Changes in the synaptic apparatus of the associative area (field 5b) of the cat cerebral cortex following destruction of the lateral posterior thalamic nucleus

Quantity of normal and degenerating axonal terminals (AT) in area 5b of the associative cortex of cat was studied 2, 4 and 30 days after the electrolytic lesion of the thalamic nucleus lateralis posterior (LP). It was established that the quantity of degenerating AT in area 5b two and four days after the LP lesion was 7.4 and 7.2%, respectively. Four days after LP lesions the quantity of normal AT diminished by 16.4%, which was a direct result of excitatory AT degeneration. 40% of vanished AT formed synapses on dendrites and 60%--on spines. The quantity of axosomatic synapses in area 5b did not change after the LP lesion. Consequently, neurons in area 5b receive direct excitatory, predominantly axospinal input from LP. 30 days after LP lesions the total number of AT in area 5b was lower by 100% as compared with the control. Results and peculiarities of reinnervation in area 5b after degeneration of thalamocortical synapses are discussed.     

The neuronal reactions of the parietal associative cortex (area 5) in the cat during internal inhibition of the instrumental food reflex

Neuronal responses of the parietal associative cortex were studied in cat during differentiation, extinction and conditional inhibition of the placing reflex. It has been shown that this cortical area is involved in triggering and realization of the conditioned movement.     

The characteristics of the synaptic apparatus of the primary auditory area (A1) of the cat cerebral cortex

Quantitative and qualitative comparative studies of the synaptic apparatus in different layers of the primary auditory cortex (AI) in cat were performed using an electron microscope. The total average density of axonal terminal profiles in this area was 255 terminals per 1000 microns2 of the slice area. These profiles occupied 8.9% of the total studied area of slices. 45.3% of axonal terminals in area AI formed synapses on spines, 48.5%--on dendrites and 6.2%--on neuronal somata. 83.9% of synapses were asymmetric, 16.1%--symmetric. The number of synapses in 1 mm3 of the neural tissue in area AI estimated by stereological methods was 322.8 x 10(6).     

Responses of the neurons of the parietal associative cortex of the cat brain to indifferent and conditioned acoustic stimuli

Neuronal responses of the associative cortex (field 5) to indifferent and to conditioned sound stimuli were studied. The number of neurons responding to the conditioned sound during classical reflex increased two times. A percentage of inhibitory responses of neurons to conditioned acoustic stimulus during placing reflex grew. Neurons were found which responded to the conditioned sound only in the absence of the conditioned movement during instrumental food reflex. Despite the fact that the associative cortex participates in the analysis of sensory signals and in evaluation of their biological significance, its main functional property is its involvement in the initiation of behavioral reaction to the conditioned stimulus.     

Reactions of neurons of the parietal association cortex (field 5) of the cat brain during the performance of conditioned reflex movement

Responses of 160 neurons of the cat parietal cortex were studied during food instrumental reflex (placing reaction). 49% of them changed the discharge frequency before the movement initiation. Activation responses prevailed. Three main types of neurons involved in different ways into the process of realization of conditioned placing reaction were revealed. The obtained data evidence for the participation of the parietal cortex not only in realization of the conditioned placing reflex but also in its triggering.     

Distribution of norepinephrine-containing neurons projecting to the parietal associative cortex and spinal cord in the cat locus coeruleus

The distribution of neurons projecting to the parietal associative cortex and spinal cord in the locus coeruleus (LC) of the cat was examined by the retrograde transport of the horseradish peroxidase (HRP) and catecholamine histofluorescence technique. It was demonstrated that neurons projecting to the parietal cortex were localized predominantly in the dorsal LC and the most dense labeling was observed at the coronal section corresponding to frontal plane P-1. Neurons projecting to the spinal cord were concentrated in the ventral LC and the largest quantity of labeled neurons was found in the frontal plane P-3. Microinjections of HRP into the parietal cortex and spinal cord also caused intense retrograde labeling of neurons in the reticular formations of mesencephalon, pons varolii and medulla oblongata. Coeruleo-cortical and coeruleo-spinal projection neurons formed in the LC two partially overlapping cellular populations which could not be differentiated according to morphologic characteristics of their cells. It was concluded that the parietal associative cortex of cat as well as spinal cord had direct afferent inputs both from the LC and from the reticular formation.     

Thalamic projections to areas 5a and 5b of the parietal cortex in the cat: a retrograde horseradish peroxidase study

The cytoarchitecture of areas 5a and 5b of the cat's parietal cortex was re-examined and the afferent connections from the thalamus were investigated using the horseradish peroxidase (HRP) retrograde transport technique. Single or multiple small injections of the enzyme were made in different points of these areas in the rostral sectors of the lateral and middle suprasylvian gyri. The cytoarchitecture of the cortical region affected by the injections was carefully assessed in each case, and the labeled neurons found in the thalamus were plotted on projection drawings of each histological section. A prominent projection to area 5a arises from the posterior (Po) and ventral lateral (VL) complexes; less substantial projections originate in the ventral anterior nucleus (VA), the lateral intermediate complex (LI), and the central lateral nucleus (CL). Projections to area 5b (and to the laterally adjacent area suprasylviana anterior) mainly arise from LI, the dorsal part of VL, and the caudodorsal part of VA and CL; a moderate projection was also found from Po, the pulvinar, and the lateral dorsal complex. The main conclusions of this study are as follows. The shape and extent of areas 5a and 5b show notable variations when only their projection on the convoluted cortical surface is considered; however, they are relatively constant when plotted on unfolded cortical maps. The thalamic neurons projecting to areas 5a and 5b are organized according to a loose topographic plan, particularly noticeable in Po, VL and LI. In general, the rostral portion of this cortex (5a) receives projections from more ventral regions of the thalamus (mainly Po and VL), whereas the caudal part (5b) has connections from more dorsal regions (mainly LI and VA-VL). Moreover, the medial portions of these areas receive projections from lateral and ventral parts of the thalamic nuclei, whereas more dorsal and medial sectors of the thalamus project to the lateral portions of areas 5a and 5b. When labeled thalamic cell populations resulting from cases with single injections in neighboring cortical loci were compared, no abrupt changes of labeling were observed; rather, we generally observed gradual transitions and overlaps, even across nuclear boundaries. When only layers I and II of the cortex received the HRP, the number of labeled neurons and the intensity of their labeling decreased, their location in the thalamus was more restricted, and the mean size of the labeled cells was significantly smaller than that of the neurons labeled in the same regions after deep HRP injections.     

Connections between pyramidal neurons in layer 5 of cat visual cortex (area 17)

The structural features of two physiologically-characterised pyramidal neurons (PC1 and PC2) closely situated in layer 5b in the visual cortex (area 17) of a single cat were studied using a combination of electrophysiological and anatomical techniques. Both PC1 and PC2 had exceptionally large somata (30-40 microns in diameter). On the basis of this and other morphological features cell PC1 was classified as a Meynert cell. PC1 possessed a very large (2.75 degrees X 4.50 degrees) binocularly driven standard complex receptive field. PC2 was also binocularly driven with a small, B-type receptive field. Both cells had the same preference for the direction and orientation of visual stimuli. PC1 and PC2 could be antidromically activated from stimulating electrodes positioned above the dorsal lateral geniculate nucleus with a response latency indicating that these cells probably innervated the visual tectum or pretectum. In addition to corticoefferent axons, the two neurons possessed extensive intracortical axon arbors that ramified extensively in layers 5 and 6 of the medial and lateral banks of the lateral gyrus in area 17. Axon collaterals from both PC1 and PC2 also innervated a small common target region in area 18. A total of 313 boutons from the axonal arbors of PC1 and PC2 were examined in the electron microscope. All of the identified synaptic junctions were found to establish Gray type 1 asymmetrical contacts. The combined ultrastructural data for both neurons indicated that 80% of boutons were onto dendritic spine heads, with 14%, 6%, and 1% onto small-, medium-, and large-calibre dendritic shafts, respectively. The spectrum of postsynaptic targets showed little variation with respect to lamina, distance from somata, or cortical area. Other large pyramidal neurons in layer 5 and spiny neurons in layer 6 were identified as receiving synaptic input from either PC1 or PC2. Using a computer graphics system, rotations of the bouton distributions revealed the existence of a clustered innervation of layers 5 and 6 in areas 17 and 18 derived from the two identified neurons. The bouton distributions strongly resembled the tangential pattern described previously for the functional slab-like organisation of the cortex. The results provide a morphological basis for the clustered intrinsic connectivity of pyramidal cells in layers 5 and 6 of the cat visual cortex. Furthermore, the results indicate the widespread excitatory influence of large pyramidal neurons on other cells projecting subcortically to sites dealing with visually guided behavior.     

GABA-ergic structures in intact and chronically isolated association cortex of the cat brain (field 5)

Distribution of GABAergic structures in intact and neuronally isolated associative cortex (field 5) of the cat was studied using histochemical GABA-transaminase reaction two and three weeks after isolation. An overwhelming majority of GABAergic fibres of the neuropile and synaptic terminals were found to be formed by axons of few GABAergic interneurons. Only a small portion of them belongs to afferent axons of the extracortical origin. GABAergic interneurons can be divided into short-axon ones, making contacts within the isolated slat, and long-axon neurons forming horizontal connections with more distant cortical neurons. GABAergic axons give numerous projections to the soma and proximal parts of dendrites of many pyramidal neurons, containing no GABA-transaminase, and of stellate neurons which are of GABAergic and non-GABAergic mediatory nature. It is suggested that the influence of some GABAergic neurons on the others may ensure intracortical spatial regulation of inhibitory processes.     

Subcortical projections of the inferior parietal cortex (area 7) in the stump-tailed monkey

The efferent projections of the neocortex on the lateral convexity of the inferior parietal lobe (area 7 of Brodmann) were examined using the anterograde transport of tritiated amino acids. Multiple injections of ³H-leucine and ³H-proline were placed within the three cytoarchitecturally distinct zones that lie along the exposed surface of the inferior parietal lobe (IPL). The subcortical projections resulting from these injections were studied. Prominent projections were seen in the thalamus (medial and lateral pulvinar), brainstem (dorsolateral and ventral pontine nuclei), and basal ganglia (caudate and putamen) with less dense label over the thalamic intralaminar nuclei, pretectal complex, superior colliculus, reticular nucleus of the thalamus, suprageniculate nucleus, lateral posterior nucleus, oral pulvinar, and claustrum. In many of these cases there was a topographical relationship apparent with regard to the injections placed along the rostral-caudal dimension of the IPL. There is a striking reciprocal arrangement in the afferent and efferent projection systems of the IPL. The functional relevance of both the topography and the efferent projections of the IPL is discussed.     

Excitation and inhibition in parietal associative cortex neurons during stimulation of thalamic associative nuclei]

Responses of the parietal association cortex neurons to stimulation of lateralis dorsalis (LD) and lateralis posterior (LP) thalamic nuclei were investigated. Primary IPSP were observed in 62.5% of cases after stimulation of LD and in 79.6% of cases after LP stimulation. Latent periods of EPSP and IPSP were longer in case of LD stimulation rather than after LP stimulation. EPSP amplitude to association nuclei stimulation increased and decreased gradually, while amplitude of IPSP increased quickly and decreased slowly. Amplitude of EPSP which has appeared during IPSP development was larger than in case of the resting potential. Amplitude and duration of EPSP of the certain neuron after stimulation of different nuclei were rather similar. It is supposed that with stimulation of different association nuclei the same cortical inhibitory interneurons are involved. IPSP duration is shorter as compared to inhibition of the background impulse activity. A supposition is advanced that such a difference in duration is determined primarily by properties of these neurons.     

Projections from the parietal cortex to the brain stem nuclei in the cat, with special reference to the parietal cerebro-cerebellar system

Direct projections from the anterior portions of the parietal cortex of the cat to the brain stem nuclei, especially those sending fibers to the cerebellum, were investigated by the Nauta-Gygax and Fink-Heimer methods. Following unilateral lesions of the anterior portions of the middle suprasylvian and/or lateral gyri, a significant amount of pericellular degeneration was found almost entirely ipsilaterally in the rostral levels of the red nucleus and its vicinities, and in the pontine nuclei. Projection fibers to the pontine nuclei appeared to extend over several longitudinal, columnar zones in the pontine gray. Fibers from the anterior portion of the lateral gyrus were observed mainly in the paramedian and lateral nuclei, and those from the middle suprasylvian gyrus in the ventral, paramedian and lateral nuclei. Degeneration in the nucleus reticularis tegmenti pontis of Bechterew was slight, and found bilaterally with ipsilateral predominance. The significance of the anterior portion of the parietal cortex of the cat as a link of cerebro-cerebellar loops was discussed.     

Organization of corticothalamic projections from parietal cortex in cat

Corticothalamic projections from areas 5a, 5b, and 7 of cat parietal cortex were studied with autoradiographic techniques. Each cortical area was identified by its cytoarchitectural characteristics and the patterns of termination were related to the thalamic nuclear groups. Injections of 3H-leucine in cortical area 5a were associated with terminal labeling primarily in the spinal recipient zone of the ventral lateral nucleus (VLsp) and the medial division of the posterior group (POm). The corticothalamic projections of area 5a are loosely topographically organized; medial parts of 5a project heavily to rostral and lateral parts of VLsp and sparsely to POm, while lateral parts of 5a project to more medial and caudal parts of VLsp and heavily to POm. Cortical area 5b projects primarily to the rostral portions of the lateral posterior nucleus (LP). These projections also appear to be topographically organized. The part of area 5b on the marginal gyrus projects to more ventral parts of rostral LP, while area 5b on the middle suprasylvian gyrus projects to more dorsal and lateral parts of rostral LP. Cortical area 7 projects to LP and the pulvinar (Pul). Rostral parts of area 7 project heavily to dorsal and lateral parts of LP and lightly to Pul; more caudal portions of area 7 projects relatively more heavily to Pul. The reticular, central lateral, and paracentral nuclei also receive projections, especially from the suprasylvian gyrus. The results are discussed with regard to putative sensory response characteristics of these cortical areas and to general thalamocortical organization.     

The connections of the parietal cortex with the lateral suprasylvian gyrus (the Clare-Bishop field) and the auditory cortex in the cat

It has been shown that a parietal projection to the Clare--Bishop area is moderate and organized in a topographic manner. Associative fibres of area 5 terminate in the anterior part of the Clare--Bishop area, which corresponds to the intermediate and anterior part of the posterior suprasylvian sulcus belt. Area 7 projects to the posterior part of the intermediate and posterior suprasylvian sulcus belt. Area 5 and 7 send a few fibres to the auditory cortex. Associative fibres of area 5 terminate in the middle ectosylvian and sylvian gyri: areas 22, 50. Area 7 is connected only with the superior extremity of the middle ectosylvian gyrus or of areas 22, 50.     

The retinotopic organization of area 17 (striate cortex) in the cat.

The location and retinotopic organization of visual areas in the cat cortex were determined by systematically mapping visual cortex in over 100 cats. The positions of the receptive fields of single neurons or small clusters of neurons were related to the locations of the corresponding recording sites in the cortex to determine the representations of the visual field in these cortical areas. In this report, the first of a series, we describe the organization of area 17. A single representation of the cat's entire visual field corresponds closely to the cytoarchitectonically defined area 17. This area has the largest cortical surface area (380 mm2) and the highest cortical magnification factor (3.6 mm2/degree2 at area centralis) of all the cortical areas we have studied. There was perfect agreement between the borders of area 17 determined electrophysiologically and cytoarchitecturally. This area contains a first order transformation of the visual hemifield in which every adjacent point in the visual field is represented as an adjacent point in the cortex. Some variability exists among cats in the extent and retinotopic representation of the visual field in area 17.     

Histochemical localization of synaptic zinc in the developing cat visual cortex

The terminal boutons of many neurons in the telencephalon are known to contain a vesicle-bound, chelatable pool of zinc (Zn²⁺) that can be selectively visualized with histochemical procedures. In this paper, the normal laminar, areal, and ultrastructural distribution of histochemically reactive zinc in the visual cortex of the adult cat as well as its development from birth are described. In the adult cat visual cortex, intense zinc staining was found in layers I, II, III, and V, with layer VI staining only lightly. The primary geniculostriate input zone, layer IV, was conspicuously distinguished by the relative absence of zinc. This distinct pattern was restricted only to areas 17 and 18 and differentiated them from adjacent cortical area 19 laterally and the subadjacent cingulate cortex. The earliest zinc-positive staining in visual cortical areas 17 and 18 was first apparent by postnatal day 2 (P2) and was characterized by staining of a thin layer at the bottom of the cortical plate. By P10, and continuing through P20, synaptic zinc formed a trilaminar pattern of dense staining in areas 17 and 18, which included the top of layer I, and layers III and V. The laminar pattern of synaptic zinc in visual cortex appeared mature by P30, except that the distribution of zinc in layer IV was not uniform. This was most apparent around P50 in tangential sections through layer IV from opened and flattened cortex, where columnar patches of increased zinc staining were apparent in area 17. These columns were approximately 400 μm in diameter, with a centre-to-centre spacing of approximately 900 μm. The distribution of synaptic zinc apparently reflects the process of synaptic maturity of the cat visual cortex and appears to demarcate a particular form of columnar organization in visual cortex. © 1993 Wiley-Liss, Inc.     

Functional interaction between the associative parietal cortex and hippocampal place cell firing in the rat

The hippocampus and associative parietal cortex (APC) both contribute to spatial memory but the nature of their functional interaction remains unknown. To address this issue, we investigated the effects of APC lesions on hippocampal place cell firing in freely moving rats. Place cells were recorded from APC-lesioned and control rats as they performed a pellet-chasing task in a circular arena containing three object cues. During successive recording sessions, cue manipulations including object rotation in the absence of the rat and object removal in the presence of the rat were made to examine the control exerted by the objects or by non-visual intramaze cues on place field location, respectively. Object rotations resulted in equivalent field rotation for all cells in control rats. In contrast, a fraction of place fields in APC-lesioned rats did not rotate but remained stable relative to the room. Object removal produced different effects in APC-lesioned and control rats. In control rats, most place fields remained stable relative to the previous object rotation session, indicating that they were anchored to olfactory and/or idiothetic cues. In APC-lesioned rats, a majority of place fields shifted back to their initial, standard location, thus suggesting that they relied on uncontrolled background cues to maintain place field stability. These results provide strong evidence that the hippocampus and the APC cooperate in the formation of spatial memory and suggest that the APC is involved in elaboration of a hippocampal map based on proximal landmarks.     

Changes in the responsive ability of parietal associative cortex neurons to stimulation of associative thalamic nuclei during development of inhibition]

Neuronal responses of area 7 of the parietal associative cortex (PAC) to paired stimulation of the nLD and nLP thalamic nuclei were investigated by extra- and intracellular recording methods. It was found out that testing stimulation of the associative nuclei resulted in inhibition, facilitation or absence of clear neuronal responses. Inhibition was observed more often then other types of reactions. The intracellular recording has shown that inhibition of responses to testing stimuli developed during the initial phase of IPSP, while in the repolarization phase a facilitation was possible. The reversion of a Cl- -dependent component of IPSP did not lead to inhibition of responses to the testing stimulation in that period. It is suggested that inhibition of responses to the testing stimulus occurs in the presence of inward Cl- -dependent current, while facilitation of responses may be a result of activation of the late EPSPs.