Plastic reorganizations of the receptive fields of neurons of the auditory cortex and the medial geniculate body induced by microstimulation of the auditory cortex.

The receptive fields of neurons (RFs) whose activity was simultaneously recorded at several loci of the auditory cortex (AC) and in the medial geniculate body (MGB) were investigated before and after intracortical microstimulation (ICMS). Three types of neurons of the AC and MGB were distinguished on the basis of the character of the RFs: mono-, bi-, and polymodal. The RFs of the neighboring neurons in the AC (or in the MGB) could differ, while the RFs of remote neurons of the AC could be similar. The microstimulation of the AC could lead to changes in the RFs of neurons in the stimulated locus and neighboring loci of the AC, as well as in the loci of the MGB tonotopically associated with them. It is hypothesized that long-term modification of the efficiency of synaptic transmission between different elements of the cortex-thalamus-cortex circuit which arises as the result of the circulation of impulses along this chain during ICMS may be the mechanism underlying the observed changes in the RFs of AC and MGB neurons.

Long-term posttetantic changes in the reaction of neighboring neurons in microsegments of the cat motor cortex.

The long-term postsynaptic changes of mono- and polysynaptic reactions of neighboring neurons of the MC were investigated following conditioning tetanization of different afferent inputs. Modifiable synapses were found both in the cells investigated and in local neuronal circuits which included the cells, i.e., possibly in interneurons. Alternating and concurrent conditioning of thalamocortical and corticocortical input showed that, depending upon the modality of the conditioned input, the tetanization parameters, the character of the distribution of the afferents, as well as on the character of local circuits which include excitatory and inhibitory interneurons, the effectiveness of synaptic inputs to neighboring neurons varies diversely, as a result of which a specific pattern of interneuronal connections forms in a microsegment of cortex, a pattern which may be maintained over the course of tens of minutes. It was found that modifiable synapses of different types may function simultaneously in one and the same micronetwork. The investigation may be of interest in developing models of memory and learning.

Neurobiology of the integrative activity of the brain: some properties of long-term posttetanic heterosynaptic depression in the motor cortex of the cat.

It has been demonstrated that long-term posttetanic heterosynaptic depression (LTHD), manifested in the form of a prolonged decrease in the probability of monosynaptic responses of the cell to stimulation of that afferent pathway which was not activated during conditioning tetanization of another input, takes place in the neocortex, as it does in the hippocampus. LTHD is characterized by such properties as its long-term character, cooperativity, and nonspecificity of input. LTHD in the nonconditioned input and long-term posttetanic potentiation or long-term posttetanic homosynaptic depression in the conditioned input may develop both in parallel or independantly of one another. It is hypothesized on the basis of the results obtained that LTHD (as is the case with LTP and LTD) is a calcium-dependant phenomenon, and that the achievement of a specific level of depolarization of the membrane in the region of the disposition of the inactive synapses is required for its occurrence. "Contrasting," i.e., a relative increase in the efficiency of transmission in the activating synapse, may be effected through LTHD; LTHD may be one of the mechanisms underlying forgetting.

[The plastic reorganization of the neuronal receptive fields of the auditory cortex and medial geniculate body evoked by microstimulation of the auditory cortex]. / Plasticheskie perestroiki retseptivnykh polei neironov slukhovoi kory i medial'nogo kolenchatogo tela, vyzvannye mikrostimuliatsiei slukhovoi kory.

The effects of intracortical microstimulation (ICMS) on receptive fields (RFs) of simultaneously recorded auditory cortex (A1) and medial geniculate body (MGB) neurons were examined in rats under ketamine. Three types of neurons were distinguished according to the configuration of their RFs: mono-, be-, and polymodal. It was shown that RFs of adjacent neurons may be different and Rfs of distant (up to 1 mm in rostro-caudal direction) neurons may be similar. ICMS was able to produce changes in RF structure of A1 neurons at and near the stimulation side and also in homotopically related regions of MGB. RFs of homotopically related A1 and MGB neurons mostly changed in a similar way after ICMS. We suggest that the mechanism underlying the observed RF changes of A1 and MGB neurons in related to ICMS-induced long-term changes (LTP and LTD) in synaptic efficacy of the cortex-thalamus-cortex loop.

[Long-term posttetanic changes in the reaction of adjacent neurons in microareas of the cat motor cortex]. / Dlitel'nye posttetanicheskie izmeneniia reaktsii sosednykh neironov v mikrouchastkakh motoronoi kory koshki.

The long-term posttetanic changes of mono- and polysynaptic reactions of the neighbouring neurons in the microareas of the motor cortex were studied before and after conditioning tetanization of different afferent inputs to these neurons. Consecutive or conjoint conditioning of the thalamo-cortical and the cortico-cortical inputs could induce different changes in the efficiency of the synaptic inputs to the neighbouring neurons the effect depending on the modality of the conditioned input, the parameters of tetanization, the pattern of distribution of the afferent terminals on the dendritic tree and the character of the local circuits formed by the neurons under study and connected with them excitatory and inhibitory interneurons. As a result of such changes the new patterns of interneuronal connections could appear and be stable for tens of minutes. It was shown that in the same neuronal network the different types of modifiable synapses could function simultaneously. The results might be used for construction of models of learning and memory.

[The properties of long-term posttetanic heterosynaptic depression in the cat motor cortex]. / O nekotorykh svoistvakh dlitel'noi posttetanicheskoi geterosinapticheskoi depressii v motornoi kore koshki.

It is shown that in the neocortex (like in the hippocampus) the long-term heterosynaptic depression (LTD) may exist. The effect appears as a long-term decrease of the probability of the monosynaptic responses in the afferent input to the recorded neuron after the conditioning tetanization of the other afferent input to the same neuron. The main properties of the heterosynaptic LTD are its longevity, cooperativity and lack of input specificity. The heterosynaptic LTD in the unconditioned input and the LTP or the homosynaptic LTD in the conditioned input can develop in parallel or independently of each other. It is supposed that the heterosynaptic LTD (like the LTP and the homosynaptic LTD) is Ca-dependent phenomenon and for its induction a certain level of depolarization of the membrane under the inactive (during conditioning) synapses must be achieved. The heterosynaptic LTD may provide for the "contrasting", i.e. the relative increase of the efficiency of the activated synapses, and probably be effective in such a phenomenon as forgetting.

[Prolonged homosynaptic depression of the impulse reactions of the motor cortex neurons in the cat]. / Dlitel'naia gomosinapticheskaia depressiia impul'snykh reaktsii neironov motornoi kory koshki.

It is shown that in the motor and the visual cortices of the cat the homosynaptic long-term posttetanic depression (LTD) of monosynaptic impulse responses of the cortical neurons may be induced in the tetanized input. Homosynaptic LTD appears as a decrease of the probability of the monosynaptic discharges or an increase in the latency of the monosynaptic responses. The cortical homosynaptic depression possesses the same properties as the hippocampal LTD, namely, the longevity, input specificity, cooperativity, and associativity. The possible mechanisms of the homosynaptic LTD induction are discussed. The effect may be determined, on the one hand, as Ca-dependent phenomenon, and on the other hand, as the LTP of monosynaptic reactions of the input inhibitory interneurons. It is supposed that the homosynaptic LTD of the impulse reactions of the cortical neurons may be one of the basic mechanisms in certain learning tasks, such as habituation or extinction.

[Distribution of efferent neurons of different types in the motor cortex of the cat]. / O raspredelenii éfferentnykh neironov razlichnykh tipov v motornoi kore koshki.

Distribution of efferent neurones of different categories was studied in the cat sensorimotor cortex: pyramidal and extrapyramidal systems and cortico-thalamic cells, whose axons end in the ventroposterolateral nucleus and medial geniculate body. In the major part of cortical microareas (88%) several output cells were found which could belong equally to one or several different categories. In some vertical cortical tracks no output neurones of the investigated categories were discovered. In tracks with efferent neurones, cortical areas with a definite set of output cells usually represented cylinders of 500-700 mcm in height. Separate microareas differed from each other by sets of fast or slow cells.

[Correlation between the amplitude of action potentials and the rate of conduction along axons of the output cells of the sensomotor cortex]. / O korreliatsii mezhdu amplitudnoi potentsialov deistviia i skorost'iu provedeniia vozbuzhdeniia po aksonam vykhodnykh kletok sensomotornoi kory.

Antidromic responses of neighbouring neurones in micro-areas of the sensorimotor cortex to the stimulation of fibers of the pyramidal tract as well as of the red nucleus and thalamic nuclei VPL and MGB, were studied in acute experiments on unanesthetized immobilized cats. Depending on the velocity of conduction along the axon, the neurones of all the categories were divided into fast and slow cells. When examining the two neuronal groups most differing in AP amplitude (N1 and N3), it was found that N1 neurones were mainly fast-conducting and N3 neurones-- slow-conducting. The conclusion is made that at multineuronal recording, each of the examined categories of the output neurones is characterized by positive correlation between AP amplitude and the axon conduction velocity and consequently, the size of the cell.

[Comparative analysis of the latent periods of adjacent neurons in microareas of the visual cortex]. / Sravnitel'nyi analiz latentnykh periodov reaktsii blizlezhashchikh neironov v mikrouchastkakh zritel'noi kory.

Latencies of responses to heteromodal stimuli were determined for neurones of the cat visual cortex recorded with different amplitudes of action potentials (AP). Latencies of responses of three neighbouring neurones recorded with one microelectrode were compared. It was shown that in the case of light stimulation, the neurone recorded with a smaller AP amplitude, as a rule entered the reaction before the neurone recorded with a greater AP amplitude; in response to acoustic stimulation, the reverse relationship was observed. In most of the microareas in which response latencies of the neighbouring cells differed slightly, crosscorrelograms revealed action of "the common input". Comparative analysis has shown that an afferent wave excites each of the microarea cells separately.

[Functional organization of neurons in different microportions of the visual cortex in the cat]. / O funktsional'noi organizatsii neironov v razlichnykh mikrouchastkakh zritel'noi kory koshki.

Evoked spike activity of neighbouring neurones in microareas of the cortical visual field 18 was studied in non-anaesthetized non-immobilized cats. In most of the micropools (MP) either all neighbouring cells reacted to the presented stimulus or they all did not react. A tendency has been detected to grouping of neurones with similar convergent properties. In a considerable part of MPs in which activity of the neighbouring cells was synchronized by the action of "common input", the responses of all the studied cells in the MP were similar. In the MPs with inhibitory interactions, some neurones produced inhibitory, and other--excitatory responses. The existence of several types of functionally differing MPs is suggested.

[Convergent properties of 3 types of visual cortex neurons recorded with various action potential amplitudes]. / Konvergentnye svoistva trekh tipov neironov zritel'noi kory, registriruemykh s raznymi amplitudami potentsialov deistviia.

Responses of visual cortex neurones recorded with large (N1), mean (N2) and small (N3) amplitudes of action potentials to photic, acoustic and electro-cutaneous stimuli were studied. It was shown that N1 neurones as differing from N3 were predominantly polymodal, often reacted with inhibitory responses and their background activity was characterized by random distribution of interstimuli intervals. Inputs for heteromodal stimuli were found to differ in N1 and N3 neurones. The results suggest existence of functional heterogeneity of neurones, recorded with different action potential amplitudes.

[Comparative analysis of the background activity of adjacent neurons in microsegments of the visual cortex of wakeful cats]. / Sravnitel'nyi analiz fonovoi aktivnosti sosednikh neironov v mikrouchastkakh zritel'noi kory bodrstvuiushchikh koshek.

By the shape of autocorrelation histograms six types of background activity have been defined in neurones of the field 18 of the visual cortex in non-anaesthetized non-immobilized cats. In the spike activity of neurones recorded with a high amplitude of action potentials, burst discharges and periods of activity inhibition were observed more often than in neurones recorded with a small amplitude. It is suggested that similarity of spike activity of neighbouring neurones observed in cortical microareas is due to similarity of afferent flows to these neurones.

[Cooperative activity of adjacent visual cortex neurons in alert cats]. / Sovmestnaia deiatel'nost' blizlezhashchikh neironov zritel'noi kory bodrstvuiushchikh koshek.

By means of auto- and crosscorrelation analysis, background activity of neighbouring neurones was studied in the visual cortex of alert cats. Four types of units differing in spike activity were found; each cortical microarea could consist of neurones of one or of different types. Analysis of configuration of mutual correlation histograms revealed four types of interdependent relations between the neighbouring neurones; definite types of spike activity of individual neurones were characteristic for each type of relations. Study of microsystems in which all or part of the neurones were united by the activity of a "common source" led to the assumption that the source was located outside the microsystems.