Temporal and intensity coding of pain in human cortex

Exploring the pain "neuromatrix"

A considerable number of functional imaging studies have demonstrated the involvement of multiple central regions during the experience of pain. These regions process information in circuits that can broadly be assumed to process the affective, sensory, cognitive, motor, inhibitory, and autonomic responses stimulated by a noxious event. The concept of a "neuromatrix" for pain processing is, therefore, well supported. There is, however, scant evidence for any particular regional or circuit dysfunction during clinical pain. To be clinically useful, functional imaging may have to step beyond the generalities of the neuromatrix.

Up-regulation of metabotropic glutamate receptor 3 mRNA expression in the cerebral cortex of monoarthritic rats

Metabotropic glutamate receptors (mGluR) have been shown to play a role in the modulation of acute and inflammatory pain. Additionally, we have recently detected time-dependent changes in the mRNA expression of several mGluR subtypes in thalamic nuclei of monoarthritic (MA) rats. In the present study, mGluR1, -3, -4, and -7 subtype mRNA expression was analyzed by in situ hybridization with radioactively labelled oligonucleotide probes in cerebral cortical regions of normal and MA rats at 2, 4, and 14 days of the disease. The mGluR1, -4, and -7 mRNAs were at background level in normal rats and did not change in MA animals. In contrast, mGluR3 mRNA expression was abundant in normal rats and was significantly increased in cortical areas of MA rats at all time points. Higher changes were detected bilaterally at 4 days, predominantly in layers IV/V, in the motor, primary, and secondary somatosensory cortices (average increases of 50-75%), but maximum rises occurred in the contralateral cingulate cortex (+138%). No changes were detected in the auditory cortex. The present data show an up-regulation of mGluR3 mRNA expression in the motor, somatosensory, and limbic cortices of MA rats. This possibly reflects the occurrence of central mechanisms counteracting the increased transmission of nociceptive input arising from the inflamed paw and the impaired motor behavior of these rats. Changes in the cingulate cortex may be related to the motivational-affective component of nociception.

Mapping of early and late human somatosensory evoked brain potentials to phasic galvanic painful stimulation

In the present study, we modeled the spatiotemporal evolution of human somatosensory evoked cortical potentials (SEPs) to brief median-nerve galvanic painful stimulation. SEPs were recorded (-50 to +250 ms) from 12 healthy subjects following nonpainful (reference), slight painful, and moderate painful stimulations (subjective scale). Laplacian transformation of scalp SEPs reduced head volume conduction effects and annulled electric reference influence. Typical SEP components to the galvanic nonpainful stimulation were contralateral frontal P20-N30-N60-N120-P170, central P22-P40, and parietal N20-P30-P60-P120 (N = negativity, P = positivity, number = latency in ms). These components were observed also with the painful stimulations, the N60, N120, P170 having a longer latency with the painful than nonpainful stimulations. Additional SEP components elicited by the painful stimulations were parietomedian P80 as well as central N125, P170 (cP170), and P200. These additional SEP components included the typical vertex negative-positive complex following transient painful stimulations. Latency of the SEP components exclusively elicited by painful stimulation is highly compatible with the involvement of A delta myelinated fibers/spinothalamic pathway. The topography of these components is in line with the response of both nociceptive medial and lateral systems including bilateral primary sensorimotor and anterior cingulate cortical areas. The role of attentive, affective, and motor aspects in the modulation of the reported SEP components merits investigation in future experiments.

Long-lasting effect evoked by tonic muscle pain on parietal EEG activity in humans

OBJECTIVE

To explore EEG changes evoked by tonic experimental muscle pain compared to a non-painful vibratory stimulus.

METHODS

Thirty-one EEG channels were recorded before, during and after painful and non-painful stimulation. Pain was induced in the left brachioradialis muscle by injection of hypertonic (5%) saline. The vibratory stimulus was applied to the skin area overlying the brachioradialis muscle. The power of the major frequency components of the EEG activity (FFT, fast Fourier transform) was quantified and t-maps between the different experimental conditions were evaluated in frequency domain.

RESULTS

The main effect of muscle pain, compared to non-painful stimulation, was a significant and long-lasting increase of delta (1-3 Hz) power and an alpha-1 (9-11 Hz) power increase over the contralateral parietal locus. This finding could suggest a decreased excitability of the primary somatosensory cortex during muscle pain. The main effect of vibration, compared to its unstimulated baseline, consisted in an increase of beta-1 (14-20 Hz) power in the right frontal region.

CONCLUSIONS

Our data demonstrate significant and specific topographic EEG changes during tonic muscle pain. Since these modifications differ from those produced by an unstimulated baseline and during non-painful tonic stimulation, they might reflect mechanisms involved in the processing of nociceptive and adverse tonic stimuli.

Neurophysiology and functional neuroanatomy of pain perception

The traditional view that the cerebral cortex is not involved in pain processing has been abandoned during the past decades based on anatomic and physiologic investigations in animals, and lesion, functional neuroimaging, and neurophysiologic studies in humans. These studies have revealed an extensive central network associated with nociception that consistently includes the thalamus, the primary (SI) and secondary (SII) somatosensory cortices, the insula, and the anterior cingulate cortex (ACC). Anatomic and electrophysiologic data show that these cortical regions receive direct nociceptive thalamic input. From the results of human studies there is growing evidence that these different cortical structures contribute to different dimensions of pain experience. The SI cortex appears to be mainly involved in sensory-discriminative aspects of pain. The SII cortex seems to have an important role in recognition, learning, and memory of painful events. The insula has been proposed to be involved in autonomic reactions to noxious stimuli and in affective aspects of pain-related learning and memory. The ACC is closely related to pain unpleasantness and may subserve the integration of general affect, cognition, and response selection. The authors review the evidence on which the proposed relationship between cortical areas, pain-related neural activations, and components of pain perception is based.

Functional imaging of brain responses to pain. A review and meta-analysis (2000)

Brain responses to pain, assessed through positron emission tomography (PET) and functional magnetic resonance imaging (fMRI) are reviewed. Functional activation of brain regions are thought to be reflected by increases in the regional cerebral blood flow (rCBF) in PET studies, and in the blood oxygen level dependent (BOLD) signal in fMRI. rCBF increases to noxious stimuli are almost constantly observed in second somatic (SII) and insular regions, and in the anterior cingulate cortex (ACC), and with slightly less consistency in the contralateral thalamus and the primary somatic area (SI). Activation of the lateral thalamus, SI, SII and insula are thought to be related to the sensory-discriminative aspects of pain processing. SI is activated in roughly half of the studies, and the probability of obtaining SI activation appears related to the total amount of body surface stimulated (spatial summation) and probably also by temporal summation and attention to the stimulus. In a number of studies, the thalamic response was bilateral, probably reflecting generalised arousal in reaction to pain. ACC does not seem to be involved in coding stimulus intensity or location but appears to participate in both the affective and attentional concomitants of pain sensation, as well as in response selection. ACC subdivisions activated by painful stimuli partially overlap those activated in orienting and target detection tasks, but are distinct from those activated in tests involving sustained attention (Stroop, etc.). In addition to ACC, increased blood flow in the posterior parietal and prefrontal cortices is thought to reflect attentional and memory networks activated by noxious stimulation. Less noted but frequent activation concerns motor-related areas such as the striatum, cerebellum and supplementary motor area, as well as regions involved in pain control such as the periaqueductal grey. In patients, chronic spontaneous pain is associated with decreased resting rCBF in contralateral thalamus, which may be reverted by analgesic procedures. Abnormal pain evoked by innocuous stimuli (allodynia) has been associated with amplification of the thalamic, insular and SII responses, concomitant to a paradoxical CBF decrease in ACC. It is argued that imaging studies of allodynia should be encouraged in order to understand central reorganisations leading to abnormal cortical pain processing. A number of brain areas activated by acute pain, particularly the thalamus and anterior cingulate, also show increases in rCBF during analgesic procedures. Taken together, these data suggest that hemodynamic responses to pain reflect simultaneously the sensory, cognitive and affective dimensions of pain, and that the same structure may both respond to pain and participate in pain control. The precise biochemical nature of these mechanisms remains to be investigated.

A Review of Functional Imaging of the Brain and Pain

Functional imaging of the brain is a current reality using positron emission tomography and functional magnetic imaging. This article reviews many of the reports that have emerged in the past several years using these techniques in the analysis of pain experience. The areas of the brain that appear to be functioning during the experience of pain are discussed, and the variances in findings between studies are described. The implications of the findings are noted. Although much has been learned through these techniques, it is clear that further research is needed before clinicians can use these diagnostic studies for therapeutic purposes.

Functional Magnetic Resonance Imaging of Pain Consciousness: Cortical Networks of Pain Critically Depend on What is Implied by "Pain"

Brain imaging studies, using primarily functional magnetic resonance imaging (fMRI), are reviewed. These studies are aimed at developing imaging approaches that can be used in the clinical setting to investigate clinically relevant pain states. To this end, our recent studies indicate that by taking advantage of the temporal variations in pain perception, we are able to identify cortical regions that may be uniquely involved in pain consciousness. This procedure in turn becomes a general approach with which clinical pain states can be studied. Preliminary results are shown in patients suffering from chronic reflex sympathetic dystrophy (RSD) and chronic back pain. The review emphasizes that different experimental pain states, and chronic and acute clinical pain states, seem to involve dramatically different networks, the details of which remain to be worked out. It is concluded that these procedures need to be applied in the larger clinical setting in which multicentered studies may be conducted to begin building the brain pain network atlas.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

We examined whether cerebral activation to two different intense and painful stimuli could be detected using functional magnetic resonance imaging (fMRI) in alpha-chloralose anesthetized rats. Experiments were performed using a 9.4 T magnet and a surface coil centered over the forebrain. A set of gradient echo images were acquired and analyzed using our software based on fuzzy cluster analysis (EvIdent). Following the injection of 50 microl of formalin (5%) into the forepaw we observed a regional increase in signal intensity in the MR images in all animals. Anterior cingulate cortex, frontal cortex and sensory-motor cortex were some of the regions that activated frequently and often bilaterally. Surprisingly, activation appeared sequentially, often occurring first in either the right or the left hemisphere with a separation of seconds to minutes between peak activations. Morphine pre-treatment (1 mg/kg, i. v.) delayed and/or reduced the intensity of the activation resulting in a decrease in the overall response. Following episodes of intense electrical stimulation, produced by two brief stimulations (15 V, 0. 3 ms, 3 Hz) of the forepaw, activation was observed consistently in the sensory-motor cortex contralateral to the stimulation. Activation also occurred frequently in the anterior cingulate cortex, ipsilateral sensory-motor cortex and frontal cortical regions. All these regions of activation were markedly reduced during nitrous oxide inhalation. Treatment with morphine resulted in an inhibition of the activation response to electrical stimulation in most regions except for sensory-motor cortex. Thus, electrical and chemical noxious stimuli activated regions that are known to be involved in the central processing of pain and morphine modified the activation observed. fMRI combined with appropriate exploratory data analysis tools could provide an effective new tool with which to study novel analgesics and their effects on the CNS processing of pain in animal models.

Injury threshold: whiplash-associated disorders

OBJECTIVES

To review current knowledge and recent concepts of the causes of injuries after minor impact automobile collisions and to acquaint those who treat these types of injuries with possible injury thresholds and mechanisms that may contribute to symptoms.

DATA SOURCES

A review of literature involving mechanisms of injury, tissue tensile threshold, and neurologic considerations was undertaken. A hand-search of relevant engineering, medical/chiropractic, and computer Index Medicus sources in disciplines that cover the variety of symptoms was gathered.

RESULTS

Soft-tissue injuries are difficult to diagnose or quantify. There is not one specific injury mechanism or threshold of injury. With physical variations of tissue tensile strength, anatomic differences, and neurophysiologic considerations, such threshold designation is not possible.

CONCLUSIONS

To make a competent assessment of injury, it is important to evaluate each patient individually. The same collision may cause injury to some individuals and leave others unaffected. With the variability of human postures, tensile strength of the ligaments between individuals, body positions in the vehicle, collagen fibers in the same specimen segment, the amount of muscle activation and inhibition of muscles, the size of the spinal canals, and the excitability of the nervous system, one specific threshold is not possible. How individuals react to a stimulus varies widely, and it is evident peripheral stimulation has effects on the central nervous system. It is also clear that the somatosensory system of the neck, in addition to signaling nociception, may influence the control of neck, eyes, limbs, respiratory muscles, and some preganglionic sympathetic nerves.

Pathobiology of visceral pain: molecular mechanisms and therapeutic implications V. Central nervous system processing of somatic and visceral sensory signals

Somatic and visceral sensation, including pain perception, can be studied noninvasively in humans with functional brain imaging techniques. Positron emission tomography and functional magnetic resonance imaging have identified a series of cerebral regions involved in the processing of somatic pain, including the anterior cingulate, insular, prefrontal, inferior parietal, primary and secondary somatosensory, and primary motor and premotor cortices, the thalamus, hypothalamus, brain stem, and cerebellum. Experimental evidence supports possible specific roles for individual structures in processing the various dimensions of pain, such as encoding of affect in the anterior cingulate cortex. Visceral sensation has been examined in the setting of myocardial ischemia, distension of hollow viscera, and esophageal acidification. Although knowledge regarding somatic sensation is more extensive than the information available for visceral sensation, important similarities have emerged between cerebral representations of somatic and visceral pain.

Pain perception and response: central nervous system mechanisms

Although several decades of studies have detailed peripheral and ascending nociceptive pathways to the thalamus and cerebral cortex, pain is a symptom that has remained difficult to characterize anatomically and physiologically. Positron emission tomography (PET) and functional magnetic imaging (fMRI) have recently demonstrated a number of cerebral and brain stem loci responding to cutaneous noxious stimuli. However, intersubject variability, both in the frequency and increased or decreased intensity of the responses, has caused uncertainty as to their significance. Nevertheless, the large number of available imaging studies have shown that many areas with recognized functions are frequently affected by painful stimuli. With this evidence and recent developments in tracing central nervous system connections between areas responding to noxious stimuli, it is possible to identify nociceptive pathways that are within, or contribute to, afferent spino-thalamo-cortical sensory and efferent skeletomotor and autonomic motor systems. In this study it is proposed that cortical and nuclear mechanisms for pain perception and response are hierarchically arranged with the prefrontal cortex at its highest level. Nevertheless, all components make particular contributions without which certain nociceptive failures can occur, as in pathological pain arising in some cases of nervous system injury.