tract nucleus, the adjoining portion of the lateral tegmental field, and the rostral section of the retrofacial nucleus. High threshold (group Aγ and Aδ) afferent fibers also terminate in these nuclei, but are distributed over a wider area than low-threshold afferent projections in the solitary tract nucleus and the lateral tegmental field. The reverse applied to the retrofacial nucleus. Terminals of high threshold (group Aδ) afferents extended to the caudal trigeminal nerve spinal tract nucleus, and possibly to the dorsal motor nucleus of the vagus nerve at obex level.

Modulation of the somatosensory evoked potentials by the input information originating from the gastrocnemius and sural nerves in the dog.

Input-output characteristics of the somatosensory system were studied in the dog. Somatosensory evoked potentials were recorded from the scalp at stimulations of the sural and gastrocnemius nerves. There was a sigmoid relationship between the stimulus strength and the amplitude of the sural nerve action potential, which was best described by a logistic equation. The early waves (P1, N1) increased especially at low threshold stimulations of the sural nerve; then they remained either unchanged or decreased at high threshold stimulations within the Group II and III ranges. The late waves (P2, N2) especially increased at high threshold Group II stimulations of the sural nerve, and increased further at strong stimuli. Similar changes were observed at stimulations of the gastrocnemius nerves. The attenuation of the early waves was explained by "gating" mechanisms acting from the sensorimotor cortex onto the sensory relay nuclei. The increase of the late waves at especially high threshold Group II and III stimulations was accounted for by the multisynaptic diffuse projection from the reticular formation and/or the nonspecific thalamic nuclei to the somatosensory cortex. In a three weeks old pup, stimulation of the gastrocnemius nerves evoked somatosensory responses with similar morphology observed in adult dogs, but the latencies of the evoked potential components were relatively long, presumably as a result of the uncompleted myelination in the somatosensory afferent system.

Response characteristics of neurons in the ventral posterior lateral nucleus of the monkey thalamus.

The activity of 132 neurons in the caudal part of the ventral posterior lateral nucleus (VPLc) of the thalamus was recorded from 23 anesthetized monkeys. All single thalamic units that could be excited by electrical search stimuli applied to the contralateral sciatic nerve were investigated. Responses of these cells to mechanical, thermal, and electrical stimuli applied in the periphery indicated that at least half of the sampled cells were nociceptive. Based on responses to graded mechanical stimuli applied to the periphery, 110 of the sampled cells that received a predominant input from cutaneous receptive fields were classified. There were 56 low-threshold, 39 wide dynamic range, and 15 high-threshold cells. The same neurons were also classified into five mechanical types based on a cluster analysis: types 1-5 contained 25, 34, 17, 10, and 24 cells, respectively. The fact that about half the population of cells belonged to either the wide dynamic or the high threshold group (or mechanical types 3-5) suggested that a large population of VPLc neurons respond to mechanical nociceptive stimuli either exclusively or preferentially. Responses of 63 thalamic neurons were tested to noxious heat pulses applied to their cutaneous receptive fields with a contact thermostimulator. Of these, 47 cells were excited, whereas only 16 cells did not respond. The peripheral nerve that innervated the receptive field of each of 82 thalamic neurons was stimulated with graded strengths to activate A fibers only or both A and C fibers. All tested cells responded to peripheral A fiber volleys. In addition, 42 of these cells responded to peripheral C fiber volleys. The C fiber responses could be either short lasting (a few hundreds of milliseconds) or long lasting (up to several seconds). The recording sites of 80 cells were reconstructed. Of these, 78 were in the VPLc nucleus and the remaining two were in the reticular nucleus of the thalamus. No obvious relationship between the response characteristics and the locations of the cells within the VPLc nucleus was found. Sampled thalamic units had a variety of sources of input from the periphery, including both cutaneous and/or deep tissue receptive fields. The majority of the cells, however, had exclusively cutaneous receptive fields. The sizes of the cutaneous receptive fields were often very small, so that nearly half (41%) of the receptive fields of cells sampled occupied an area of skin smaller than half the foot.(ABSTRACT TRUNCATED AT 400 WORDS)