Effects of partial bulbar section on inhibition of jaw-opening reflexes induced by central gray matter stimulation in the cat

The effects of severing the spinal trigeminal tract and its caudal nucleus on high-threshold jaw-opening reflex elicited by tooth pulp stimulation were investigated during experiments on cats under chloralose-Nembutal anesthesia. Low-threshold jaw-opening reflex produced by stimulating the A--infraorbital nerve at an intensity 2–3 thresholds in relation to the most excitable fibers on this nerve was also observed, as well as suppression of these reflexes induced by central gray matter stimulation. It was found that spinal trigeminal tract section produces a 8–52% increase in high-threshold reflex. The amplitude of low-threshold reflex either remained unchanged or showed a slight tendency to rise or fall. Brief stimulation of the central gray matter produced a 100% decrease in high-threshold reflex in intact animals compared with a 40–60% decrease after section of the trigeminal tract. Protracted stimulation of the central gray brought about an 80% decline in high-threshold reflex in intact animals as against 25–30% after section. The degree to which brief stimulation of the central gray produced depression of low-threshold stimulation remained unchanged by trigeminal tract section. Protracted stimulation of the central gray matter brought about a 25–50% reduction in low-threshold reflex in intact animals and a reduction of 75% in three animals and 15–20% in four animals. This implied that the caudal nucleus of the spinal trigeminal tract exerts a more substantial influence on the process of high- than low-threshold reflex inhibition when the central gray matter is stimulated.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 3, pp. 362–368, May–June, 1987.     

Functional characteristics and topography of spinal pathways conducting high- and low-threshold startle reflexes

Electrophysiological and topographical properties of the spinal tract systems involved in two functional types of startle reflexes were studied in chloralose anesthetized cats: a high-threshold reflex produced by intense peripheral nerve stimulation (spinobulbo-spinal, SBS, reflex) and a low-threshold reflex evoked by tactile (T-reflex) and acoustic (A-reflex) stimulation. Maximum conduction velocity of descending transmission of the high-threshold reflex, at 30 m/sec, was perceptibly lower than that of low-threshold reflexes, at 85 m/sec for T-reflex and 100 m/sec for A-reflex. Mean conduction velocity for SBS and T-reflexes were 40.2 and 70.8 m/sec respectively. Perceptible differences were also found in the topography of spinal and especially ascending pathways of these reflexes. It was established by partial spinal cord destruction that accomplishment of T-reflex depended on the integrity of ascending pathways of the dorsal and dorsolateral funiculi and the SBS reflex on preservation of the dorsolateral, ventrolateral and (partially) of ventral funiculi. Descending pathways of the reflexes under study were revealed mainly in the ventrolateral and ventral funiculi and those of the SBS reflex mainly in the first of these. Findings also show the noticeable similarity between the organization of both T- and A-reflex descending pathways. The functional organization of the spinal pathways of a variety of startle reflexes is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 4, pp. 486–496, July–August, 1986.     

Neurochemical mechanisms in inhibitory effects of stimulation of the periaqueductal gray matter on high-threshold reflex activity of the reticular formation

A study was made of microinjections of antagonists of various neuromodulators on the dynamics of inhibition of the spino-bulbo-spinal reflexes which were evoked by stimulation of the central gray matter (PAG) in rats anesthetized with chloralose. Injections were made into the reticular gigantocellular nucleus (GN), which is the basic supraspinal center of this reflex. Administering methysergide (a blocker of serotonin receptors) was accompanied by significant (two to four times) diminution of inhibition evoked by PAG stimulation with a short, high-frequency series of stimuli. Long inhibition caused by long, rhythmic stimulation of the PAG was diminished less significantly: from 6–10 to 2.5–4 min. When the opiate receptors of the GN neurons were blocked with naloxone, duration of inhibition was reduced by two to five times. The most clearly expressed diminution of both types of inhibitions was noted with injections of haloperidol, an antagonist of catecholamines. Our data indicate that evidently all of these neuromediator (neuromodulator) systems participate in inhibition of high-threshold, reflex activity of the reticular formation evoked by stimulation of the PAG, but their participation in this process is unequal.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 23, No. 4, pp. 455–463, July–August, 1991.     

Reticular structures in the cat brain involved in startle reflexes to somatic stimuli of various modalities

Characteristics and reticular mechanisms of two types of late reflex responses, namely startle reflexes evoked by weak (tactile) and strong (high-threshold electrical stimulation of nerves) stimulation, were studied in cats anesthetized with chloralose. The peripheral and central organization of these responses, described as "tactile" and "spino-bulbo-spinal" reflexes, were subjected to comparative analysis. Definite rostro-caudal differentiation of the reticular structures involved in the formation of reflexes of these types was discovered with the aid of destructive lesions of the brain-stem reticular formation (RF). Reflexes of the first type relay mainly at the level of the pontine RF, whereas those of the second type relay mainly at the level of the medullary RF. This result was confirmed by reversible blockade of RF by injection of small doses of sodium chloride and procaine solutions into it. The functional significance of this differentiation of RF is discussed in the context of its motor and sensory functions.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 17, No. 3, pp. 380–390, May–June, 1985.     

Effects of calcium channel blockers on spino-bulbo-spinal reflexes evoked by activation of the high-threshold and low-threshold somatic afferents

Effects of the calcium channel blockers nifedipine and verapamil on spino-bulbo-spinal (SBS) reflexes were studied in rats anesthetized with a mixture of urethane and chloralose; the reflexes evoked by activation of the high-threshold and low-threshold (tactile) somatic afferents were tested. The blockers were microinjected into then. reticularis gigantocellularis, the main structure where the SBS reflexes are relayed. Nifedipine (10^–4 M) evoked fast (with a latency of 1–2 min) and significant (20%–60%) suppression of both the high-threshold and low-threshold SBS reflexes. The reflexes recovered within 40 to 90 min. Lower concentrations of nifedipine (10^–5 M) either did not modify both types of the SBS reflexes (in 25% of experiments) or facilitated these reflexes by 16%–40% within 15–20 min. Microinjections of verapamil affected the SBS reflexes in a similar manner, but the effects were shorter. Possible ways by which the L-type calcium channels are involved in the transmission of afferent impulsation of different modalities (nociceptive and non-nociceptive) are discussed.Neirofiziologiya/Neurophysiology, Vol. 26, No. 4, pp. 270–275, July–August, 1994.     

Inhibition of jaw opening reflexes by stimulation of the central gray matter and raphe nuclei in cats

The effect of stimulation of the mesencephalic central gray matter and raphe nuclei on jaw opening reflexes evoked by excitation of high-threshold (dental pulp) and low-threshold (A-alpha) fibers of the infraorbital nerve afferents was studied in cats anesthetized with chloralose and pentobarbital. The jaw opening reflex evoked by stimulation of the dental pulp was shown to be effectively suppressed by conditioning stimulation of the central gray matter and raphe nuclei. The reflex evoked by stimulation of low-threshold infraorbital nerve afferents also was depressed (but less deeply and for a shorter period than the reflex evoked by stimulation of the dental pulp) during stimulation of the raphe nuclei and caudal zone of the central gray matter, but was unchanged after stimulation of the points located in the rostral zone of the central gray matter. Application of single stimuli or bursts of five stimuli with a frequency of 100 Hz had no effect on the reflexes studied. Short-term stimulation with a burst of 10–20 stimuli with a following frequency of 200–400 Hz led to inhibition of the reflexes, which lasted 450–1000 msec. Long-term stimulation of the central gray matter and raphe nuclei for 30 sec with a frequency of 50 Hz caused inhibition of jaw opening reflexes evoked by stimulation of both high- and low-threshold afferents for 60 min. Impulses from the central gray matter and raphe nuclei thus have a mainly inhibitory action on the jaw opening reflex evoked by stimulation of high-threshold afferents, but they act less effectively on the reflex evoked by stimulation of low-thres-hold afferents. The duration of inhibition depends essentially on the parameters of stimulation.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 374–387, May–June, 1984.     

Modulation of segmental reflex reactions during actual locomotion in rats

High- and low-threshold reflex segmental reflex reactions produced by stimulating the dorsal root at different stages of the locomotor cycle were investigated during locomotor swimming motions in white rats. Findings show respectively considerable inhibition of and a pronounced increase in extensor and flexor lowthreshold reflex reactions during the absence and presence of activity in the associated muscles. Low-threshold stimulation produced no outstanding effect on the shaping of the muscles' own activity and hence failed to affect time course or amplitude parameters of locomotor movements. Changes in reflex reactions to high-threshold stimulation during the locomotor cycle largely resembled changes in these responses to low-threshold stimulation, although development of highthreshold reactions differed from that of low-threshold response in affecting the parameters of locomotor activity in the associated muscle, while likewise altering frequency parameters of the locomotor rhythm. The physiological significance and mechanisms possibly underlying modulations in the efficacy of afferent peripheral influences during locomotion are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 326–333, May–June, 1988.     

Role of the serotoninergic system in inhibition of jaw-opening reflex induced by stimulating the midbrain central gray matter

The effects were studied in waking cats of brief stimulation (20 stimuli at a rate of 400 Hz) of the central gray matter (CGM) and dorsal raphe nucleus (DRN) on high-threshold jaw-opening reflex (HJOR) evoked by tooth pulp stimulation during blockade of serotonin synthesis produced by application of 300 mg/kg parachlorophenylalanine (PCPA) i.p. Inhibitory effects of CGM and DRN stimulation had already declined in comparison with post-stimulation (but pre-PCPA) level within 24 h after PCPA application; 96 h afterwards, inhibition of HJOR induced by CGM and DRN stimulation had become only minimal: amplitude of the reflex had declined to 30–35% and duration of inhibitory effects ws 200–250 msec. It is therefore deduced that serotonin contributes to the HJOR depression induced by CGM and DRN stimulation and the possible involvement of other neuromodulators in this effect is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 45–52, January–February, 1989.     

Efferent activity in the phrenic nerve during startle reflex in chloralose anesthetized cats

Efferent activity was investigated in the phrenic nerve during startle reflex manifesting as somatic nerve discharges (lower intercostal nerves and the nerve endings) in chloralose anesthetized cats. Inhibition (usually of short duration, lasting 23–36 msec) of inspiration activity was found to be the main component of response in the phrenic nerve in the shaping of "low threshold" startle reflex produced by acoustic and tactile stimuli and stimulation of low threshold peripheral afferents. Reflex discharge prevailed amongst the response patterns produced in the phrenic nerve by stimulating high threshold afferents, i.e., early (propriospinal) and late (suprasegmental, arising from stimulating intercostal nerve) or late only (when stimulating the hindlimb nerves). Two patterns of late response could be distinguished, one on inspiration (found in roughly 3 out of 4 experiments) and other on exhalation — the respiratory homologs of somatic startle reflex. Response pattern is described throughout the respiratory cycle. Structure and respiratory modulation of reflex responses produced in the phrenic nerve by stimulating bulbar respiratory structure are also examined. Possible neurophysiological mechanisms underlying phrenic response during the shaping of startle reflex are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 4, pp. 473–482, July–August, 1987.     

Respiratory modulation of the startle reflex in cats

Phase respiratory influences on reflex after-discharges in response to stimulation of the segmental nerves as well as tactile and acoustic stimulation in the limb and intercostal nerves — physiological analogs of startle reflexes (SR) were studied in unanesthetized (decerebrate) or chloralose-anesthetized cats. It was found that the level of these reflexes in the inhalation phase of respiration was 8–58% lower than during exhalation. The difference between the inhalation and exhalation phases was determined for different types of reflexes and under varying experimental conditions. Evidence was obtained that respiratory modulation of reflexes occurs mainly at the level of suprasegmental (reticular) mechanisms. Clear distinctions could be drawn between the pattern of reflex modulation in the lower intercostal nerves and those of the limbs. Findings would lead to the conclusion that the likely mechanisms underlying suprasegmental respiratory influences on these reflexes differ, as does the organization of their reticular centers.S. V. Kurashov Medical Institute, Ministry of Public Health of the RSFSR, Kazan'. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 593–603, September–October, 1986.     

Mechanisms underlying inhibition of nociceptive jaw-opening reflex produced by stimulating limbic structures

The comparative effectiveness of the inhibitory influence of tetanic (100 Hz) stimulation of the hypothalamus, amygdala, and limbic cortex on electromyograph (EMG) response in m. digastricus produced by electrical stimulation of tooth pulp afferents was investigated in cats anesthetized by a mixture of chloralose and Nembutal. It was found that the most pronounced inhibition of nociceptive EMG response was produced by stimulating the medial and lateral structures of the hypothalamus, a less intense response by stimulating central and medial amygdaloid nuclei, and the least reaction by stimulation of different areas of the limbic cortex. It was shown that mechanisms underlying inhibition of the jaw-opening reflex produced by hypothalamic tetanic stimulation is unconnected with the concomitant increase in blood pressure. The inhibitory action of hypothalamic tetanic stimulation following blood pressure stabilization persists; this would suggest a primary but not baroafferent mechanism underlying inhibition of activity in trigeminal nucleus nociceptive neurons. Pronounced and protracted depression of jaw-opening reflex takes place following long-term pressor response produced by injecting noradrenaline intravenously. The same pronounced and protracted pressor response occurs under the effects of angiotensin but without any noticeable change in the amplitude of nociceptive EMG response. Hypothalamic and noradrenergic mechanisms of pain sensitivity are discussed.L. A. Orbeli Institute of Physiology, Academy of Sciences of the Armenian SSR, Erevan. Translated from Neirofiziologiya, Vol. 19, No. 6, pp. 825–832, November–December, 1987.     

电刺激猫中脑导水管周围灰质和中缝大核对脊髓背角神经元伤害性感受反应的抑制

1.在氯醛糖麻醉的猫上,观察了电刺激中脑导水管周围灰质(PAG)和中缝大核(NRM)对脊髓腰段背角神经元传入活动的影响。2.按照对刺激的反应型式,在背角记录到非伤害性低阈值传入、广动力范围、伤害性热敏以及高阈值传入诱发的自发放电抑制等四类神经元。3.刺激 PAG和 NRM对记录到的多数背角神经元皮肤传入反应有明显抑制效应,而对自发放电抑制性神经元产生去抑制。4.比较刺激两脑区的抑制效应:NRM 作用较PAG 强;PAG 活动对背角伤害性反应抑制的选择性较 NRM强;阿片肽拮抗剂-纳洛酮拮抗NRM刺激的抑制。5.这些结果提示PAG和NRM对脊髓的下行抑制,可能有一部分是通过不同神经机制实现的。     

Reticular structures and reticulospinal pathways participating in the initiation and inhibition of spino-bulbo-spinal activity

Reflex discharges in intercostal nerves and activity of reticulospinal fibers of the ventral and lateral funiculi, evoked by stimulation of the reticular formation and of the splanchnic and intercostal nerves were investigated in cats anesthetized with chloralose (50 mg/kg). Brain-stem neuronal structures participating in the "relaying" of spino-bulbo-spinal activity were shown to lie both in the medial zones of the medullary and pontine reticular formation and in its more lateral regions; they include reticulospinal neurons and also neurons with no projection into the spinal cord. Structures whose stimulation led to prolonged (300–800 msec) inhibition of reflex spino-bulbo-spinal activity were widely represented in the brain stem, especially in the pons. Analogous inhibition of this activity was observed during conditioning stimulation of the nerves. Reticulospinal fibers of the ventral (conduction velocity 16–120 m/sec) and lateral (17–100 m/sec) funiculi were shown to be able to participate in the conduction of spino-bulbo-spinal activity to spinal neurons. In the first case fibers with conduction velocities of 40–120 m/sec were evidently most effective. Evidence was obtained that prolonged inhibition of this activity can take place at the supraspinal level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Institute of Normal and Pathological Physiology, Slovak Academy of Sciences, Bratislava, Czechoslovakia. Translated from Neirofiziologiya, Vol. 8, No. 4, pp. 373–383, July–August, 1976.     

Suppression of postsynaptic response in trigeminal motoneurons by stimulating the midbrain central gray matter

The effects of stimulating the midbrain central gray matter (CGM) on motoneuronal response in trigeminal nerves were investigated in anesthetized cat. It was found that stimulating the CGM did not induce postsynaptic response in these motoneurones. Conditioning stimulation of the CGM brought about suppression of motoneuronal postsynaptic response to stimulation of tooth pulp and high threshold infraorbital nerve afferents without affecting motoneuronal antidromic response and jaw-opening reflex as induced by stimulating the caudal nucleus of the spinal trigeminal tract. It was thus concluded that stimulating the CGM exerts no direct effect on motoneurons but does have an influence on postsynaptic response — a result of modulation of the afferent spike flow at interneuronal level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 543–549, July–August, 1990.     

Changes in postsynaptic responses in spinal motoneurons during repetitive stimulation of the locus coeruleus

Experiments on cats anesthetized with chloralose showed that repetitive stimulation of the locus coeruleus is accompanied by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons. The effect appeared 600 msec after the beginning of stimulation and reached its maximum after 1500–2000 msec. Repetitive stimulation of the locus coeruleus did not change the membrane potential and did not affect EPSPs or IPSPs evoked by stimulation of low-threshold muscle afferents; EPSPs due to activation of high-threshold cutaneous and muscle afferents likewise remained unchanged. Repetitive stimulation of more central regions of the brain stem was accompanied not only by a decrease in IPSPs evoked by stimulation of flexor reflex afferents in extensor motoneurons, but also by a decrease in amplitude of EPSPs arising in response to stimulation of these same afferents in flexor motoneurons. These effects were not connected with activation of monoaminergic structures, for unlike effects arising during stimulation of the locus coeruleus, they were also found in previously reserpinized animals.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 1, pp. 51–59, January–February, 1982.     

Synaptic activation of thoracic spinal interneurons by reticulospinal pathways

Synaptic processes of 119 thoracic spinal interneurons (T10–11) were investigated in anesthetized cats in response to stimulation of the medial and central zones of the gigantocellular nucleus in the medulla and the ventral columns of the spinal cord. Fast (90–130 m/sec) reticulospinal fibers running in the ventral column were found to produce monosynaptic or disynaptic excitation of interneurons of Rexed's layers VII–VIII, which are connected monosynaptically with group I muscle afferents, and interneurons excited both by group I muscle afferents and low-threshold cutaneous afferents. In most neurons of layer IV, connected monosynaptically with low-threshold cutaneous afferents, and in neurons of layers VII–VIII excited by afferents of the flexor reflex no marked postsynaptic processes were observed during stimulation of the reticular formation. Excitatory, inhibitory, and mixed PS Ps during activation of reticulospinal fibers were found in 14 neurons, high-threshold afferents in which evoked predominantly polysynaptic IPSPs. Seventeen neurons activated monosynaptically by reticulospinal fibers and not responding to stimulation of segmental afferents were found in the medial part of the ventral horn (layers VII–VIII).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 566–578, November–December, 1972.     

Effects of thalamic sensory relay nucleus stimulation on the jaw-opening reflex in response to tooth-pulp stimulation in the cat

The effects of stimulation of the thalamic sensory relay nucleus (TSRN, nucleus ventralis posteromedialis) on the jaw-opening reflex (JOR) in response to tooth pulp stimulation were compared with the effects of stimulation of the periaqueductal gray (PAG) and nucleus raphe magnus (NRM) in the cat. After stimulation of the TSRN, PAG and NRM, the JOR was inhibited. However, while the inhibitory effects of PAG and NRM stimulation lasted for more than 500 ms and were antagonized by the opiate antagonist, naloxone, the inhibitory effects of TSRN stimulation lasted for approximately 100 ms and were resistant to naloxone. These findings suggest that although TSRN stimulation exerts descending inhibitory effects on segmental nociceptive activity, similarly to PAG or NRM stimulation, the descending inhibitory pathways mediating the effect of TSRN stimulation may be largely distinct physiologically as well as pharmacologically from those mediating the effect of PAG and NRM stimulation.     

Involvement of bulbar respiratory and non-respiratory neurons in onset of expiration reflex

It was shown that excitation of high- and low-threshold superior laryngeal afferents triggers reflexes of varying complexity in a considerable proportion of non-respiratory neurons during experiments on cats anesthetized by Nembutal involving stimulation-induced expiration reflex. Systemic alterations in background firing activity were noted during this reflex in "respiratory" neurons; reflex reaction setting in as a result of low-threshold laryngeal afferent activation was also recorded in 22.4% of this group. Oligo- and polysynaptic excitatory connections were found between low-threshold laryngeal afferents and inspiratory beta neurons, P-cells, and laryngeal muscle motoneurons as opposed to inhibitory connections with inspiratory gammaneurons. This article discusses involvement of the neurons investigated in mechanisms of inspiratory inhibition, closure of the vocal chords, and adaptive decline in breathing rate occurring during expiration reflex.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 22, No. 5, pp. 670–680, September–October, 1990.     

Postnatal Temporal,Spatial and Modality Tuning of Nociceptive Cutaneous Flexion Reflexes in Human Infants

Cutaneous flexion reflexes are amongst the first behavioural responses to develop and are essential for the protection and survival of the newborn organism. Despite this, there has been no detailed, quantitative study of their maturation in human neonates. Here we use surface electromyographic (EMG) recording of biceps femoris activity in preterm (<37 weeks gestation, GA) and term (≥37 weeks GA) human infants, less than 14 days old, in response to tactile, punctate and clinically required skin-breaking lance stimulation of the heel. We show that all infants display a robust and long duration flexion reflex (>4 seconds) to a single noxious skin lance which decreases significantly with gestational age. This reflex is not restricted to the stimulated limb: heel lance evokes equal ipsilateral and contralateral reflexes in preterm and term infants. We further show that infant flexion withdrawal reflexes are not always nociceptive specific: in 29% of preterm infants, tactile stimulation evokes EMG activity that is indistinguishable from noxious stimulation. In 40% of term infants, tactile responses are also present but significantly smaller than nociceptive reflexes. Infant flexion reflexes are also evoked by application of calibrated punctate von Frey hairs (vFh), 0.8–17.2 g, to the heel. Von Frey hair thresholds increase significantly with gestational age and the magnitude of vFh evoked reflexes are significantly greater in preterm than term infants. Furthermore flexion reflexes in both groups are sensitized by repeated vFh stimulation. Thus human infant flexion reflexes differ in temporal, modality and spatial characteristics from those in adults. Reflex magnitude and tactile sensitivity decreases and nociceptive specificity and spatial organisation increases with gestational age. Strong, relatively non-specific, reflex sensitivity in early life may be important for driving postnatal activity dependent maturation of targeted spinal cord sensory circuits.     

Effects of Stimulation of the Periaqueductal Gray and <Emphasis Type="Italic">Locus Coeruleus</Emphasis> on Postsynaptic Reactions of Cat Somatosensory Cortex Neurons Activated by Nociceptors

In cats, we studied the influences of stimulation of the periaqueductal gray (PAG) and locus coeruleus (LC) on postsynaptic processes evoked in neurons of the somatosensory cortex by stimulation of nociceptive (intensive stimulation of the tooth pulp) and non-nociceptive (moderate stimulations of the infraorbital nerve and ventroposteromedial nucleus of the thalamus) afferent inputs. Twelve cells activated exclusively by nociceptors and 16 cells activated by both nociceptive and non-nociceptive influences (hereafter, nociceptive and convergent neurons, respectively) were recorded intracellularly. In neurons of both groups, responses to nociceptive stimulation (of sufficient intensity) looked like an EPSP-spike-IPSP (the latter, of significant duration, up to 200 msec) complex. Electrical stimulation of the PAG (which could itself evoke activation of the cortical neurons under study) resulted in long-term suppression of synaptic responses evoked by excitation of nociceptors (inhibition reached its maximum at a test interval of 600 to 800 msec). We observed a certain parallelism between conditioning influences of PAG activation and effects of systemic injections of morphine. Isolated stimulation of LC by a short high-frequency train of stimuli evoked primary excitatory responses (complex EPSPs) in a part of the examined cortical neurons, while in other cells high-amplitude and long-lasting IPSP (up to 120 msec) were observed. Independently of the type of the primary response to PAG stimulation, the latter resulted in long-term (several seconds) suppression of the responses evoked in cortical cells by stimulation of the nociceptive inputs. The mechanisms of modulatory influences coming from opioidergic and noradrenergic brain systems to somatosensory cortex neurons activated due to excitation of high-threshold (nociceptive) afferent inputs are discussed.Neirofiziologiya/Neurophysiology, Vol. 37, No. 1, pp. 61–73, January–February, 2005.