Postsynaptic effects evoked by activation of long propriospinal pathways in cervical spinal neurons in cats

Effects induced in motoneurons and interneurons of the cervical enlargements of the cat spinal cord by stimulation of the lateral and ventral funiculi at the lower thoracic level were studied under conditions producing degeneration of fibers of descending brain systems. Stimulation of this sort evoked PSPs (mainly of mixed character) in 57 of 90 motoneurons tested. In nine motoneurons the primary response consisted of monosynaptic EPSPs evoked by activity of fibers of the lateral funiculus, and in the rest it consisted of polysyanptic (at least disynaptic) EPSPs and IPSPs. Polysynaptic effects arising in the neuron in response to stimulation of the lateral and ventral funiculi usually differed only quantitatively. The intensity of excitatory synaptic action on motoneurons of the proximal muscle (especially thoracid) was much greater than that on motoneurons of distal muscles. Nearly all motoneurons with no synaptic action belonged to the latter group. Stimulation of the lateral and ventral funculi facilitated synaptic action induced in motoneurons by stimulation of high-threshold segmental afferents and led to excitation of interneurons located in the vectral quadrant, and had no effect on interneurons in the dorsal regions of gray matter. These effects are regarded mainly as the result of excitation of long ascending propriospinal pathways in the cervical parts of the cord; it is also postulated that some of them are evoked by the arrival of activity along collaterals of descending propiospinal pathways to the neurons in this region.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 11, No. 4, pp. 339–347, July–August, 1979.     

猫脊髓背索双投射神经元轴突分叉部位的电生理学研究

猫腰骶髓背角神经元对同侧颈髓背索和背外侧索的逆向刺激发生反应。这些神经元对刺激背索抑或背外侧索的逆向反应,在C_7-T_4节段的背索和背外侧索被分隔开时消失。其中,以背索刺激引起的逆向反应的消失机率居多。背索刺激的阈强度、不应期和逆向反应潜伏期等参数多高于背外侧索的有关参数。结果提示,脊髓背角脊颈束-背索突触后神经元轴突在下颈上胸节段分叉;分叉后的背外侧索分支的直径多粗于背索分支。     

Localization and properties of reticulospinal neurons with axons descending in the dorsolateral parts of the lateral funiculi

The localization of reticulospinal neurons responding antidromically to stimulation of fibers in the dorsolateral parts of the lateral funiculi (shown previously to be the principal collector of fibers conveying bulbar pressor influences) was determined in experiments on anesthetized and curarized cats. Most of these neurons were found to occupy the medioventral portions of the medulla, but they were concentrated in the rostral portions of the gigantocellular and ventral nuclei of the reticular formation. The velocity of conduction of excitation along axons of most reticulospinal neurons was 10–50 m/sec. Reflex responses to stimulation of the sciatic nerve with a latent period of 10–40 msec were found in 35 of 125 such cells. Stimulation of the sinus nerve did not activate them. Spontaneous activity occurred in 29 reticulospinal neurons; the mean firing rate of the various cells varied from 5 to 20/sec.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 6, No. 3, pp. 266–272, May–June, 1974.     

Activation of bulbar reticular neurons by visceral afferents

Transmission of impulsation from the visceral nerves to the bulbar reticular formation in the cat is effected by two systems of conductive pathways. The first (fast-conducting) is formed by the spinoreticular pathways of the lateral funiculi. This system activates reticular neurons with a latent period of 8–10 msec. The second system is made up of the polysynaptic pathways of the ventrolateral and ventral funiculi. Activation of reticular neurons through this pathway requires 10–20 msec or longer. Transection of the dorsal funiculi and the dorsal portion of the lateral funiculi causes no material changes in the synaptic activation of reticular neurons by visceral nerves.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 1, No. 2, pp. 177–185, September–October, 1969.     

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 responses of propriospinal neurons to stimulation of the stepping strip of the dorsolateral funiculus in cats

Synpatic responses were recorded extracellularly from single neurons at levels T12–T13 in response to microstimulation of the stepping strip of the dorsolateral funiculus in the thoracic and cervical portions of the spinal cord in cats decerebrated at the precollicular level [4]. The latent periods of these responses increased when the distance between the stepping point and recording point exceeded 20 mm, and when two stimuli had to be applied in order to evoke responses. Axons of neurons respondingtrans-synaptically to stimulation of the stepping strip were located on the boundary between the lateral and ventral funiculi close to the gray matter. Antidromic responses of neurons sending their axons in the caudal direction were found on average in 37% of cases, provided that the distance between the recording and stimulation points did not exceed 40 mm. Half of the neurons which generated synaptic responses to stimulation of spinal stepping points could be excited also by a series of three to six stimuli, applied to the mesencephalic locomotor point [15]. It is suggested that the neurons discovered may participate in the spread of activity toward the generator of stepping movements of the hind limb.Institute for Problems in Information Transmission, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 17, No. 2, pp. 270–278, March–April, 1985.     

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.     

Primary afferent depolarization due to selective activation of propriospinal pathways

Depolarization of primary afferent terminals in the lumbosacral portion of the spinal cord evoked by selective activation of propriospinal pathways was investigated in anesthetized cats. The strongest depolarization developed as a result of activation of short (two to five segments) propriospinal pathways in the lateral funiculus; stimulation of the long propriospinal pathways of this funiculus also induced depolarization, but of lower amplitude. Stimulation of propriospinal pathways of the ventral funiculi was ineffective. Significant primary afferent depolarization developed only following the use of a series of stimuli and strong stimulation of the propriospinal pathways. Excitation of these pathways caused depolarization of afferent terminals of both cutaneous and muscular nerves, including muscular sensory fibers of group Ia, although in the latter case its intensity was low. Neuronal mechanisms involved in the generation of this depolarization and its possible functional role are dicussed.     

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.     

Medullary reticular neurons in the Japanese toad: morphologies and excitatory inputs from the optic tectum

1. To elucidate the neural mechanisms that mediate visual responses of optic tectum (OT) to medullary and spinal motor systems, we analyzed medullary reticular neurons in paralyzed Japanese toads (Bufo japonicus). We examined their responses to electrical stimulation of OT, and stained some neurons intracellularly. Responses to stimulation of the glossopharyngeal nerve (IX) were also analyzed. 2. Extracellular single unit recording revealed excitatory responses of medullary neurons to OT and IX stimulation. Among 92 units encountered, 79 responded to OT stimuli, 10 to IX stimuli, and 3 to both. Some units responded to successive stimuli of short intervals with relatively stable lags. 3. Intracellular recording and staining experiments revealed morphologies of reticular neurons that received excitatory inputs from OT. Thirteen units were identified after complete reconstruction of somata and dendrites. Neurons in the nucleus reticularis medius received excitatory inputs from bilateral OT. They had wide dendrites in ventral, ventrolateral and lateral funiculi, and single axons descending in the ipsilateral ventral funiculus as far caudally as the cervical spinal cord. Some collaterals of these axons projected directly to the hypoglossal and spinal motor nuclei. Some neurons in other medullary nuclei (nuc. reticularis superior, pretrigeminal nucleus, nuc. reticularis inferior, and nuc. tractus spinalis nervi trigemini) also responded to the OT stimulation. 4. Activities in bilateral OT converge onto medullary reticular neurons, which may directly control medullary and spinal motor systems.     

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.     

Inhibitory effects of reticulospinal fibers of the lateral funiculus on neurons of thoracic spinal reflex pathways

Experiments on anesthetized cats with partial transection of the spinal cord showed that reticulo-spinal fibers in the ventral part of the lateral funiculus participate in the inhibition of polysynaptic reflexes evoked by stimulation of the ipsi- and contralateral reticular formation. The reticulo-fugal wave in the ventrolateral funiculus evoked comparatively short (up to 70 msec) IPSPs in some motoneurons of the internal intercostal nerve investigated and at the same time evoked prolonged (up to 500 msec) inhibition of IPSPs caused by activation of high-threshold segmental afferents. This wave also led to the appearance of IPSPs in 14 of 91 (15.5 %) thoracic spinal interneurons studied. The duration of these IPSPs did not exceed 100 msec; meanwhile, segment excitatory responses of 21 of 43 interneurons remained partly suppressed for 120–500 msec. It is concluded that the inhibitory action of the lateral reticulo-spinal system on segmental reflexes is due to several synaptic mechanisms, some of them unconnected with hyperpolarization of spinal neurons. The possible types of mechanisms of this inhibition are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 162–172, March–April, 1978.     

Synaptic responses of medulla oblongata neurons to stimulation of the spinal stepping atria in the cat

Synaptic responses of medulla oblongata (bulbar) neurons to microstimulation of stepping points in the spinal dorsolateral funiculi were recorded in decrebrate cats. Upon stimulation of the stepping point both in the ipsi- and contralateral funiculi, 40% of the neurons generated synaptic responses; the remaining cells responded to stimulation of only one stepping point. A part of the bulbar neurons responds to stimulation of stepping points both at the C₂ and Th₁₂ level. The latent periods of the synaptic responses of the bulbar neurons to stimulation of the stepping point at the C₂ level were in the 2–10 msec range. The data obtained indicate that bulbar neurons, along with propriospinal neurons, can participate in evoking locomotion upon stimulating the stepping point.Institute of Information Transmission Problems, Academy of Sciences of the USSR, Moscow. M. V. Lomonosov State University, Moscow. Translated from Neirofiziologiya, Vol. 23, No. 3, pp. 328–333, May–June, 1991.     

Study of neuronal structures of the brain stem which produce rapid reactions in reticulo-spinal pathways

The total electrical discharges of the ventral and ventrolateral funiculi of the 11th thoracic segment of the spinal cord in response to stimulation of different points of the pons varolii and the medulla oblongata between frontal planes P-2 and P-12 were investigated in decerebrated cats. Regions were found corresponding to the location of the oral and caudal reticular nuclei of the pons and the rostral section of the reticular giant cell nucleus whose stimulation caused a short-latent discharge in the ventral funiculus (latent period 1.6–1.9 msec) with maximum amplitude. It was concluded that such a discharge is due to the direct stimulation of a rather homogeneous group of fast-conducting reticulo-spinal fibers which terminate mainly in the ventral funiculus and which have an average rate of conduction of 131 m /sec. When the same regions were stimulated, the short-latet discharge of the ventro-lateral funiculus had a considerably lower amplitude and was accompanied by a powerful, late discharge with a latent period of about 3.3 msec. It was assumed that this funiculus consists mainly of fibers with a lower rate of conduction which originate in a broader region of the medial RF of the brain stem. Stimulation of contralateral regions of the reticular formation of the medulla oblongata evoked only a weak short-latent discharge in the ventral funiculus; the late component of the discharge in this case was considerably stronger.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 3, No. 3, pp. 274–283, May–June, 1971.     

Role of propriospinal neurons in inhibition of the afferent flow

The effects of stimulation of the dorsal funiculus on dorsal surface potentials (DSPs) of the spinal cord evoked by stimulation of a peripheral nerve and on antidromic action potentials (AAPs) evoked by stimulation of terminal branches of primary afferent fibers and recorded from the afferent nerve or dorsal root, were investigated in acute experiments on spinal cats and on cats anesthetized with pentobarbital and chloralose. Stimulation of the dorsal funiculus led to biphasic inhibition of the N₁-component of the DSP with maxima at the 15th–30th and 60th–80th milliseconds between the conditioning and testing stimuli. Maximal reinforcement of the AAP was found with these intervals. Bilateral division of the dorsal funiculi between the point of application of the conditioning stimuli and the point of recording the DSP abolished the first wave of inhibition of the DSP and the reinforcement of the AAP. After total transection of the cord above the site of conditioning stimulation the picture was unchanged. It is concluded that the initial changes in DSP and AAP are due to activation of the presynaptic inhibition mechanism by antidromic impulses traveling along nerve fibers running in the dorsal funiculus. Repeated inhibition of the DSP, like reinforcement of the AAP, can possibly be attributed to activation of similar inhibitory mechanisms through the propriospinal neurons of the spinal cord.Dnepropetrovsk State University. Translated from Neirofiziologiya, Vol. 5, No. 4, pp. 401–405, July–August, 1973.     

Confluence of barosensory and nonbarosensory inputs at neurons in the ventrolateral medulla in rabbits

In urethane-anesthetized rabbits, 209 spontaneously active neurons that responded to stimulation of aortic nerve A fibers were found within the ventrolateral medulla (VLM). The neurons, termed barosensory VLM neurons, were inhibited, except for three instances, by stimulation of A fibers. Forty-seven percent of barosensory VLM neurons tested (74 of 159) were activated antidromically by electrical stimulation of the dorsolateral funiculus at the C2 level. Activity of barosensory VLM neurons was enhanced by stimulation of carotid body chemoreceptors or the posterior hypothalamic area, whereas it was diminished by increases in arterial pressure elicited by injection of phenylephrine. Barosensory VLM neurons responded variously to stimulation, with two to three pulses at 40 or 100 Hz, of spinal afferents of cutaneous and muscle origins and the spinal trigeminal complex. Although stimulation of one group of somatosensory fibers could evoke different patterns of neuronal responses consisting of excitatory and inhibitory components, the following responses were most often encountered. Group II cutaneous afferents caused an inhibition. Recruitment of group III afferents brought about a brief excitatory component preceding it. Activation of group IV cutaneous fibers added a long latency excitatory component. Excitation of groups III and IV muscle afferents most often resulted in an inhibition, whereas stimulation of the spinal trigeminal complex elicited various combinations of excitatory and inhibitory components. These results are consistent with the view that neurons in the ventrolateral medulla receive barosensory and nonbarosensory inputs from various peripheral and central sources and participate in the control of sympathetic vasomotor activity and arterial pressure.     

Interaction between responses evoked by acoustic and somatosensory stimuli in neurons of the magnocellular part of the medial geniculate body

Interaction between responses to acoustic clicks and to electrodermal stimulation of the contralateral forelimb was investigated in 78 neurons in the magnocellular part of the medial geniculate body of curarized cats. Of this number, 33 neurons responded by discharges both to clicks and to electrodermal stimulation, 25 responded to clicks only, and 20 to electrodermal stimulation only, or to stimulation of the dorsal funiculus of the spinal cord. Conditioning stimulation evoked inhibition of the response to the testing stimulus in 32 of 33 neurons responding by spike discharges to both clicks and electrodermal stimulation. Electrodermal stimulation inhibited responses to clicks in all the neurons tested which responded only to clicks, whereas clicks evoked inhibition of responses to electrodermal stimulation (or to stimulation of the dorsal funiculus) in only four of the 20 neurons which responded to these types of stimulation only. It is suggested that inhibition of excitability arising in neurons of the magnocellular part of the medial geniculate body during interaction between auditory and somatosensory afferent volleys is based on postsynaptic inhibition.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 12, No. 4, pp. 368–374, July–August, 1980.     

Character of activation of medullary reticulospinal neurons by collaterals of pyramidal fibers

The character of activation of medullary reticulospinal neurons by collaterals of pyramidal fibers was investigated in cats anesthetized with pentobarbital (40 mg/kg) or a mixture of chloralose (45 mg/kg) and pentobarbital (15 mg/kg). The experiments were carried out on animals after preliminary destruction of the contralateral red nucleus and division of the ipsilateral dorsolateral fasciculus in segment C₄. A conditioning technique showed that pre- and postsynaptic effects arising in the medullary gigantocellular nucleus to stimulation of the cortex and of the isolated dorsolateral funiculus are due to activation of collaterals of pyramidal fibers projecting into the brain stem. In most reticulospinal neurons tested, stimulation of the fasciculus induced monosynaptic EPSPs. Their generation was due to influences transmitted via fast- and slow-conducting pyramidal fibers. Pyramidal fibers with different conduction velocities are distributed irregularly in the pyramidal tract in the cervical region of the spinal cord. Mainly slowly-conducting fibers are found in its medial zones and fast-conducting pyramidal fibers in its lateral zones. The results are evidence that in cats fibers of the pyramidal tract, running into the spinal cord, can activate medullary reticulospinal neurons directly.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 9, No. 5, pp. 495–503, September–October, 1977.     

Responses of units in the magnocellular part of the medial geniculate body to acoustic and somatosensory stimulation

Responses of 150 neurons in the magnocellular part of the medial geniculate body to clicks and to electrodermal stimulation of the contralateral forelimb were investigated in cats immobilized with myorelaxin. Of the total number of neurons 65% were bimodal, 16.6% responded only to clicks, and 15.4% only to electrodermal stimulation. The unitary responses were excitatory (spike potentials) and inhibitory (inhibition of spontaneous activity). Responses beginning with excitation occurred more frequently to stimulation by clicks than to electrodermal stimulation, whereas initial inhibition occurred more often to electrodermal stimulation. The latent period of the initial spike potentials in response to clicks and to electrodermal stimulation was 5–27 and 6–33 (mean 11.6 and 16.2) msec respectively. Positive correlation was found between the latent periods of spike potentials recorded in the same neurons in response to clicks and to electrodermal stimulation, and also to electrodermal stimulation and to stimulation of the dorsal funiculus of the spinal cord. It is concluded that the magnocellular division of the medial genicculate body is a transitional structure between the posterior ventral nucleus and the parvocellular division of the medial geniculate body, and that in addition, it is connected more closely with the auditory than with the somatosensory system. It is suggested that the somatosensory input into the magnocellular division of the medial geniculate body is formed mainly by fibers of the medial lemniscus.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 10, No. 2, pp. 133–141, March–April, 1978.     

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.