Organization of the projections from the posterior parietal cortex to the rostral and caudal regions of the motor cortex of the cat

The posterior parietal cortex (PPC) is an important source of input to the motor cortex in both the primate and the cat. However, the available evidence from the cat suggests that the projection from the PPC to those rostral areas of the motor cortex that project to the intermediate and ventral parts of the spinal gray matter is relatively small. This leaves in question the importance of the contribution of the PPC to the initiation and modulation of voluntary movements in the cat. As this anatomical evidence is not entirely compatible with the physiological data, we reinvestigated the PPC projection to the motor cortex by injecting dextran amine tracers either into the proximal or distal representations of the forelimb in the rostral motor cortex, into the representation of the forelimb in the caudal motor cortex, or into the hindlimb representation. The results show strong projections from the PPC to each of these regions. However, projections to the rostral motor cortex were observed primarily from the caudal bank of the ansate sulcus and the adjacent gyrus, whereas those to the caudal motor cortex were generally located more rostrally. There was also evidence of some topographic organization with the distal limb being located progressively more laterally and rostrally in the PPC than the areas projecting to more proximal regions. In contrast to previous anatomical investigations, these results suggest that the PPC can potentially modulate motor activity via its strong projection to the more rostral regions of the motor cortex.     

Organization of cortical and subcortical projections to medial prefrontal cortex in the cat

We have analyzed the cortical and subcortical afferent connections of the medial prefrontal cortex (MPF) in the cat with the specific aim of characterizing subregional variations of afferent connectivity. Thirteen tracer deposits were placed at restricted loci within a cortical district extending from the proreal to the subgenual gyrus. The distribution throughout the forebrain of retrogradely labeled neurons was then analyzed. Within the thalamus, retrogradely labeled neurons were most numerous in the mediodorsal nucleus and in the ventral complex. The projection from each region exhibited continuous topography such that more medial thalamic neurons were labeled by tracer from more ventral and posterior cortical deposits. Marked retrograde labeling without any sign of topographic order occurred in a narrow medioventral sector of the lateroposterior nucleus. Several additional thalamic nuclei contained small numbers of labeled neurons. In a subset of nuclei closely affiliated with the limbic system (the parataenial, paraventricular, reuniens, and basal ventromedial nuclei), retrograde labeling occurred exclusively after deposits at extremely ventral and posterior cortical sites. Within the amygdala, retrogradely labeled neurons occupied the anterior basomedial nucleus, the posterior basolateral nucleus, and a narrow strip of the lateral nucleus immediately adjoining the basolateral nucleus. The number of labeled neurons was greater after more ventral deposits. Very ventral deposits resulted in extensive labeling of the cortical amygdala. Within the cerebral cortex, the distribution of labeled neurons depended on the location of the tracer deposit. Comparatively dorsal deposits produced prominent retrograde transport to the anterior and posterior cingulate areas, to the agranular insula, and to lateral prefrontal cortex. Comparatively ventral deposits gave rise to prominent labeling of the hippocampal subiculum, various parahippocampal areas, and prepiriform cortex. On the basis of afferent connections, it is possible to divide the cat's medial prefrontal cortex into an infralimbic component, MPFil, marked by strong afferents from prepiriform cortex and the cortical amygdala, and a dorsal component, MPFd, without afferents from these structures. Further, within MPFd, it is possible to define an axis, running from ventral and posterior to dorsal and anterior levels, along which limbic afferents gradually become weaker and projections from cortical association areas gradually become stronger.     

Organization of cortical and subcortical projections to anterior cingulate cortex in the cat

The aim of the experiments reported here was to identify cortical and subcortical forebrain structures from which anterior cingulate cortex (CGa) receives input in the cat. Deposits of retrograde tracers were placed at nine sites spanning the anterior cingulate area and patterns of retrograde transport were analyzed. Thalamic projections to CGa, in descending order of strength, originate in the anteromedial nucleus, lateroposterior nucleus, ventroanterior nucleus, rostral intralaminar complex, reuniens nucleus, mediodorsal nucleus, and laterodorsal nucleus. Minor and inconsistent ascending pathways arise in the paraventricular, parataenial, parafascicular, and subparafascicular thalamic nuclei. The basolateral nucleus of the amygdala, the hypothalamus, the nucleus of the diagonal band, and the claustrum are additional sources of ascending input. Cortical projections to CGa, in descending order of strength, derive from posterior cingulate cortex, prefrontal cortex, motor cortex (areas 4 and 6), parahippocampal cortex (entorhinal, perirhinal, postsubicular, parasubicular, and subicular areas), insular cortex, somesthetic cortex (areas 5 and SIV), and visual cortex (areas 7p, 20b, AMLS, PS and EPp). In general, the limbic, sensory, and motor afferents of CGa are weak. The dominant sources of input to CGa are other cortical areas with high-order functions. This finding calls into question the traditional characterization of cingulate cortex as a bridge between neocortical association areas and the limbic system.     

Correlations between thalamic projections to different areas of the parietal association cortex of the cat

Projections of thalamic neurons to parietal association cortex of cat were examined by means of the retrograde axonal transport of fluorescent dyes (primuline and fast blue). It has been demonstrated that a dorsal part of the pulvinar (PL) and a dorsal part of the caudal area of the lateral posterior nucleus (LP) projected mostly to the middle suprasylvian gyrus (MSSG), while a ventral part of PL and a ventral part of the rostral area of LP--to the rostral suprasylvian gyrus (RSSG). Double labelled neurons were found in PL, LP, suprageniculatus, anterior ventral, ventral lateral as well as in the central lateral, paracentral and central medial nuclei after injections of two different dyes into MSSG and RSSG. Topic organization of sources of cortical projections from the PL--LP complex can probably provide a high level of discrimination of visual signals by single cortical neurons. At the same time during integration of information of a different subcortical origin RSSG and MSSG act, probably, in concord to a considerable extent, that suggests insufficient differentiation of RSSG and MSSG corresponding approximately to cortical areas 5 and 7 of cat.     

Contralateral corticothalamic projections from MI whisker cortex: potential route for modulating hemispheric interactions

Rat whisking behavior is characterized by high amounts of bilateral coordination in which whisker movements on both sides of the face are linked. To elucidate the neural substrate that might mediate this bilateral coordination, neuronal tracers were used to characterize the bilateral distribution of corticothalamic projections from primary motor (MI) cortex. Some rats received tracers in the MI whisker region, whereas others received tracers in the MI forepaw region. The MI whisker region projects bilaterally to the anteromedial (AM), ventromedial (VM), and ventrolateral (VL) nuclei, and to parts of the intralaminar nuclei. By contrast, the MI forepaw region sends virtually no projections to the contralateral thalamus. Consistent with these findings, bilateral injections of different tracers into the MI whisker region of each hemisphere produced tracer overlap on both sides of the thalamus. Furthermore, MI whisker projections to the contralateral thalamus terminate in close proximity to the thalamocortical neurons that project to the MI whisker region of that contralateral hemisphere. The terminal endings of the contralateral corticothalamic projections contain small synaptic varicosities and other features that resemble the modulator pathways described for other corticothalamic projection systems. In addition, tracer injections into AM, VM, and VL revealed dense clusters of labeled neurons in layer VI of the medial agranular (Agm) zone, which corresponds to the MI whisker region. These results suggest that projections from the MI whisker region to the contralateral thalamus may modulate the callosal interactions that are presumed to play a role in coordinating bilateral whisking behavior.     

Transcallosal non-pyramidal cell projections from visual cortex in the cat

Non-pyramidal cells with transcallosal projections were identified in the area 17/18 border region of the cat by retrograde transport of horseradish peroxidase injected into border region of the opposite hemisphere. From several hundred neurons filled with a Golgi-like diaminobenzidine (DAB) reaction product, seven cells were identified by their radially oriented smooth dendrites as possible non-pyramidal cells. Following thin-sectioning and examination with the electron microscope, four of the neurons proved to be layer IV spiny stellate cells with incompletely filled dendritic spines, and two proved to be layer III pyramidal cells with an incompletely labelled apical dendrite and dendritic spines. The remaining neuron was a non-pyramidal cell whose essentially smooth dendrites were covered with synapses, and whose cell body formed both symmetric and asymmetric synapses with presynaptic terminals. To better assess how many non-pyramidal cells might be labelled, thin sections of the area 17/18 border were surveyed using material processed with tetramethylbenzidine (TMB), and another five labelled non-pyramidal cells with transcallosal projections were identified by the needle-like crystals of TMB reaction product they contained. During the study it became evident that both the DAB and TMB reaction products in the lightly labelled neurons tended to be associated with granules that are 0.5 microns or larger in diameter and that had the characteristics of lysosomes. These granules are also visible in the light microscope as dark puncta. The numbers of puncta in profiles of pyramidal and of non-pyramidal cells in layers II/III and IVa of the area 17/18 border region and in the control acallosal region of area 17 were counted and compared. These comparisons revealed that labelled transcallosally projecting non-pyramidal cells may constitute 10-32% of the non-pyramidal cell population at the area 17/18 border region. Similar values were also obtained for pyramidal cells in this region. Consequently, it is concluded that significant numbers of non-pyramidal cells have axons that project through the corpus callosum to the contralateral hemisphere.     

Thalamocortical and corticothalamic connections of multiple functional domains in posterior parietal cortex of galagos

In prosimian galagos, the posterior parietal cortex (PPC) is subdivided into a number of functional domains where long-train intracortical microstimulation evoked different types of complex movements. Here, we placed anatomical tracers in multiple locations of PPC to reveal the origins and targets of thalamic connections of four PPC domains for different types of hindlimb, forelimb, or face movements. Thalamic connections of all four domains included nuclei of the motor thalamus, ventral anterior and ventral lateral nuclei, as well as parts of the sensory thalamus, the anterior pulvinar, posterior and ventral posterior superior nuclei, consistent with the sensorimotor functions of PPC domains. PPC domains also projected to the thalamic reticular nucleus in a somatotopic pattern. Quantitative differences in the distributions of labeled neurons in thalamic nuclei suggested that connectional patterns of these domains differed from each other.     

Contralateral corticothalamic projections from area 6 in the raccoon

Contralateral corticothalamic projections from cytoarchitectonic area 6 in the raccoon were studied using the autoradiographic tracing technique. Following injections of tritiated amino acids, accumulations of silver grains were present over both the ipsilateral and contralateral ventral medial, central lateral, paracentral, central medial, parafascicular and mediodorsal thalamic nuclei. These nuclei are also known to receive a number bilateral subcortical motor inputs. The additional presence of bilateral area 6 inputs suggests that these thalamic nuclei may be critically invovled in the bilateral control of movement.     

Cortical projections of posterior parietal cortex in owl monkeys.

Efferent cortical projections of posterior parietal cortex were determined by degeneration and autoradiographic methods in owl monkeys. Intraregional connections were to the immediate surround of the injection or lesion site, and to distinct foci within the posterior parietal region. The extraregional ipsilateral connections were with (1) previously established subdivisions of visual association cortex (the Dorsomedial Area, the Medial Area, the Dorsolateral Area, and the Middle Temporal Area), (2) other locations in caudal neocortex, and (3) frontal cortex. The callosal projections were to separate foci in posterior parietal cortex of the contralateral cerebral hemisphere. The separate foci of both ipsilateral and contralateral terminations in posterior parietal cortex raise the possibility that this region contains more than one functional subdivision. The connections with visual association cortex suggest a role for parietal cortex in visual behavior. Other foci in caudal neocortex indicate the possible locations of additional subdivisions of association cortex.     

Thalamic projections to visually responsive regions of parietal cortex

Thalamic efferents to visually responsive regions of parietal cortex of the cat were investigated by experimental retrograde tracing techniques. Two classes of photically evoked potentials recorded from the middle suprasylvian gyrus could be distinguished on the basis of waveform and latency. Type 1 responses were recorded from the Clare-Bishop area at the lateral border of the suprasylvian gyrus and Type 2 responses were recorded from association response areas on the crown of the suprasylvian gyrus. Studies of retrograde degeneration in kittens and adult cats and retrograde transport of intracortically injected horseradish peroxidase indicate that afferents to the electrophysiologically identified Clare-Bishop area originate in the lateroposterior nucleus, the posterior nucleus, and the medial interlaminar nucleus of the lateral geniculate body. Afferents to electrophysiologically identified association response areas originate in the lateroposterior nucleus, ventroanterior nucleus, the pulvinar, the laterodorsal nucleus, and the central lateral nucleus. Studies of orthograde degeneration following placement of parietal lesions indicate a close reciprocity of corticothalamic and thalamocortical projections.     

Neonatal auditory cortex lesions result in aberrant crossed corticotectal and corticothalamic projections in rats

Ablation of one auditory cortex at birth in rats results in the formation of aberrant crossed projections from the intact hemisphere. These aberrant projections extend throughout most of the corticorecipient zone of the contralateral inferior colliculus and can be traced as far rostral as the contralateral medial geniculate nucleus. The aberrant crossed corticothalamic axons arise from layer V pyramidal neurons in the intact auditory cortex.     

Organization of direction preferences in cat visual cortex

Single unit recordings were made from the visual cortex of 5 adult cats. Visual stimuli were used to determine the stimulus orientation and direction of movement preferred by cortical cells. Analysis of the sequence of neurons recorded along each electrode penetration and their direction preferences indicates that neurons preferring similar directions of movement are clustered together in the cortex.     

Organization of corticocortical connections in the parietal cortex of the rat

An analysis based on Nissl, anterograde degeneration, and sucinic dehydrogenase histochemical techniques reveals that there are two distinct regions of parietal cortex which are characterized by different cytoarchitectonic features and anatomical connections. The “granular” cortical zone possesses a well-defined fourth layer composed of small, densely-packed cells, receives dense projections from the ventral posterior nucleus of the thalamus, and is essentially free of callosal inputs. “Agranular” cortical areas which surround or lie embedded within the granular zone lack a well-defined fourth layer, receive sparse projection from the ventral posterior nucleus, but send and receive extensive callosal projections. These findings suggest that thalamic and callosal projections to the parietal cortex maintain a pattern of areal segregation. The granular cortical zone, which apparently corresponds to SmI, projects ipsilaterally to motor cortex, SmII, and adjacent agranular areas. The superficial layers of the granular cortex also project heavily upon the underlying layer V. This intracortical projection is not organized in discrete clusters within the “barrel field” cortex. This suggests that the specialized organization of thalamic afferents and granule cells within the “barrel field” is not maintained in the intracortical circuitry of this region.     

Organization of visual cortical projections to the claustrum in the cat

The projection patterns from different visual areas of the parieto-occipital cortex to the claustrum were studied autoradiographically in cats. When [3H]proline was injected into 17, 18, 19 or Clare-Bishop areas, the label was transported to an area restricted to the dorsal and caudal parts of the claustrum without any suggestion of retinotopic organization. Injection in each of these visual areas resulted in individual patterns of projection but with overlapping fields of termination, a pattern similar to corticocaudate projection. When injected into area 7, a region shown to have neurons involved in visuomotor mechanisms, the label was transported to the same area as that of the visual projection. These and other findings suggest that claustrum may be reciprocally and topographically connected with the cerebral cortex.     

Organization of the projections from the pericruciate cortex to the pontomedullary reticular formation of the cat: A quantitative retrograde tracing study

Dextran-amines were used as retrograde tracers to investigate the organization of cortical projections to different cytoarchitectonic regions of the pontomedullary reticular formation of the cat. Injections into the nucleus reticularis pontis oralis resulted in labelling of neurones in the proreus cortex and area 6a|iB of the premotor cortex, with little labelling in the motor cortex (area 4). This labelling was predominantly ipsilateral to the injection site. In contrast, injections into the nucleus reticularis pontis caudalis (NRPc), nucleus reticularis gigantocellularis (NRGc), and nucleus reticularis magnocellularis (NRMc) resulted in bilateral labelling—primarily in areas 6aβ, 6aγ, and in the rostromedial region of area 4—with little labelling in the proreus cortex. In general, the cortical projections to the caudal NRGc and the NRMc were larger than those to the NRPc. More than 25% of the total projections to each of the latter three reticular regions arose from the medial part of area 4. Labelling in the hindlimb regions of area 4 was largest following the NRMc injections and smallest after injections in the NRPc. The projections to the NRPc originated from more medial parts of areas 4 and 6 than did the projections to the caudal region of the NRGc. These results suggest that areas 4 and 6 may be able to differentially activate different regions of the pontomedullary reticular formation depending on the movement that is made and perhaps also on the context of that movement. J. Comp. Neurol. 388:228–249, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Focal projections of cat auditory cortex to the pontine nuclei

The pontine nuclei (PN) receive projections from the auditory cortex (AC) and they are a major source of mossy fibers to the cerebellum. However, they have not been studied in detail using sensitive neuroanatomical tracers, and whether all AC areas contribute to the corticopontine (CP) system is unknown. We characterized the projection patterns of 11 AC areas with WGA-HRP. We also compared them with their corticothalamic and corticocollicular counterparts. A third objective was to analyze the structure of the CP axons and their terminals with BDA. Both tracers confirm that all AC areas projected to lateral, central, and medial ipsilateral pontine divisions. The strongest CP projections were from nontonotopic and polymodal association areas. Preterminal fibers formed single terminal fields having many boutons en passant as well as terminal endings, and there was a specific morphological pattern for each pontine target, irrespective of their areal origin. Thus, axons in the medial division had a simpler terminal architecture (type 1 terminal plexus); both the central and lateral pons received more complex endings (type 2 terminal plexus). Auditory CP topographical distribution resembled visual and somatosensory CP projections, which preserve retinotopy and somatotopy in the pons, respectively. However, the absence of pontine tonotopy suggests that the AC projection topography is unrelated to tonotopy. CP input to the medial and central pons coincides with the somatosensory and visual cortical inputs, respectively, and such overlap might subserve convergence in the cerebellum. In contrast, lateral pontine input may be exclusively auditory.     

The organization of thalamic projections to the parietal cortex of the Virginia opossum

The right/left ratio for the number of neurons in individual pairs as well as series of pairs of normal rat and guinea pig thoracic dorsal root ganglia have been investigated. After perfusion of the animals with aldehyde fixatives, the 13 pairs of thoracic ganglia were embedded in resin, serially sectioned, and stained with azur methylene blue. Counts of neuronal nucleoli (with correction for possible split nucleoli) were used as an indicator of the number of neurons in each ganglion. In individual pairs the right/left difference varied between 0 and 47% in the rat and 0 and 26% in the guinea pig. There appeared to be no particular level with a preference for large right/left differences. However, when the total number of neurons from the 13 ganglia on one side were compared with the corresponding figure from the other side, the right/left differences were reduced to less than 3% in the rat and less than 2% in the guinea pig. These results show that the organization of the thoracic spinal nerve unit is markedly asymmetrical at the segmental level, but strictly symmetrical in terms of the total number of dorsal root ganglion cells. The marked asymmetry in individual ganglion pairs necessitates caution when evaluating the effect of unilateral experimental manipulations on the number of neurons in thoracic spinal ganglia.