REACTIONS OF NEURONS OF THE MEDIODORSAL NUCLEUS OF THE THALAMUS TO THE STIMULATION OF PERIPHERAL NERVES
R. A. Bagdasaryan and L. G. Vaganyan
UDC 612.822.3+612.826
The reactions of neurons of the associative mediodorsal nucleus of the thalamus to a solitary stimulation of the radial, sciatic, and splanchnic nerves were studied in conditions of an acute experiment in anesthetized and immobilized cats. There were 91.85, 90.56, and 81.35% reactive neurons, respectively. They were mainly concentrated in the parvocellular division of the nucleus. A high degree of convergence (82.6%) of somatic and visceral signals was found and their interaction was o_fthe reciprocal blocking type.
As the mediodorsal nucleus is an associative structure, it plays an important role in the perception and integration of heteromodal afferent streams arriving along various afferent channels [5, 7, 10-13]. Systematic investigations of the neuronal organization of the somatic, and especially the visceral afferent system, in the nucleus in question have not been undertaken up until the present time. The representation of the visual and auditory systems has been studied in greater detail [5, 10, 11]. The present study was devoted to the investigation of the projection of the radial, sciatic, and splanchnic nerves, as well as to the elucidation of the degree of convergence and character of the interaction of the extero- and interoceptive signals in the neurons of the mediodorsal nucleus.
METHODS
The experiments were carried out in 35 adult cats anesthetized with a mixture of chloralose and nembutal (50 and 10 mg/kg, respectively) and immobilized by d-tubocurarine. The simultaneous recording of the extracellular impulse activity and the focal potential was carried out by means of glass microelectrodes filled with a 4% solution of a procion dye. The stereotaxic atlas of Jasper and Aimone-Marsan was used. The potentials were photographed in a frame-by-frame tape transport mechanism mode from the screen of an $8-11 storage double-beam oscillograph. The stimulation of the peripheral nerves was accomplished through bipolar silver electrodes. Suprathreshold stimuli, at a voltage of 10-25 V, and with a duration of 0.1-0.3 msec, were utilized. The localization of the tip of the microelectrode was determined in sections of the brain on the basis of electrophoretic markers of a procion dye.
INVESTIGATION
RESULTS
Reactions of Neurons to Stimulation of the Somatic Nerves. The results of the investigation showed the high capacity of the neurons of the mediodorsal nucleus to respond in the presence of excitation of the afferent system of the somatic nerves: of 233 neurons tested, 91.85% reacted to stimulation of the radial nerve, and 90.56% to stimulation of the sciatic nerve (Fig. 1A). The region of the somatic representation includes the parvocellular division of the nucleus in which the majority of somatically activated neurons is concentrated (Fig. 1B). The majority of the reacting units were "silent" (76.82%); those neurons which were characterized by an extremely low level of spontaneous activity were also included in this group. A comparatively high level of background impulse activity was observed in the remaining neurons.
Laboratory of the Evolution of the Functions of the Subcortical Structures of the Brain, L. A. Orbeli Institute of Physiology, Academy of Sciences of the ArmSSR Erevan. Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 76, No. 11, pp. 1528-1537, November, 1990. Original article submitted February 2, 1990.
0009-3122/91/2701-0153512.50 9
Plenum Publishing Corporation
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Fig. 1. Reactivity and latent periods of neurons of the mediodorsal nucleus of the thalamus in response to stimulation of the radial (RN) and sciatic (SN) nerves. A) Percent relationship of reacting (1) and non-reacting (2) neurons; n = 233. Along the vertical: number of neurons. B) Sections of frontal planes of the mediodorsal nucleus on which the localization of reacting units is indicated by dots. C) Histograms of the distribution of latent periods of the reactions of neurons to stimulation of the radial and sciatic nerves. Along the abscissa: latent periods, msec; along the ordinate: number of neurons. A high degree of similarity was established in the reactions of the neurons upon a solitary stimulation of both somatic nerves. The responses of the neurons of the "silent" group were primarily of the phasic excitatory type. Tonic and phasictonic reactions were recorded significantly less frequently. A secondary activation was observed following the initial phasic excitation in 8 neurons during stimulation of the radial nerve and in 11 units upon stimulation of the sciatic nerve. The interval between them was 341 _+99.33 and 327 + 98.44 msec on the average. A response in the form of a discharge of from 2 to 5 impulses (75.1 and 76A.4%, respectively) was the most typical reaction of neurons during stimulation of both the radial and the sciatic nerves. Solitary (16.55 and 12.04%) and more prolonged discharges of from 6-14 spikes (8.35 and 11.52%) were recorded significantly less frequently. The average value of the duration of the evoked discharges was 16.63 _+0.74 and 16.53 + 0.76 msec. The pulse repetition frequency in the grouped responses was equal to 265.8 + 9.72 and 264 + 9.24 in I sec, respectively. For the background-active neurons which react to stimulation of the radial and sciatic nerves, a phasic activation with subsequent complete suppression of spontaneous impulse activity was the characteristic response (73.91 mad 74.36%, respectively). The remaining neurons responded with initial suppression. The duration of the post-activation suppression in response to stimulation of the radial nerve was 569.59 + 38.49 msec on the average, and in response to stimulation of the sciatic nerve, 614.39 + 44.35 msec. The temporal parameters of the initial inhibition had the following values: 549.25 _+45.5 and 534 -+ 66.4 msec. The latent period of the excitatory reactions of the neurons varied within quite wide limits (Fig. 1C). The average value of the latent periods upon stimulation of the radial nerve was 33.95 _+0.96 msec, and of the sciatic nerve, 37.89 -+ 1.15 msec. The histogram in the figure does not contain the value of the latent periods of the reactions of eight "silent" neurons which reacted with significantly later activation. Their latent period ranged from 376 to 1488 msec (893 + 155.95 msec) and from 426 to 1848 msec (965 + 183.4 msec), respectively, in response to stimulation of the nerves of the fore and hind extremities.
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Pig. 2. Reactivity and latent periods of neurons in response to stimulation of the splanchnic nerve. A) Sections of frontal planes of the mediodorsal nucleus on which the localization of reacting units is indicated by dots. B) Percent relationship of reactive (1) and nonreacting (two columns) neurons; n = 193. Along the vertical: number of neurons. C) Histograms of the distribution of latent periods of the reactions of neurons. Remaining designations as in Fig. I. A n
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Fig. 3. Diagram of the percent relationship of convergent neurons. Neurons: (A) 1) Triconvergent, 2) biconvergent, 3) uniconvergent; n = 182. b) R + Sc + Sp, reacting to stimulation of all peripheral three nerves: radial (R), sciatic (Sc), and splanchnic (Sp); R + So, reacting to stimulation of both somatic nerves; R and Sc, to stimulation of one of the somatic nerves; n = 182. Focal potentials in response to the stimulation of the somatic nerves had mainly a bi- and triphasic configuration, primarily with an initial positive deviation. The amplitude and duration of these potentials in response to stimulation of the radial nerve were, on the average, 114.7 +_ 5.65 rtV and 75.89 + 2.98 msec. Approximately the same values were obtained upon stimulation of the sciatic nerve (110.23 + 5.98 ~tV and 66.85 + 2.71 msec). The discharge of the neurons correlated in the main with the initial phase of the potential.
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Fig. 4. Convergence of somatic and visceral impulse activity in "silent" (A) and background (B) neurons of the mediodorsal nucleus. Neurons: (A) 1) Background, stimulation of nerve: 2, 5) radial; 3, 6) sciatic; 4, 7) splanchnic. 5-7) Superimposition of five sweeps of the beam. Calibration: 20 msec, I00 ~tV in A, and 200 msec in B. The functional properties of the neurons in question were studied in conditions of paired and frequency stimulation of the somatic nerves. No differences of any kind in the degree of fatigue of the neurons during the stimulation of both nerves were identified. At a stimulation frequency of 2 Hz, complete suppression of the reaction of the neuron and disappearance of the focal potential took place following the initial response. The long duration of the cycle of recovery of the reactions also corresponded to the low lability of the neurons. The complete recovery of the capacity to respond of the nem~ns following the conditioning stimulation commenced on the average after 1064.5 + 63.2 and 1075.07 _+99.1 msec. Reactions of Neurons to Stimulation of the Splanchnic Nerve. Of 193 investigated units, 81.35% reacted to visceral signals (Fig. 213). The region in which they were recorded corresponded to those divisions of the nucleus in which somatoactivated neurons were recorded (Fig. 2A). In the case of a solitary stimulation of the splanchnic nerve, phasic excitatory, tonic, and phasic-tonic reactions were also identified. However, the ratio of the types of reactions, as in the case of somatic stimulation, was shifted in the direction of the predominance of phasic excitatory over the other types: solitary stimulation of the splanchnic nerve elicited phasic excitatory responses in in a group of "silent" neurons. A secondary activation was recorded in a small portion of neurons following the early response. The interval between the early and late responses somewhat exceeded that found in the case of somatic stimulation, and on the average was equal to 481.175 + 137.07 msec. Stimulation of the splanchnic nerve in eight neurons induced late neuronal reactions with a latent period Of 383-1947 msec (899.3 + 197 msec on the average). The phasic activation was expressed in the occurrence of single, short, and prolonged discharges. However, as in the ease of somatic stimulation, a significant prevalence of short discharges of from 2 to 5 impulses (67.89%) 156
over single discharges (15.32%) and group responses of from 6-10 spikes (16.79%) were observed. The duration of the evoked discharges was on the average 19.97 + 1.29 msec, while the pulse repetition frequency in these discharges was 247.8 5:10.7 in 1 sec. The background-active neurons reacted in the majority of cases (69%) to stimulation of the splanchnic nerve with initial excitation and a subsequent phase of complete suppression, the duration of which was 576.2 + 48.6 msee on the average. An initial inhibition of spontaneous activity was recorded in the remaining neurons (31%) over the course of 373.5 +_.56.33 msec on the average. The maximal and minimal values of the latent period of the excitatory reaction were 15 and 121 msec, respectively (46.2 +_.1.74 msec on the average). Comparison of the temporal parameters of the reactions shows that the average value of the latent period of the response in the case of stimulation of the splanchnic nerve is greater than that which takes place during stimulation of the somatic nerves. Detailed analysis of the latent period distribution revealed that the majority of neurons were brought into activation 20-50 msec after the application of the stimulus (Fig. 2C). Analysis of the recorded visceral focal potentials showed that they have a similar configuration to that of the somatic potential. However, their amplitude-temporal indices differ somewhat. Thus, visceral focal potentials have a comparatively long latent period and a lower amplitude period. The amplitude and duration of the potentials was 95.51 + 7.06 t~V and 71.7 +_3.67 msec on the average. The reactions of the neurons more frequently correspond to the initial phase of the potential. When the frequency of stimulation is increased up to 1.5 Hz, the amplitude of the visceral focal potentials decreases almost by 50% of the initial value and vanishes at a frequency of 2 Hz. In the case of paired stimulation of the splanchnic nerve, the capacity of the neurons to respond is restored, by comparison with somatically activated neurons, at longer intervals, equal to 1738.4 5:199.96 msec on the average. Convergence and Interaction of Somatic and Visceral Signals. The neurons of the mediodorsal nucleus were characterized by a pronounced capacity for the convergence of somatic and visceral impulse activity: of 182 neurons tested, 86.26% responded to all three peripheral stimulations, 8.79% selectively reacted to stimulation of both somatic nerves, and 4.95%, to stimulation of one of the somatic nerves (Fig. 3A, B). Homogeneity of the types and "patterns" of responses of the convergent somatovisceral neurons was observed in the overwhelming majority of cases. Examples of convergence of the peripheral influences are shown in Fig. 4. In oscillograrn A, a silent neuron (1) reacts with a similar phasic excitatory type of reaction to stimulation of the radial (2, 5), sciatic (3, 6), and splanchnic (4, 7) nerves. A background-active neuron (1) in oscillogram B generates a complex response sequence consisting of several phases, in response to stimulation of the peripheral nerves: an initial phasic activation, following which a period of complete suppression of spontaneous activity appears with subsequent restoration of the initial background. The presence of a high degree of convergence facilitated processes of their interaction. This took place by the reciprocal blocking type of interaction, the total duration of which varied over wide range, and depended upon the type of conditioning stimulus. If the preceding stimulation was applied to a somatic nerve, the test response of splanchnic origin was completely suppressed in 96.97% of cases, and only in 3.03% was a partial blockade observed, manifested in an increase in the latent period and a decrease in the number of spikes in the evoked discharges. The capacity of the neurons to respond recovered in the intervals of 750--6000 msec. Complete restoration took place in the majority of the neurons (61.54%) in the 3000 msec interval. In the case of backward tracking of the stimulus, the somatic responses tested were completely blocked in 72.05% of cases, partially blocked in 14.7%, and were not subjected to the influence in 13.25% of cases. The intervals during which complete restoration of the test responses took place were 600-4000 msec. However, the response capacity of the main portion of the neurons (71.2%) was completely restored in the interval of 1000 msec, i. e., earlier than the application of the test stimulation of the splanchnic nerve. An example of the interaction of the peripheral signals is shown in Fig. 5. It must be noted that paired stimulation of two somatic nerves was also used in the experiments. And an inhibitory type of interaction was observed in this case as well. However, the test response recovered comparatively more quickly by comparison with the heteromodal interaction, in intervals from 300 up to 3500 msec.
DISCUSSION
OF R E S U L T S
Comparative analysis of the data obtained demonstrates the high degree of reactivity of the neuronal populations of the mediodorsal nucleus in response to solitary stimulation of the radial (91.85%), sciatic (90.56%), and splanchnic (81.35%) nerves. The majority of the reacting units are located in the ventral, central, and lateral regions, i. e., in the parvocellular division of the nucleus, and are distributed diffusely in iL Our data regarding the low amplitude and the lengthiness of the tern157
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Y~ Fig. 5. Interaction of somatic and visceral signals in a neuron of the mediodorsal nucleus. Upper three oscillograms, reactions of convergent neuron to separate stimulation of the radial (R), sciatic (Sc), and splanchnic (Sp) nerves. R + Sp, Sp + R, Sc + Sp, Sp + Sc, paired stimulation of the corresponding nerves. The intervals (in msec) between the paired stimuli are indicated by the figures. Calibration: 20 msec. poral parameters of the somatic and visceral focal potentials also point to the less compact distribution of the neurons. This has been also confirmed by morphological investigations of Scheibel and co-authors [17], who demonstrated the diffuse distribution of the cells in which the terminals of the ascending afferent systems terminate. In reference to the question of the localization of the neurons with somatic inputs, it should be noted that our data are in agreement with the published data that attest to a similar location of the neuronal elements which react to electrodermal stimulation of the fore and hind extremities [11]. Analysis of the distribution of the neurons reacting to different afferent signals show that discrete zones of the representations of the nerves under investigation are lacking in the mediodorsal nucleus. This contradicts the investigations of Durinyan [7] which demonstrated, in experiments using an evoked potentials recording technique, the existence of a defined orderly arrangement of somatic and visceral afferent projections in this nucleus. This disparity can be explained evidently by the use of ~ferent investigatory methods. The large variability of the latent periods of the neuronal reactions upon stimulation of the peripheral nerves points to the heterogeneity of the pathways of transmission of these influences. It has been established at the present time that associative nuclei of the thalamus have direct connections with ascending afferent systems [5, 7, 11, 15-18]. In particular, the afferent supply of the nucleus includes influx along the fibers of the medial loop lemniscus and the spinothalamic tract. In addition to the direct pathway, somatic and visceral impulse activity may reach the structure under investigation from the somatosensory relay nucleus of the thalamus, from nonspecific thalamic structures, and from the reticular formations of the brain stem. According to the data of Kirzon and Kaplan [8], up to 72% of neurons of the mediodorsal nucleus react to stimulation of the reticular formation of the medulla and the brain stem, as well as of the central gray substance; the majority of these are excitatory reactions. Taking note of the fact that the neurons of the nucleus in question experience intense and diverse influences from the above-mentioned subcortical structures, with which the ascending afferent system has powerful connections, it can be hypothesized that the somatic and visceral streams of impulse activity reach the nucleus along both direct and complexly organized pathways. The presence of these pathways admits of the possibility of oligo- and polysynaptic activation of neurons upon stimulation of the peripheral nerves. The different types of reactions of the "silent" and background-active neurons observed in our experiments probably point to the presence of several types of cells and different types of synaptic contacts, regarding the existence of which convincing data are cited in morphological investigations [1, 11, 15, 17, 18]. The results of our experiments have shown that a wide overlapping of the representations of the somatic and visceral systems is observed in the mediodorsal nucleus, as it is in other subeortical structures [2-7, 10, 11], which is confmned by the convergence of the somatovisceral signals in a significant number of neurons (82.6%). The high degree of convergence is a 158
necessary precondition for the interaction of these signals, which takes place in accordance with the reciprocal blocking interaction type. The investigation of the interaction revealed varying degrees of reciprocal influence of the peripheral impulse activities~ Preceding stimulation of the somatic nerves exerts a more effective suppressant influence, whereas the conditioning stimulation of the splanchnic nerve could either not exert an influence or partially block the test somatic responses. The various degrees of interaction evidently are determined by the varying densities of the innervation of the nucleus by somatic and visceral afferents, while the identical type of reciprocal influence evidently points to the similar character of the synaptic processes developing in the neurons. Since it is known that convergence and interaction of the somatovisceral signals is already accomplished at the level of the interneurons of the spinal cord, there are grounds for believing that these two processes are the reflection of processes taking place at lower-lying levels [9, 10]. However, the possibility of encountering peripheral fignals for the fu'st time at the level of the thalamus cannot be excluded [7, 9]. At the same time, naturally, the corrective influence of descending cortical projections on the functional state of structures of lower-lying levels is of great importance. Our data have shown that in addition to excitatory reactions, inhibitory reactions are also recorded. The latter were identified in background-active neurons in the form of initial and secondary suppression of spontaneous activity, while they were identified in the "silent" neurons in the form of a complete blockade of the test responses when the process of interaction is studied by the method of paired stimulation of homogeneous and heterogeneous nerves. In extracellular recording conditions, the question regarding the nature of the suppression is very complex. Nevertheless, basing ourselves on the data of intraceUuiar investigations [11, 16], we can hypothesize that it is the result of postsynaptic inhibition arising during the participation of specialized inhibitory interneurons. The possibility of the participation of the mechanism of presynaptic inhibition as well is not excluded; the substrate of presynaptic inhibition is comprised of axoaxonal synapses found in substantial numbers in the associative structures of the thalamus [1]. When the data obtained are compared with the results of our previously published studies [2, 3, 6, 14], a definite similarity is revealed in the neuronal organization of the representations of the somatic and visceral afferent systems in the mediodorsal nucleus and in the centrum medianum of the thalamus. This confm-ns the existing notion [7] according to which both of these types of formations are regarded as a secondary switching system associated with the transmission of integrated information along slow-conducting somatic and visceral systems. Thus, having compared our data with those in the literature, we can assume that in addition to the dominant somatosensory input, as is generally accepted, the viscerosensory system also makes a significant contribution to the afferentation of the mediodorsal nucleus. In addition, the parvocellular division of the nucleus is the site of reception and integration of the extero- and interoceptive information.
LITERATURE CITED 1.
2. 3.
4. 5.
6. 7. 8. 9.
O.S. Adrianov, V. P. Babmindra, L. A. Kukuev, and G. A. Tolchenova, "The structural characterization of the thalamocortical interactions," in: The Evolution of the Functions of the Parietal Lobes of the Brain [in Russian], Leningrad (1973), pp. 7--43. A.A. Airapetyan, L. G. Vaganyan, and R. A. Bagdasaryan, "The reactions of neurons of the posterolateral nucleus of the thalamus to the stimulation of peripheral nerves," Fiziol. Zhurn. SSSR, 72, No. 3, 323-329 (1986). A.A. Airapetyan, L. G. Vaganyan, and I. G. Tatevosyan, "The convergence and interaction of somatic and visceral impulse activity in neurons of the ventral posterolateral nucleus of the thalamus," Fiziol. Zhurn. SSSR, 68, NO. 7, 976984 (1982). O.G. Baldavadzhyan, The Viscerosomatic Afferent Systems of the Thalamus [in Russian], Leningrad (1985). A.S. Batuev, L. A. Vasireva, and O. P. Tairov, "The functions of the thalmoparietal associative system of the brain of mammals," in: The Evolution of the Functions of the Parietal Lobes of the Brain [in Russian], Leningrad (1973), pp. 44--117. L.G. Vaganyan, R. A. Bagdasaryan, and D. S. Karapetyan, "The neuronal organization of the viseerosomatic afferent systems of the centrum medianum of the thalamus," Fiziol. Zhurn. SSSR, 71, No. 1, 72-79 (1985). R.A. Durinyan, The Central Structure of Afferent Systems [in Russian], Leningrad (1965). M.V. Kirzon and A. Ya. Kaplan, "The role of the reticular nucleus in the integration of extrathalamic influences regulating the sensory stream," Doklady Akad. Nauk SSSR, 236, No. 2, 481--483 (1977). P.G. Kostyuk and N. N. Preobrazhenskii, The Mechanisms of the Integration of Visceral and Somatic Afferent Signals [in Russian], Leningrad (1975), pp. 23-179. 159
10. 11. 12. 13. 14.
15. 16. 17. 18.
160
V.C. Raitses, The Mechanisms of the Interaction of Internal and External Analyzers [in Russian], Leningrad (1980), pp. 8-28. F.N. Serkov and V. N. Kazakov, TheNeurophysiology of the Thalamus [in Russian], Kiev (1980), pp. 117-151. H. Encabo and R. Volkind, "Evoked somatic activity in nucleus medialis: a microelectrode study," EEG and Clin. Neurophysiol., 25, No. 3, 252-258 (1968). H. Encabo, D. Cosarinsky, R. Glaucspiel, and S. Epstein, "Medial dorsalis thalamie responses masked in the background activity: a statistical analysis of spike discharge," Exp. Neurol., 41, No. 2, 260-270 (1973). A.A. Hayrapetian, L. G. Vahanian, I. G. Tatevosian, and R. A. Baghdasarian, "Comparative analysis of neuronal mechanisms of somatovisceral representation in specific and association thalamic nuclei," Intern. J. Neurosci., 22, No. 3--4, 183-184 (1984). W . J . H . Nauta and D. G. Whitlock, "An anatomical analysis of the non-specific thalamic projection system," in: Brain Mechanisms and Consciousness. A Symposium, Oxford (1954), pp. 81-116. D.P. Purpura and R. J. Shofer, "IntraceUular recording from thalamic neurons during reticulo-cortical activation," J. Neurophysiol., 26, No. 3,494-505 (1963). Mo E. Scheibel and A. B. Scheibel, "Patterns of organization in specific and non-specific thalamic fields," in: The Thalamus, New York (1966), pp. 13-16. A.E. Walker, "Internal structure and afferent-efferentrelations of the thalamus," in: The Thalamus, New York (1966), pp. 1-12.
R. A. Bagdasaryan and L. G. Vaganyan
UDC 612.822.3+612.826
The reactions of neurons of the associative mediodorsal nucleus of the thalamus to a solitary stimulation of the radial, sciatic, and splanchnic nerves were studied in conditions of an acute experiment in anesthetized and immobilized cats. There were 91.85, 90.56, and 81.35% reactive neurons, respectively. They were mainly concentrated in the parvocellular division of the nucleus. A high degree of convergence (82.6%) of somatic and visceral signals was found and their interaction was o_fthe reciprocal blocking type.
As the mediodorsal nucleus is an associative structure, it plays an important role in the perception and integration of heteromodal afferent streams arriving along various afferent channels [5, 7, 10-13]. Systematic investigations of the neuronal organization of the somatic, and especially the visceral afferent system, in the nucleus in question have not been undertaken up until the present time. The representation of the visual and auditory systems has been studied in greater detail [5, 10, 11]. The present study was devoted to the investigation of the projection of the radial, sciatic, and splanchnic nerves, as well as to the elucidation of the degree of convergence and character of the interaction of the extero- and interoceptive signals in the neurons of the mediodorsal nucleus.
METHODS
The experiments were carried out in 35 adult cats anesthetized with a mixture of chloralose and nembutal (50 and 10 mg/kg, respectively) and immobilized by d-tubocurarine. The simultaneous recording of the extracellular impulse activity and the focal potential was carried out by means of glass microelectrodes filled with a 4% solution of a procion dye. The stereotaxic atlas of Jasper and Aimone-Marsan was used. The potentials were photographed in a frame-by-frame tape transport mechanism mode from the screen of an $8-11 storage double-beam oscillograph. The stimulation of the peripheral nerves was accomplished through bipolar silver electrodes. Suprathreshold stimuli, at a voltage of 10-25 V, and with a duration of 0.1-0.3 msec, were utilized. The localization of the tip of the microelectrode was determined in sections of the brain on the basis of electrophoretic markers of a procion dye.
INVESTIGATION
RESULTS
Reactions of Neurons to Stimulation of the Somatic Nerves. The results of the investigation showed the high capacity of the neurons of the mediodorsal nucleus to respond in the presence of excitation of the afferent system of the somatic nerves: of 233 neurons tested, 91.85% reacted to stimulation of the radial nerve, and 90.56% to stimulation of the sciatic nerve (Fig. 1A). The region of the somatic representation includes the parvocellular division of the nucleus in which the majority of somatically activated neurons is concentrated (Fig. 1B). The majority of the reacting units were "silent" (76.82%); those neurons which were characterized by an extremely low level of spontaneous activity were also included in this group. A comparatively high level of background impulse activity was observed in the remaining neurons.
Laboratory of the Evolution of the Functions of the Subcortical Structures of the Brain, L. A. Orbeli Institute of Physiology, Academy of Sciences of the ArmSSR Erevan. Translated from Fiziologicheskii Zhurnal SSSR imeni I. M. Sechenova, Vol. 76, No. 11, pp. 1528-1537, November, 1990. Original article submitted February 2, 1990.
0009-3122/91/2701-0153512.50 9
Plenum Publishing Corporation
153
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Fig. 1. Reactivity and latent periods of neurons of the mediodorsal nucleus of the thalamus in response to stimulation of the radial (RN) and sciatic (SN) nerves. A) Percent relationship of reacting (1) and non-reacting (2) neurons; n = 233. Along the vertical: number of neurons. B) Sections of frontal planes of the mediodorsal nucleus on which the localization of reacting units is indicated by dots. C) Histograms of the distribution of latent periods of the reactions of neurons to stimulation of the radial and sciatic nerves. Along the abscissa: latent periods, msec; along the ordinate: number of neurons. A high degree of similarity was established in the reactions of the neurons upon a solitary stimulation of both somatic nerves. The responses of the neurons of the "silent" group were primarily of the phasic excitatory type. Tonic and phasictonic reactions were recorded significantly less frequently. A secondary activation was observed following the initial phasic excitation in 8 neurons during stimulation of the radial nerve and in 11 units upon stimulation of the sciatic nerve. The interval between them was 341 _+99.33 and 327 + 98.44 msec on the average. A response in the form of a discharge of from 2 to 5 impulses (75.1 and 76A.4%, respectively) was the most typical reaction of neurons during stimulation of both the radial and the sciatic nerves. Solitary (16.55 and 12.04%) and more prolonged discharges of from 6-14 spikes (8.35 and 11.52%) were recorded significantly less frequently. The average value of the duration of the evoked discharges was 16.63 _+0.74 and 16.53 + 0.76 msec. The pulse repetition frequency in the grouped responses was equal to 265.8 + 9.72 and 264 + 9.24 in I sec, respectively. For the background-active neurons which react to stimulation of the radial and sciatic nerves, a phasic activation with subsequent complete suppression of spontaneous impulse activity was the characteristic response (73.91 mad 74.36%, respectively). The remaining neurons responded with initial suppression. The duration of the post-activation suppression in response to stimulation of the radial nerve was 569.59 + 38.49 msec on the average, and in response to stimulation of the sciatic nerve, 614.39 + 44.35 msec. The temporal parameters of the initial inhibition had the following values: 549.25 _+45.5 and 534 -+ 66.4 msec. The latent period of the excitatory reactions of the neurons varied within quite wide limits (Fig. 1C). The average value of the latent periods upon stimulation of the radial nerve was 33.95 _+0.96 msec, and of the sciatic nerve, 37.89 -+ 1.15 msec. The histogram in the figure does not contain the value of the latent periods of the reactions of eight "silent" neurons which reacted with significantly later activation. Their latent period ranged from 376 to 1488 msec (893 + 155.95 msec) and from 426 to 1848 msec (965 + 183.4 msec), respectively, in response to stimulation of the nerves of the fore and hind extremities.
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msec
Pig. 2. Reactivity and latent periods of neurons in response to stimulation of the splanchnic nerve. A) Sections of frontal planes of the mediodorsal nucleus on which the localization of reacting units is indicated by dots. B) Percent relationship of reactive (1) and nonreacting (two columns) neurons; n = 193. Along the vertical: number of neurons. C) Histograms of the distribution of latent periods of the reactions of neurons. Remaining designations as in Fig. I. A n
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% ~
100
3
EO
M !
I
/
za
/
zo R + S o + Sp
R + Sc
R
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Fig. 3. Diagram of the percent relationship of convergent neurons. Neurons: (A) 1) Triconvergent, 2) biconvergent, 3) uniconvergent; n = 182. b) R + Sc + Sp, reacting to stimulation of all peripheral three nerves: radial (R), sciatic (Sc), and splanchnic (Sp); R + So, reacting to stimulation of both somatic nerves; R and Sc, to stimulation of one of the somatic nerves; n = 182. Focal potentials in response to the stimulation of the somatic nerves had mainly a bi- and triphasic configuration, primarily with an initial positive deviation. The amplitude and duration of these potentials in response to stimulation of the radial nerve were, on the average, 114.7 +_ 5.65 rtV and 75.89 + 2.98 msec. Approximately the same values were obtained upon stimulation of the sciatic nerve (110.23 + 5.98 ~tV and 66.85 + 2.71 msec). The discharge of the neurons correlated in the main with the initial phase of the potential.
155
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l
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s
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3
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4
B
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Fig. 4. Convergence of somatic and visceral impulse activity in "silent" (A) and background (B) neurons of the mediodorsal nucleus. Neurons: (A) 1) Background, stimulation of nerve: 2, 5) radial; 3, 6) sciatic; 4, 7) splanchnic. 5-7) Superimposition of five sweeps of the beam. Calibration: 20 msec, I00 ~tV in A, and 200 msec in B. The functional properties of the neurons in question were studied in conditions of paired and frequency stimulation of the somatic nerves. No differences of any kind in the degree of fatigue of the neurons during the stimulation of both nerves were identified. At a stimulation frequency of 2 Hz, complete suppression of the reaction of the neuron and disappearance of the focal potential took place following the initial response. The long duration of the cycle of recovery of the reactions also corresponded to the low lability of the neurons. The complete recovery of the capacity to respond of the nem~ns following the conditioning stimulation commenced on the average after 1064.5 + 63.2 and 1075.07 _+99.1 msec. Reactions of Neurons to Stimulation of the Splanchnic Nerve. Of 193 investigated units, 81.35% reacted to visceral signals (Fig. 213). The region in which they were recorded corresponded to those divisions of the nucleus in which somatoactivated neurons were recorded (Fig. 2A). In the case of a solitary stimulation of the splanchnic nerve, phasic excitatory, tonic, and phasic-tonic reactions were also identified. However, the ratio of the types of reactions, as in the case of somatic stimulation, was shifted in the direction of the predominance of phasic excitatory over the other types: solitary stimulation of the splanchnic nerve elicited phasic excitatory responses in in a group of "silent" neurons. A secondary activation was recorded in a small portion of neurons following the early response. The interval between the early and late responses somewhat exceeded that found in the case of somatic stimulation, and on the average was equal to 481.175 + 137.07 msec. Stimulation of the splanchnic nerve in eight neurons induced late neuronal reactions with a latent period Of 383-1947 msec (899.3 + 197 msec on the average). The phasic activation was expressed in the occurrence of single, short, and prolonged discharges. However, as in the ease of somatic stimulation, a significant prevalence of short discharges of from 2 to 5 impulses (67.89%) 156
over single discharges (15.32%) and group responses of from 6-10 spikes (16.79%) were observed. The duration of the evoked discharges was on the average 19.97 + 1.29 msec, while the pulse repetition frequency in these discharges was 247.8 5:10.7 in 1 sec. The background-active neurons reacted in the majority of cases (69%) to stimulation of the splanchnic nerve with initial excitation and a subsequent phase of complete suppression, the duration of which was 576.2 + 48.6 msee on the average. An initial inhibition of spontaneous activity was recorded in the remaining neurons (31%) over the course of 373.5 +_.56.33 msec on the average. The maximal and minimal values of the latent period of the excitatory reaction were 15 and 121 msec, respectively (46.2 +_.1.74 msec on the average). Comparison of the temporal parameters of the reactions shows that the average value of the latent period of the response in the case of stimulation of the splanchnic nerve is greater than that which takes place during stimulation of the somatic nerves. Detailed analysis of the latent period distribution revealed that the majority of neurons were brought into activation 20-50 msec after the application of the stimulus (Fig. 2C). Analysis of the recorded visceral focal potentials showed that they have a similar configuration to that of the somatic potential. However, their amplitude-temporal indices differ somewhat. Thus, visceral focal potentials have a comparatively long latent period and a lower amplitude period. The amplitude and duration of the potentials was 95.51 + 7.06 t~V and 71.7 +_3.67 msec on the average. The reactions of the neurons more frequently correspond to the initial phase of the potential. When the frequency of stimulation is increased up to 1.5 Hz, the amplitude of the visceral focal potentials decreases almost by 50% of the initial value and vanishes at a frequency of 2 Hz. In the case of paired stimulation of the splanchnic nerve, the capacity of the neurons to respond is restored, by comparison with somatically activated neurons, at longer intervals, equal to 1738.4 5:199.96 msec on the average. Convergence and Interaction of Somatic and Visceral Signals. The neurons of the mediodorsal nucleus were characterized by a pronounced capacity for the convergence of somatic and visceral impulse activity: of 182 neurons tested, 86.26% responded to all three peripheral stimulations, 8.79% selectively reacted to stimulation of both somatic nerves, and 4.95%, to stimulation of one of the somatic nerves (Fig. 3A, B). Homogeneity of the types and "patterns" of responses of the convergent somatovisceral neurons was observed in the overwhelming majority of cases. Examples of convergence of the peripheral influences are shown in Fig. 4. In oscillograrn A, a silent neuron (1) reacts with a similar phasic excitatory type of reaction to stimulation of the radial (2, 5), sciatic (3, 6), and splanchnic (4, 7) nerves. A background-active neuron (1) in oscillogram B generates a complex response sequence consisting of several phases, in response to stimulation of the peripheral nerves: an initial phasic activation, following which a period of complete suppression of spontaneous activity appears with subsequent restoration of the initial background. The presence of a high degree of convergence facilitated processes of their interaction. This took place by the reciprocal blocking type of interaction, the total duration of which varied over wide range, and depended upon the type of conditioning stimulus. If the preceding stimulation was applied to a somatic nerve, the test response of splanchnic origin was completely suppressed in 96.97% of cases, and only in 3.03% was a partial blockade observed, manifested in an increase in the latent period and a decrease in the number of spikes in the evoked discharges. The capacity of the neurons to respond recovered in the intervals of 750--6000 msec. Complete restoration took place in the majority of the neurons (61.54%) in the 3000 msec interval. In the case of backward tracking of the stimulus, the somatic responses tested were completely blocked in 72.05% of cases, partially blocked in 14.7%, and were not subjected to the influence in 13.25% of cases. The intervals during which complete restoration of the test responses took place were 600-4000 msec. However, the response capacity of the main portion of the neurons (71.2%) was completely restored in the interval of 1000 msec, i. e., earlier than the application of the test stimulation of the splanchnic nerve. An example of the interaction of the peripheral signals is shown in Fig. 5. It must be noted that paired stimulation of two somatic nerves was also used in the experiments. And an inhibitory type of interaction was observed in this case as well. However, the test response recovered comparatively more quickly by comparison with the heteromodal interaction, in intervals from 300 up to 3500 msec.
DISCUSSION
OF R E S U L T S
Comparative analysis of the data obtained demonstrates the high degree of reactivity of the neuronal populations of the mediodorsal nucleus in response to solitary stimulation of the radial (91.85%), sciatic (90.56%), and splanchnic (81.35%) nerves. The majority of the reacting units are located in the ventral, central, and lateral regions, i. e., in the parvocellular division of the nucleus, and are distributed diffusely in iL Our data regarding the low amplitude and the lengthiness of the tern157
R
Ill
R+
Sp
Sc
Ib
it
'IP
Sp + R
Sp
:I Sc + Sp
Sp + Sc
t00 -'"
l~
300
Y~ Fig. 5. Interaction of somatic and visceral signals in a neuron of the mediodorsal nucleus. Upper three oscillograms, reactions of convergent neuron to separate stimulation of the radial (R), sciatic (Sc), and splanchnic (Sp) nerves. R + Sp, Sp + R, Sc + Sp, Sp + Sc, paired stimulation of the corresponding nerves. The intervals (in msec) between the paired stimuli are indicated by the figures. Calibration: 20 msec. poral parameters of the somatic and visceral focal potentials also point to the less compact distribution of the neurons. This has been also confirmed by morphological investigations of Scheibel and co-authors [17], who demonstrated the diffuse distribution of the cells in which the terminals of the ascending afferent systems terminate. In reference to the question of the localization of the neurons with somatic inputs, it should be noted that our data are in agreement with the published data that attest to a similar location of the neuronal elements which react to electrodermal stimulation of the fore and hind extremities [11]. Analysis of the distribution of the neurons reacting to different afferent signals show that discrete zones of the representations of the nerves under investigation are lacking in the mediodorsal nucleus. This contradicts the investigations of Durinyan [7] which demonstrated, in experiments using an evoked potentials recording technique, the existence of a defined orderly arrangement of somatic and visceral afferent projections in this nucleus. This disparity can be explained evidently by the use of ~ferent investigatory methods. The large variability of the latent periods of the neuronal reactions upon stimulation of the peripheral nerves points to the heterogeneity of the pathways of transmission of these influences. It has been established at the present time that associative nuclei of the thalamus have direct connections with ascending afferent systems [5, 7, 11, 15-18]. In particular, the afferent supply of the nucleus includes influx along the fibers of the medial loop lemniscus and the spinothalamic tract. In addition to the direct pathway, somatic and visceral impulse activity may reach the structure under investigation from the somatosensory relay nucleus of the thalamus, from nonspecific thalamic structures, and from the reticular formations of the brain stem. According to the data of Kirzon and Kaplan [8], up to 72% of neurons of the mediodorsal nucleus react to stimulation of the reticular formation of the medulla and the brain stem, as well as of the central gray substance; the majority of these are excitatory reactions. Taking note of the fact that the neurons of the nucleus in question experience intense and diverse influences from the above-mentioned subcortical structures, with which the ascending afferent system has powerful connections, it can be hypothesized that the somatic and visceral streams of impulse activity reach the nucleus along both direct and complexly organized pathways. The presence of these pathways admits of the possibility of oligo- and polysynaptic activation of neurons upon stimulation of the peripheral nerves. The different types of reactions of the "silent" and background-active neurons observed in our experiments probably point to the presence of several types of cells and different types of synaptic contacts, regarding the existence of which convincing data are cited in morphological investigations [1, 11, 15, 17, 18]. The results of our experiments have shown that a wide overlapping of the representations of the somatic and visceral systems is observed in the mediodorsal nucleus, as it is in other subeortical structures [2-7, 10, 11], which is confmned by the convergence of the somatovisceral signals in a significant number of neurons (82.6%). The high degree of convergence is a 158
necessary precondition for the interaction of these signals, which takes place in accordance with the reciprocal blocking interaction type. The investigation of the interaction revealed varying degrees of reciprocal influence of the peripheral impulse activities~ Preceding stimulation of the somatic nerves exerts a more effective suppressant influence, whereas the conditioning stimulation of the splanchnic nerve could either not exert an influence or partially block the test somatic responses. The various degrees of interaction evidently are determined by the varying densities of the innervation of the nucleus by somatic and visceral afferents, while the identical type of reciprocal influence evidently points to the similar character of the synaptic processes developing in the neurons. Since it is known that convergence and interaction of the somatovisceral signals is already accomplished at the level of the interneurons of the spinal cord, there are grounds for believing that these two processes are the reflection of processes taking place at lower-lying levels [9, 10]. However, the possibility of encountering peripheral fignals for the fu'st time at the level of the thalamus cannot be excluded [7, 9]. At the same time, naturally, the corrective influence of descending cortical projections on the functional state of structures of lower-lying levels is of great importance. Our data have shown that in addition to excitatory reactions, inhibitory reactions are also recorded. The latter were identified in background-active neurons in the form of initial and secondary suppression of spontaneous activity, while they were identified in the "silent" neurons in the form of a complete blockade of the test responses when the process of interaction is studied by the method of paired stimulation of homogeneous and heterogeneous nerves. In extracellular recording conditions, the question regarding the nature of the suppression is very complex. Nevertheless, basing ourselves on the data of intraceUuiar investigations [11, 16], we can hypothesize that it is the result of postsynaptic inhibition arising during the participation of specialized inhibitory interneurons. The possibility of the participation of the mechanism of presynaptic inhibition as well is not excluded; the substrate of presynaptic inhibition is comprised of axoaxonal synapses found in substantial numbers in the associative structures of the thalamus [1]. When the data obtained are compared with the results of our previously published studies [2, 3, 6, 14], a definite similarity is revealed in the neuronal organization of the representations of the somatic and visceral afferent systems in the mediodorsal nucleus and in the centrum medianum of the thalamus. This confm-ns the existing notion [7] according to which both of these types of formations are regarded as a secondary switching system associated with the transmission of integrated information along slow-conducting somatic and visceral systems. Thus, having compared our data with those in the literature, we can assume that in addition to the dominant somatosensory input, as is generally accepted, the viscerosensory system also makes a significant contribution to the afferentation of the mediodorsal nucleus. In addition, the parvocellular division of the nucleus is the site of reception and integration of the extero- and interoceptive information.
LITERATURE CITED 1.
2. 3.
4. 5.
6. 7. 8. 9.
O.S. Adrianov, V. P. Babmindra, L. A. Kukuev, and G. A. Tolchenova, "The structural characterization of the thalamocortical interactions," in: The Evolution of the Functions of the Parietal Lobes of the Brain [in Russian], Leningrad (1973), pp. 7--43. A.A. Airapetyan, L. G. Vaganyan, and R. A. Bagdasaryan, "The reactions of neurons of the posterolateral nucleus of the thalamus to the stimulation of peripheral nerves," Fiziol. Zhurn. SSSR, 72, No. 3, 323-329 (1986). A.A. Airapetyan, L. G. Vaganyan, and I. G. Tatevosyan, "The convergence and interaction of somatic and visceral impulse activity in neurons of the ventral posterolateral nucleus of the thalamus," Fiziol. Zhurn. SSSR, 68, NO. 7, 976984 (1982). O.G. Baldavadzhyan, The Viscerosomatic Afferent Systems of the Thalamus [in Russian], Leningrad (1985). A.S. Batuev, L. A. Vasireva, and O. P. Tairov, "The functions of the thalmoparietal associative system of the brain of mammals," in: The Evolution of the Functions of the Parietal Lobes of the Brain [in Russian], Leningrad (1973), pp. 44--117. L.G. Vaganyan, R. A. Bagdasaryan, and D. S. Karapetyan, "The neuronal organization of the viseerosomatic afferent systems of the centrum medianum of the thalamus," Fiziol. Zhurn. SSSR, 71, No. 1, 72-79 (1985). R.A. Durinyan, The Central Structure of Afferent Systems [in Russian], Leningrad (1965). M.V. Kirzon and A. Ya. Kaplan, "The role of the reticular nucleus in the integration of extrathalamic influences regulating the sensory stream," Doklady Akad. Nauk SSSR, 236, No. 2, 481--483 (1977). P.G. Kostyuk and N. N. Preobrazhenskii, The Mechanisms of the Integration of Visceral and Somatic Afferent Signals [in Russian], Leningrad (1975), pp. 23-179. 159
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V.C. Raitses, The Mechanisms of the Interaction of Internal and External Analyzers [in Russian], Leningrad (1980), pp. 8-28. F.N. Serkov and V. N. Kazakov, TheNeurophysiology of the Thalamus [in Russian], Kiev (1980), pp. 117-151. H. Encabo and R. Volkind, "Evoked somatic activity in nucleus medialis: a microelectrode study," EEG and Clin. Neurophysiol., 25, No. 3, 252-258 (1968). H. Encabo, D. Cosarinsky, R. Glaucspiel, and S. Epstein, "Medial dorsalis thalamie responses masked in the background activity: a statistical analysis of spike discharge," Exp. Neurol., 41, No. 2, 260-270 (1973). A.A. Hayrapetian, L. G. Vahanian, I. G. Tatevosian, and R. A. Baghdasarian, "Comparative analysis of neuronal mechanisms of somatovisceral representation in specific and association thalamic nuclei," Intern. J. Neurosci., 22, No. 3--4, 183-184 (1984). W . J . H . Nauta and D. G. Whitlock, "An anatomical analysis of the non-specific thalamic projection system," in: Brain Mechanisms and Consciousness. A Symposium, Oxford (1954), pp. 81-116. D.P. Purpura and R. J. Shofer, "IntraceUular recording from thalamic neurons during reticulo-cortical activation," J. Neurophysiol., 26, No. 3,494-505 (1963). Mo E. Scheibel and A. B. Scheibel, "Patterns of organization in specific and non-specific thalamic fields," in: The Thalamus, New York (1966), pp. 13-16. A.E. Walker, "Internal structure and afferent-efferentrelations of the thalamus," in: The Thalamus, New York (1966), pp. 1-12.
