Analysis of long cortical evoked potentials

Evoked potentials arising in the motor cortex in response to its direct stimulation (dendritic and slow negative potentials), to stimulation of the ventrolateral (primary response) and intralaminar (nonspecific response) thalamic nuclei, and to stimulation of the pyramidal tracts (antidromic response), and also postsynaptic responses of neurons corresponding to them were studied in acute experiments on curarized cats. Evoked potentials arising in response to direct cortical stimulation and also to stimulation of the specific and nonspecific thalamic nuclei and pyramidal tracts were recorded from the same point of the motor cortex, and the corresponding intracellular responses were recorded from the same neuron. Slow negative potentials arising under these conditions of stimulation and the IPSPs corresponding to them were shown to have an identical time course. The results show that slow negative potentials are a reflection of hyperpolarization of pyramidal neurons. It is suggested that the individual components of responses evoked by direct stimulation of the cortex and thalamic nuclei have a common genesis.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 115–121, March–April, 1982.     

Action of strychine on evoked potentials and postsynaptic responses of cat sensomotor cortical neurons

Acute experiments on immobilized cats lightly anesthetized with pentobarbital showed that application of strychine to the cortical surface inhibits slow negative potentials arising during direct and primary responses of the sensomotor cortex and corresponding IPSPs in pyramidal neurons. Iontophoretic applications of strychine blocks predominantly the early component of the IPSP, during which the input resistance under normal conditions is significantly less than during the late component of the IPSP, indicating that these components differ in their genesis. It is concluded that individual components of cortical evoked potentials have a common genesis, and that the slow negative potential is the dipole reflection of the IPSP in pyramidal neurons; the early component of the IPSP, moreover, is generated as a result of activation of axo-somatic inhibitory synapses, whereas the late component is generated as a result of activation of axo-dendritic synapses. The mediators in these inhibitory synapses may be different.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 480–487, July–August, 1984.     

Neuronal and synaptic mechanisms of cortical inhibition

The latent periods, amplitude, and duration of IPSPs arising in neurons in different parts of the cat cortex in response to afferent stimuli, stimulation of thalamocortical fibers, and intracortical microstimulation are described. The duration of IPSPs evoked in cortical neurons in response to single afferent stimuli varied from 20 to 250 msec (most common frequency 30–60 msec). During intracortical microstimulation of the auditory cortex, IPSPs with a duration of 5–10 msec also appeared. Barbiturates and chloralose increased the duration of the IPSPs to 300–500 msec. The latent period of 73% of IPSPs arising in auditory cortical neurons in response to stimulation of thalamocortical fibers was 1.2 msec longer than the latent period of monosynaptic EPSPs evoked in the same way. It is concluded from these data that inhibition arising in most neurons of cortical projection areas as a result of the arrival of corresponding afferent impulsation is direct afferent inhibition involving the participation of cortical inhibitory interneurons. A mechanism of recurrent inhibition takes part in the development of inhibition in a certain proportion of neurons. IPSPs arise monosynaptically in 2% of cells. A study of responses of cortical neurons to intracortical microstimulation showed that synaptic delay of IPSPs in these cells is 0.3–0.4 msec. The length of axons of inhibitory neurons in layer IV of the auditory cortex reaches 1.5 mm. The velocity of spread of excitation along these axons is 1.6–2.8 msec (mean 2.2 msec).A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 3, pp. 394–403, May–June, 1984.     

Effect of neurotropic drugs on cortical evoked potentials

In acute experiments on unanesthetized, curarized cats and rabbits and also on animals anesthetized with chloralose, recordings were made of direct cortical and transcallosal responses, responses in the pyramids of the medulla to peripheral stimulation and stimulation of the motor cortex, primary responses in area S-I, and interzonal somatomotor responses. The effect of narcotics on these cortical responses was shown to persist under conditions partially or completely excluding effects mediated through the reticular formation and other subcortical structures (intracarotid injection of the drugs or their local application to the cortex, experiments after premesencephalic section or on the isolated cortex). Neuroleptics have only a slight effect on these cortical evoked responses, mainly due to their blocking action on the reticular formation. Tranquilizers of the benzodiazepine series are active against the cortical responses studied, and this effect is due to their direct action on the cortex.Institute of Pharmacology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 3, No. 6, pp. 582–591, November–December, 1971.     

Habituation of direct cortical responses in cats

Gradual diminution of direct responses of various cortical areas was studied in acute experiments on cats under barbiturate anesthesia. During repetitive stimulation of the surface of cortical association areas these responses underwent all changes characteristic of the habituation phenomenon. Direct cortical responses in primary projection areas were distinguished by great stability, and habituation was absent in this case. After neuronal isolation of an island of projection cortex, habituation of responses began to develop there just as in the cortical association areas. The possible causes of differences between changes in direct cortical responses in different areas are discussed. It is suggested that projection areas receive strong activating influences from deep brain structures, which exert a sensitizing effect on the direct responses.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 13, No. 3, pp. 241–246, May–June, 1981.     

Involvement of calcium-dependent potassium conductance in membrane hyperpolarization processes at the pyramidal cell membrane in the cat sensorimotor cortex

In acute experiments on cats immobilized with myorelaxants intracellular injection of Cs ions reduced IPSP amplitude and postburst hyperpolarization of pyramidal cells in the sensorimotor cortex. Intracellular injection of ethylene glycol tetra-acetic acid produced a similar effect. The latter substance barely altered the lead front of neuronal after-hyperpolarization arising after a single action potential, while the lag front was inhibited. In conclusion, calcium-dependent potassium conductance may play an important part in the genesis of IPSP and of post-burst and after-hyperpolarization at the membrane of pyramidal cells of the cat sensorimotor cortex.I. S. Beritashvili Institute of Physiology, Academy of Sciences of the Georgian SSR, Tbilisi. Translated from Neirofiziologiya, Vol. 20, No. 3, pp. 383–389, May–June, 1988.     

Spread of excitation in upper layers of the cat auditory cortex with participation of intracortical interneuronal connections

In chronically isolated slabs of the cat auditory cortex with additional transection of lower layers and preservation of the structural integrity of one, two, or three upper layers of cortex just under the pial membrane, impulse responses of slab neurons to stimulation applied at the additionally undercut section were studied. High effectiveness of axodendritic and axospinal excitatory contacts formed by nerve elements of intracortical origin in upper cortical layers was demonstrated. The participation of geniculocortical fibers in spread of excitation in the cortex through synaptic contacts in layer I with dendrites of underlying-layer pyramidal neurons is discussed. The capacity for generation of polysynaptic excitation responses by the neurons indicates preservation of complex interneuronal interactions in the isolated cortex slab preparations with their undercut lower layers.I. I. Mechnikov State University of Odessa, Odessa. Translated from Neirofiziologiya, Vol. 23, No. 1, pp. 80–87, January–February, 1991.     

Modulation of pyramidal response in the rat sensorimotor cortex after combined stimulation of the lateral hypothalamus and the sensorimotor cortex

It was shown during experiments on unrestrained rats that rhythmic stimulation of the pyramidal tract produced a statistically significant increase in the functional activity of neuronal populations of the sensorimotor cortex, manifesting as potentiation of the primary, positive phase of pyramidal cortical response. Combined rhythmically matched stimulation of the pyramidal tract and of the lateral hypothalamus leads to statistically significant enhancement in potentiation of the positive phase of pyramidal cortical response compared with effects produced independently of hypothalamic involvement. When stimulation of the pyramidal tract and the lateral hypothalamus are combined with stimulation applied at the same periodicity to the sensorimotor cortex, a further statistically significant enhancement in potentiation of the positive phase of pyramidal cortical response is seen in addition to the potentiating effect produced by hypothalamic stimulation.Institute for Brain Research of the All-Union Scientific Center of Mental Health, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 18, No. 3, pp. 367–373, May–June, 1986.     

Generation and spread of direct responses of the intact and chronically isolated cortex

Mechanisms of generation and conduction of direct cortical responses were studied in acute experiments on unanesthetized curarized cats with an intact cortex or with a chronically isolated strip of cortex. All the properties of these responses of the intact and isolated cortex of the suprasylvian gyrus were found to be identical. Inhibition of muscarinic cholinergic receptors in the region of the cortex between the stimulating and recording electrodes prevents or considerably reduces the conduction of excitation through this area, which is explained by the polysynaptic mechanism of conduction of the direct response. Cortical neuronal networks over which direct responses spread are anisotropic in direction. Application of the anticholinergic drug benactyzine to the cortex inhibits, whereas application of the anticholinesterase agent galanthamine increases not only the original direct cortical responses, but also responses previously reversed by application of -aminobutyric acid or pentobarbital or by coagulation of the cortical surface. Both deep and surface components of the direct response are thus generated by cholinergic structures in the cortex itself.Institute of Physiology, Academy of Medical Sciences of the USSR, Siberian Branch, Novosibirsk. Translated from Neirofiziologiya, Vol. 7, No. 5, pp. 451–457, September–October, 1975.     

Analysis of the organization of afferent inputs in the projection cortex

Fast fluctuations in the evoked potentials (EP) at a local point of the projection cortex following stimulation of different pathways may reflect the activity of pyramidal neurons of different cortical layers. Analysis shows that the afferent and interarea projections to the somatic sensory cortex terminate on different neurons which can be regarded as relay neurons for a given pathway. Each group of neurons has its own system of inhibition for selective control of impulses coming along this pathway at the cortical level.Institute of Normal and Pathological Physiology, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 2, No. 4, pp. 368–372, July–August, 1970.     

Raised excitability produced in cortical neurons by reinforced stimulation of the lateral hypothalamus

Response was recorded in the pyramidal tract (PT) under three experimental situations modelling the shaping of conditioned reflex (CR) during experiments on unrestrained but unanesthetized rabbits. The first paradigm consisted of direct stimulation of two points on the sensorimotor cortex, the second of the same stimuli combine with electrical stimulation (used as additional reinforcement) of the lateral hypothalamus (LH), and the third of LH stimulation in response to a rise occurring in PT response to above control level (modelling instrumental CR). An overall increase in the monosynaptic wave indicative of altered efficacy of synaptic connections was most commonly observed under all these conditions. Increase in the component directly reflecting pyramidal neuronal excitation appeared significantly more pronounced in the second and third than in the first experimental paradigm. The data obtained would point to reinforced efficacy of excitatory synaptic connections as the principal mechanism of CR, while the changed quality of electrical excitability at the membrane of cortical neurons reflects mechanisms underlying the contribution of reinforcement triggered by LH activation in cortical reordering of the motivational/emotional component of reinforcement.Higher Nervous Activity and Neurophysiology Research Institute, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 21, No. 6, pp. 805–811, November–December, 1989.     

Activation of various areas of the ventral horn of lumbar segments during stimulation of the motor cortex

Focal potentials (FP) in segments L₆–L₇ of the ventral horn, evoked by stimulation of the motor cortex with series of stimuli of threshold magnitude for the flexor nerve response, were studied in acute experiments on cats. Appreciable differences were found to exist between the FP arising in the medial zone (layer VIII of Rexed) and those in the inner and outer parts of the lateral zone (layer IX). The FP of the medial zone appear earlier than in other zones (with a latency of 5–12 msec); they are multiphasic, negative components predominating over the positive ones. The FP from the inner part of layer IX possess the largest amplitude (up to 500 µV), a latency of 7–13 msec, a large first negative phase, and marked late positivity. Positive — negative FP (latency 9–15 msec) of small amplitude are recorded from the outermost portion of the ventral horn. The FP of the three zones mentioned above differ also with respect to other functional criteria. The FP of the medial zone are assumed to reflect the realization at the segmental level of the extrapyramidal component of descending cortical activity, the FP of both lateral zones reflecting reciprocal interrelations between postsynaptic processes in the motoneurons of flexor and extensor nuclei during implementation of a cortical motor reaction.I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 3, No. 2, pp. 175–184, March–April, 1971.     

Spread of excitation in the upper layers (I–III) in an auditory cortex slab isolated from the cat

Research was performed on an auditory cortex slab isolated from unanesthetized immobilized cats after severing of the lower layers, preserving intact a bridge one, two, or three upper cortical layers under the pial membrane; the spike response of neurons on this slab to stimuli applied at the other side of this additional incision was observed. It was found that stimulation applied to level IV produces an excitatory wave which spreads to the upper cortical layers, leading to mono- and polysynaptic activation of neurons at all levels via the apical dendrite of pyramidal cells. Antidromic activation of layer IV neurons together with an especially high proportion of monosynaptic response was observed with the three upper cortical layers remaining intact. A possibly crucial role in the horizontal spread of excitation within the cerebral cortex of a major bundle of axons from cortical interneurons is discussed in this article.I. I. Mechnikov University, Odessa. Translated from Neirofiziologiya, Vol. 20, No. 4, July–August, 1988, pp. 546–553.     

Responses of neurons in cat primary somatosensory cortex to repetitive stimulation of vibrissae

Extra- and intracellular responses of neurons in the primary somatosensory cortex to repetitive mechanical stimulation of the vibrissae at different frequencies were studied in unanesthetized curarized adult cats. Unlike responses to electrical stimulation of the combined afferent input (the infraorbital nerve) spike discharges of neurons in response to vibrissal stimulation can reproduce rather higher frequencies of stimulation and their initial character changes more often in the course of the repetitive series. Most cortical neurons were characterized by limitation of the area of their peripheral receptive fields with an increase in the frequency of adequate repetitive stimulation. A group of cortical neurons was distinguished by its ability to respond to high-frequency stimulation and to generate burst discharges. Comparison of the frequency characteristics of spike responses of these cells and of inhibitory synaptic action in other cortical neurons led to the conclusion that this group of cells thus distinguished may be inhibitory cortical neurons. The role of interaction between excitatory and inhibitory processes arising in cortical neurons during repetitive stimulation of different areas of their receptive fields is discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 14, No. 2, pp. 164–171, March–April, 1982.     

Mechanisms of the effect of diazepam on paroxysmal electrical activity of an isolated cortical slab in cats

The effect of diazepam on paroxysmal global electrical activity of a neuronally isolated slab of auditory cortex and on inhibitory responses of its neurons due to intracortical electrical stimulation was investigated in cats. Diazepam (2 mg/kg, intravenously) caused inhibition of paroxysmal electrical activity and increased the number of inhibited neurons in both the acutely isolated slab and three weeks after isolation, compared with the intact cortex. However, the number of disynaptic responses was reduced under these circumstances, especially in the long-isolated slab. It is postulated that diazepam exerts its action through activation of GABA-ergic inhibitory neurons, by synchronizing inhibition and increasing the duration of the IPSPs. The action of diazepam is manifested first, probably, in the initial links of cortical neuron chains.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 17, No. 1, pp. 3–10, January–February, 1985.     

Cortico-pyramidal and cortico-extrapyramidal synaptic influences on lumbar motoneurons in monkeys

Postsynaptic potentials evoked by stimulation of the motor cortex or pyramids before and after acute pyramidotomy were investigated in the lumbar motoneurons of monkeys. In response to activation of fibers of the pyramidal tract monosynaptic EPSPs predominated in motoneurons innervating the distal muscles of the hind limbs. Monosynaptic EPSPs in the motoneurons of the distal muscles had a significantly higher amplitude and could be evoked by weaker stimuli than EPSPs in the motoneurons of the proximal muscles. Cortico-motoneuronal EPSPs in the motoneurons of the distal muscles had a less marked frequency potentiation than EPSPs with monosynaptic segmental delay in the motoneurons of the proximal muscles. Cortico-extrapyramidal synaptic responses appeared in the pyramidotomized monkeys during intensive repetitive stimulation of the motor cortex in motoneurons of both distal and proximal muscles. These effects, transmitted by descending projections of the brain stem, may be responsible for the partial preservation of cortical motor control after pyramidotomy.I. M. Sechenov Institute of Evolutionary Physiology and Biochemistry, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 4, No. 6, pp. 587–596, November–December, 1972.     

Characteristics of inhibition in receptive fields of the cat visual cortex

Unit responses of neurons of zone 17 in the cat striate cortex to stripes of different widths were studied. Changes in the number of spikes during different time intervals (cuts) from the beginning of the response were analyzed in relation to stimulus area. Comparison of the results with those obtained by the study of receptive fields of the lateral geniculate body showed a significant difference in the dynamics of inhibition between cortical and geniculate receptive fields. Similar results were obtained when cortical unit responses to simultaneous and consecutive appearance of two stripes in the receptive field, one in the excitatory zone and the other at the inhibitory periphery, were studied. Evidence of the longer duration of cortical inhibition also was obtained by the same technique. When both stripes were placed in the excitatory center of the field another feature of cortical inhibition was revealed: its dependence on the order of stimulus application. If the order of stimulus application coincided with the optimal direction of movement of the stripe for the given field, the unit response to the next stimulus was strongly facilitated by the action of the stimulus applied previously. Application of stimuli in the opposite order invoked inhibition. The sensitivity of inhibition to the order of stimulus application was observed in the center of the field; it diminished toward the periphery, where application of the stimuli in any order evokes inhibition of the response.Medical Academy, Sofia, Bulgaria, I. P. Pavlov Institute of Physiology, Academy of Sciences of the USSR, Leningrad. Translated from Neirofiziologiya, Vol. 9, No. 4, pp. 339–346, July–August, 1977.     

Habituation of postsynaptic unit responses of the cat motor cortex to stimuli of different modalities

Habituation (extinction) of postsynaptic unit responses of the cat motor cortex to repetitive electrodermal, photic, acoustic, and combined bimodal stimulation was investigated by intracellular recording. Habituation was shown by a decrease in the number of spikes per grouped discharge and a decrease in the amplitude and duration of the EPSPs, and sometimes IPSPs, on repetition of the stimulus. The way in which the course of habituation depends on the modality and duration of stimulation (at a constant frequency of 1/sec) is examined. Habituation of postsynaptic responses to sensory stimuli is observed with neurons of different functional groups, namely identified neurons of pyramidal tract and unidentified neurons, some of which were evidently pyramidal neurons and interneurons. The hypothesis is put forward that the habituation of PSPs of the cortical neurons is based on processes taking place mainly at the subcortical level.A. A. Zhdanov Leningrad State University. Translated from Neirofiziologiya, Vol. 4, No. 5, pp. 545–553, September–October, 1972.     

Neuronal responses of a chronically isolated slab of auditory cortex to intracortical stimulation during paroxysmal electrical activity in cats

Responses of 155 neurons 3 weeks after neuronal isolation of a slab of auditory cortex (area AI) to single intracortical stimulating pulses at the level of layer IV were studied in unanesthetized, curarized cats during paroxysmal electrical activity evoked by series of high-frequency (10–20 Hz) electrical stimulation by a current 2–5 times above threshold for the direct cortical response. In response to such stimulation a discharge of paroxysmal electrical activity, lasting from a few seconds to tens of seconds, appeared in the slab. As a rule it consisted of two phases — tonic and clonic. This indicates that cortical neurons can form both phases of paroxysmal cortical activity. Depending on behavior of the neurons during paroxysmal electrical activity and preservation of their ability to respond to intracortical stimulation at this time, all cells tested in the isolated slab were divided into four groups. Their distribution layer by layer and by duration of latent periods was studied. Two-thirds of the neurons tested were shown to generate spike activity during paroxysmal discharges whereas the rest exhibited no such activity. A special role of neurons in layer II in generation of paroxysmal activity in the isolated slab was noted. The view is expressed that at each moment functional neuronal circuits, independent of each other, exist in the slab and also, evidently in the intact cortex, which can interact with one another when conditions change.I. I. Mechnikov Odessa State University. Translated from Neirofiziologiya, Vol. 16, No. 1, pp. 3–11, January–February, 1984.     

Electrophysiological research into afferent connections of embryonic neocortex allotransplants grafted into the association cortex of adult rats

Grafts of the rat fetal neocortex at the 17–18th day of gestation were placed in the cavity made by aspiration in the primary visual or somatosensory cortex of adult rats. Findings from electrophysiological research performed 3–3.5 months after this transplant showed that neurons of this transplant responded to sensory stimulation specific to the cortical regions replaced by the transplant in 50% of animals. This response was evoked by stimulating local receptive fields displaying a topical organization pattern in a proportion of the animals. Neuronal response in the transplant indicated that the usual field of vision previously existing on the replaced portions of visual cortex had been restored. Electrical stimulation applied locally to a number of brain structures showed that the transplants received afferent inputs from the thalamic nucleus normally projecting to the cortical region replaced by the graft, as well as from homotopic sites on the contralateral cortex. Latencies and time course of neuronal response to stimulating these regions of the host brain resemble those observed in the normal. Afferent inputs from the host brain to cortical transplants thus emulate normal cortical input. Possible mechanisms underlying reinnervation of the grafts are discussed.N. I. Vavilov Institute of General Genetics, Academy of Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 20, No. 4, July–August, 1988, pp. 448–456.