Physiological properties of tooth pulp-driven neurons in the first somatosensory cortex (SI) of the cat

S1 Employs Feature-Dependent Differential Selectivity of Single Cells and Distributed Patterns of Populations to Encode Mechanosensations

The primary somatosensory (S1) cortex plays an important role in the perception and discrimination of touch and pain mechanosensations. Conventionally, neurons in the somatosensory system including S1 cortex have been classified into low/high threshold (HT; non-nociceptive/nociceptive) or wide dynamic range (WDR; convergent) neurons by their electrophysiological responses to innocuous brush-stroke and noxious forceps-pinch stimuli. Besides this “noxiousness” (innocuous/noxious) feature, each stimulus also includes other stimulus features: “texture” (brush hairs/forceps-steel arm), “dynamics” (dynamic stroke/static press) and “intensity” (weak/strong). However, it remains unknown how S1 neurons inclusively process such diverse features of brushing and pinch at the single-cell and population levels. Using in vivo two-photon Ca²⁺ imaging in the layer 2/3 neurons of the mouse S1 cortex, we identified clearly separated response patterns of the S1 neural population with distinct tuning properties of individual cells to texture, dynamics and noxiousness features of cutaneous mechanical stimuli. Among cells other than broadly tuned neurons, the majority of the cells showed a highly selective response to the difference in texture, but low selectivity to the difference in dynamics or noxiousness. Between the two low selectivity features, the difference in dynamics was slightly more specific, yet both could be decoded using the response patterns of neural populations. In addition, more neurons are recruited and stronger Ca²⁺ responses are evoked as the intensity of forceps-pinch is gradually increased. Our results suggest that S1 neurons encode various features of mechanosensations with feature-dependent differential selectivity of single cells and distributed response patterns of populations. Moreover, we raise a caution about describing neurons by a single stimulus feature ignoring other aspects of the sensory stimuli.

Stimulation of dural vessels excites the SI somatosensory cortex of the cat via a relay in the thalamus

Aim

We carried out experiments in cats to determine the thalamo-cortical projection sites of trigeminovascular sensory neurons.

Methods

1) We stimulated the middle meningeal artery (MMA) with C-fibre intensity electrical shocks and made field potential recordings over the somatosensory cortical surface. 2) We then recorded neurons in the ventroposteromedial (VPM) nucleus of the thalamus in search of neurons which could be activated from the skin, MMA and superior sagittal sinus. 3) Finally, we attempted to antidromically activate the neurons found in stage 2 by stimulating the responsive cortical areas revealed in stage 1.

Results

VPM neurons received trigeminovascular input, input from the V1 facial skin and could also be activated by electrical stimulation of the somatosensory cortex. VPM neurons activated from the cortex responded with short and invariant latencies (6.7 ± 7.7 msec mean and SD). They could follow high rates of stimulation and sometimes showed collision with orthodromic action potentials.

Conclusions

We conclude that somatosensory (SI) cortical stimulation excites trigeminovascular VPM neurons antidromically. In consequence, these VPM neurons project to the somatosensory cortex. These findings may help to explain the ability of migraineurs with headache in the trigeminal distribution to localise their pain to a particular region in this distribution.

Cortical representation area of human dental pulp

To elucidate the dental pulp-representing area in the human primary somatosensory cortex and the presence of A-beta fibers in dental pulp, we recorded somatosensory-evoked magnetic fields from the cortex in seven healthy persons using magnetoencephalography. Following non-painful electrical stimulation of the right maxillary first premolar dental pulp, short latency (27 ms) cortical responses on the magnetic waveforms were observed. However, no response was seen when stimulation was applied to pulpless teeth, such as devitalized teeth. The current source generating the early component of the magnetic fields was located anterior-inferiorly compared with the locations for the hand area in the primary somatosensory cortex. These results demonstrate the dental pulp representation area in the primary somatosensory cortex, and that it receives input from intradental A-beta neurons, providing a detailed organizational map of the orofacial area, by adding dental pulp to the classic "sensory homunculus".

Local subcutaneous and muscle pain impairs detection of passive movements at the human thumb

Activity in both muscle spindle endings and cutaneous stretch receptors contributes to the sensation of joint movement. The present experiments assessed whether muscle pain and subcutaneous pain distort proprioception in humans. The ability to detect the direction of passive movements at the interphalangeal joint of the thumb was measured when pain was induced experimentally in four sites: the flexor pollicis longus (FPL), the subcutaneous tissue overlying this muscle, the flexor carpi radialis (FCR) muscle and the subcutaneous tissue distal to the metacarpophalangeal joint of thumb. Tests were conducted when pain was at a similar subjective intensity. There was no significant difference in the ability to detect flexion or extension under any painful or non-painful condition. The detection of movement was significantly impaired when pain was induced in the FPL muscle, but pain in the FCR, a nearby muscle that does not act on the thumb, had no effect. Subcutaneous pain also significantly impaired movement detection when initiated in skin overlying the thumb, but not in skin overlying the FPL muscle in the forearm. These findings suggest that while both muscle and skin pain can disturb the detection of the direction of movement, the impairment is site-specific and involves regions and tissues that have a proprioceptive role at the joint. Also, pain induced in FPL did not significantly increase the perceived size of the thumb. Proprioceptive mechanisms signalling perceived body size are less disturbed by a relevant muscle nociceptive input than those subserving movement detection. The results highlight the complex relationship between nociceptive inputs and their influence on proprioception and motor control.

A comparative magnetoencephalographic study of cortical activations evoked by noxious and innocuous somatosensory stimulations

We recorded somatosensory-evoked magnetic fields and potentials produced by painful intra-epidermal stimulation (ES) and non-painful transcutaneous electrical stimulation (TS) applied to the left hand in 12 healthy volunteers to compare cortical responses to noxious and innocuous somatosensory stimulations. Our results revealed that cortical processing following noxious and innocuous stimulations was strikingly similar except that the former was delayed approximately 60 ms relative to the latter, which was well explained by a difference in peripheral conduction velocity mediating noxious (Adelta fiber) and innocuous (Abeta fiber) inputs. The first cortical activity evoked by both ES and TS was in the primary somatosensory cortex (SI) in the hemisphere contralateral to the stimulated side. The following activities were in the bilateral secondary somatosensory cortex (SII), insular cortex, cingulate cortex, anterior medial temporal area and ipsilateral SI. The source locations did not differ between the two stimulus modalities except that the dipole for insular activity following ES was located more anterior to that following TS. Both ES and TS evoked vertex potentials consisting of a negativity followed by a positivity at a latency of 202 and 304 ms, and 134 and 243 ms, respectively. The time course of the vertex potential corresponded to that of the activity of the medial temporal area. Our results suggested that cortical processing was similar between noxious and innocuous stimulation in SI and SII, but different in insular cortex. Our data also implied that activities in the amygdala/hippocampal formation represented common effects of noxious and tactile stimulations.

Response properties and organization of nociceptive neurons in area 1 of monkey primary somatosensory cortex

The organization and response properties of nociceptive neurons in area 1 of the primary somatosensory cortex (SI) of anesthetized monkeys were examined. The receptive fields of nociceptive neurons were classified as either wide-dynamic-range (WDR) neurons that were preferentially responsive to noxious mechanical stimulation, or nociceptive specific (NS) that were responsive to only noxious stimuli. The cortical locations and the responses of the two classes of neurons were compared. An examination of the neuronal stimulus-response functions obtained during noxious thermal stimulation of the glabrous skin of the foot or the hand indicated that WDR neurons exhibited significantly greater sensitivity to noxious thermal stimuli than did NS neurons. The receptive fields of WDR neurons were significantly larger than the receptive fields of NS neurons. Nociceptive SI neurons were somatotopically organized. Nociceptive neurons with receptive fields on the foot were located more medial in area 1 of SI than those with receptive fields on the hand. In the foot representation, the recording sites of nociceptive neurons were near the boundary between areas 3b and 1, whereas in the hand area, there was a tendency for them to be located more caudal in area 1. The majority of nociceptive neurons were located in the middle layers (III and IV) of area 1. The fact that nociceptive neurons were not evenly distributed across the layers of area 1 suggested that columns of nociceptive neurons probably do not exist in the somatosensory cortex. In electrode tracks where nociceptive neurons were found, approximately half of all subsequently isolated neurons were also classified as nociceptive. Low-threshold mechanoreceptive (LTM) neurons were intermingled with nociceptive neurons. Both WDR and NS neurons were found in close proximity to one another. In instances where the receptive field shifted, subsequently isolated cells were also classified as nociceptive. These data suggest that nociceptive neurons in area 1 of SI are organized in vertically orientated aggregations or clusters in layers III and IV.

Effects of conditioning stimulation of the central amygdaloid nucleus on tooth pulp-driven neurons in the cat somatosensory cortex (SI)

To study the limbic control of nociception, we examined the effect of conditioning stimulation of the central amygdaloid nucleus (ACE) on tooth pulp-driven (TPD) neurons in the first somatosensory cortex (SI). Cats were anesthetized with N(2)O-O(2) (2:1) and 0.5% halothane, and immobilized with tubocurarine chloride. The tooth pulp test stimulus was applied by a single rectangular pulse (0.5 ms in duration and 3-5 times the threshold intensity for the jaw-opening reflex). Conditioning stimuli to the ACE consisted of trains of 33 pulses (300 microA) delivered at 330 Hz at intervals of 8-10 s. In 35 out of 61 of the slow (S)-type TPD neurons with latencies of more than 20 ms, conditioning stimulation in the ACE, especially in the medial division, markedly reduced the firing response to the pulpal stimulation. The inhibition of the firing rate in the S-type neurons was 74% of the control. In these S-type neurons, the neurons that were inhibited had significantly longer latencies compared to the non-inhibited neurons (45.0 +/- 17.6 ms, n = 32 vs. 34.8 +/- 10.5 ms, n = 26). In contrast, the ACE conditioning stimulation affected only one out of 18 fast-type TPD neurons with latencies of less than 20 ms. In addition, ACE stimulation had no effect on the spontaneous discharges of either S-type or F-type neurons. The ACE inhibitory effect on S-type neurons was not diminished by naloxone administration (1 mg/kg, I.V. ), while the blockade of histamine H(1)-receptor by diphenhydramine hydrochloride (0.5 mg/kg, I.V.) partially reversed the inhibitory effect. These results suggest that the ACE inhibits ascending nociceptive information to the SI and that this inhibition is mediated in part by histamine (H(1)) receptors. It seems likely that the antinociceptive effect is a neurophysiological basis for stress-induced analgesia (SIA).

Primary somatosensory cortical neuronal activity during monkey's detection of perceived change in tooth-pulp stimulus intensity

To elucidate the functional properties of primary somatosensory cortical neurons for the perception of tooth-pulp sensation, neuronal activity was recorded from the primary somatosensory cortex (SI) in awake behaving monkeys. Monkeys were trained to detect changes in tooth-pulp stimulus intensity applied to the upper canine or incisor tooth pulp. Stimulus intensities applied to the tooth pulp were multiples of the threshold intensity for the jaw opening reflex (1.0 T) elicited by tooth-pulp stimulation. When monkeys pressed a button, baseline electrical pulses (V1: 0.5 T, 1.0 T, 2.0 T, or 3.0 T) were applied to the tooth pulp. After 4-8 s, a V2 stimulus (0.3 T, 0.5 T, 1.0 T, or 2.0 T) was added to V1. Percent escapes at V1 stimulus intensity of 0.5 T and 1.0 T were approximately 10%, 22% at 2.0 T, and 40% at 3.0 T (total of 1,997 trials). A total of 862 single units were recorded from the SI. Thirty-seven SI neurons responded to electrical stimulation of the tooth pulp (tooth-pulp-driven neurons; TPNs), 139 SI neurons responded to tactile stimulation of the lateral face area, 90 to upper lip and 99 to lower lip, 44 to tongue and 102 to periodontal membrane, whereas 351 SI neurons were not responsive to tactile stimulation of the orofacial regions. Thirty of 37 TPNs were recorded long enough to test with V1 stimuli ranging from 0.5 T to 3.0 T. Eleven of 30 TPNs linearly increased their firing frequency following increases in stimulus intensity (encoding TPNs), whereas 19 did not (nonencoding TPNs). Mean first spike latency of encoding TPNs was 24.8 +/- 1.7 ms (n = 11), that of nonencoding TPNs was 23.6 +/- 1.5 ms (n = 19), and that of unclassified TPNs was 24.7 +/- 3.7 ms (n = 7). TPNs were distributed in the areas 1-2, 3a, and 3b within the oral projection area and the transition zone between the face and oral projection areas of the SI. All of them received inputs from the intraoral structures, facial skin, or both. The firing frequency of eight encoding and nonencoding TPNs was correlated with detection latency at stimulus intensities of 0.5 and 1.0 T. On the other hand, when the baseline stimulus was increased to 2.0 T and 3.0 T, the discharge of most TPNs did not increase in firing frequency with the reduction in detection latency. These results indicate that the discharge rates of some SI TPNs are correlated with detection latency at near-noxious threshold and noxious stimulus intensities. These findings suggest that some TPNs are involved in the sensory-discriminative aspect of tooth-pulp sensation in the near-pain threshold and pain ranges.

A stereotaxic method of anatomical localization by means of H(2)15O positron emission tomography applicable to the brain activation study in cats: registration of images of cerebral blood flow to brain atlas

In the neuronal activation study of normal animals, precise anatomical correlation, preferentially to a detailed brain atlas, is required for the activation foci co-registration. To obtain precise regional correlation between H(2)15O-PET images and the brain atlas, a method of stereotaxic image reorientation was applied to an activation study with vibrotactile stimulation. Cats anesthetized with halothane underwent repeated measurements of regional cerebral blood flow (rCBF) in the resting condition and during vibration of the right forepaw. The image set was adjusted three-dimensionally to the atlas. The postmortem brain was sectioned according to the atlas planes. The activated areas were determined by the stimulus-minus-resting subtraction images, and the areas were projected to the atlas. The PET images of the cat brain were compatible both to the postmortem brain slices and to the brain atlas. The activation foci obtained from the subtraction images corresponded to the area around the coronal sulcus, which is electrophysiologically known as the primary sensory area as described in the atlas. There were precise regional correlations between the PET image and anatomy in a PET activation study of the cat by means of stereotaxic image reorientation.

Anatomic evidence of nociceptive inputs to primary somatosensory cortex: relationship between spinothalamic terminals and thalamocortical cells in squirrel monkeys

This study examined anatomic pathways that are likely to transmit noxious and thermal cutaneous information to the primary somatosensory cortex. Anterograde and retrograde labeling techniques were combined to investigate the relationship between spinothalamic (STT) projections and thalamocortical neurons in the squirrel monkey (Saimiri sciureus). Large injections of diamidino yellow (DY) were placed in the physiologically defined hand region of primary somatosensory cortex (hSI), and wheat germ agglutinin-horseradish peroxidase (WGA-HRP) was injected in the contralateral cervical enlargement (C5-T1). Both DY-labeled neuronal cell bodies and HRP-labeled STT terminal-like structures were visualized within single thalamic sections in each animal. Quantitative analysis of the positions and numbers of retrogradely labeled neurons and anterogradely labeled terminal fields reveal that: 1) ventral posterior lateral (VPL), ventral posterior inferior (VPI), and central lateral (CL), combined, receive 87% of spinothalamic inputs originating from the cervical enlargement; 2) these three nuclei contain over 91% of all thalamocortical neurons projecting to hSI that are likely to receive STT input; and 3) these putatively contacted neurons account for less than 24% of all thalamic projections to hSI. These results suggest that three distinct spinothalamocortical pathways are capable of relaying nociceptive information to the hand somatosensory cortex. Moreover, only a small portion of thalamocortical neurons are capable of relaying STT-derived nociceptive and thermal information to the primary somatosensory cortex.

Cortical influences on neurons of the lateral reticular nucleus responding to noxious stimuli

The effect of frontoparietal sensorimotor (FPSM) cortex stimulation on both the spontaneous and the noxious evoked activity of neurons in the lateral reticular nucleus (LRN) was tested in barbiturate-anesthetized rats. Ninety-three LRN neurons that responded to a noxious heat stimulus (HS) were recorded (72% antidromically fired from the cerebellum). Of these, 66 neurons altered their spontaneous firing rates in response to cortical stimulation. Two patterns of responses were found: either an excitation followed by a suppression of spontaneous activity (52 neurons), or a pure suppression of spontaneous activity lasting 50-400 msec (14 neurons). In 46 of these neurons, it was found that cortical stimulation reduced HS-evoked activity to near the baseline level. Furthermore, it was found that when applied after a prolonged cortical stimulation, the HS was ineffective. It is concluded that FPSM cortex can influence nociceptive information in LRN neurons that respond to its stimulation, possibly interfering with the mechanisms underlying stimulation-produced analgesia (SPA). In this context, it is proposed that the cortex can modulate the activity of LRN neurons that activate, through local loops, a descending antinociceptive system and also a separate projection system to the cerebellum.

Characteristics of tooth pulp-driven neurons in the posterior group of the cat thalamus

This investigation was designed to determine the responses of neurons in the posterior group of nuclei (PO) to tooth pulp stimulation. Eighteen tooth pulp-driven (TPD) neurons were recorded in 9 cats anesthetized with nitrous oxide and halothane, 14 of them in the medial part (POM) and the remainder in the lateral part (POL) of the posterior nuclei. These TPD neurons also responded to non-noxious tactile stimuli of the orofacial region of the body. Most TPD neurons responded with a short latency of less than 20 ms to tooth pulp stimulation (mean 13.5 +/- 5.9). The number of teeth having afferents to these neurons was 4-8 (mean 6.7 +/- 1.3).

The projection pathway from the tooth pulp to the ipsilateral first somatosensory cortex (SI) in the cat

About 40 per cent of tooth pulp-driven (TPD) neurones received afferent fibres from the ipsilateral tooth pulp. This ipsilateral afferent pathway was investigated in cats anaesthetized with nitrous oxide and halothane. The subcortical temperature of the brain was lowered to about 28 degrees C by perfusion of cold water within a thermode. Cooling the homotopic area contralateral to the recording site caused little change in the firing rate of short-latency TPD neurones upon ipsilateral pulp stimulation (n = 13). Microinjection (1-2 microliters) of 1 per cent lidocaine into the ipsilateral nucleus ventralis posteromedialis (VPM) caused significant diminution in the firing rate of short-latency TPD neurones (n = 11) to ipsilateral stimulation but not of long-latency TPD neurones (n = 8). About 35 per cent (13/35) of the TPD neurones distributed in the medial shell region of the VPM proper responded with short latency to ipsilateral pulp stimulation. These findings suggest that the ipsilateral input to short-latency TPD neurones in the oral area is carried via projection fibres from the ipsilateral VPM but not via commissural ones, and that the ipsilateral input to long-latency neurones is probably relayed in a site other than the ipsilateral VPM.