Actitud ante las caídas de los ancianos. Parte I: “El anciano que se cae” / Attitudes concerning falls in the elderly. Part I: "The elderly person who falls"

Se trata de una revisión acerca de las caídasen el anciano: La importancia del problema, elequilibrio y las caídas, epidemiología de las caídasy consecuencias de las mismas.Qué hacer con un anciano que sufre caídas.Cuáles son sus factores de riesgo, cómo evaluarlosy cómo hacer intervención en prevención

The article deals with falls in the elderly: the relevanceof the problem, balance and falls, as wellas the epidemiology of falls and their consequences.There is some discussion of what to do withthe elderly person who suffers from falls, whichare the main risk factors, how these may be evaluatedand how we can intervene towards prevention

Actitud ante las caídas de los ancianos. Parte II: “El anciano que se-nos-cae” / Attitudes Concerning Falls in the Elderly. Part II: “The elderly person who falls while under our care”

Nuestra experiencia personal y el análisis delas caídas ocurridas en los ancianos de las ResidenciasGeriátricas y del Hospital de Media-LargaEstancia de Fundación Matía, nos llevan a reflexionarsobre las caídas en los usuarios muydiscapacitados, incapaces de caminar de formaautónoma.Ello, nada tiene que ver con el concepto “Caídas-Síndrome Geriátrico” sino más bien conunos adecuados cuidados gerontológicos, con lanecesidad de clasificar el grado de riesgo y ensu caso con las adecuadas medidas de restricciónfísica

Our personal experience and analysis of thefalls suffered by elderly people in Fundación Matía´s Geriatric Nursing Homes and Medium-Longterm Care Hospital leads us to reflect upon fallsof severely disabled users who are unable towalk autonomously.This has little to do with the concept of “GeriatricSyndrome – Falls”, but rather with adequategerontological care, as well as the need to establishthe level of risk and, adequate, physical restraintmeasures when needed

Cholinergic modulation of the picrotoxin-induced electrocorticographical events and behavioral "pain-like" symptoms at somatomotor cortical level in the rat.

In this study, we examined the modulation by acetylcholine of electrocorticographical (ECoG) ictal events and spontaneous pain-like behaviors following cortical application of the GABA(A) antagonist picrotoxin in the awake rat. Distilled water as vehicle, the cholinomimetic substance eserine, and the general muscarinic antagonist atropine were microinjected 10 min before the second microinjection of 2 microg picrotoxin into the hind paw region of the somatomotor cortex (SmI). Under these conditions, we observed that eserine (physostigmine, 1 microg, 10 microg, and 20 microg) did not consistently modify the number of the picrotoxin-induced ECoG spikes and bursts, but instead produced a massive enhancement of the number of hind paw licks compared with vehicle at 10 microg and, to a lesser extent, the number of the stereotyped "turn-in" and "neglected" paws following picrotoxin. In contrast, atropine (1 microg, 10 microg, and 20 microg) increased the number of the picrotoxin-induced spikes and bursts at 10 microg and, at all doses, decreased the number of the picrotoxin-induced pain-like symptoms. Statistically significant changes for the number of paw lifts, licks, and "turn-in" paws were observed only with 10 microg. These results tend to show that epilepsy and pain are not strictly related to each other and also emphasize the cortex as a target for interactions between GABA and acetylcholine relative to "central" pain.

Picrotoxin produces a "central" pain-like syndrome when microinjected into the somato-motor cortex of the rat.

In this study, we report the possibility of producing marked electrocorticographic changes and "pain-like" reactions, when the GABAA antagonist picrotoxin is microinjected unilateraly into the rat somato-motor Sml cortex in the region of the hind paw. After the microinjection, we observed continuous seizure isolated spikes, spikes-and-waves, bursts, and pain-like reactions, almost exclusively confined to the hind paw. These reactions considered of lifting off the floor, licking of the paw palm or digits, biting, paw tremors, and a peculiar paw position that we called "turn-in" paw. We also noted other behaviors, such as "limping," "neglected" paw, or rearing. The "pain-like" character of these manifestations was suggested by the fact that similar qualitative and quantitative data occurred consequent to the administration of 2.5% diluted formalin into the palm of the hind paw in different rats. Bringing together the electrocorticographic events and the behavioral reactions produced by Sml picrotoxin indicated that there was no obvious correlation between the phenomena, except that the tremor was always associated with the bursts. Sensory denervation of the hind paw, produced by sciatic and saphenous nerve transections, did not significantly modify either the ictal activity or the behavior. Finally, microinjection of naloxone prior to picrotoxin did not change the cortical events, but greatly diminished the "pain-like" reactions. All these results favor the cortical microinjection of a GABAA receptor antagonist as a good rat model for studying pain of "central" origin. They emphasize the possible role of the Sml cortex in such a phenomenon, and the deficit of cortical GABAergic processing, which can include an opioid link.

The "plantar test" apparatus (Ugo Basile Biological Apparatus), a controlled infrared noxious radiant heat stimulus for precise withdrawal latency measurement in the rat, as a tool for humans?

In the present study, we precisely and automatically measured the withdrawal latency to noxious radiant heat application in unrestrained male rats and in human subjects of both sexes, by means of the "plantar test" apparatus (Ugo Basile Biological Apparatus). The infrared light stimulus of this tool was applied underneath the hindpaws of rats and the middle fingers of human subjects. With one right and one left stimulation every 10 min, we observed a decrease in latency over a 40-min testing period in rats; the latency reached a mean value of 5.08 +/- 0.25 sec after 40 min with a 36-W stimulus, which corresponded to 46.5 degrees C. In pilot experiments, also performed on rats, we showed that the opiate morphine (10 mg/kg, i.p.) produced remarkable increases of the withdrawal latency only in "naive" animals (i.e., ones that had never experienced the plantar test stimulus) and not in animals "habituated" to it. Among humans, we noted gender differences, such as less sensitivity to the infrared noxious radiant heat for women, particularly during the menstrual period. A difference from rats was that there was no significant latency modification along the 40-min testing period for either women or men, with a mean latency of 5.61 +/- 0.18 sec (47.5 degrees C) for the women and 4.39 +/- 0.10 sec (45.5 degrees C) for the men. These data confirm the reliability of the plantar test in rats, and demonstrate the possible use of an infrared source in human subjects as a noxious heat stimulus; the withdrawal reaction to this stimulus is emphasized as a good index of nociception in humans.

Does barbiturate anesthesia modify the neuronal properties of the somatosensory thalamus? A single-unit study related to nociception in the awake-pentobarbital-treated rat.

By means of extracellular recordings, we studied thalamic ventrobasal complex neurons of rats tested first awake, and then anesthetized with pentobarbital. In both conditions, we found two groups of units in both states. The first group, displaying a spontaneous bursting activity, was not obviously responding to peripheral stimuli. Another group, displaying a single-spike activity, was almost exclusively activated by innocuous and/or noxious and innocuous mechanical stimuli. Still in this group, units specifically driven by noxious stimuli were only found under pentobarbital. These data, different from classical findings, emphasize the interest of the awake preparation in order to study nociceptive cellular mechanisms at the thalamic level.

Single-unit recordings at dorsal raphe nucleus in the awake-anesthetized rat: spontaneous activity and responses to cutaneous innocuous and noxious stimulations.

In this study, we recorded the single-unit activity of the dorsal raphe nucleus (DRN) in rats tested first awake and, a few days later, anesthetized with sodium pentobarbital and recorded again. This was achieved by means of a small chronically implanted device supporting a 25 micron platinum-iridium wire as the recording electrode. In both the awake and anesthetized conditions, and in agreement with most of the studies performed at the DRN level, we found that a vast majority of the units, displaying small amplitude and long-duration action potentials, possessed a low level of spontaneous activity (0.2-4 Hz). Among these units, found in greater number under pentobarbital, it was possible to establish that this activity was regular or irregular, in accordance with the literature reports. However, as opposed to these studies, we determined that the 'regularity' was relative, only noticeable in more or less prolonged phases of activity. In particular, we never recorded the so-called 'clock-like' activity, largely reported as an unambiguous criterion for selecting the serotoninergic neurons. In both the awake and anesthetized conditions, the responses of the DRN neurons to peripheral mechanical innocuous and noxious stimulations were observed in only one-half of the units recorded and were weak in comparison to other results that we obtained at the nucleus raphe magnus level in previous studies. When present, these responses were excitation or inhibition, occurring during or after the stimulus application. These results question the direct involvement of the DRN in acute nociception.

Are ventromedial medulla neuronal properties modified by chronic peripheral inflammation? A single-unit study in the awake, freely moving polyarthritic rat.

In the present work, we recorded the neuronal properties of the ventromedial medulla, a brainstem structure involved in the descending spinal control systems related to nociception, in awake, freely moving healthy and polyarthritic rats. These animals were rendered polyarthritic with a subcutaneous administration of the Freund's adjuvant into the tail, and studied at 20 and 30 days post-inoculation. At the ventromedial medulla level, the single-unit activities were recorded by means of a chronically implanted device supporting a 50 microns platinum-iridium wire as the recording electrode. With a total of 308 recorded neurons, we determined that in both healthy rats, i.e. animals having received mineral oil only and arthritic rats, there were ventromedial medulla units with common physiological properties, but also changes. In agreement with the results from anesthetized arthritic rats at spinal and thalamic levels, the systematic analysis of the responses to light touch and mechanical shock revealed that the 'multimodal, multireceptive' units, excited by innocuous and noxious stimuli, were much more responsive to both modalities in arthritic rats. Approximately 7% of these neurons displayed a 'paroxysmal' spontaneous activity, also reported in the literature for other structures. In addition, we recorded a significant number of neurons inhibited or excited-inhibited by innocuous and noxious cutaneous stimulations, and a few with a regular spontaneous activity, also responding, which has never been the case in healthy rats. We conclude that a peripheral chronic inflammation, such as arthritis, can produce changes of the ventromedial medulla neuronal properties, as compared to healthy animals. Consequently, in addition to its classical role in the spinal control of nociception, the ventromedial medulla is able to develop some form of plasticity in the case of persistent pain of peripheral origin.

The GABAA receptor antagonist picrotoxin induces a 'pain-like' behavior when administered into the thalamic reticular nucleus of the behaving rat: a possible model for 'central' pain?

In this study, we reported that the microinjection of the GABAA antagonist picrotoxin into the rat thalamic reticular nucleus produced a 'pain-like' behavior. This behavior was primarily characterized by repetitive lifting off the hindpaw from the floor contralateral to the injection site, sometimes accompanied by extension of the leg and maximal fingers separation. Surprisingly, these manifestations were not occurring when picrotoxin was applied to the ventrobasal complex itself, alternatively producing 'wet-dog' shakes. These data show that the local administration of picrotoxin is a relevant approach for studying pain of 'central' origin and complex GABAergic modulatory mechanisms within the thalamic sensory complex.

Thalamic VPM nucleus in the behaving monkey. III. Effects of reversible inactivation by lidocaine on thermal and mechanical discrimination.

1. The present study evaluates the necessity of the ventroposterior medial thalamic nucleus (VPM) for discrimination of the intensity of noxious heating, innocuous cooling, and innocuous tactile (airpuff) stimulation of the maxillary skin. 2. Two rhesus monkeys were trained to detect small differences (< 1.0 degrees C) in the intensity of noxious heat (near 46 degrees C) and innocuous cold (near 30 degrees C) as well as differences in the force of an airpuff applied to the skin over the maxilla. As a control the monkeys also detected small differences in the intensity of a white light. Lidocaine hydrochloride (2%) was microinjected into regions of thalamus where single-unit recordings had identified neuronal responses to the noxious heating and/or cooling stimuli. The effectiveness of the anesthetic blockade was monitored by multiunit recordings using microelectrodes positioned 1-3 mm from the orifice of the injection cannula. The monkey's ability to detect near-threshold changes in stimulus intensity was compared before and after each injection. 3. During six experimental sessions, single injections of 1-4 microliters lidocaine near the dorsomedial border of VPM did not significantly alter the monkey's ability to detect small changes in the intensity of noxious heat, cool, airpuff, or visual stimuli despite neurophysiological evidence that spontaneous neuronal activity was blocked within parts of VPM. 4. During three experiments, dual simultaneous microinjections of lidocaine (delivered through 2 microcannulae separated by approximately 1 mm) resulted in profound deficits in noxious heat discrimination, with lesser deficits in cool and airpuff discrimination; visual discrimination was never altered. Monitoring of adjacent microelectrodes revealed that although activity ventral to the injection sites was blocked, activity in medial thalamic nuclei, implicated in nociceptive processing, was probably not altered by these injections. 5. These data suggest that VPM is important for the perception of noxious and innocuous thermal stimuli as well as for the perception of tactile stimuli. However, considering the ineffectiveness of small single microinjections of lidocaine, it appears that some critical proportion of VPM must be inactivated to disrupt thermal or tactile discrimination, possibly because of overlapping receptive field properties of neurons in different areas of the nucleus.

Involvement of ventromedial medulla "multimodal, multireceptive" neurons in opiate spinal descending control system: a single-unit study of the effect of morphine in the awake, freely moving rat.

In the present work, we have studied the effects of systemic morphine on the electrophysiological properties of ventromedial medulla (VMM) neurons in the awake, freely moving rat. By means of a chronically implanted single-unit recording device, a drug delivery catheter, and the use of controlled innocuous and noxious cutaneous stimuli, we were able to study precisely the spontaneous and evoked VMM neuronal activities. We have particularly focused our attention upon the VMM "multimodal, multireceptive" units, excited by non-noxious and noxious stimuli (VMM MULT ON), which we have already determined as the neuronal class potentially involved in nociceptive processes at VMM level. We found that morphine (3 mg/kg, i.v.) does not affect the spontaneous activity of these neurons whereas their responses to noxious heat are strongly attenuated (70%), over a prolonged period (about 2 hr) associated with an increase in the response latency. This action of morphine appears to be pharmacologically specific since it is dose dependent to some extent, and is reversed by 0.3 mg/kg of naloxone. In parallel with this pharmacological specificity, we have also demonstrated a preferential physiological effect since the response of the VMM MULT ON neurons to light touch application is not affected by morphine. This specificity is emphasized by the fact that morphine does not modify the activity of the other VMM neuronal groups such as the units unresponsive to any kind of peripheral stimuli, and does not reveal "new" neuronal classes such as those we have found in previous studies after barbiturate administration. The differential effect upon the noxious versus innocuous inputs of these units produced by the opiate reinforces their participation in nociceptive processing since similar effects have been reported in well-known nociceptive somatosensory structures such as the dorsal horn of the spinal cord. Furthermore, although the precise mechanisms of action have not yet been determined, the spinal projection of the VMM MULT ON neurons, previously demonstrated by our group, suggests their involvement in an opiate descending spinal control system of nociception. Although speculative, one can imagine either a direct facilitatory MULT ON spinal effect being attenuated by morphine (disfacilitation), or a morphine-induced disinhibition of inhibitory GABAergic neurons acting upon the MULT ON neurons.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. I. Comparison of the single-unit types in the same awake and pentobarbital-treated rats.

By means of single-unit recordings, as we have already performed in other studies, we have found that in the awake, drug-free, freely moving rat, there is only one neuronal class potentially involved in nociception and its control at the ventromedial medulla level (VMM, a structure involved in the spinal descending control systems of nociception): the 'multireceptive multimodal' units. These neurons are always activated by very light mechanical (air puff, light touch) and mechanical (pinch, pin-prick) or thermal noxious stimuli, in addition to an auditory stimulus. During identical VMM penetrations, performed in the same animals tested first awake and then anesthetized a few days later with 30 mg/kg of i.p. pentobarbital, we once again found the 'multireceptive multimodal' units, but this time with physiological properties that were strongly modified: in particular, we noted a disappearance of the nociceptive responses consecutive to a strong noxious heat pulse application (36-51 degrees C), associated sometimes with a reduction of the responses due to innocuous stimulation. This is in agreement with the classical effects of barbiturates. In light of previous observations reported in the literature devoted to the VMM physiology in the anesthetized rat, the most important observation in our study was that, with pentobarbital anesthesia, we recorded 'new' neuronal classes as compared to the awake condition. In these classes, which appeared to be qualitatively similar to those already reported under anesthesia, we found the units exclusively driven by innocuous stimulation (excited for the majority), the units specifically driven by noxious stimulation (half excited, half inhibited) and a 'multireceptive multimodal' group inhibited or excited-inhibited by non-noxious and noxious stimuli (half of the multireceptive group). All these data demonstrate that barbiturate anesthesia strongly modifies the VMM physiology in relation to nociception. Furthermore, since our results, that were obtained in anesthetized rats, were qualitatively identical to those described in the literature under similar experimental conditions, they raise the question of the appropriateness of using a barbiturate anesthetic in order to study the cellular mechanisms related to nociception at this level. In addition, these findings indicate that the obtention of only one neuronal class in the awake, drug-free, freely moving rat (the excited 'multireceptive' neurons) is not due to an experimental bias, which strongly emphasizes the reliability of using awake animals. However, it remains to be determined by which mechanisms pentobarbital 'distorts' the VMM physiology as compared to the normal, standard physiological conditions of the awake animal.

Drastic changes of ventromedial medulla neuronal properties induced by barbiturate anesthesia. II. Modifications of the single-unit activity produced by Brevital, a short-acting barbiturate in the awake, freely moving rat.

In the preceding study, we have found that pentobarbital, a powerful barbiturate substance, strongly modified the ventromedial medulla (VMM) physiology in relation to nociception: indeed, in the same rats, during time-separated similar VMM penetrations, we have recorded, under pentobarbital, 'new' neuronal groups as compared to the awake state, such as the units exclusively driven (excited or inhibited) by cutaneous innocuous or noxious stimulations and the multimodal multireceptive neurons inhibited by non-noxious and noxious stimuli. Still under pentobarbital, we have also recorded the same units found as the rats were awake, i.e., the multimodal multireceptive neurons exclusively excited by various innocuous and noxious stimuli. However, the spontaneous and nociceptive activities of these units were strongly modified as compared to awake animals. Using Brevital (a short-acting barbiturate substance) administration, we have, in the present study, tried to understand the mechanisms underlying these drastic modifications. In particular, one of the questions was whether or not the 'new' neuronal classes recorded under anesthesia resulted from a modification of the physiological properties of the unique VMM neuronal group potentially involved in nociception in awake animals: the multimodal multireceptive units. By following the VMM neuronal activities either before and after or after Brevital administration until recovery from anesthesia, we have determined that the units exclusively driven by innocuous stimulation might result from a modification of the multimodal multireceptive neurons. Alternatively, the multireceptive units inhibited by peripheral stimulations are possibly totally different neurons, silent when the animals are awake.

Back-firing of ventromedial medulla neurons from the spinal cord in awake, freely moving rats.

In the awake, freely moving rat we showed, by means of single-unit recordings and antidromic spinal cord activation, that at the ventromedial medulla level, in these particular experimental conditions, the 'multimodal, multireceptive' units excited by auditory, cutaneous non-noxious and noxious stimuli are possibly involved in a spinal descending control system. These neurons were back-fired from the medial part of the lateral funiculus, hence they probably projected to the dorsal and intermediate horn of the cord, and not to the ventral horn, which emphasizes a role in the control of nociception. Due to their convergent properties, these units are probably involved in nonspecific aspects of nociception such as alertness or stress.

A single-unit recording system, contact thermal probe and electromechanical stimulator for studying cellular mechanisms related to nociception at brain stem level of awake, freely moving rats.

The purpose of this paper is to describe a simple, light-weight (3 g) device bearing a fine platinum-irridium Teflon-coated wire (50 microns) used to record single-unit activity extracellularly at brain stem level in the totally conscious freely moving rat. The up and down movements of the electrode through a guide cannula are insured by a small nut and a spring; the distance between the electrode and the end of the guide cannula is measured with a nut index. The system is directly connected to an amplifier (no FET or preamplifier) and allows for long term recordings necessary for a complete neuronal characterization and pharmacological experiments. The device is easy to make, entirely recoverable, and can be implanted from an animal to another. Further improvements are possible such as tungsten microelectrodes and telemetric or microinjection systems. In order to study some neuronal brain stem mechanisms involved in nociception, we have also designed a contact thermal probe and an electromechanical stimulator. The thermode is stuck to the shaved skin on the back of the rat, allowing heat pulses up to 51 degrees C to be applied. The mechanical stimulator is used manually and delivers reproducible innocuous stimuli to the skin. The fact that both types of stimulations are driven electrically enables the elaboration of cumulated peristimulus histograms which will reflect the neuronal activities in response to the application of noxious and non noxious stimuli.

Single unit activity at ventromedial medulla level in the awake, freely moving rat: effects of noxious heat and light tactile stimuli onto convergent neurons.

In this study, we recorded single unit activity at the ventromedial medulla (VMM) level in the awake, freely moving rat. In agreement with previous work under the same conditions, we found a vast majority of neurons which possess heterosensory and heterosegmental inputs ('convergent'). These units are activated either by auditory or mechanical innocuous and noxious stimuli applied all over the body surface. The activation threshold of these neurons is very low since light stimulation such as air puff produce intense bursts. In addition to this highly represented neuronal class, we also find another consistent VMM group of neurons which fire in relation to precise or generalized body movements. The main result of the present work is that, in addition to auditory and mechanical inputs, a relatively high proportion of VMM convergent neurons are activated by noxious heat pulses between 43 and 51 degrees C. In this range, it was possible to obtain stimulus-response functions with 2 degrees C steps only when a skin twitch reflex produced by the heat was present, also encoding the temperature intensity. In comparison to the VMM activations produced by an intense noxious heat pulse such as 51 degrees C, either auditory or controlled light touch stimuli induced a more robust response in terms of maximum frequency of discharge. Differential properties of VMM neurons in relation to innocuous and noxious information were also found using repetitive stimulation: although a strong and fast habituation of the 51 degrees C responses was observed, this phenomenon was not present for light touch induced activations. We propose that these differential properties might reflect separate pathways reaching the VMM, the one carrying innocuous information possibly relayed through the dorsal column nuclei. Although obtaining stimulus-response functions might implicate the VMM convergent neurons in the sensory-discriminative aspect of pain, their massive heterosensory and heterosegmental inputs favor a role in more general processes such as alertness or stress. Also, due to massive convergent properties, the involvement of this neuronal class in specific bulbospinal descending control systems of nociceptive information is questionable, Finally, our results obtained in the awake, freely moving rat strongly differ from the anesthetized preparation in that we found neither nociceptive specific units nor neurons inhibited by noxious peripheral stimulations largely described in this approach.

Responses of monkey medullary dorsal horn neurons during the detection of noxious heat stimuli.

1. We examined the activity of thermally sensitive trigeminothalamic neurons and nonprojection neurons in the medullary dorsal horn (trigeminal nucleus caudalis) in three monkeys performing thermal and visual detection tasks. 2. An examination of neuronal stimulus-response functions, obtained during thermal-detection tasks in which noxious heat stimuli were applied to the face, indicated that wide-dynamic-range neurons (WDR, responsive to innocuous mechanical stimuli with greater responses to noxious mechanical stimuli) could be subclassified based on the slope values of linear regression lines. WDR1 neurons exhibited significantly greater sensitivity to noxious heat stimulation than WDR2 neurons or nociceptive-specific neurons (NS, responsive only to noxious stimuli). 3. In one behavioral task, the monkeys detected 1.0 degrees C increases in noxious heat from preceding noxious heat stimuli ranging from 44 to 48 degrees C. WDR1, WDR2, and NS neurons increased their discharge frequency as a function of the intensity of the first noxious heat temperature (T1) as well as the final temperature (T2). The responses of WDR1 neurons were greater than those produced by WDR2 or NS neurons across all the temperatures examined. The order of stimulus presentation affected the responses of WDR1 neurons to 1.0 degrees C increases in the noxious heat range but not those of WDR2 or NS neurons. 4. In a second behavioral task, the monkeys detected small increases in noxious heat (0.2-0.8 degrees C) from a first temperature of 46 degrees C. Although the responses of all three classes of neurons were monotonically related to stimulus intensity, WDR1 neurons exhibited greater sensitivity to small temperature increases than either WDR2 or NS neurons. 5. Subpopulations of all three classes of neurons exhibited responses that were independent of thermal stimulus parameters or sensory modality and that only occurred during the behavioral task. These task-related responses were time-locked to specific behavioral events associated with trial initiation and trial continuation. 6. These data provide evidence that a subpopulation of WDR neurons is the dorsal horn cell type most sensitive to small increases in noxious heat in the 45-49 degrees C temperature range and provides the most information about stimulus intensity. The findings support the view that nociceptive neurons have the capacity to precisely encode stimulus features in the noxious range and that WDR neurons are likely to participate in the monkeys' ability to perceive the intensity of such stimuli.

The correlation of monkey medullary dorsal horn neuronal activity and the perceived intensity of noxious heat stimuli.

1. We examined the relationship between the activity of medullary dorsal horn nociceptive neurons and the monkeys' ability to detect noxious heat stimuli. In two different detection tasks, the temperature of a contact thermode positioned on the monkey's face increased from 38 degrees C to temperatures between 44 and 48 degrees C (T1). After a variable time period, the thermode temperature increased an additional 0.2-1.5 degrees C (T2), and the monkeys' detection speed from the onset of T2 was determined. We previously have established that detection speed is a measure of the perceived intensity of noxious thermal stimuli. Nociceptive neurons were classified as wide-dynamic-range (WDR, responsive to innocuous mechanical stimuli with greater responses to noxious mechanical stimuli) and nociceptive-specific (NS, responsive only to noxious stimuli). WDR neurons were subclassified as WDR1 and WDR2 based on the higher slope values of the stimulus-response functions of WDR1 neurons. The monkeys were trained to detect small increases in noxious heat, and their detection speeds were correlated with the responses of WDR1, WDR2, and NS neurons. 2. Detection speeds to T2 temperatures of 1.0 degrees C from preceding T1 temperatures of 45 and 46 degrees C were faster during a preceding ascending series of stimuli than during a descending series. Similarly, the peak discharge frequencies of WDR1 neurons in response to the same stimuli were greater during the ascending series of T2 temperatures. In contrast, the responses of WDR2 and NS neurons showed no significant differences during the ascending and descending series of stimuli. 3. Detection speeds following 0.4, 0.6, and 0.8 degrees C T2 stimuli were higher when the preceding T1 temperature was 46 degrees C as compared with detection speeds to the identical stimuli when the preceding T1 temperature was 45 degrees C. WDR1 neurons also exhibited a significant increase in peak discharge frequency to these same T2 stimuli when the preceding T1 temperature was 46 degrees C. In contrast, the neuronal activity of WDR2 and NS neurons did not differ on 45 and 46 degrees C T1 trials. 4. The relationship between detection speed and neuronal peak discharge frequency was examined in response to different pairs of T1 and T2 stimuli when T1 was either 45 or 46 degrees C. There was a significant correlation between detection speed and neuronal discharge for WDR1 and WDR2 neurons. No correlation was observed for NS neurons. 5. The magnitude of neuronal activity on correctly detected and nondetected trials was compared when T1 was 46 degrees C and T2 was 0.2 degree C.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of ventromedial medulla neurons in response to noxious and non-noxious stimuli in the awake, freely moving rat: a single-unit study.

The spontaneous and evoked activities of ventromedial medulla (VMM) neurons have been recorded in the chronic, awake, freely moving rat. The vast majority of neurons located at the level of the nucleus raphé magnus exhibited an irregular and variable (2-16 Hz) spontaneous activity and were activated by either cutaneous or auditory stimuli. Within this convergent neuronal class the neurons were activated by either cutaneous noxious and non-noxious inputs. The threshold for cutaneous activation was likely very low since a majority of units responded to air puffs, but the application of controlled brushing and pin-prick revealed that the VMM convergent neurons responded more for the noxious mechanical stimulation. Similar findings were found with pinch application. For both innocuous and noxious stimuli, the cutaneous receptive field was extremely extensive (almost all of the body); however, the application of the controlled brushing showed that for this innocuous stimulation, the most sensitive regions were the tail, back, snout and vibrissae and, to a lesser extent, the flank and paws. Preliminary experiments indicated that both the spontaneous and evoked activities of VMM convergent neurons were inhibited during stressful manipulations such as scruff lifting or defense reactions. These data contrast with other studies on VMM single unit recordings in anesthetized rats since the majority of these studies did not emphasize the VMM convergent group; in addition, with one exception, we did not find neurons exclusively driven by noxious inputs. Without excluding a role of the VMM convergent group in pain descending control systems, we proposed that this neuronal class is perhaps also involved in pain transmission or in general processess such as alertness and stress. Experiments are proposed in order to precisely determine the involvement of the VMM convergent neurons in alertness versus sensory discriminative aspects of nociception in the awake, freely moving rat.