Effects of stimulating the periaqueductal gray matter on high- and low-threshold startle reflexes

The effects of stimulating the periaqueductal gray matter (PAG) on two types of startle reflex (spino-bulbo-spinal reflex produced by intensive stimulation of the peripheral nerves and low-threshold tactile spino-reticulo-spinal reflex) as well as high-threshold jaw-opening reflex arising in response to tooth pulp stimulation were investigated in cats anesthetized with chloralose. Simulating most PAG test sites led to pronounced inhibition of jaw-opening reflex, profound depression of spino-bulbo-spinal reflex, and moderate inhibition of tactile reflexes. The facilitatory effect of stimulating a number of PAG sites on the latter reflexes was demonstrated. Effects of PAG stimulation fell into two classes: brief, measurable in hundreds of msec and more prolonged, measured in minutes and seconds. Findings would indicate certain differences between the effects of PAG stimulation low-threshold (non-nociceptive) and high-threshold (nociceptive) startle reflexes, of which the possible mechanisms are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 21, No. 1, pp. 71–78, January–February, 1989.     

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.     

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.     

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.     

Synaptic activation of thoracic interneurons by reticulospinal fibers of the lateral funiculus

Synaptic responses of different functional groups of interneurons in segments T10 and T11 to stimulation of the ipsilateral and contralateral medullary reticular formation were investigated in anesthetized cats with only the ipsilateral lateral funiculus remaining intact. Activation of reticulospinal fibers of the lateral funiculus with conduction velocities of 30–100 m/sec was shown to induce short-latency and, in particular, monosynptic EPSPs in all types of cells tested: in interneurons excited by group Ia muscle afferents, in cells activated only by high-threshold cutaneous and muscle afferents (afferents of the flexor reflex), in cells activated mainly by descending systems, and, to a lesser degree, in neurons connected with low-threshold cutaneous afferents. These cell populations are located mainly in the central and lateral parts of Rexed's lamina VII. Most neurons in laminae I–V of the dorsal horn, except six cells located in the superficial layers of the dorsal horn, received no reticulofugal influences. The functional organization of connections of the lateral reticulospinal tract with spinal neurons is discussed and compared with the analogous organization of the medial reticulospinal tract, and also of the "lateral" (cortico- and rubrospinal) descending systems.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 150–161, March–April, 1978.     

Peculiarities of the synaptic adaptation of the intermediate neurons of the thoracic segments of the spinal cord by cutaneous,muscle, and visceral afferents

The responses of the interneurons of the thoracic segments of the spinal cord to stimulation of the intercostal and splanchnic nerves were studied on decerebrated and narcotized cats. It was established that the neurons of different layers of the gray matter (according to Rexed) differ substantially in type of afferent inputs. Cells in laminae I–III and IV are activated chiefly by somatic afferents: primarily high-threshold in laminae I–III and low-threshold in lamina IV. The neurons of lamina V and most of the neurons of laminae VII and VIII respond to stimulation of high-threshold somatic afferents (cutaneous fibers of the A group and muscle afferents of groups II and III), as well as visceral afferents of group A, conducting impulses at a rate of 9–35 m/sec. Cells of laminae VII and VIII, monosynaptically activated by muscle afferents of group I, do not respond to stimulation of the visceral afferents. The peculiarities of the "functional" laminar organization of the thoracic segments of the spinal cord are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 6, pp. 563–572, November–December, 1970.     

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.     

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.     

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.     

Effects of stimulating the central gray matter on neuronal response in the cat medial thalamic nuclei

This study was performed on the effects of stimulating the midbrain central gray matter (CGM) on neuronal response in the association medial thalamic nuclei evoked by stimulation of A-alpha and A-delta fibers of the infraorbital nerve and the caudal nucleus of the spinal trigeminal tract (CN STT) and tooth pulp stimulation using cats anesthetized by thiopental-chloralose as experimental animals. Stimulating the CGB with trains of stimuli was found to evoke an excitatory response in a percentage of the neurons tested, in which latency fluctuated between 15 and 40 msec. Applying conditioned stimuli to the CGM caused suppression of response to afferent impulses in neurons belonging to the "convergent" and "low" and "high" threshold groups. Responses induced by stimulating tooth pulp and A-delta fibers showed 100% inhibition as compared with 86% during A-alpha fiber and infraorbital nerve stimulation. The fact that stimulating the CGM produces an inhibitory effect on the response of thalamic neurons evoked by stimulation of both peripheral afferents and the CN STT would indicate that the CGM can exert a direct action on thalamic neuronal activity.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 5, pp. 660–665, September–October, 1987.     

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.     

Activity of lumbosacral interneurons during fictitious scratching

Activity of lumbosacral spinal interneurons was studied during fictitious scratching in decerebrate, immobilized cats. Neurons whose activity changed during fictitious scratching were located in the substantia intermedia lateralis and ventral horn. Among these neurons cells were distinguished whose activity was modulated in rhythm with motor discharges to different muscles (61.6%) and cells which were activated tonically (21.4%) or inhibited tonically (17%). By correlation of activity with discharges to corresponding muscles the rhythmically activated neurons were divided into "aiming" (36.6%) and "scratching" (25%). Neurons whose activity was unchanged during fictitious scratching also were observed. These cells were located mainly in the more dorsal regions of gray matter. Neurons to which wide convergence of excitatory influences from high-threshold cutaneous and muscular afferents was observed were mainly placed in the "aiming" group. "Scratching" neurons, compared with "aiming," more often received inputs only from low-threshold cutaneous or high-threshold muscular afferents. Group Ia interneurons were activated in phase with the corresponding motoneurons. Passive displacement of the limb in a forward direction predominantly inhibited spike activity of the "aiming" and potentiated activity of the "scratching" neurons. The neuronal organization of the spinal scratch generator is discussed on the basis of the results.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 57–66, January–February, 1981.     

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.     

Presynaptic mechanisms of changes in segmental responses accompanying the formation of a spinal locomotor generator in the cat

Using unanesthetized and decorticated (or decerebrated at level A 13) cats, it was found that spinalization leads to depolarization of the central terminals of primary afferents and an increase in the N₁ component of dorsal surface potential and dorsal root potential (DRP) produced by stimulating the low-threshold cutaneous and muscle afferents. Other effects include an increase in early polysynaptic responses and DRP produced by stimulation of high-threshold muscle afferents, a reduction in the intensity of interneuron activation in the nucleus interpositus mono- and polysynaptically connected with primary afferents, and a rise in the activity of n. interpositus interneurons di- and oligo-synaptically connected with afferent terminals. Changes in the opposite direction were produced by injecting DOPA into spinal animals. The connection between changes in the state of the segmental neuronal apparatus of the lumbosacral spinal cord and the level of spinal locomotor generator activity is discussed in the light of the findings obtained.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 18, No. 5, pp. 669–678, 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.     

Populations of motor units of the human soleus muscle supplying H reflex generation: Electrophysiological analysis

Variations in the amplitude of H and M responses of them. soleus related to the variation in intensity of stimulation of then. tibialis comm. were evaluated in five persons with different ratios of the maximum H and maximum M response amplitudes (from 0.27 to 0.75). A decrease in amplitude of the H reflex accompanied by an increase of M response is supposed to be determined by collision of ortho- and antidromically conducted spikes in motoneuronal axons; this makes it possible to quantify the participation of various motoneuronal populations differing in activation thresholds of their axons in H reflex generation. The H response in individuals with a low ratio of the maximum H and M response amplitudes was shown to be due primarily to the involvement of high-threshold motoneurons. When the ratio between the above-mentioned maximum EMG potentials was high, all populations of motoneurons, except very low-threshold ones, participated in the H reflex generation. In all cases, only a portion of high-threshold motoneurons was involved in H activity, which contradicts the so-called size principle.Neirofiziologiya/Neurophysiology, Vol. 25, No. 6, pp. 417–420, November–December, 1993.     

Descending and ascending influences on neurons of the zona incerta

Characteristics of responses of single neurons in zona incerta to electrical stimulation of the amygdala and central gray matter of the midbrain were compared in acute experiments on rats. Analysis of the results showed that stimulation of the central gray matter evokes predominantly excitation (86%), where stimulation of the amygdala invokes inhibition (64%). Convergence of impulses from the amygdala and central gray matter was observed on 58 of 174 responding neurons. Most frequently (57%) the responses were opposite in direction. Stimulation of the amygdala invoked inhibitory effects and stimulation of the central gray matter activation effects. Possible functional relation in the amygdala-zona incerta-central gray matter system are discussed and the role of competitive interactions of flows of impulses descending from the amygdala and ascending from the central gray matter of neurons of zona incerta is examined.Institute of Physiology, Siberian Branch, Academy of Medical Sciences of the USSR, Novosibirsk. Translated from Neirofiziologiya, Vol. 12, No. 1, pp. 46–52, January–February, 1980.     

Effect of vasopressin on neuronal responses of theNucl. Caudalis of the spinal trigeminal tract in rats

Effect of vasopressin on responses of individual neurons of thenucl. caudalis of the spinal trigeminal tract was studied on rats under urethan anesthesia; the responses were evoked by nociceptive (stimulation of the tooth pulp) or non-nociceptive (stimulation of Aa fibers of the infraorbital nerve) afferent activation. After injection of 10 nM vasopressin into the recording zone, responses evoked by stimulation of the tooth pulp were suppressed in all studied neurons of the high-threshold group; the same was true as to responses induced by stimulation of the tooth pulp and infraorbital nerve in most neurons of the convergent group. At the same time, vasopressin did not change the responses evoked by stimulation of Aa fibers of the infraorbital nerve in neurons of the low-threshold group. Possible involvement of vasopressin in the process of pain suppression is discussed.     

Effect of noradrenalin and serotonin on synaptic transmission in the rat spinal cord

Experiments on superfused isolated spinal cord preparations from rats aged 8–13 days showed that noradrenal in and serotonin have only a weak effect on monosynaptic reflex discharges but a substantial effect on polysynaptic motoneuronal discharges: noradrenalin potentiates whereas serotonin inhibits them. Both amines inhibit dorsal root potentials evoked by stimulation of high-threshold afferents. Potentiation of polysynaptic motoneuronal discharges induced by noradrenalin is connected with hyperpolarization of high-threshold afferents due to inhibition of the function of neurons in the substantia gelatinosa, and with increased excitability of interneurons participating in the generation of motoneuronal discharges. Serotonin inhibits polysynaptic motoneuronal discharges through its direct depolarizing effect on terminals of high-threshold afferents and depression of interneuron activity responsible for these discharges. Adrenergic and serotonin receptors, mediating these effects of noradrenalin and serotonin, were subjected to pharmacologic analysis.A. M. Gor'kii Donetsk Medical Institute. Translated from Neirofiziologiya, Vol. 14, No. 3, pp. 241–247, May–June, 1982.     

Effect of nociceptive stimulation of signal transmission from low-threshold cutaneous afferents in the somatosensory system

In cats anesthetized with chloralose nociceptive heating of the skin of the foot to 44–60°C led to a two- to fourfold increase in amplitude of primary cortical responses to direct stimulation of neurons of the spinocervical tract receiving information from the heated area of skin, but did not affect primary responses evoked by stimulation of axons of these neurons in the dorsolateral funiculus, and actually inhibited the response to stimulation of the nerve innervating the heated area of skin. Inhibition was accompanied by depolarization of central terminal of low-threshold fibers of this nerve: During heating the amplitude of the antidromic discharges evoked in the nerve by stimulation of its presynaptic endings in the spinal cord was increased two- to threefold. After abolition of presynaptic depolarization with picrotoxin (0.2–0.7 mg/kg, intravenously) or as a result of asphyxia, nociceptive heating acquired the ability to facilitate primary responses arising as a result of stimulation of the nerve also. The amplitude of the responses was increased under these circumstances by 3–20 times. It is concluded that acute nociceptive stimulation causes such powerful presynaptic inhibition of impulse transmission from low-threshold fibers of the cutaneous nerve that it virtually abolishes the facilitating effect of nociceptive impulses on sensory neurons of the spinal cord. It is suggested that it is this inhibitory mechanism which prevents the development of hyperalgesia during acute nociceptive stimulation.Institute of General Pathology and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 13, No. 6, pp. 621–627, November–December, 1981.