The organization and postnatal development of the commissural projection of the rat somatic sensory cortex

Homotopic and heterotopic callosal afferents of caudal inferior parietal lobule in Macaca mulatta

We have examined callosal-axon neurons giving rise to homotopic and heterotopic callosal projections to caudal inferior parietal lobule (area PG) in Macaca mulatta, identifying these neurons by means of retrograde axonal transport of horseradish peroxidase. The labeled neurons in the homotopic region occur predominantly in layers IIIB and V.A moderate number are also seen in layer VI, a smaller number of layer IV, and rare cells occur in layer II. These neurons occupy a region very similar in outline to the injection area, and though variable in density in the horizontal plane, are continuously distributed in this plane. The heterotopic neurons are seen in the contralateral cingulate gyrus, continuing caudally into medial parietal cortex, in the cortex of the superior temporal and occipitotemporal sulci, in the caudal superior temporal gyrus, and in the caudal inferior parietal lobule, behind the homotopic area. These same regions on the ipsilateral side contain labeled neurons of origin of ipsilateral association projections to area PG. For other ipsilateral labeling was found. A review of the literature on heterotopic callosal connections of a particular generalization of this conclusion: The callosal heterotopic connections of a particular cortical area are made with regions which on the ipsilateral side have associated connections with that area, though usually not with all of such regions.

On the evolution and growth of lateralization

This is a welcome attempt to seek common biological principles underlying laterality in animals and humans. I do not think it is wholly convincing, but this is partly because many of the results from nonhuman species are not yet firmly established or understood. I suspect that the author's characterization of lateralization, with the left hemisphere supposedly specialized for communicative functions and the right for spatial and affective functions, will require modification. For instance, it now seems fairly clear that the left hemisphere in humans plays a general role in the production and perception of sequences not restricted to communicative acts (Craig 1980; Kimura 1979), and indeed some of the examples of left-hemispheric specialization listed in Table 2 are not obviously communicative. Even so, there are some fairly striking parallels between humans and nonhumans with respect to the pattern of lateralization, and one suspects that common principles are operating. At the same time, in the enthusiastic search for functional asymmetries, we should not overlook the striking degree of bilateral symmetry that characterizes the brains of all animals, including humans.

Distribution of cyclic nucleotide phosphodiesterase in mouse brain

Seventy-one regions of mouse brain, and many subdivisions of some of these, were analyzed for cyclic nucleotide phosphodiesterase. The samples were dissected from lyophilized frozen sections. Since the average sample weighed only 25 ng (20 X 75 X 75 mu3), regions as small as the locus ceruleus could be analyzed. Activities in gray areas ranged 40-fold from a high in the pars reticulata of the substantia nigra to a low in the deep cerebellar nuclei. The activity in fiber tracts also varied about 40-fold, and on a lipid-free dry weight basis was similar to the activity in the gray matter where the fibers originated. The rank order for gray regions was basal ganglia, amygdala, hippocampus, cerebral cortex, most of the diencephalic nuclei, nuclei of the pons, cerebellum, and nuclei of the medulla.

Cerebral predominance in the monkey?

The issues raised by Dr. Denenberg are complex, not least because the apparent communalities of hemispheric specialization among birds, rodents, monkeys, and human beings are also associated with negative instances (e.g., for parrots, Nottebohm 1976; for Macaco, other than fuscata, Petersen et al. 1978). To extend the available evidence I would like to refer to the preliminary findings of Garcha et al. (1980) on the monkey.

Hemispheric laterality and an evolutionary perspective

Denenberg rightly stresses the importance of studying ethologically meaningful species-specific behavior in animals, and makes the interesting distinction between lateralization at an individual and at a population level. However, in the case of man, I believe Denenberg is wrong in arguing that lateralization in the individual increases with maturation. The overall evidence nowadays tends very much to the contrary. Moreover, with respect to a population, why should it become lateralized? If there is indeed an advantage for the individual in hemispheric specialization, why should the direction of such specialization be so consistent across a majority of individuals, whether human or, as Denenberg points out, other members of the phylum? Is there an evolutionary advantage in most animals' sharing the same direction, or is it a necessary consequence of some other preexisting, more fundamental anatomical, biochemical, or physical property of the organism and its constituents? If the former, why are not all members of the species, rather than just a majority, lateralized in the same direction? (Or, to put it another way, what is the evolutionary advantage to the species or individual of dimorphism, of retaining a minority who polarize in the opposite direction?) If the latter - i.e., if lateralization is a necessary consequence of some prior state - then there should not be any dimorphism, exceptions, or minority members, unless they are somehow disadvantaged in consequence. Indeed, there is some evidence of a cognitive deficit in sinistrals, though it is disputed (see Bradshaw 1980 for review), and others have even suggested that the species as a whole may benefit in some way from such an uneven dimorphism (Levy 1974), but what evidence is there for such propositions with respect to rats, apes, monkeys, or chicks? This is an issue that should be addressed in any general model that includes laterality in animals. [See Corhallis & Morgan: “On the Biological Basis of Human Laterality” BBS 1(2) 1978.]

The ontogeny of the distribution of callosal projection neurons in the rat parietal cortex

The ontogeny of callosal projection neurons in the rat parietal cortex was examined using the retrograde and anterograde transport of horseradish peroxidase (HRP), as well as Golgi and Nissl stains. From postnatal day 0 (PND 0) to early PND 4, the callosal projection neurons are distributed as two continuous horizontal bands of cells which extend throughout the subplate in layers Va and Vc-upper VIa. Neurons within the cortical plate (CP), however, do not transport HRP from a contralateral injection site until PND 3 to early PND 4, when a few cells at the lower CP border are generally labeled. However, by late on PND 4, and more consistently by PND 5, several changes in the distribution of callosal projection neurons take place. First, cells at all levels of the CP become labeled in a sequential fashion, from the lower border upward. Second, gaps, or areas devoid of HRP, become apparent in layer IV of the barrel field area. Third, in the cortical areas containing the gaps, as well as in other areas which are destined not to be callosally connected in the adult, there is a noticeable decrease in the number of cells labeled with HRP. This decrease continues through PND 15 and possibly into adulthood. The foregoing developmental events are compared to cortical maturation as seen in both Golgi- and Nissl-stained material. By PND 15, the basic adult pattern of callosal projection neurons is established. The neurons reside mainly in layers III and Va, with fewer in layers II and Vc-upper VIa, and fewer still in the other cortical layers. They are aligned in vertical arrays in discrete areas of the cortex.

The early development of corticobulbar and corticospinal systems. Studies using the North American opossum

The North American opossum is born 12 days after conception and is therefore available for experimental manipulation in an immature state. We have used the opossum to study the growth of cortical axons into the brainstem and spinal cord and have obtained evidence that such growth occurs in an orderly fashion. Cortical axons reach the ventral mesencephalon 12 days after birth and some of them have grown into the caudal medulla where they decussate by 23 days. At the latter stage immature cortical axons also distribute to the midbrain tegmentum, the basilar pons, the inferior olive and the hilum of the nucleus cuneatus. Cortical axons first enter the spinal cord about 30 days after birth where they are present in the white matter before growing into the dorsal horn. The forelimb placing reaction does not develop until well after cortical axons have reached cervical levels. Axons from the cerebral cortex grow into the spinal cord before there is evidence for cortical innervation of either the red nucleus or the bulbar reticular formation and well before pyramidal cells of the neocortex are mature. The relatively late development of corticospinal and corticobulbar systems contrasts markedly with the early growth of bulbospinal axons.

Autoradiographic tracing of developing subcortical projections of the occipital region in fetal rabbits

Thirty rabbit embryos and two neonates (E18-P1) received micropipette injections of 3H-Leucine into the occipital region of one hemisphere and were killed after 0.5--5 hours. Incorporated tracer was demonstrated by autoradiography of serial sections of the brains. The first axons were seen in the intermediate zone of the developing cerebral cortex, on day E20, and by day E22 they reached the internal capsule. The entire cortico-peduncular bundle and a short branch of the superficial (thalamic) bundle were labeled on day E24. On day E25, additional branches directed to claustrum, thalamus (deep bundle), and cerebellum were distinguished. By day E28 the first indications of terminal field development were observed. One day before birth (E30), the neonate pattern of subcortical pathways was fully established and silver grain condensations were present over most of the subcortical target areas. Subcortically, the axons followed preferentially preexisting fiber tracts. There was a period of at least 2--3 days between the arrival of the supplying bundles at the target sites and the onset of terminal field formation: The axon bundles grew first towards more distal targets, and even beyond, before terminal fields developed proximally. Transient axon bundles reaching the cerebellar paraflocculus and traveling along the pyramidal tract and the external and extreme capsules failed to form terminal fields and disappeared around birth. The data suggest that growth of long axonal tracts and the development of terminal fields are separate phenomena possibly regulated by different mechanisms.

Absence of callosal collaterals derived from rat corticospinal neurons. A study using fluorescent retrograde tracing and electrophysiological techniques

In rat the presence of axon collaterals from corticospinal neurons to the contralateral hemisphere has been investigated by means of anatomical and electrophysiological techniques. Anatomical Experiments. Several combinations of fluorescent retrograde tracers were used. In eight rats injections of Evans Blue, "True Blue", "Fast Blue" or DAPI-Primuline were made in areas 10, 6, and 4 and in the most medial part of the S1 granular cortex of one hemisphere, 1.5 mm below cortical surface. These injections were combined with injections of "Fast Blue", DAPI-Primuline, "Granular Blue", "Nuclear Yellow", or Bisbenzimide in the ipsilateral corticospinal tract in the C2 segment. Survival times of the animals varied according to the tracers used. In the non-injected hemisphere the retrogradely labeled corticospinal neurons were present in layer V of especially areas 10, 6, 4 and the medial portion of the S1 granular cortex. However, the retrogradely labeled callosal neurons in these areas were present in all layers except layer I. The labeled callosal and corticospinal neurons in layer V were intermingled and frequently situated very close to one another. However, with none of the tracer combinations were double labeled neurons observed. Electrophysioloogical Experiments. In six rats, layer V neurons of hindlimb-sensorimotor cortex were tested for antidromic responses to stimulation of contralateral corticospinal tract (CST) and corpus callosum (CC). Eighty-five CST neurons were identified, none of which responded antidromically to CC shocks. Eighty-two layer V neurons were identified which responded antidromically to CC shocks, but none of them responded antidromically to CST shocks. CC shocks elicited strong synaptic responses in CST neurons and vice versa. Depth measures indicated extensive intermingling of CST and CC neurons. From both sets of findings it was concluded that, in rat, CST neurons do not give rise to callosal collaterals.

Commissural columns in the sensory-motor cortex of monkeys

Callosally projecting cells and the terminal ramifications of their axons were identified in the monkey sensory-motor cortex by retrograde and anterograde labeling techniques, often by double labeling cells and axons in the same animal. Bundles of callosal fibers terminate in small column-like zones 0.5-1 mm wide in the motor cortex (area 4) and in the first (SI) and second (SII) somatic sensory areas. Such columns are aligned in register to form elongated strips extending mediolaterally in the long axes of the pre- and postcentral gyri. Significant portions of area 4, SI and SII, in regions corresponding to the representations of the hand and foot, are not callosally connected. The cells of origin of callosal fibers in SI are largely confined to layer IIIB and form columns and strips corresponding to the above. In connected zones of SI, the callosal connection is reciprocal and precisely point-to point. This and the laminar distribution of the terminal ramifications of callosal fibers (to layers I-IV) suggest that callosal fibers may arise from the terminate upon exactly homotopic, column-like groups of layer IIIB pyramidal cells. Commissurally projecting cells and their terminal ramifications are not limited to particular architectonic fields or particular parts of fields in SI. All architectonic fields of SI project heterotopically to the contralateral SII.

The bilaminar and banded distribution of the callosal terminals in the posterior neocortex of the rat

After callosal sectioning, the callosal connections of the posterior neocortex of the rat cerebral hemisphere were demonstrated using the Fink-Heimer technique. Serial frozen sections of the whole brains were cut in transverse, horizontal, and tangential planes. In tissue sections, degenerating terminals were concentrated in two distinct laminae within the depth of the cortex. In addition the terminals had a patchy distribution. The degeneration was marked on projection drawings of serially arranged sections, and subsequent reconstruction showed the terminal degeneration to be distributed in bands. Five dorsoventrally oriented bands of terminals were present in areas 39, 41 and 36 collectively, and a rostrocaudal band in area 20. In area 17 terminations were apparently absent except at its borders with areas 18, 18a and 7. The degenerating callosal terminals within these areas produced a circumferential band around area 17. The findings are discussed with respect to the significance of these patterns of corticocortical connections.

Distribution of norepinephrine and dopamine in cerebral cortical areas of the rat

Concentrations of norepinephrine and dopamine were determined using enzyme isotope assay in 27 microdissected cerebral cortical areas of the rat. A detailed map is presented for microdissection of rat cerebral cortex. Norepinephrine was found in low but still measurable quantities throughout the cortex. Differences between cortical areas are also low. Relatively highest levels were demonstrated in the pyriform, insular and entorhinal cortices. The distribution of dopamine was found to be uneven with a maximal regional difference of 1:24. Concentration of dopamine was in all areas lower than that of norepinephrine. The highest dopamine concentration (2,4 ng/mg protein) was measured in the rostral pyriform cortex but other mesocortical (cingulate, frontal, insular and entorhinal) dopaminergic areas also contained relatively high amounts. Except for the caudal occipital and caudal entorhinal cortices all regions studied contained measurable quantities of dopamine. Its low concentration relative to norepinephrine (below 15%) suggests that in the cortical areas studied dopamine is present as the precursor of norepinephrine.

Sets of neurons in somatic cerebral cortex of the cat and their ontogeny

The process of set formation is briefly reviewed and five monothetic schemes for classification of neurons in the somatic cerebral cortex are described. Criteria for evaluation of neuronal sets are presented and applied to the five different monothetic classification schemes. Classification by size and distribution of peripheral receptive fields orders existing data on cortical neurons better than classification by possession of an axon in the pyramidal tract, by modality, by lability of receptive field, or by 'lemniscal properties'; however, no monothetic scheme orders all the data. A useful polythetic scheme, using s and m terminology is suggested. The ontogeny of the cerebral cortex is reviewed in detail. It is suggested that sa neurons are Golgi type II neurons while m neurons are Golgi type I neurons. The hypothesis is presented that wide-field or m neurons develop and are recognizable before small-field or sa neurons in ontogeny. Evidence regarding this hypothesis is indirect, often conflicting, but suggestive that the hypothesis may be correct. The idea that m neurons may also be phylogenetically older than sa neurons is presented and shown to be consistent with ontogenetic data and interpretations.

Rehabilitation following early malnutrition in the rat: body weight, brain size, and cerebral cortex development

Sprague-Dawley rats were malnourished by giving their mothers an 8% casein diet starting at day 10 of gestation, while controls were fed a 24% casein diet. Starting at postnatal day 20 (P20), rehabilitation of the malnourished animals was attempted by: (1) feeding both mother and young a 24% casein diet, (2) leaving the pups with their mothers until they were 40 days old, and (3) reducing the litter size from 8 to 4 pups. Observations were made on aldehyde-perfused tissue from animals 20, 40 and 70 days old. The somatosensory cortex from one hemisphere was embedded in Araldite, and that from the other side was processed fro Golgi staining. At 20 days of age the body weight of the malnourished animals was 21% that of the controls, but at 70 days it was no longer different. The anterior-posterior length, the width, and the height of the cerebral hemispheres were also significantly reduced at P20, but the differences had disappeared by P70. The thickness of area 3 of the cerebral cortex was measured in 1 micron sections. It was significantly reduced in the malnourished animals at P20, but at P40, following rehabilitation, the difference was no longer statistically significant. In tangential 1 micron sections the fraction of the volume of tissue occupied by neuropil was measured in layers II through IV. At P20 it was significantly reduced only in the upper half of layers II/III of the malnourished animals; at P40 this difference was no longer present. The mean volume of upper layer II/III cell bodies was estimated and found to be significantly reduced in the experimental animals at P20 but not at P40. In the Golgi preparations, pyramidal cells in upper layer II/III were studied. Their estimated volume, as well as the thickness of their basal dendrites, was significantly reduced in the 20 day malnourished animals, but not in the rehabilitated animals. These results show that animals severely malnourished until 20 days of age can reach normal body weight and attain cerebral hemispheres of normal size when proper nutrition is provided. The effects of malnutrition on the cerebral cortex of these animals are most apparent in upper layer II/III which, during the time of nutritional restriction, is the least developed of the cortical layers. However, when proper nutrition is provided, the cerebral cortex may attain normal morphology.

Development of the noradrenergic innervation of neocortex

The development of the noradrenaline (NA)-neuron innervation of rat neocortex was studied by fluorescence histochemistry, high affinity uptake of [3H-]NA, and biochemical assay of regional NA content. Fluorescence histochemistry indicates that NA axons enter areas of developing neocortex prenatally and the innervation matures rapidly during early postnatal life. Frontal and lateral neocortical areas are the first to be innervated followed by occipital and parietal areas. All cortical layers receive innervation. The distribution and density of neocortical NA innervation achieves the adult pattern by the end of the first postnatal week. High affinity uptake studies confirm the observations from fluorescence histochemistry and show a very rapid maturation of the NA axon innervation with adult levels of uptake occurring by postnatal day 9. Following birth, there is a brief rise in NA content from PO to P2 in all neocortical areas. NA content then drops to low levels in all areas by P4. This is followed by a gradual increase in NA content in all areas occuring over several months. This pattern of development of NA axon innervation of neocortex demonstrates that the density and distribution of NA axons in developing neocortex matures much earlier than shown in previous studies whereas the NA content of the developing axonal plexus achieves adult levels later in postnatal life.

An electron microscope study of the early postnatal development of the visual cortex of the hooded rat

Synaptic plasticity in response to environmental events has been clearly demonstrated in the visual cortex of the rat, but no detailed data concerning the course of early synaptogenesis in this area are available. In this study, synaptogenesis in the visual cortex of hooded rats at 1, 3, 5, 7 and 10 postnatal days of age (P1-P10) was examined with electron microscopy. The cortex was divided into the molecular layer, the superficial layers (II-IV) and deep layers (V-VI). In the visual cortex at P1, very few synapses are present in the molecular and deep layers and virtually none in the yet undifferentiated layers II-IV that compose the cortical plate at this age. The synapses that are present are axodendritic and often symmetrical with little membrane thickening and few vesicles. Axosomatic synapses were seen as early as P3 but very rarely. There are marked increases in axodendritic synaptic density and maturity with increasing age. By P7 and P10, many synapses appear mature in form and the majority can be classified a symmetrical. Axospinal synapses first appeared at P7 and were more frequent by P10. However, this classification was somewhat uncertain since no spinal apparatus was detected. Rate of synaptogenesis appeared to increase over the ages studied and showed no signs of leveling off except in the deep layers. Synaptic length was extremely variable and did not change systematically with age.

Somatotopic organization of mechanosensory projections to SII cerebral neocortex in the raccoon (Procyon lotor)

The pattern of projections of peripheral receptors to the neocortex in the second somesthetic receiving area (SII) was mapped in raccoons. The purpose was to determine if the projection area of peripheral receptors to the forepaw area in the SII region is disproportionally enlarged as it is in SI. Tungsten microelectrode recording procedures were used to map thoroughly the inferior wall of the suprasylvian sulcus for regions responsive to mechanical stimulation of peripheral receptors. The results show that: 1. The forepaw area in SII shows an enlargement commensurate with that found in the SI. This suggests that those factors that are selective for tactile acuity of the raccoon forepaw were operating in the evolution of SII as they were in SI. 2. The somatotopic organization of mechanoreceptive projections to SII is reversed mediolaterally compared to previous descriptions of this arrangement in other mammals: projections form axial structures lie medially and those from apical structures lie laterally along the inferior bank of the suprasylvian sulcus in the raccoon.

Organization of corticocortical connections in the parietal cortex of the rat

An analysis based on Nissl, anterograde degeneration, and succinic 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.

Developmental organization of raphe serotonin neuron groups in the rat

The pre- and early postnatal development of serotonin neurons in the rat brainstem was studied using the fluorescence histochemical method. The technique utilized does not require drug pretreatment to visualize an intense serotonin fluorophore localized in neuronal perikarya, dendrites, and axons. All the serotonin neuron groups develop as bilateral nuclei which extend from the midbrain through the medulla. Six of the nine groups undergo a midline fusion from embryonic day 18 (E 18) through postnatal day 6 (P 6) in a rostrocaudal gradient. Cells of the nucleus raphe dorsalis fuse first (by P 1), whereas the serotonin neurons located in nucleus raphe pallidus do not fuse until P 6. This gradient is comparable to the one described for the first observable fluorescence in the serotonin neurons groups. After final cell division, the serotonin neurons undergo a primary migration from the ventricular zone along the midline, where they are situated during embryogenesis, and a secondary migration extending into postnatal life which concludes with fusion in the midline. The bilateral origins of the serotonin cell groups are maintained in the adult. This is expressed by the apparent ipsilateral projections of some of the raphe neurons determined recently in our laboratory utilizing autoradiographic and horseradish peroxidase techniques.

Intracortical connectivity of architectonic fields in the somatic sensory, motor and parietal cortex of monkeys

Anterograde and retrograde transport methods were used to study the corticocortical connectivity of areas 3a, 3b, 1, 2, 5, 4 and 6 of the monkey cerebral cortex. Fields were identified by cytoarchitectonic features and by thalamic connectivity in the same brains. Area 3a was identified by first recording a short latency group I afferent evoked potential. Attempts were made to analyze the data in terms of: (1) routes whereby somatic sensory input might influence the performance of motor cortex neurons; (2) possible multiple representations of the body surface in the component fields of the first somatic sensory area (SI). Apart from vertical interlaminar connections, two types of intracortical connectivity are recognized. The first, regarded as "non-specific," consists of axons spreading out in layers I, III and V-VI from all sides of an injection of isotope; these cross architectonic borders indiscrimininately. They are not unique to the regions studied. The second is formed by axons entering the white matter and re-entering other fields. In these, they terminate in layers I-IV in one or more mediolaterally oriented strips of fairly constant width (0.5--1 mm) and separated by gaps of comparable size. Though there is a broadly systematic topography in these projections, the strips are probably best regarded as representing some feature other than receptive field position. Separate representations are nevertheless implied in area 3b, in areas 1 and 2 (together), in areas 3a and 4 (together) and in area 5; with, in each case, the representations of the digits pointed at the central sulcus. Area 3b is not connected with areas 3a or 4, but projects to a combined areas 1 and 2. Area 1 is reciprocally connected with area 3a and area 2 reciprocally with area 4. The connectivity of area 3a, as conventionally identified, is such that it is probably best regarded not as an entity, but as a part of area 4. Areas identified by others as area 3a should probably be regraded as parts of area 3b. Parts of area 5 that should be more properly considered as area 2, and other parts that receive thalamic input not from the ventrobasal complex but from the lateral nuclear complex and anterior pulvinar, are also interconnected with area 4. More posterior parts of area 5 are connected with laterally placed parts of area 6. A more medial part of area 6, the supplementary motor area, occupies a pivotal position in the sensory-motor cortex, for it receives fibers from areas 3a, 4, 1, 2 and 5 (all parts), and projects back to areas 3a, 4 and 5.

Neuronal differentiation in somatosensory cortex of the rat. I. Relationship to synaptogenesis in the first postnatal week

Newborn (P-0 and P-1) through 6-day-old (P-6) rats were studied using light microscopic (Golgi) and ultrastructural methods. Previous studies demonstrated that early-formed synapses are concentrated at specific cortical depths, i.e. in strata. The present study shows that the synaptic stratum in the marginal zone corresponds to a dense fiber plexus and few somata (Cajal-Retzius cells). Axons in this zone almost exclusively form synapses on distal branches of dendrites originating in deeper lamina. In newborn neocortex there is a second synaptic stratum located deep to the cortical plate. It contains numerous axosomatic and axoproximal dendritic synapses as well as the most highly differentiated somata and proximal dendrites. By age P-6 there are 3 synaptic strata; one each in the marginal zone, cortical plate and 'subplate' layers. For all 3 strata a neuron's most differentiated dendrites are directed towards, traverse or run within, the nearest synaptic stratum. We conclude that, throughout the first postnatal week, the most mature dendrites of a given neuron generally occur at depths where synapse density is highest. At P-0 the most mature somata are similarly related to synaptic density.

Use of cortical circuits during focal penicillin seizures: an autoradiographic study with [14C]deoxyglucose

Autoradiography with [14C]deoxyglucose was used to study the architectural pattern of glucose utilization in the motor cortex of rats during focal penicillin seizures. The seizure focus was characterized by a well circumscribed area whose metabolic activity was increased 2-3 times normal. This was tightly surrounded by cortex that was normal or slightly depressed. The posterior third of the focus showed an increase in glucose utilization in a columnar pattern with particular accentuation of activity in lamina V. There was a loss of normal activity in lamina IV within the focus and in somatosensory and occipital cortex far behind the focus. This depression was particularly prominent in the ipsilateral barrel field. Increased metabolic activity was found in a small area in contralateral homotopic cortex, in lamina Vb with columns extending above this from lamina IV to the surface. Glucose utilization was accentuated 1.2-1.8 fold in the ipsilateral secondary somatosensory area, but was normal in the contralateral cortex. The intensity of focal seizures was increased by the intracortical injection of more penicillin or by giving intravenous metrazol. Both of these methods resulted in an increase in the size of the focus as determined with [14C]deoxyglucose. This was most prominent on the lateral border in lamina I-II and V. In addition, there was an accentuation of the columnar pattern in the posterior part of the focus, ipsilateral somatosensory cortex, and contralateral motor cortex. The architectural pattern of glucose utilization in the cortex during focal seizures is discussed with reference to corticocortical, commissural, and corticothalamic circuits that have been identified by others in anatomical studies. Superimposed on this structure are physiological principles of recurrent excitation, lateral spread, and surround inhibition that characterize basic electrophysiological mechanisms of epilepsy, and influence the intensity of activity within the architectural design.

The forms of non-pyramidal neurons in the visual cortex of the rat

Rapid Golgi preparations from area 17 of young adult rats have been studied to determine the morphology and distribution of non-pyramidal neurons. Such cells were observed in all of the cellular laminae of the cortex, but were particularly prevalent in layers IV and V. Non-pyramidal neurons were categorized according to two features: (1) dendritic projection pattern, and (2) abundance of dendritic spines. Dendritic patterns were classified as multipolar, bitufted, and bipolar, and spine patterns as spinous, sparsely spinous, and spine-free. Spinous dendrites were associated only with multipolar neurons, while sparsely spinous and spine-free dendrites were each associated with cells of all three non-pyramidal dendritic patterns. The most frequently observed non-pyramidal cell types were multipolar cells of the spine-free and sparsely spinous varieties. All of the general cell types encountered have been described in the literature on non-pyramidal neurons, indicating the lack of any unique forms in rat area 17. An analysis of the dendritic projections of individual non-pyramidal neurons through particular cortical laminae made possible an evaluation of common sources of dendrites present in the neuropil of each layer. Non-pyramidal cell axons were impregnated only in small numbers. Spinous multipolar axons invariably exhibited a descending main branch, while the axons of bipolar neurons were distributed in a narrow vertical field. Axonal patterns of remaining cell types, including Golgi type II arborizations, did not appear to correlate consistently with dendritic morphology. Axons of the basket cell type and "horsetail" axons associated with double bouquet cells of Cajal's original type were not impregnated.

The representation of the body surface in somatosensory area I of the grey squirrel

Microelectrode mapping methods were used to determine the organization of primary somatosensory cortex, SmI, in grey squirrels. A systematic representation of the contralateral body surface was found within somatic konicortex. This primary representation differs from maps of SmI in other mammals in at least two significant ways. The first way in which SmI of squirrels differs from the organization reported for other mammals is that SmI of squirrels contains a double representation of the hand and parts of the forearm. The glabrous skin of the digits is represented twice in a mirror image fashion joined at the finger tips. The hairy skin of the digits, wrist, and parts of the forearm are also represented twice, once on each side of the joined representations of the glabrous skin. A second unique feature of SmI of squirrels is that there is a small region of cortex completely surrounded by SmI that was unresponsive to light cutaneous stimuli under our recording conditions. This unresponsive zone is easily identified in brain sections by architectonic features that deviate from sensory koniocortex and approach motor cortex. A third significant finding was that the back is rostral to the belly in the representation of the trunk in SmI of squirrels. This is the reverse of the orientation reported elsewhere for SmI of mammals, but corresponds to the orientation of the trunk representation in Area 3b of owl monkeys (Kaas et al., '78; Merzenich et al., '78). This similarity supports an earlier contention that the representation of the body in Area 3b of primates is the homolog of SmI in other mammals (Merzenich et al., '78).

Association and commissural fiber systems of the olfactory cortex of the rat

The association and commissural fiber systems arising in the olfactory cortical areas caudal to the olfactory peduncle (the piriform cortex, nucleus of the lateral olfactory tract, anterior cortical nucleus of the amygdala, periamygdaloid cortex and entorhinal cortex) have been studied utilizing horseradish peroxidase as both an anterograde and a retrograde axonal tracer. In the piriform cortex two sublaminae within layer II (IIa and IIb) layer III have been found to give rise to distinctly different projections. Retrograde cell labeling experiments indicate that the association fiber projection from layer IIb is predominatnly caudally directed, while the projection from layer III is predominantly rostrally directed. Cells in layer IIa project heavily to areas both caudal and rostral to the piriform cortex. The commissural fibers from the piriform cortex are largely restricted in their origin to layer IIb of the anterior part of the piriform cortex and in their termination on the contralteral side to the posterior part of the piriform cortex and adjacent olfactory cortical areas. A projection to the olfactory bulb has also been found to arise from cells in layers IIb and III of the ipsilateral piriform cortex, but not in layer IIa. In addition to those from the piriform cortex, association projections have also been found from other olfactory cortical areas. The nucleus of the lateral olfactory tract has a heavy bilateral projection to the medial part of the anterior piriform cortex and the lateral part of the olfactory tubercle (as well as a lighter projection to the olfactory bulb); both the anterior cortical nucleus of the amygdala and the periamygdaloid cortex project ipsilaterally to several olfactory cortical areas. The entorhinal cortex has been found to project to the medial parts of the olfactory tubercle and the olfactory peduncle. The olfactory tubercle is the only olfactory cortical area from which no association fiber systems (instrinsic or extrinsic) have been found to originate. A broad topographic organization exists in the distribution of the fibers from several of the olfactory areas. This is most obvious in the anterior part of the olfactory cortex, in which fibers from the more rostral areas (the anterior olfactory nucleus and the anterior piriform cortex) terminate in regions near the lateral olfactory tract, while those from more caudal areas (the posterior piriform cortex and the entorhinal cortex) terminate in areas further removed, both laterally and medially, from the tract. Projection to olfactory areas from the hypothalamus, thalamus, diagonal band, and biogenic amine cell groups have been briefly described.

Autoradiographic maps of regional brain glucose consumption in resting, awake rats using (14C) 2-deoxyglucose

The 2-deoxy-D-[14C]-glucose (2-DG) autoradiographic method for determining regional brain glucose consumption has been applied successfully by a number of workers for mapping the alterations of brain glucose consumption which occur in association with experimental alterations of brain functional activity. This paper provides a framework for the interpretation of these and further studies by presenting: (1) the pattern of regional brain glucose consumption in the normal, resting, awake rat; (2) the anatomical identities of brain structures which on autoradiographs appear only as regional variations of optical density. For this purpose, a series of 2-DG autoradiographs of coronal brain sections from an injected animal is compared with adjacent labeled Nissl sections.

Somatotopic and columnar organization in the corticotectal projection of the rat somatic sensory cortex

Single injections of tritiated amino acids into the first somatic sensory area (SI) of the rat neocortex result in axoplasmically transported labeling of the stratum griseum intermidiale and stratum griseum profundum of the ipsilateral superior colliculus. The terminal labeling in these layers takes the form of multiple, column-like patches. The SI projection is somatotopically organized with the face and head representations projecting to an extensive anterolateral part of the colliculus and the limb representations projecting to a restricted posterolateral part. Injections of horseradish peroxidase into the superior colliculus result in retrograde labelling of corticotectal cells in the superficial part of layer VB of SI and of the second somatic sensory area (SII).