The Trigeminal System

Rhinarium skin structure and epidermal innervation in selected mammals

The glabrous skin around the nostrils in mammals is called a rhinarium or planum nasale. Rhinarium skin has multiple epidermal domes that are generally assumed to form a tactile surface. The rhinarium is innervated by a branch of the trigeminal nerve which is associated with stimuli such as touch, chemical irritants and temperature. In this study, our aim was to correlate variation in rhinarium skin sensory innervation with different feeding behaviors while also covering a broad systematic spectrum. Using histological and immunohistological methods, we studied skin morphology, nerve fiber density and nerve fiber distribution in the rhinarium epidermal domes of four species: cow, ring-tailed lemur, brown bear, and dog, that all exhibit different feeding behaviors. All species share similar traits in rhinarium skin morphology, but glands were only found in cow rhinarium skin. The most substantial differences were observed in the innervation pattern. Mechanosensory skin organs were found only in the ring-tailed lemur. Dog epidermal domes possess a pronounced central dermal papilla containing a nerve bundle in its top, close to the skin surface. The abundance of free epidermal nerve fibers in epidermal domes of all species, suggest that the rhinarium skin is a sensory surface, that can be used to detect fine touch, chemical irritants or temperature. In the species where the whole epidermal dome was examined, the intraepidermal nerve fiber density is higher in the central part of the domes. The nerve distribution and the central positioning of a single gland duct in cow and the dermal papilla top organ in dog indicates that each epidermal dome can be considered a functional unit. The observed differences in innervation hint at different sensory functions of rhinaria in mammals that may be correlated to feeding behavior.

EXCITABILITY PROPERTIES OF TRIGEMINAL GANGLION NEURONS

The firing properties of small neurons (with diameters of soma less than 25 µm) were investigated using patch-clamp technique in whole-cell configuration in primary culture of trigeminal ganglia (TG) of postnatal rats. TG neurons were divided into three groups according to their firing responses to long-lasting depolarizing pulses: adaptive neurons (AN) characterized by a strongly adaptive responses; tonic neurons (TN) characterized by a multiple tonic firing; neurons with a delay before initiation of AP generation, namely, NDG. AN, TN and NDG also differed in AP electrophysiological and pharmacological characteristics. TN was distinguished by responses to hyperpolarization and the greatest value of input resistance. TN, AN and NDG were characterized by different active properties (amplitude of action potential and afterhyperpolarization, reobase, threshold). Each group of neurons was characterized by heterogeneity of AP duration and of frequency properties for TN. The application of tetrodotoxin (TTX) (250 nM) resulted in full or partial inhibition of AP generation and some neurons had TTX – insensitive firing responses. Neurons that were not affected by TTX had markedly longer AP. TTX had no effect on electrical activity of some AN and NDG. Based on sensitivity to TTX and their electrophysiological properties, AN and NDG seem to be C-fiber nococeptors.

Involvement of trigeminal mesencephalic nucleus in kinetic encoding of whisker movements

In previous experiments performed on anaesthetised rats, we demonstrated that whisking neurons responsive to spontaneous movement of the macrovibrissae are located within the trigeminal mesencephalic nucleus (Me5) and that retrograde tracers injected into the mystacial pad of the rat muzzle extensively labelled a number of Me5 neurons. In order to evaluate the electrophysiological characteristics of the Me5-whisker pad neural connection, the present study analysed the Me5 neurons responses to artificial whisking induced by electrical stimulation of the peripheral stump of the facial nerve. Furthermore, an anterograde tracer was injected into the Me5 to identify and localise the peripheral terminals of these neurons in the mystacial structures. The electrophysiological data demonstrated that artificial whisking induced Me5 evoked potentials as well as single and multiunit Me5 neurons responses consistent with a direct connection. Furthermore, the neuroanatomical findings showed that the peripheral terminals of the Me5 stained neurons established direct connections with the upper part of the macrovibrissae, at the conical body level, with fibres spiralling around the circumference of the vibrissae shaft. As for the functional role of this sensory innervation, we speculated that the Me5 neurons are possibly involved in encoding and relaying proprioceptive information related to vibrissae movements to other CNS structures.

Projections from the dorsal peduncular cortex to the trigeminal subnucleus caudalis (medullary dorsal horn) and other lower brainstem areas in rats

This study has revealed direct projections from the dorsal peduncular cortex (DP) in the medial prefrontal cortex (mPfC) to the trigeminal brainstem sensory nuclear complex and other lower brainstem areas in rats. We first examined the distribution of mPfC neurons projecting directly to the medullary dorsal horn (trigeminal subnucleus caudalis [Vc]) and trigeminal subnucleus oralis (Vo) which are known to receive direct projections from the lateral prefrontal cortex (insular cortex). After injections of the retrograde tracer Fluorogold (FG) into the rostro-dorsomedial part of laminae I/II of Vc (rdm-I/II-Vc), many neurons were labeled bilaterally (with an ipsilateral predominance) in the rostrocaudal middle level of DP (mid-DP) and not in other mPfC areas. After FG injections into the lateral and caudal parts of laminae I/II of Vc, or the Vo, no neurons were labeled in the mPfC. We then examined projections from the mid-DP by using the anterograde tracer biotinylated dextranamine (BDA). After BDA injections into the mid-DP, many axons and terminals were labeled bilaterally (with an ipsilateral predominance) in the rdm-I/II-Vc, periaqueductal gray and solitary tract nucleus, and ipsilaterally in the parabrachial nucleus and trigeminal mesencephalic nucleus. In addition, the connections of the mid-DP with the insular cortex were examined. Many BDA-labeled axons and terminals from the mid-DP were also found ipsilaterally in the caudalmost level of the granular and dysgranular insular cortex (GI/DI). After BDA injections into the caudalmost GI/DI, many axons and terminals were labeled ipsilaterally in the mid-DP. The projections from the mid-DP to the rdm-I/II-Vc and other brainstem nuclei suggest that mid-DP neurons may regulate intraoral and perioral sensory processing (including nociceptive processing) of rdm-I/II-Vc neurons directly or indirectly through the brainstem nuclei. The reciprocal connections between the mid-DP and caudalmost GI/DI suggest that this regulation may involve mid-DP interactions with the caudalmost GI/DI neurons.

Transient receptor potential channels encode volatile chemicals sensed by rat trigeminal ganglion neurons

Primary sensory afferents of the dorsal root and trigeminal ganglia constantly transmit sensory information depicting the individual’s physical and chemical environment to higher brain regions. Beyond the typical trigeminal stimuli (e.g. irritants), environmental stimuli comprise a plethora of volatile chemicals with olfactory components (odorants). In spite of a complete loss of their sense of smell, anosmic patients may retain the ability to roughly discriminate between different volatile compounds. While the detailed mechanisms remain elusive, sensory structures belonging to the trigeminal system seem to be responsible for this phenomenon. In order to gain a better understanding of the mechanisms underlying the activation of the trigeminal system by volatile chemicals, we investigated odorant-induced membrane potential changes in cultured rat trigeminal neurons induced by the odorants vanillin, heliotropyl acetone, helional, and geraniol. We observed the dose-dependent depolarization of trigeminal neurons upon application of these substances occurring in a stimulus-specific manner and could show that distinct neuronal populations respond to different odorants. Using specific antagonists, we found evidence that TRPA1, TRPM8, and/or TRPV1 contribute to the activation. In order to further test this hypothesis, we used recombinantly expressed rat and human variants of these channels to investigate whether they are indeed activated by the odorants tested. We additionally found that the odorants dose-dependently inhibit two-pore potassium channels TASK1 and TASK3 heterologously expressed In Xenopus laevis oocytes. We suggest that the capability of various odorants to activate different TRP channels and to inhibit potassium channels causes neuronal depolarization and activation of distinct subpopulations of trigeminal sensory neurons, forming the basis for a specific representation of volatile chemicals in the trigeminal ganglia.

Spatial mapping of electrotactile sensation threshold and intensity range on the human tongue: initial results

We have developed a novel, tongue-based electrotactile brain-machine interface. Variability of the tactile sensation intensity across the stimulated area, however, limits the amount of reliable information transmission. We have conducted an experiment to characterize local sensitivity across the region stimulated by the array. From this data we have constructed an isointensity algorithm to compensate for the variability in electrotactile sensation levels across the stimulated area of the tongue.

Convergence of spinal trigeminal and cochlear nucleus projections in the inferior colliculus of the guinea pig

In addition to ascending auditory inputs, the external cortex of the inferior colliculus (ICX) receives prominent somatosensory inputs. To elucidate the extent of interaction between auditory and somatosensory representations at the level of IC, we explored the dual projections from the cochlear nucleus (CN) and the spinal trigeminal nucleus (Sp5) to the inferior colliculus (IC) in the guinea pig, using both retrograde and anterograde tracing techniques. Injections of retrograde tracers into ICX resulted in cell-labeling primarily in the contralateral DCN and pars interpolaris and caudalis of Sp5. Labeled cells in DCN were either fusiform or multipolar cells, whereas those in Sp5 varied in size and shape. Injections of anterograde tracers into either CN or Sp5 resulted in terminal labeling in ICX primarily on the contralateral side. Most projection fibers from Sp5 terminated in a laminar pattern from ventromedial to dorsolateral within the ventrolateral ICX, the ventral border of IC, and the ventromedial edge of IC (collectively termed "the ventrolateral border region of IC," ICXV). Less dense anterograde labeling was observed in lateral and rostral ICX. Injecting different tracers into both Sp5 and CN confirmed the overlapping areas of convergent projections from Sp5 and CN in IC: The most intense dual labeling was seen in the ICXV, and less intense dual labeling was also observed in the rostral part of ICX. This convergence of projection fibers from CN and Sp5 provides an anatomical substrate for multimodal integration in the IC.

Brainstem projections from recipient zones of the anterior ethmoidal nerve in the medullary dorsal horn

Stimulation of the anterior ethmoidal nerve or the nasal mucosa induces cardiorespiratory responses similar to those seen in diving mammals. We have utilized the transganglionic transport of a cocktail of horseradish peroxidase conjugates and anterograde and retrograde tract tracing techniques to elucidate pathways which may be important for these responses in the rat. Label was seen throughout the trigeminal sensory complex after the horseradish peroxidase conjugates were applied to the anterior ethmoidal nerve peripherally. Reaction product was most dense in the medullary dorsal horn, especially in laminae I and II. Injections were made of biotinylated dextran amine into the recipient zones of the medullary dorsal horn from the anterior ethmoidal nerve, and the anterogradely transported label documented. Label was found in many brainstem areas, but fibers with varicosities were noted in specific subdivisions of the nucleus tractus solitarii and parabrachial nucleus, as well as parts of the caudal and rostral ventrolateral medulla and A5 (noradrenergic cell group in ventrolateral pons) area. The retrograde transport of FluoroGold into the medullary dorsal horn after injections into these areas showed most neurons in laminae I, II, and V. Label was especially dense in areas which received primary afferent fibers from the anterior ethmoidal nerve. These data identify potential neural circuits for the diving response of the rat.

Brainstem reflex circuits revisited

Our current understanding of brainstem reflex physiology comes chiefly from the classic anatomical-functional correlation studies that traced the central circuits underlying brainstem reflexes and establishing reflex abnormalities as markers for specific areas of lesion. These studies nevertheless had the disadvantage of deriving from post-mortem findings in only a few patients. We developed a voxel-based model of the human brainstem designed to import and normalize MRIs, select groups of patients with or without a given dysfunction, compare their MRIs statistically, and construct three-plane maps showing the statistical probability of lesion. Using this method, we studied 180 patients with focal brainstem infarction. All subjects underwent a dedicated MRI study of the brainstem and the whole series of brainstem tests currently used in clinical neurophysiology: early (R1) and late (R2) blink reflex, early (SP1) and late (SP2) masseter inhibitory reflex, and the jaw jerk to chin tapping. Significance levels were highest for R1, SP1 and R2 afferent abnormalities. Patients with abnormalities in all three reflexes had lesions involving the primary sensory neurons in the ventral pons, before the afferents directed to the respective reflex circuits diverge. Patients with an isolated abnormality of R1 and SP1 responses had lesions that involved the ipsilateral dorsal pons, near the fourth ventricle floor, and lay close to each other. The area with the highest probabilities of lesion for the R2-afferent abnormality was in the ipsilateral dorsal-lateral medulla at the inferior olive level. SP2 abnormalities reached a low level of significance, in the same region as R2. Only few patients had a crossed-type abnormality of SP1, SP2 or R2; that of SP1 reached significance in the median pontine tegmentum rostral to the main trigeminal nucleus. Although abnormal in 38 patients, the jaw jerk appeared to have no cluster location. Because our voxel-based model quantitatively compares lesions in patients with or without a given reflex abnormality, it minimizes the risk that the significant areas depict vascular territories rather than common spots within the territory housing the reflex circuit. By analysing statistical data for a large cohort of patients, it also identifies the most frequent lesion location for each response. The finding of multireflex abnormalities reflects damage of the primary afferent neurons; hence it provides no evidence of an intra-axial lesion. The jaw jerk, perhaps the brainstem reflex most widely used in clinical neurophysiology, had no apparent topodiagnostic value, probably because it depends strongly on peripheral variables, including dental occlusion.

Pathfinding and growth termination of primary trigeminal sensory afferents in the embryonic rat hindbrain

Axons of the trigeminal ganglion convey sensory information from mechanoreceptors, thermoreceptors, and nociceptors in the face and nasal mucosa, then terminate on several groups of neurons including the principal sensory nucleus and the nuclei of the spinal trigeminal tract. To understand guidance mechanisms during the development of trigeminal sensory axons (TA) in the embryonic brain, we first investigated the growth pattern of TA in relation to organization in the hindbrain using flat whole-mount preparation from rat. We found that the primary TA from the trigeminal ganglion entered the brainstem and grew longitudinally within the hindbrain. Whereas descending axons ran just medial to the primary vestibular axons to innervate the spinal nucleus, ascending axons stayed near the entry point. In flat whole-mount culture, the TA extended both ascending and descending branches as they do in vivo. Rostral hindbrain was found to be a less permissive substrate for the TA compared to caudal hindbrain. In addition, the nonpermissive property of the ventral hindbrain substrate restricted the invasion of TA along the entire length of the hindbrain. Thus, cooperation of absolute and relative permissiveness of the substrate plays important roles in the guidance of TA to their targets.

Movement-related modulation of vibrotactile detection thresholds in the human orofacial system

By virtue of the direct coupling between circumoral skin and the underlying orofacial musculature, mechanosensation associated with precise orofacial force control may contribute significantly to processes associated with perception, proprioception, and sensorimotor control in this region. The purpose of this study was to assess lower lip (LL) vibratory detection thresholds of adult subjects during the simultaneous performance of a visually guided and continuous lip motor control task. Vibrotactile inputs were delivered to the right LL vermilion at test frequencies of 5, 10, 50, 150, 250, and 300 Hz. The psychophysical detection task was performed simultaneously with the three force control conditions: a null-force baseline condition, an active force control task performed with the right index finger, and an active force control task performed with the lip musculature. For the active tasks, subjects were instructed to use their analog force signal (lip or finger) to continuously perform a visually guided precision force task by tracking a 2 Hz sinusoidally moving target calibrated to a peak-to-peak force load of 0.2 N. Both the analog force signal and the target-tracking signal were displayed in real-time on an oscilloscope. Results showed a statistically significant elevation of LL vibrotactile detection thresholds for test frequencies below 50 Hz during the simultaneous performance of the lip force control task. Disassociating the site of motor control from the location of sensory stimulation (active control task) was effective in normalizing the elevations in LL vibrotactile thresholds, thus demonstrating that the threshold elevation during the lip force task was not solely an artifact of the added attentional load resulting from combining a perceptual task with a motor control requirement. These findings are discussed in relation to published reports of movement-related sensory gating in limb systems and the possible significance that this phenomenon may have for perception and proprioception in the orofacial system.

Comparative analysis of the chemical neuroanatomy of the mammalian trigeminal ganglion and mesencephalic trigeminal nucleus

A characteristic peculiarity of the trigeminal sensory system is the presence of two distinct populations of primary afferent neurons. Most of their cell bodies are located in the trigeminal ganglion (TG) but part of them lie in the mesencephalic trigeminal nucleus (MTN). This review compares the neurochemical content of central versus peripheral trigeminal primary afferent neurons. In the TG, two subpopulations of primary sensory neurons, containing immunoreactive (IR) material, are identified: a number of glutamate (Glu)-, substance P (SP)-, neurokinin A (NKA)-, calcitonin gene-related peptide (CGRP)-, cholecystokinin (CCK)-, somatostatin (SOM)-, vasoactive intestinal polypeptide (VIP)- and galanin (GAL)-IR ganglion cells with small and medium-sized somata, and relatively less numerous larger-sized neuropeptide Y (NPY)- and peptide 19 (PEP 19)-IR trigeminal neurons. In addition, many nitric oxide synthase (NOS)- and parvalbumin (PV)-IR cells of all sizes as well as fewer, mostly large, calbindin D-28k (CB)-containing neurons are seen. The majority of the large ganglion cells are surrounded by SP-, CGRP-, SOM-, CCK-, VIP-, NOS- and serotonin (SER)-IR perisomatic networks. In the MTN, the main subpopulation of large-sized neurons display Glu-immunoreactivity. Additionally, numerous large MTN neurons exhibit PV- and CB-immunostaining. On the other hand, certain small MTN neurons, most likely interneurons, are found to be GABAergic. Furthermore, NOS-containing neurons can be detected in the caudal and the mesencephalic-pontine junction portions of the nucleus. Conversely, no immunoreactivity to any of the examined neuropeptides is observed in the cell bodies of MTN neurons but these are encircled by peptidergic, catecholaminergic, serotonergic and nitrergic perineuronal arborizations in a basket-like manner. Such a discrepancy in the neurochemical features suggests that the differently fated embryonic migration, synaptogenesis, and peripheral and central target field innervation can possibly affect the individual neurochemical phenotypes of trigeminal primary afferent neurons.

Vertical dimension. Part 2: the changes in electrical activity of the cervical muscles upon varying the vertical dimension

This study was conducted in order to determine the effect of vertical dimension variation on the electromyographic (EMG) activity of the sternocleidomastoid and trapezius muscles. The study was performed on 15 healthy subjects. Basal tonic electromyographic (BT-EMG) recordings were performed by placing surface electrodes on the left sternocleidomastoid and trapezius muscles. BT EMG activity was recorded upon varying the vertical dimension every five millimeters from vertical dimension of occlusion to 45 millimeters of jaw opening (series 1), following the habitual opening path. Afterward, BT-EMG activity was recorded every millimeter from vertical dimension of occlusion to 4 mm, and then every two millimeters from four to ten millimeters (series 2). In series 1, a significant increase of BT-EMG activity was observed in both muscles (simple logarithmic regression analysis). In series 2, a significant increase was observed in the sternocleidomastoid muscle whereas trapezius muscle did not present a significant change. BT-EMG behavior of the sternocleidomastoid muscle in series 2 could be relevant when dentists increase vertical dimension by means of intermaxillary appliances during a short-term period. Moreover, these results add further information to the concept of the interrelatedness between the different components of the cranio cervical-mandibular system.

Relationship between craniomandibular disorders and poor posture

The purpose of this research was to show that a relationship between craniomandibular disorders (CMD) and postural abnormalities has been repeatedly postulated, but still remains unproven. This study was intended to test this hypothesis. Twenty-five CMD patients (mean age 28.2 years) were compared with 25 gender and age matched controls (mean age 28.3 years) in a controlled, investigator-blinded trial. Twelve postural and ten muscle function parameters were examined. Measurements were separated into three subgroups, consisting of those variables associated with the cervical region, the trunk in the frontal plane, and the trunk in the sagittal plane. Within these subgroups, there was significantly more dysfunction in the patients, compared to control subjects (Mann-Whitney U test p < 0.001, p < 0.05, p < 0.01). Postural and muscle function abnormalities appeared to be more common in the CMD group. Since there is evidence of the mutual influence of posture and the craniomandibular system, control of body posture in CMD patients is recommended, especially if they do not respond to splint therapy. Whether poor posture is the reason or the result of CMD cannot be distinguished by the data presented here.

Afferent input to the medullary dorsal horn from the contralateral face in rat

Sensory deficits on the contralateral face in Wallenberg's lateral medullary syndrome (WS) may be due to an involvement of the crossing contralateral trigeminothalamic tract. Alternatively, neurons within the medullary dorsal horn (MDH) get input from the contralateral face. MDH neurons supplying the supraorbital nerve area in rat were recorded by electrophysiological techniques. In this first study on contralateral projections, about 60% of the neurons received excitatory afferent input from the contralateral face as well as the ipsilateral supraorbital area. Thus, contralateral sensory deficits in WS may be due to an involvement of these neurons.

Suppressive effect of vagal afferents on the activity of the trigeminal spinal neurons related to the jaw-opening reflex in rats: involvement of the endogenous opioid system

The purpose of the present study is to test the hypothesis that via the endogenous pain control system, vagal afferent input modulates the activity of the trigeminal spinal nucleus oralis (TSNO) related to the tooth pulp (TP)-evoked jaw-opening reflex (JOR). Extracellular single-unit recordings were made from 36 TSNO units responding to TP electrical stimulation with a constant temporal relationship to a digastric electromyogram (dEMG) signal in 26 pentobarbital-anesthetized rats. The activity of 36 TSNO neurons and the amplitude of the dEMG increased proportionally during 1.0-3.5 times the threshold for JOR. Some of these neurons (4 out of 5) were also excited by chemical stimulation (bradykinin, 1-2 microl, 1 mM) of TP. In 31 out of 36 TSNO neurons (86%), their activities during tooth pulp stimulation were suppressed by conditioning stimulation of the right vagus nerve. The suppressive effect of vagal afferent stimulation occurred at conditioning-test intervals of 20-150 ms after the onset of the stimulation, and its maximal suppressive effect occurred at approximately 50 ms. The mean time course of this suppressive effect paralleled that of the dEMG. After administration of naloxone (0.5 and 1.0 mg/kg, i.v.), an opiate receptor blocker, the suppressive effect on the activity of TSNO neurons (6 out of 8) was significantly attenuated at the conditioning-test interval of 50 ms compared to the control (p < 0.01). These results suggested that vagal afferent input inhibits nociceptive transmission in the TSNO related to TP-evoked JOR and this inhibitory effect may occur via the endogenous opioid system in rats.

Characterization of blink reflex interneurons by activation of diffuse noxious inhibitory controls in man

The blink reflex consists of an early, pontine R1-component and a late, medullary R2-component. R1 and R2 can be evoked by innocuous stimuli, but only the R2 also by painful heat, suggesting that the R2 is mediated by wide dynamic range neurons (WDR) of the spinal trigeminal nucleus. Remote noxious stimuli suppress the activity in WDR neurons via activation of diffuse noxious inhibitory controls (DNIC), whereas low-threshold mechanoreceptive neurons (LTM) are unaffected. In order to characterize the trigeminal interneurons of R1 and R2 we investigated the modulation of the blink reflex by remote painful heat. The blink reflex was elicited in 11 healthy subjects by innocuous electrical pulses applied to the left supraorbital nerve. The remote, painful heat stimuli were applied by a Peltier type thermode to the left volar forearm. Remote painful heat of 44 to 46 degreesC significantly suppressed the R2 by 15% (p<0.01), while the R1 remained unchanged. These results provide further evidence that the R2 is mediated by medullary WDR neurons and the R1 by pontine LTM neurons.

Role of respiratory and non-respiratory neurones in the region of the NTS in the elaboration of the sneeze reflex in cat

Extracellular recordings were made in the dorsal respiratory group (DRG) and adjacent reticular formation following single-shock stimulation of the anterior ethmoidal nerve (AEN) and during sneeze evoked by repetitive stimulation of the AEN in nembutal-anaesthetized, curarized and ventilated cats. These neurones were characterised according to (i) their activity during the respiratory cycle (as inspiratory augmenting or decrementing (I Aug or I Dec), expiratory augmenting or decrementing (E Aug or E Dec), silent or tonic), and (ii) their axonal projection (bulbospinal or non-bulbospinal-non-vagal (BS or NBS-NV)). Following single-shock stimulation of the AEN, most of the inspiratory neurones were transiently inhibited, whereas E Aug neurones were activated and E Dec neurones were activated and then inhibited. Silent neurones responded with a multispike or a paucispike pattern. Following repetitive stimulation of the AEN and during the resulting sneeze reflex, I Aug neurones increased their activity in parallel with the phrenic activity, I Dec neurones fired at the onset and at the end of the inspiration, E Dec and some silent neurones fired either during the compressive phase or after the expulsive phase, whereas E Aug and some silent neurones fired during the expulsive phase. We conclude that sneeze involves a reconfiguration of the central respiratory drive which uses, at least partly, the respiratory network to trigger a non-ventilatory defensive motor act.

Nociceptive masseter inhibitory reflexes evoked by laser radiant heat and electrical stimuli

Electrical stimulation of the mental nerve evokes two suppression periods SP1 and SP2 in masseter muscle activity bilaterally. In order to investigate a possible nociceptive origin of the suppression periods, we compared the reflex responses evoked by electrical stimulation and by selective activation of nociceptors in hairy skin using painful infrared laser stimuli. The SP was elicited during more than 90% maximal voluntary contraction. Thresholds for detection, pain, and SP in the mental nerve area were determined by the method of limits. A suppression period was evoked by laser stimuli in nine of ten subjects bilaterally. The mean onset latency was 46.9 ms, the mean duration 58.9 ms. The electrical threshold of SP1 (9 mA) was 7.7 x I(0), about 20% smaller than I(P), and significantly higher than I(SP2) (4.7 mA). The onset latencies and durations were 11.7 ms and 21 ms for SP1, and 45 ms and 42.7 ms for SP2 (stimulus intensity 2 x I(P)). The mean difference in onset latencies between laser SP and electrically evoked SP1 was 35.1 +/- 6.2 ms, which closely matches the nociceptor response latency to a laser heat pulse. Based on the threshold and the onset latency we conclude that at least SP1 and laser SP are nociceptive in origin and mediated by group III fibers.

Timecourse of development of the wallaby trigeminal pathway. I. Periphery to brainstem

The development of the vibrissae and their innervation and the maturation of the brainstem trigeminal sensory nuclei have been studied in the wallaby, Macropus eugenii, from birth to adulthood. At birth, developing vibrissal follicles consist of solid epidermal pegs surrounded by dermal condensations. The developing follicles and adjacent skin are innervated by trigeminal afferents. Ten days after birth the follicle contains a dermal papilla and the deep vibrissal nerve can be recognised. A hair cone is present at postnatal day (P) 30 and hairs are apparent on the skin surface by P35. By P63 the deep vibrissal nerve can be seen innervating Merkel cells in the outer root sheath; in addition, the first signs of the blood sinus can be recognised. Innervation of the inner conical body and lanceolate and lamellated receptors supplying the mesenchymal sheath and waist region are not seen until P119, when the follicle resembles that seen in the adult. At birth, central processes of the trigeminal ganglion cells have entered the trigeminal tract and extend from the rostral pons to the upper cervical cord. Labelling with a carbocyanine dye at P0 shows afferents extending medially from the tract into the trigeminal subnuclei at all levels. At this stage the trigeminal nuclei appear as areas of increased cell density in the lateral brainstem. By P30-40 the four subnuclei can be distinguished on the basis of shape, cytoarchitecture, and succinic dehydrogenase reactivity. Adult morphology is not fully established until P210. In mature animals, nucleus principalis contains closely packed, polymorphic cells, frequently aligned parallel to thick fibre bundles that traverse the nucleus obliquely. Subnuclei oralis and interpolaris contain sparsely distributed, medium to large cells, randomly oriented, as well as prominent rostrocaudally directed fibre bundles. Subnucleus caudalis consists of the marginal layer, substantia gelatinosa, and magnocellular layers as described in other species. Patches of increased succinic dehydrogenase or cytochrome oxidase reactivity, presumably corresponding to the vibrissae, are present in subnuclei principalis, interpolaris, and caudalis in developing and adult animals, although the pattern is less clear than in rats. The brainstem patches are first seen at P40, approximately 6 weeks before the corresponding vibrissal-related pattern develops in the cortex. This suggests that the onset of patch formation may be regulated independently at different levels of the pathway.

Anatomical organization of the limb premotor network in the turtle (Chrysemys picta) revealed by in vitro transport of biocytin and neurobiotin

The in vitro turtle brainstem-cerebellum preparation has been a valuable tool in the study of central motor programs. In the present study, we investigate the anatomical organization of the turtle rubrocerebellar limb premotor network and its sensory connections in vitro by combining the rapid anterograde and retrograde transport of neurobiotin and biocytin with the extended viability of the isolated turtle brainstem-cerebellum. These compounds retrogradely labeled soma, dendrites, and axons, and orthogradely labeled axons and, to a lesser extent, terminals. The chelonian red nucleus receives a dense input from the contralateral lateral cerebellar nucleus and projects heavily to the contralateral spinal cord. Rubral axons sparsely innervate the lateral cerebellar nucleus and project heavily to the lateral reticular nucleus. Lateral reticular axons heavily innervate the lateral cerebellar nucleus before terminating in the pars lateralis of the cerebellar cortex as mossy fibers. These prominent, recurrent loops among the lateral cerebellar nucleus, red nucleus, and lateral reticular nucleus constitute the turtle rubrocerebellar limb premotor network. Sensory inputs to the red nucleus originate in the contralateral dorsal column nuclei, the principal trigeminal nucleus, and the spinothalamic system. These sites project bilaterally to the lateral reticular nucleus. The lateral cerebellar nucleus receives a contralateral input from the dorsal column nuclei. The red nucleus projects sparsely to the dorsal column nuclei. The red nucleus also receives an ipsilateral descending projection from the suprapeduncular nucleus, located in the diencephalon, and an ascending input from the rostral rhombencephalic reticular formation. An ipsilateral descending pathway originating in the red nucleus is likely to be the rubro-olivary tract.

Central projections of the sensory innervation to the middle cerebral artery in the squirrel monkey

To analyze the brainstem projections of the innervation to the middle cerebral artery (MCA) in the squirrel monkey, transganglionic tracing of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) was used. After application of WGA-HRP to the middle cerebral artery (MCA), labelled cell bodies were identified in the ipsilateral trigeminal and superior cervical ganglia. In the brainstem, positive labelling indicative of preterminals and terminals occurred in a discontinuous pattern throughout the trigeminal brainstem nuclear complex. At the level of the obex, nerve terminations were identified in the nucleus tractus solitarius, nucleus motorius dorsalis nervi vagi and the nucleus nervi hypoglossi. Positive WGA-HRP profiles were also observed in the periaqueductal gray matter.

Cervical spinal cord neurons receiving sensory input from the cranial vasculature

The superior sagittal sinus, middle meningeal artery or superficial temporal artery was stimulated electrically in anaesthetized cats. Field potential recordings were used to locate areas of maximum responses in the upper cervical cord, which were then further examined for responsive single units. Short latency units responded to stimulation of the superior sagittal sinus with a mean latency of 11.9 ms. Some units also responded at longer latencies in the 200-250 ms range. Spontaneous discharge rates of some units in a dorsolateral area of the cervical cord were accelerated by iontophoretic application of glutamic or homocysteic acid to these same units. Evoked action potentials were commonly multiphasic. Dorsolateral area units commonly received convergent input from two vessels and often had receptive fields on the face and limbs. Spontaneously active cells which respond to electrical stimulation were accelerated by the local application of bradykinin to the sinus and responses of dorsolateral area units could be reversibly blocked by local application of lignocaine to the sinus. It was concluded that the dorsolateral area is a relay area for the perception of pain from cranial vessels.

The neurobiology of facial and dental pain: present knowledge, future directions

This review outlines recent research which has identified critical neural elements and mechanisms concerned with the transmission of sensory information related to oral-facial pain, and which has also revealed some of the pathways and processes by which pain transmission can be modulated. The review highlights recent advances in neurobiological research that have contributed to our understanding of pain, how acute and chronic pain conditions can develop, and how pain can be controlled therapeutically. Each section of the review also identifies gaps in knowledge that still exist as well as research approaches that might be taken to clarify even further the mechanisms underlying acute and chronic oral-facial pain. The properties of the sense organs responding to a noxious oral-facial stimulus are first considered. This section is followed by a review of the sensory pathways and mechanisms by which the sensory information is relayed in nociceptive neurones in the brainstem and then transmitted to local reflex centers and to higher brain centers involved in the various aspects of the pain experience--namely, the sensory-discriminative, affective (emotional), cognitive, and motivational dimensions of pain. Reflex and behavioral responses to noxious oral-facial stimuli are also considered. The next section provides an extensive review of how these responses and the activity of the nociceptive neurones are modulated by higher brain center influences and by stimulation of, or alterations (e.g., by trauma) to, other sensory inputs to the brain. The neurochemical processes, involved in these modulatory mechanisms are also considered, with special emphasis on the role of neuropeptides and other neurochemicals recently shown to be involved in pain transmission and its control. The final section deals with recent findings of peripheral and central neural mechanisms underlying pain from the dental pulp.

Histochemical demonstration of vibrissae-representing patchy patterns of cytochrome oxidase activity within the trigeminal sensory nuclei in the cat

Cytochrome oxidase histochemistry revealed patchy patterns of the enzyme activity in transverse sections through the caudal part of the ventral subnucleus of the principal sensory trigeminal nucleus, interpolar spinal trigeminal nucleus, and layer IV of the caudal spinal trigeminal nucleus in the cat. By the transganglionic transport method of horseradish peroxidase, the patterns were indicated to replicate the spatial array of the facial vibrissae.

Operant conditioning of somatosensory evoked potential (SEP) amplitude in rats. II. Associated changes in reflex and continuous non-timelocked movements

Animals were rewarded for increasing (uptrain) or decreasing (downtrain) the amplitude of a 30 ms surface positive component of a somatosensory evoked potential (SEP) evoked by innocuous stimulation of the spinal trigeminal tract. The reflex movement produced by the evoking stimulus had a larger amplitude in uptraining than downtraining. This change in reflex amplitude suggests that operantly conditioning SEP amplitude was correlated with a change in innocuous somatosensory activity. There was no change in continuous non-timelocked movement associated with conditioning. This latter finding suggests that SEP conditioning is not necessarily mediated by such movement.

Operant conditioning of somatosensory evoked potential (SEP) amplitude in rats. I. Specific changes in SEP amplitude and a naloxone-reversible somatotopically specific change in facial nociception

The aim of this experiment was to investigate possible endogenous opioid modulation of innocuous somatosensory activity. Somatosensory activity was measured by recording cortical somatosensory evoked potential (SEP) and reflex movement amplitude evoked by innocuous electrical stimulation of the spinal trigeminal tract in awake rats. Putative endogenous opioid activity was blocked using the opiate antagonist naloxone (1 mg/kg). The amplitude of midlatency SEP components (14-50 ms latency) increased following administration of naloxone and repeated stimulus presentations. The amplitude of these components decreased following administration of the opiate agonist morphine (3 mg/kg). An early cortical component (10 ms latency) habituated following the administration of saline but did not habituate following naloxone. Naloxone also enhanced habituation of the late SEP components (60-120 ms latency) and reflex movement evoked at higher stimulus intensities. Morphine decreased the amplitude of the early cortical component but had no consistent effect on the amplitude of the late SEP components.

Vibrissae tactile stimulation: (14C) 2-deoxyglucose uptake in rat brainstem, thalamus, and cortex

The right mystacial vibrissae of awake, adult rats were stroked at 4-6 times/second and brain regions which increased (14C) 2-deoxyglucose (2DG) uptake were mapped autoradiographically. The ventral parts of the ipsilateral spinal trigeminal nuclei pars caudalis (Sp5c), pars interpolaris (Sp5i), pars oralis (Sp5o), and the principal trigeminal sensory (Pr5) nuclei were activated. The lateral part of the ipsilateral facial (VII) nucleus (the region which innervates the vibrissae muscles) was also activated possibly via excitatory, trigeminal (Sp5c, Sp5i, Sp5o, Pr5) sensory afferents. A number of regions were activated contralateral to the sensory stimulus. Discrete patches of (14C) 2DG uptake occurred in deep layers of the superior colliculus (SCsgp). Dorsolateral and dorsomedial parts of the ventrobasal nucleus (VB), and posterior, dorsolateral parts of the reticular nucleus (R) of thalamus were activated, along with broad portions of the primary somatosensory cortex (SI) and second somatosensory cortex (SII). Though all layers of SI and SII cortex increased 2DG uptake, VB thalamic afferents to layers IV and Vc-Vla presumably accounted for the greater activation of these cortical layers during repetitive sensory stimulation of the vibrissae (RSSV). Activation of the above structures fits well with known anatomical data. However, the pattern of activation during RSSV was very different from that previously described during vibrissae motor cortex stimulation (VMIS). RSSV and VMIS both produced similar repetitive movements of all the mystacial vibrissae. However, only a few overlapping brain regions were activated during both RSSV and VMIS. These RSSV-VMIS overlap zones included Sp5o; rostral Sp5i; lateral VII; SCsgp; ventrobasal-posteromedial and ventrobasal-ventrolateral zones in thalamus; and a rostral region of SI probably anterior to the Woolsey vibrissae barrelfield in the dysgranular somatosensory (SI) cortex. Since RSSV and VMIS would both be expected to activate vibrissae proprioceptors, we have hypothesized that vibrissae proprioceptive input was processed in part in the RSSV-VMIS overlap zones. Convergence of motor-sensory inputs and other types of processing could have also occurred in these overlap zones.

Contributions of the mesencephalic dopaminergic system and the trigeminal sensory pathway to the ventral tegmental aphagia syndrome in rats

Four experiments examined the neural substrates for the aphagia and adipsia syndrome resulting from damage of the ventral tegmental region. Radiofrequency (RF) lesions at the level of the mesencephalon in rats showed that the most effective site for producing aphagia and adipsia was in an intermediate zone between the substantia nigra and the ventral tegmental area. Injection of the neurotoxin 6-hydroxydopamine (6-OHDA) into this intermediate zone led to a less severe feeding deficit, suggesting that both dopaminergic and non-dopaminergic neurons are involved in the mesencephalic aphagic syndrome. As the lemniscus trigeminalis was destroyed after the RF lesion of the intermediate zone, the hypothesis of trigeminal projection involvement was tested by lesioning (RF) the sensory trigeminal nucleus. These rats were aphagic but they recovered and their deficit was less severe than after RF lesion of the mesencephalic intermediate zone. In the last experiment, a combined 6-OHDA lesion of the mesencephalic intermediate zone and RF lesion of the trigeminal sensory nucleus led to a more severe deficit in feeding behavior than either lesion alone. These results further demonstrate that feeding behavior is under the control of a complex system involving several neural pathways.

Sensory cortical tongue representation in man

Extensive data on cortical tongue representation were analyzed in 100 patients who underwent craniotomy and cortical mapping by electrical stimulation for surgical treatment of epilepsy. As noted in the literature, the tongue is extensively represented within the central nervous system with a highly organized sensorimotor system and the data from this study corroborate a large cortical representation of the human tongue over the postcentral gyrus. The tongue was found to have a clear somatotopic organization over the postcentral area and to be represented bilaterally to a significant degree. Furthermore, the tongue appears to have an asymmetrical sensory cortical representation, as cerebral dominance for speech is more extensively represented on the dominant hemisphere. Cortical tongue mapping has proved extremely useful in determining the point of junction of the central and Sylvian sulci, a crucial landmark during surgical cortical resections.

Intracranial self-stimulation in relation to the ascending noradrenergic fiber systems of the pontine tegmentum and caudal midbrain: a moveable electrode mapping study

Chronically implanted moveable electrodes were used to map the pontine tegmentum and caudal midbrain for intracranial self-stimulation in relation to the ascending noradrenergic systems as revealed by fluorescence histochemistry. In no area tested was there a consistent correlation between the quality or the presence of self-stimulation and the degree of noradrenergic fiber density or cellular aggregation. Of particular importance was the failure to obtain self-stimulation from the locus coeruleus, despite repeated testing and extensive attempts at behavioral shaping. Those areas supporting self-stimulation included the dorsal raphe nucleus, the superior cerebellar peduncle and the mesencephalic and motor nuclei of the trigeminal nerve. These data appear to rule out activation of the ascending noradrenergic systems as an explanation of the rewarding effects of stimulation in these areas. A gustatory-visceral fiber system is suggested as an alternative possible substrate.