General plan of the thalamic projections to the prefrontal cortex in the cat

Dorsal prefrontal and premotor cortex of the ferret as defined by distinctive patterns of thalamo-cortical projections

Recent studies of the neurobiology of the dorsal frontal cortex (FC) of the ferret have illuminated its key role in the attention network, top-down cognitive control of sensory processing, and goal directed behavior. To elucidate the neuroanatomical regions of the dorsal FC, and delineate the boundary between premotor cortex (PMC) and dorsal prefrontal cortex (dPFC), we placed retrograde tracers in adult ferret dorsal FC anterior to primary motor cortex and analyzed thalamo-cortical connectivity. Cyto- and myeloarchitectural differences across dorsal FC and the distinctive projection patterns from thalamic nuclei, especially from the subnuclei of the medial dorsal (MD) nucleus and the ventral thalamic nuclear group, make it possible to clearly differentiate three separate dorsal FC fields anterior to primary motor cortex: polar dPFC (dPFCpol), dPFC, and PMC. Based on the thalamic connectivity, there is a striking similarity of the ferret's dorsal FC fields with other species. This possible homology opens up new questions for future comparative neuroanatomical and functional studies.

Tracing thalamo-cortical connections in tenrecA further attempt to characterize poorly differentiated neocortical regions, particularly the motor cortex

The hedgehog tenrec (Afrosoricidae) has a very poorly differentiated neocortex. Previously its primary sensory regions have been characterized with hodological and electrophysiological techniques. Unlike the marsupial opossum the tenrec may also have a separate motor area as far as there are cortico-spinal cells located rostral to the primary somatosensory cortex. However, not knowing its thalamic input it may be premature to correlate this area with the true (mirror-image-like) primary motor cortex in higher mammals. For this reason the tenrec's thalamo-cortical connections were studied following tracer injections into various neocortical regions. The main sensory areas were confirmed by their afferents from the principal thalamic nuclei. The dorsal lateral geniculate nucleus, in addition, was connected with the retrosplenial area and a rostromedial visual region. Unlike the somatosensory cortex the presumed motor area did not receive afferents from the ventrobasal thalamus but fibers from the cerebello-thalamic target regions. These projections, however, were not restricted to the motor area, but involved the entire somatosensorimotor field as well as adjacent regions. The projections appeared similar to those arising in the rat thalamic ventromedial nucleus known to have a supporting function rather than a specific motor task. The question was raised whether the input from the basal ganglia might play a crucial role in the evolution of the mammalian motor cortex? Certainly, in the tenrec, the poor differentiation of the motor cortex coincides with the virtual absence of an entopeduncular projection to the ventrolateral thalamus.

Prefrontal control of the amygdala

Accumulating evidence indicates that phobic and posttraumatic anxiety disorders likely result from a failure to extinguish fear memories. Extinction normally depends on a new learning that competes with the original fear memory and is driven by medial prefrontal cortex (mPFC) projections to the amygdala. Although mPFC stimulation was reported to inhibit the central medial (CEm) amygdala neurons that mediate fear responses via their brainstem and hypothalamic projections, it is unclear how this inhibition is generated. Because the mPFC has very sparse projections to CEm output neurons, the mPFC-evoked inhibition of the CEm is likely indirect. Thus, this study tested whether it resulted from a feedforward inhibition of basolateral amygdala (BLA) neurons that normally relay sensory inputs to the CEm. However, our results indicate that mPFC inputs excite rather than inhibit BLA neurons, implying that the inhibition of CEm cells is mediated by an active gating mechanism downstream of the BLA.

Cortical auditory evoked potentials in autism: a review

The question of etiology in autism remains elusive primarily due to the fact that autism does not result from a single dysfunction but is multi-faceted in nature. Investigations into etiology have ranged from identifying abnormalities in the genome to describing structural/functional brain abnormalities. Bearing in mind the risk of over-simplification, there is still utility in isolating a specific deficit to examine its etiologic contribution. It is known that individuals with autism have difficulty processing auditory information at the cortical level but this is not consistently seen subcortically. In recent years, cortical auditory processing has been extensively researched using event-related potentials (ERPs); however, these results in relation to autism have not been reviewed. This paper will examine this literature and discuss implications for future research.

Cerebral mechanisms underlying orienting of attention towards auditory frequency changes

Brain mechanisms underlying detection of auditory frequency changes were studied with event-related potentials (ERPs) in 14 human subjects discriminating visual stimuli. Scalp-current density mapping revealed bilateral components of mismatch negativity (MMN) in frontal and auditory cortices. Deviance-related activations in frontal and temporal cortex began to be significant at 94 ms and 154 ms in the right hemisphere, and at 128 ms and 132 ms in the left hemisphere. The magnitude of MMN-neuroelectric currents from the left temporal cortex correlated significantly (r = -0.56, p < 0.05) with distraction caused by MMN-eliciting deviant tones. These results suggest a complex cerebral circuitry involved in frequency change detection and strongly support the role of this circuitry in driving attention involuntarily towards potentially relevant frequency changes in the acoustic environment.

Supramedullary modulation of sympathetic vasomotor function

1. Supramedullary structures including the ventral medial prefrontal cortex (MPFC) and the midbrain cuneiform nucleus (CnF) project directly and indirectly to premotor sympatho-excitatory neurons of the rostral ventrolateral medulla (RVLM) that are critically involved in the generation of sympathetic vasomotor tone. 2. Electrophysiological studies have demonstrated that activation of depressor sites within the MPFC is associated with splanchnic sympathetic vasomotor inhibition and inhibition of the activity of RVLM sympathoexcitatory neurons. 3. Antidromic mapping and anatomical studies support the notion that a relay in the nucleus tractus solitarius is involved in the cardiovascular response to MPFC stimulation. 4. The midbrain CnF, which lies adjacent to the midbrain periaqueductal grey, is a sympathoexcitatory region of the midbrain reticular formation. Sympathoexcitatory responses evoked from the CnF are associated with short-latency excitation of RVLM neurons. 5. Cuneiform nucleus stimulation induces the expression of mRNA for the immediate early genes c-fos and NGFI-A in mid-brain, pontine and hypothalamic structures. 6. The MPFC and CnF are supramedullary structures with opposing modulatory influences on sympathetic vasomotor drive, whose roles in cardiovascular control mechanisms warrant further investigation.

Afferent connections of the lateralis medialis thalamic nucleus in the cat

We have used anterograde and retrograde horseradish peroxidase tracing methods in this study. Peroxidase injections in the lateralis medialis thalamic nucleus (LM) of the cat resulted in labeled neurons in cortical and subcortical structures that averaged 71% and 29%, respectively. Every LM sector receives abundant projections from the polymodal sylvian anterior cortical area, the reticular thalamic nucleus, and the stratum opticum and intermediate layer of the superior colliculus. Less abundant but consistent projections were detected in cingular, auditory II, lateral suprasylvian and anterior ectosylvian visual cortices, and cortical area 7. A topographical distribution of afferent connections to different LM sectors arising from other cortical and subcortical structures could be established. The ventromedial sector receives connections mainly from the insular agranular, limic and prefrontal cortical areas, as well as from brain stem structures and the contralateral pretectal region. The dorsolateral sector is mainly related to cortical areas and subcortical structures processing visual information. The existence of overlap among neuronal LM populations receiving and sending connections to and from various cortical areas suggests that this nucleus is an appropriate substrate for effective interaction between different and distant cortical areas.

Contralateral thalamic projections predominantly reach transitional cortices in the rhesus monkey

Connections between the thalamus and the cortex are generally regarded as ipsilateral, even though contralateral connections exist as well in several adult mammalian species. It is not known, however, whether contralateral thalamocortical projections reach particular cortices or whether they emanate from specific nuclei. In the rhesus monkey different types of cortices, ranging from transitional to eulaminate, vary in their cortical connectional pattern and may also differ in their thalamic connections. Because olfactory and transitional prefrontal cortices receive widespread projections, we investigated whether they are the target of projections from the contralateral thalamus as well. With the aid of retrograde tracers, we studied the thalamic projections of primary olfactory (olfactory tubercle and prepiriform cortex) and transitional orbital (areas PAII, Pro, 13) and medial (areas 25, 24, 32) areas, and of eulaminate (areas 11, 12, 9) cortices for comparison. To determine the prevalence of neurons in the contralateral thalamus, we compared them with the ipsilateral in each case. The pattern of ipsilateral thalamic projections differed somewhat among orbital, medial, and olfactory cortices. The mediodorsal nucleus was the predominant source of projections to orbital areas, midline nuclei included consistently about 25% of the thalamic neurons directed to medial transitional cortices, and primary olfactory areas were distinguished by receiving thalamic projections predominantly from neurons in midline and intralaminar nuclei. Notwithstanding some broad differences in the ipsilateral thalamofrontal projections, which appeared to depend on cortical location, the pattern of contralateral projections was consistent with cortical type rather than location. Labeled neurons in the contralateral thalamus were noted in midline, the magnocellular sector of the mediodorsal nucleus, the anterior medial and intralaminar nuclei, and ranged from 0 to 14% of the ipsilateral; they were directed primarily to olfactory and transitional orbital and medial cortices but rarely projected to eulaminate areas. Several thalamic nuclei projected from both sides to olfactory and transitional areas, but issued only ipsilateral projections to eulaminate areas. Though ipsilateral thalamocortical projections predominate in adult mammalian species, crossed projections are a common feature in development. The results suggest differences in the persistence of contralateral thalamocortical interactions between transitional and eulaminate cortices.

Electrophysiological and anatomical studies on thalamic mediodorsal nucleus projections onto the prefrontal cortex in the cat

Electrical stimulation of the mediodorsal nucleus (MD) of the thalamus elicited field potentials in the gyrus proreus (PRO), frontalis (FR), rectus (RE) and cinguli anterior (CIant) of the ipsilateral prefrontal and adjacent cortical areas in cats. The results of a laminar field potential analysis indicate that the field potentials can be regarded as a combination of deep and superficial thalamocortical responses. By injecting horseradish peroxidase (HRP) into the MD, HRP-labeled terminals were distributed in the prefrontal and adjacent cortical areas where the field potentials were elicited. Densely labeled terminals in cortical layer I were distributed where the superficial thalamocortical responses were prominent, while those in layers III-V were distributed in the areas where the deep thalamocortical responses were prominent.

Thalamic midline cell populations projecting to the nucleus accumbens, amygdala, and hippocampus in the rat

The organization of the thalamic midline efferents to the amygdaloid complex, hippocampal formation, and nucleus accumbens was investigated in the rat by means of multiple retrograde fluorescent tracing. The present findings indicate that these connections derive from separate cell populations of the thalamic midline, with a low degree of divergent collateralization upon more than one of the targets examined. The neural populations projecting to the amygdala, hippocampus, or accumbens are highly intermingled throughout the thalamic midline, but display some topographical prevalence. Midline thalamo-hippocampal cells are concentrated in the nucleus reuniens; thalamo-accumbens neurons prevail in the ventral portion of the paraventricular nucleus, and in the central medial nucleus. Thalamo-amygdaloid cells display a topographical prevalence in the rostral third of the thalamic midline and are concentrated in the dorsal part of the paraventricular nucleus and in the medial part of the nucleus reuniens. Both dorsally in the paraventricular nucleus and ventrally in the nucleus reuniens, thalamo-amygdaloid cells are located closer to the ependymal lining than the neurons projecting to the hippocampus or nucleus accumbens. Further, thalamo-amygdaloid cells, especially in the paraventricular nucleus, extend their dendritic processes in the vicinity of the ependymal lining, where they arborize profusely. These features indicate a close topographical relationship of neurons projecting to the amygdala with ependymal cells. The fairly discrete origin of midline outputs to the amygdala, hippocampus, and accumbens indicates that the flow of information is conveyed through separate channels from the thalamic midline to limbic and limbic-related targets. Together with the literature on the limbic afferents to the thalamus, these findings emphasize the relationships between the thalamus and the limbic system subserved by parallel input-output routes. However, because of the overlap of the projection cell populations, the thalamic midline may represent a locus of interaction among neurons connected with different parts of the limbic system. The functional implications of these findings are discussed in relation to the "nonspecific" thalamic system, as well as to the circuits involved in memory formation.

Thalamocortical connections of the rostral intralaminar nuclei: an autoradiographic analysis in the cat

In this study the pattern of projections from the rostral intralaminar thalamic nuclei to the cerebral cortex was examined in the cat by autoradiography. Injections of tritiated proline and leucine were placed into the central lateral, paracentral, central medial, and para-stria medullaris nuclei. After injections into the central lateral nucleus, label is present on the lateral side within the presylvian sulcus, in most of the suprasylvian gyrus, including the adjacent lateral and suprasylvian sulci, and in the posterior corner of the ectosylvian gyrus. On the medial side, label is present in the orbitofrontal (Of), precentral agranular (Prag), anterior limbic (La), retrosplenial (Rs), and postsubicular (Ps) areas, as defined by Rose and Woolsey ('48a). The cingulate gyrus also contains label throughout (part of which was defined as the "cingular area," Cg, by Rose and Woolsey, '48a). Label is also found on both banks of the splenial and cruciate sulci. In addition, label is present within the lateral gyrus, on both its lateral and medial sides. The paracentral projections are similar to the central lateral input. On the lateral side, label is found within the presylvian sulcus, suprasylvian gyrus and adjacent lateral and suprasylvian sulci, and posterior ectosylvian gyrus. Medially, label is present in the Of, Prag, La, Cg, Rs, and Ps areas, and within the cruciate and splenial sulci, and in portions of the lateral gyrus. Following injections of the central medial nucleus, label is present in the presylvian sulcus; but in contrast to the central lateral and paracentral projections, the suprasylvian gyrus is labeled only in its posterior part. The central medial nucleus also projects to the posterior lateral gyrus, both laterally and medially. Also, the central medial nucleus projects heavily to rostral cortical zones, which include the Of, Prag and La areas, cruciate sulcus, and the rostral cingulate gyrus. The para-stria medullaris nucleus projects only to the presylvian sulcus and orbitofrontal cortex laterally, but, medially, has an extensive input similar to the central lateral and paracentral projections in that label is present in the Of, Prag, La, Cg, Rs, and Ps areas, in the cruciate and splenial sulci, and in the posterior lateral gyrus. The laminar distribution of label is as follows: the central lateral, paracentral and para-stria medullaris nuclei project primarily to layers I and III, whereas the central medial nucleus projects to layers I and VI. In addition, the central lateral projection has a patchy appearance in the retrosplenial and postsubicular cortices.

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.

Topographical organization of the thalamic afferent connections to the motor cortex in the cat

The topographical distribution of the cortical afferent connections to the different subdivisions of the motor cortex (MC) was studied in adult cats. The retrograde axonal transport of horseradish peroxidase technique was used. Small single injections of the enzyme were made in the entire MC, including the hidden regions in the depth of the sulcus cruciatus. The areal location and density of the subsequent thalamic neuronal labeling were evaluated in each case. Comparison of the results obtained in the various cases shows that the following: (1) The ventral anterior-ventral lateral complex is the principal thalamic source of afferents to the MC. (2) The ventral medial, dorsal medial, the different components of the posterior thalamic group (lateral, medial, and ventral posteroinferior and suprageniculate nuclei), and the intralaminar, lateral anterior, lateral intermediate, lateral medial, and anteromedial thalamic nuclei are also thalamic sites in which neural projections to the MC arise. (3) The thalamocortical projections to the MC are sequentially organized. The connections arising from the lateral part of the thalamus end in the region of area 4 that is situated medially in the superior lip of the sulcus cruciatus and in the posterior sigmoid gyrus. The projections originating in the most medial thalamic regions terminate in that region of area 6a beta which is located in the medial part of the inferior lip of the cruciate sulcus, and in the anterior sigmoid gyrus. Moreover, the ventral thalamic areas send connections to the most anteriorly located zones of the MC, while the most dorsal thalamic ones project to the most posteriorly located parts of the MC. (4) This shift in the thalamocortical connections is not restrained by cytoarchitectonic boundaries, either in the thalamus or in the cortex. (5) The populations of thalamocortical cells which project to neighboring MC subdivisions exhibit consistent overlapping among themselves. (6) These findings suggest, moreover, that the basal ganglia and the cerebellar projections to the MC through the thalamus are arranged in a number of parallel pathways, which may occasionally overlap.