Changes in the excitability of and synaptic effects on motoneurons during formation of the scratch motion generator in the cat

Studies on immobilized decerebrate (at intracollicular level) cats in which the scratch generator had been set up following bicuculline application to the upper cervical segments of the spinal cord, showed that the state of the segmental apparatus of the lumbosacral section of the spinal cord differs substantially from that seen in the spinal animal. Direct excitability of motoneurons of the "aiming" and "scratching" muscles rises, while recurrent and reciprocal Ia inhibition of motoneurons intensifies and the influence of Ib afferents on motoneurons declines. Afferents of the flexor reflex exert a primarily inhibitory influence on motoneurons of the "aiming" muscles. This influence becomes predominantly excitatory following spinalization, while the inhibitory effects of these afferents on motoneurons of the "scratch" muscles declines. The functional significance of the changes discovered in generation of scratch routine is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 2, pp. 244–250, March–April, 1987.     

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.     

Influence of forelimb afferents on the activity of lumbar spinal-cord motoneurons

Experiments were conducted on anesthetized cats with microelectrode recording to study the synaptic responses that develop in the lumbar motoneurons on stimulation of the afferent fibers of groups II and III in the nerves of the ipsilateral and contralateral forelegs. Stimulation of these afferents evoked predominantly inhibitory postsynaptic potentials (IPSP) in the extensor motoneurons and excitatory postsynaptic potentials (EPSP) in the flexor motoneurons. A basically inhibitory change in the rhythmic background activity developed under the influence of descending impulsation. The duration of the total inhibition of "spontaneous" motoneuron activity corresponded to the duration of the inhibitory influences exerted by the forelimb flexor-reflex afferents (FRA) on the interneurons. The interaction of the descending and segmental PSP resulted in inhibition and facilitation of the segmental responses in the motoneurons. The ultimate result of this interaction was determined by the shifts in the membrane potential of the motoneuron and by the effects created in the interneurons.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 1, pp. 58–67, January–February, 1971.     

Neuronal organization of coeruleo-spinal connections in cats

Experiments on cats showed that electrical stimulation of the locus coeruleus leads to diminution of the inhibitory action of flexor reflex afferents (FRA) on the extensor monosynaptic reflex and to a decrease in amplitude of the IPSP evoked by FRA in extensor motoneurons. Injection of microdoses of aspartic acid and chlorpromazine into the locus coeruleus depresses the inhibitory effects of FRA, whereas injections of procaine and noradrenalin potentiate the inhibitory action of FRA. Data are given on the character of the descending influence of the locus coeruleus on different groups of spinal interneurons. It is concluded that depression of the inhibitory action of FRA is effected at the level of the final inhibitory interneurons of the "FRA system."A. A. Bogolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 362–374, May–June, 1984.     

Changes in segmental reflex responses during stimulation of locus coeruleus and potentiation of the action of catecholamines

Repetitive stimulation of the locus coeruleus (up to 150 µA in strength) was accompanied by marked weakening of the inhibitory action of flexor reflex afferents and of the reciprocal inhibitory action on extensor motoneurons. Meanwhile stimulation of this sort had no significant effect on direct inhibition of flexor and extensor motoneurons, on the facilitatory action of flexor reflex afferents and the reciprocal inhibitory action on flexor motoneurons and also on dorsal root potentials. Intravenously injected pyrogallol had a similar action, but its effect was much weaker after spinalization of the animals or blocking of spinal cord conduction by cold. Enhancement of the monosynaptic reflex, which also was observed after injection of pyrogallol, was characterized by different temporal parameters; the intensity of this effect was unaffected both by spinalization and by cold block. These data, and also the results of experiments with partial divisions of the spinal cord, suggest that the effects of stimulation of the locus coeruleus are the result of activity of a descending coerulo-spinal tract, running in the ventral quadrant of the spinal cord.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 1, pp. 39–47, January–February, 1981.     

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.     

Changes in presynaptic processes during tonic sub-threshold activation of spinal scratch generator in the cat

It was found during experiments on immobilized decerebrate (at intracollicular level) cats that tonic sub-threshold activation of the spinal generator of scratching action (following application of tubocurarine or bicuculline to segments C₁-C₂) was accompanied by depolarization of primary afferent terminals, a reduction in the N₁ component of dorsal surface potential produced by stimulating the cutaneous afferents, and a reduction in the amplitude of dorsal root potentials and lead-phase polysynaptic response produced in motoneurons by stimulating the cutaneous and muscle afferents. A rise or a reduction in the activity of interneurons belonging to the interstitial nucleus connected respectively mono- and di-(oligo)synaptically with the afferents occurred in parallel with this. Spinalization produced the same changes in reverse in the animal. By administering DOPA to the spinal animal, a comparison could be made of changes occurring in the state of the segmental apparatus of the lumbar section of the spinal cord during tonic sub-threshold activation of spinal scratch generators and locomotor movements.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 19, No. 2, pp. 236–243, March–April, 1987.     

Potentials of spinal motoneurons in cats with experimental tetanus

Potentials of motoneurons of the lower segments of the spinal cord were recorded with the aid of intracellular microelectrodes in experiments on cats with induced tetanus produced by injection of tetanus toxin (1500–2000 mouse LD₅₀) into the extensor muscles of the left shin. Neither afferent volleys of impulses in cutaneous and muscle nerves, nor antidromic volleys in the corresponding ventral roots, produced IPSPs in motoneurons of the extremity into which toxin was injected. The form both of antidromic peak potentials and of monosynaptic EPSPs in motoneurons in which IPSPs were blocked by tetanus toxin did not differ from the form of corresponding potentials of motoneurons in normal cats. The values of threshold depolarization for peak discharges during synaptic and direct stimulation were equal in tetanus and control motoneurons. Resistance and time constant values of the membrane in "tetanus" motoneurons did not differ from the corresponding values for "control" motoneurons.N. I. Pirogov Second Medical Institute, Moscow. Translated from Neirofiziologiya, Vol. 1, No. 1, pp. 25–34, July–August, 1969.     

Synaptic processes in thoracic motoneurons evoked by visceral impulses

Synaptic processes in various functional groups of thoracic motoneurons (segments T9–T11) were investigated in anesthetized (chloralose and Nembutal), decerebrate, and spinal cats. Visceral stimulation in animals with an intact CNS during artificial respiration evokes the development of primary (latent period under 12 msec) and secondary (latent period over 30 msec) PSPs in the motoneurons. The primary PSPs consist of early and principal components. The early component is due to excitation of group A₂ and A visceral afferents, the principal PSP to excitation of the A group. The principal component in motoneurons of the internal and external intercostal muscles and abdominal muscles is excitatory, while in motoneurons innervating the spinal muscles it is excitatory—inhibitory or inhibitory. The secondary PSPs as a rule are excitatory and are due to activation of fibers of the A group. During natural respiration the primary PSPs of motoneurons of the intercostal muscles and abdominal muscles are predominantly inhibitory. In spinal animals no secondary responses are present and the primary becomes entirely excitatory regardless of the functional class of the motoneurons. The mechanisms of reciprocal activation of thoracic motoneurons by visceral impulses in animals during artificial and natural respiration are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 4, No. 3, pp. 286–295, May–June, 1972.     

Effect of substances potentiating or inhibiting unit activity in the locus coeruleus on some types of spinal inhibition in cats

Microinjections of aspartic acid and chlorpromazine into the region of the locus coeruleus, which strengthen spontaneous unit activity in that structure, in decerebellate cats anesthetized with chloralose, led to depression of the inhibitory influence of flexor reflex afferents on extensor discharges, but did not change the facilitatory action of these afferents on flexor monosynaptic discharges and had no effect on recurrent inhibition of extensor discharges or reduced it. Microinjection of noradrenalin into this region, which depresses spontaneous unit activity in the locus coeruleus, or of procaine, which blocks action potential generation in neurons, led to potentiation of the inhibitory action of flexor reflex afferents on extensor discharges and to strengthening of recurrent inhibition, but did not affect the facilitatory action of these afferents on flexor discharges. The role of tonic descending influences of the locus coeruleus in the control of spinal inhibition evoked by flexor reflex afferents is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 247–256, May–June, 1981.     

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.     

Synaptic effects of contralateral somatic afferents on spinal motoneurons

Electrical activity of flexor and extensor alpha-motoneurons of the lumbar segments of cat's spinal cord as recorded intracellularly during electric stimulation of afferents of the contralateral posterior limb. Contralateral postsynaptic potentials (PSP) were shown to be evoked by activation of cutaneous and high-threshold muscle afferents. The high-threshold afferents of various muscle nerves participate to varying degrees in the generation of contralateral PSP. Contralateral inhibitory postsynaptic potentials (IPSP) were recorded in both flexor and extensor motoneurons along with contralateral excitatory postsynaptic potentials (EPSP). There are no fundamental differences in their distribution between flexor and extensor neurons. Inhibitory influences as a rule are predominant in both during the first 20 msec, and EPSP are predominant in the interval between 20 and 100 msec. The balance of excitatory and inhibitory pathway activity was found to be not as stable as that of the homolateral pathways.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 4, pp. 418–425, July–August, 1971.     

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.     

Pyramidal and extrapyramidal synaptic effects upon chronically deafferented motoneurons of the spinal cord of a cat

By means of intracellular recording technique, studies have been made of the electrical activity of -motoneurons of the seventh lumbar segment in cats with chronic rhizotomy of the dorsal root fibers (L₄-S₂). Postsynaptic potentials of the reticular formation of the midbrain, medulla, and ventral columns of the spinal cord were compared with the reactions recorded from nonoperated animals; these potentials were evoked by stimulation of the motor cortex, red nucleus, and Deuters' nuclei. Deafferentiation did not cause statistically reliable variations in the amplitude of the descending monosynaptic E PSPs. Extrapyramidal short-latent disynaptic E PSPs and IPSPs remained also practically unchanged, while the responses of deafferented motoneurons to cortico-spinal impulses were considerably facilitated; this effect was retained in pyramidal cats. Deafferentation was not accompanied by variations in the dependence of the discharge frequency on the depolarizing current strength or by the variation in the threshold and input resistance of the motoneuron membranes. This suggests that intensification of the pyramidal synaptic action upon deafferented motoneurons was caused by the variation on the intermediate neuronal level.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 1, No. 1, pp. 35–46, July–August, 1969.     

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.     

Effects of stimulating midbrain central gray matter on neuronal response in the caudal trigeminal nucleus to peripheral nerve stimulation

Stimulating the midbrain central gray matter (CGM) with trains of 10–20 stimuli was found to inhibit response to electrical stimulation of infraorbital nerve and tooth pulp A-alpha and A-delta afferents at 100 msec intervals in 65% of the caudal trigeminal nucleus in neurons tested during experiments on cats under chloralose-Nembutal anesthesia. Response was inhibited most effectively in convergent neurons (activated by stimulating infraorbital nerve and tooth pulp A-alpha and A-delta afferents) to stimulating tooth pulp (0.76) and, to a somewhat lesser degree, to stimulation of A-alpha afferents (0.6). For high-threshold neurons (activated by stimulating infraorbital nerve and tooth pulp A-delta afferents), success rate of inhibiting response under the effects of CGM stimulating measured 0.71 and 0.48 for low-threshold cells (activated by stimulating infraorbital nerve A-alpha afferents). Stimulating CGM produced an excitatory response in 10 caudal trigeminal nucleus neurons within 7.5–20 msec; after this neurons showed no reaction to peripheral nerve stimulation for a 200–450 msec period. The possible involvement of these neurons in pressing the mouth-opening reflex produced by CGM stimulation is discussed in this article.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 6, pp. 729–736, November-December, 1988.     

Effects of hypobaric hypoxia on the oxidative capacity of the extensor digitorum longus motor units in the rat

The fiber number, fiber type distribution, and succinate dehydrogenase activity were investigated from the fast-twitch extensor digitorum longus muscle of male rats exposed to 7 weeks of hypobaric hypoxia. The oxidative metabolic capacity of the motoneurons in the extensor digitorum longus neuron pool was also determined from quantitative histochemical analyses. The fiber number and oxidative enzyme activity of the muscle were not changed by hypoxia. An increase in the percentage of fast-twitch oxidative (FO) fibers and a concemitant decrease in the percentage of fast-twitch (F) fibers were observed in the hypoxic muscle. On the other hand, the oxidative capacity of small-to medium-sized alpha motoneurons (25–45 m average soma diameter) was increased. The increase in the oxidative capacity of small- to medium-sized motoneurons and the type shift of muscle fibers from F (low-oxidative) to FO (high-oxidative) indicate that hypoxia enhances the oxidative capacity of particular motor units in the neuron pool.     

Neurophysiological study of bulbar projections of different groups of superior laryngeal afferents

The projections of superior laryngeal afferents to bulbar neuronal structures were investigated using ortho- and antidromic testing during acute experiments on cats anesthetized with Nembutal. It emerged that endings of the most excitable (group A) afferent fibers were mostly distributed ipsilaterally within the solitary tract nucleus, the adjoining portion of the lateral tegmental field, and the rostral section of the retrofacial nucleus. High threshold (group A and A) afferent fibers also terminate in these nuclei, but are distributed over a wider area than low-threshold afferent projections in the solitary tract nucleus and the lateral tegmental field. The reverse applied to the retrofacial nucleus. Terminals of high threshold (group A) afferents extended to the caudal trigeminal nerve spinal tract nucleus, and possibly to the dorsal motor nucleus of the vagus nerve at obex level.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 81–90, January–February, 1988.     

Synaptic processes in thoracic α-motoneurons evoked by segmental afferent stimulation

Synaptic processes in various functional groups of thoracic motoneurons (Th₉-Th₁₁) evoked by stimulation of segmental nerves were investigated in anesthetized and decerebrate cats. No reciprocal relations were found between these groups of motoneurons. Only excitatory mono- and polysynaptic responses were recorded in the motoneurons of the principal intercostal nerve following stimulation of the homonymous nerve. Activation of the afferents of the external intercostal muscle and dorsal branches does not cause noticeable synaptic processes in these motoneurons; much more rarely it is accompanied by the development of low-amplitude polysynaptic EPSP's. In motoneurons of the dorsal branches, stimulation of homonymous nerves leads to the appearance of simple, short-latent EPSP's. Late responses of the IPSP or EPSP - IPSP type with a predominance of the inhibitory component were observed in most motoneurons of this type following activation of the afferent fibers of the principal intercostal nerve. In other motoneurons of the dorsal muscles, stimulation of the main intercostal nerve (and nerve of the external intercostal muscle) did not evoke apparent synpatic processes. EPSP's (mono- and polysynaptic) appeared in the motoneurons of the external intercostal muscle following stimulation of the homonymous and main intercostal nerves. Activation of the afferents of the dorsal branches was ineffective. The character of the synaptic responses of the respiratory motoneurons to segmental afferent stimulation, investigated under conditions of spontaneous respiration, was different. The characteristics of synaptic activation of thoracic motoneurons by segmental afferents under conditions of hyperventilation apnea and during spontaneous breathing of the animals are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 3, pp. 279–288, May–June, 1970.     

Postsynaptic responses of lamina II neurons of the spinal cord of the cat to activation of primary afferent input

Development of a complex response evokedin vivo in the neurons of lamina II of the spinal cord gray matter in cats by single electrical stimulation of primary afferents was simulated using mathematical models of these neurons, including the electrically excitable soma and axon and passive equivalent nonuniform dendrite. The intracellular response consisted of an excitatory postsynaptic potential (EPSP) with an action potential (AP) followed by a two-component hyperpolarization determined by the afterprocesses of hyperpolarization. The fast early hyperpolarization component appeared at the threshold stimulation of the most fast-conducting fibers; with an increase in the stimulation intensity it became superimposed on a slow later component. The direction of the early component changed after the hyperpolarizing shift of the membrane potential by 10 to 20 mV with respect to the resting level of –60÷–70 mV. The later component was abolished but not reversed even by the 50-mV shifts (to the –120-mV level). Simulation experiments showed that observedin vivo hyperpolarization-induced modification of the complex response is determined principally by a local interaction of electrotonus with synatic processes and does not depend on the behavior of the usual potential-activated sodium and potassium conductances in the soma. Inhibitory chloride synapses located on the soma and close to it represent the main source of fast early hyperpolarization, while distal dendritic potassium synapses are responsible for its late phase.Neirofiziologiya/Neurophysiology, Vol. 26, No. 5, pp. 382–390, September–October, 1994.