Anatomical organization of the primary visual cortex (area 17) of the cat. A comparison with area 17 of the macaque monkey.

Golgi and axonal transport techniques have been used to examine the organization of neurons within primary visual cortex, area 17, of the cat. This organization has been compared to that of the primate cortical area 17 as described in previous studies and it is discussed in relationship to the distribution of afferents from the dorsal lateral geniculate nucleus (dLGN). The visual cortex of the cat and monkey show strong similarities in the laminar positions of neurons projecting extrinsically and also in the restriction of spiny stellate neurons to a central lamina (lamina 4) receiving input from the dLGN. However, lamina 4B in the monkey, which contains spiny stellate neurons but does not receive direct input from the dLGN, has no direct counterpart in cat area 17. Axon projections of spiny stellate neurons in the other divisions of lamina 4 differ in cat and monkey: the small, closely packed neurons in the lowermost division of lamina 4 (4B in the cat, 4Cbeta in the monkey) project chiefly within lamina 4 in the cat whereas in the monkey they have a strong projection to lamina 3. In the cat, spiny stellate neurons of lamina 4A project upon lamina 3 whereas in the monkey those in the apparently equivalent zone, 4Calpha, project upon lamina 4B. Most non-spiny stellate neurons examined have precisely organized interlaminar axonal projections which differ from the axon trajectories of neighboring spiny neurons.     

Corticocortical efferent systems in the monkey: a quantitative spatial analysis of the tangential distribution of cells of origin

The laminar and tangential distributions of association neurons projecting from areas 4 and 6 of the frontal lobe to area 5 of the superior parietal lobule were studied in macaque monkeys by using horseradish peroxidase histochemistry. In both areas 4 and 6 association neurons were medium-large pyramidal cells of layers II and III, and pyramidal and fusiform cells of layers V-VI. Tangentially, they were distributed unevenly over the cortical surface occupying only certain parts of areas 4 and 6, including the dorsomedial part of area 6, the proximal arm region of Woolsey's M1 map, parts of the postarcuate cortex, and the supplementary motor area. Within these projection zones, the number of projection cells waxed and waned in a periodic fashion. Quantitative methods, including spectral analysis techniques, were used to characterize precisely spatial periodicities along the rostrocaudal dimension. The same quantitative analyses were used to determine the nature of the tangential distribution of corticocallosal cells of area 5 projecting to contralateral area 5. Both association and callosal spectra contained a strong component in the range of low spatial frequencies, corresponding to periods greater than 2 mm. Moreover, a consistent peak was observed in both spectra at spatial frequencies corresponding to periods ranging from 0.85 to 1.28 mm. This peak is in accord with the hypothesis of a modular organization of the cells of origin of these projection systems.     

Cells of origin and terminal distribution of corticostriatal fibers arising in the sensory-motor cortex of monkeys.

The cells of origin of the corticostriatal projection have been identified in squirrel monkeys by the use of the retrograde horseradish peroxidase method. In the subfields of the somatic sensory, motor, parietal and frontal areas of the cortex, cells projecting to the ipsilateral striatum are relatively sparsely distributed and form a group of small- to medium-sized pyramidal cells with an average somal diameter from area to area of 14-16 mum. Such cells are found only in layer V of the cortex (mainly in the more superficial parts of the layer). Since they are consistently smaller than the pyramidal cells of layer V that project to the brainstem and spinal cord and since they lie outside layer VI which gives rise to corticothalamic axons, the corticostriatal axons are unlikely to be collaterals of axons projecting to other sites. The cells of origin of the crossed corticostriatal projection are also found in layer V and are pyramidal cells with somal diameters in the same range as above. They are found only in areas 4, 8, and 6. Studies with the anterograde, autoradiographic method in rhesus, cynomologous and squirrel monkeys, indicate that the somatic sensory areas project to most of the antero-posterior extent of the ipsilateral putamen. Subareas 3a, 3b, 1 and 2 of the somatic sensory cortex project to the same region and the projection overlaps similarly extensive projections from the motor and certain other areas of the cortex. However, in each case the pattern of terminal labeling is in the form of interrupted clusters, strips and bands. A single small injection of the cortex is associated with only one or two such clusters of terminal labeling. This seems to imply that individual corticostriatal fibers end in a very restricted manner and that the terminal ramifications of fibers from one cortical area may alternate in the putamen with those arising in other areas.     

Morphology and location of tectal projection neurons in frogs: A study with hrp and cobalt-filling

Tectal projection neurons were labeled by retrograde transport of horseradish peroxidase (HRP) or cobaltic-lysine. The tracer substances were delivered iontophoretically or by pressure injection or diffusion into various regions of the brain or spinal cord. Histochemical procedures allowed identification of labeled cells projecting to the injected regions. Many neurons were filled with cobaltic-lysine, resulting in a Golgi-like staining. After cobalt injections in the diencephalon most of the labeled cells, identified as small piriform neurons, were located in layer 8 of the tectum. Two types of small piriform neurons were distinguished. Type 1 neurons have flat dendritic arborizations confined to lamina D, while the dendrites of type 2 cells may span all of the superficial tectal strata. In smaller numbers large piriform, pyramidal, and ganglionic cells of the periventricular tectal layers were labeled after diencephalic injections. Rhombencephalic cobalt and HRP injections labeled cells whose axons form the tectobulbospinal tract. The neurons most frequently labeled were large ganglionic cells. Ipsilaterally, the majority of their somata were located in layer 7, and their dendrites arborized mainly in lamina F. Con-tralaterally, labeled ganglionic cell somata occupied the top of layer 6, and most of their dendritic end-branches entered lamina B. The possible functional significance of this anatomical arrangement is discussed. After tectal cobalt injections the topography of the tectoisthmic projection and the terminals of tectal efferent fibers in the diencephalon and brainstem were observed. It is concluded that the organization of frog tec-tofugal pathways is very similar to that of mammals.     

The distribution of neocortical projection neurons in the locus coeruleus

The present study was conducted to examine the spatial organization of locus coeruleus (LC) neurons that project to rat cerebral cortex. Long-Evans hooded rats received unilateral pressure injections of horseradish peroxidase (HRP) in either frontal (n = 6) or sensorimotor (n = 11) or occipital (n = 7) cortex to determine the intranuclear location of LC neurons which project to specific neocortical regions. Coronal and sagittal sections (40-100 micron) through the LC were examined by light microscopy after carrying out the tetramethyl benzidine reaction and staining with neutral red. The locations of retrogradely labeled cells were recorded on a three-dimensional biological coordinate system maintained by a computer linked to the light microscope. LC neurons labeled from cerebrocortical injections of HRP were primarily located in the ipsilateral and to a lesser extent (fewer than 5% of total labeled cells) in the contralateral nucleus. Coeruleocortical projection neurons were concentrated in the caudal three-fifths of the dorsal division of the ipsilateral LC. Within this portion of the nucleus, HRP-filled neurons were distributed so that individual groups of cells projecting to occipital or sensorimotor or frontal cortex were coarsely aligned in a dorsal to ventral array, respectively. Moreover, in the sagittal plane of the nucleus the pattern of labeling was spatially graded so that the subset of neurons projecting to the occipital cortex was displaced more caudally in the LC than the groups of cells sending axons to sensorimotor or frontal cortex. Only the frontal area of the cortex received a projection from both dorsal and ventral divisions of the ipsilateral LC. Computer-assisted analysis of the data further suggested that neocortical projection neurons in the dorsal LC are loosely organized into two groups which run rostrocaudally through the core of the caudal nucleus. The zone of labeling resulting from injections confined to the neocortical gray matter overlapped with but was not coextensive with that observed following injections into the caudate, hippocampus, and cerebellum. These results suggest that partially overlapping subsets of LC cells might independently influence separate populations of neurons within noradrenergic terminal fields of the neocortex.     

Spinal lamina I neurons projecting to the parabrachial area of the cat midbrain

We have observed a population of lamina I neurons in the cat that has projections to the parabrachial area (parabrachial and cuneiform nuclei). A subpopulation of these neurons also projects to the contralateral thalamus. The majority of projecting cells responded exclusively to noxious stimuli, a few wide-dynamic-range neurons were also observed. Conduction velocities for antidromic activation from the midbrain ranged from 1 to 18 m/s. We stained 14 cells intracellularly with horseradish peroxidase. These findings suggest that a major nociceptive projection pathway originating in lamina I and terminating in the parabrachial area exists in the cat.     

Intrinsic connections of macaque striate cortex: afferent and efferent connections of lamina 4C

We have studied the intrinsic organization of macaque striate cortex by tracing the pattern of horseradish peroxidase (HRP)-labeled axons and cell bodies produced by microinjections of HRP into single cortical laminae. Both anterograde and retrograde transport results were used to examine: (1) the pattern of projections from lamina 4C to the superficial layers; (2) the projection from lamina 4C to deeper cortical layers; and (3) the projections to lamina 4C from other cortical laminae. Laminae 4C alpha and 4C beta differ in their pattern of projections to the superficial layers of striate cortex. Axons from neurons in lamina 4C beta ascend through lamina 4B without giving off collaterals and terminate in lamina 4A and in the base of lamina 3. By contrast, axons from neurons in lamina 4C alpha terminate in lamina 4B and less densely in the 4A/3B region. The projection from lamina 4C beta to lamina 4A is particularly dense and is distributed in a patchy fashion immediately above each injection site. The projection from lamina 4C beta to lamina 3B appears less dense and more widespread; we estimate that individual 4C beta axons may spread laterally for more than 400 micron. Furthermore, the pattern of HRP-labeled cell bodies in lamina 4C beta following injections into laminae 4A and 3B provides evidence for a subdivision within 4C beta. These injections always produce a large number of labeled neurons in the upper part of lamina 4C beta, whereas the lower portion contains few labeled neurons that are located immediately under the center of the injection site. Both lamina 4C alpha and lamina 4C beta also contribute less dense projections to the deeper layers of cortex. Lamina 4C beta projects mainly to lamina 6, whereas lamina 4C alpha contributes axon terminals to both lamina 5A and lamina 6. Neurons in lamina 6 provide the bulk of the intracortical projections to lamina 4C. The axons of these neurons are fine in caliber and have a delicate side-spine morphology that is quite distinct from lateral geniculate axon arbors. Neurons in lamina 5A also project onto lamina 4C, but the projections of these neurons appear concentrated in lamina 4C alpha. These results confirm or refine many conclusions about intrinsic connections of striate cortex drawn from Golgi material and suggest new patterns of connections not suspected from previous work.     

Spatial organization of thalamocortical and corticothalamic projection systems in the rat SmI barrel cortex

Axonal tracing techniques were used to examine the distribution of corticothalamic projection neurons in relation to the organization of the thalamocortical recipient zones in the whisker representation of the rat first somatic sensory cortex. Following injection of horseradish peroxidase into the physiologically defined vibrissa area in the ventrobasal complex of the thalamus, labeling in the cortex had a columnar appearance. Dense patches of anterograde labeling were located within the centers of the layer IV barrels and extended superficially through lamina III; the septa between barrels contained considerably less reaction product. Retrogradely labeled neurons were observed in lower layer V and layer VI where they were concentrated preferentially deep to the barrel centers. Regions deep to the septa displayed less overall labeling and a lower relative number of thalamic projecting neurons. Zones having the larger numbers of retrogradely labeled cells also contained terminallike labeling of either corticothalamic or thalamocortical origin. Following an injection that included the posterior group medial to the ventrobasal complex, anterograde labeling in layer IV was located largely in the septa. In conjunction with previous findings concerning the origin and termination of other projection systems in the barrel cortex, these results suggest that a vibrissal column contains a central core zone intimately linked with the ventrobasal thalamus that is bounded by narrower regions of more diverse inputs and outputs that form an interface between adjacent cortical columns.     

Topography of neurons of the raccoon main cuneate nucleus with projections to the inferior colliculus

The origin of the projection from the raccoon main cuneate nucleus to the inferior colliculus has been demonstrated by means of retrograde transport of horseradish peroxidase. Neurons of the polymorphic division of the main cuneate nucleus project to the inferior colliculus via the medial lemniscus. Neurons projecting to the inferior colliculus are only found caudal to the obex and do not overlap significantly with the distribution of cuneocerebellar neurons as in the cat.     

Interlaminar connections and pyramidal neuron organisation in the visual cortex,area 17, of the Macaque monkey

The patterns of arborisation of apical dendrites of different varieties of pyramidal neurons in area 17 differ and are characteristic for each cell type. They appear to serve as a means of collating within one neuron information derived directly from several different laminae. These different patterns of apical dendrite arborisation provide dendritic links which relate closely to the laminar distribution of axons of the spiny stellate neurons as well as the pyramidal neurons themselves. The axons of spiny stellate neurons lying in laminae IVCβ and IVA (Lund, '73)—Which receive information from parvocellular geniculate layers — project heavily to the lower half of lamina III (IIIB) and to a narrow zone at the top of lamina V (VA); laminae IIIB and VA are in turn linked by a specific variety of pyramidal neuron, with basal dendritic field in lamina VI, whose apical dendrite has marked lateral branching only in laminae VA and IIIB (where it terminates). Pyramidal neurons with basal dendritic field in laminae VA (with vestigial apical dendrite) or in IIIB have recurrent axon projections to lamina IIIA and above (the descending axon projection of lamina IIIB pyramids is principally to lamina VA itself). The pyramidal neurons of laminae IIIA and above have axons which distribute in the same upper laminae as their dendtritic fields and a descending axon projection to lamina VB. Pyramidal neurons with basal dendritic field on lamina VB have an apical dendrite which, if not vestigal, arborises in IIIA or above; their axons in some cases project to the superior colliculus or may be exclusively, or in addition, recurrent, distributing collaterals within laminae VB, VI and in IIIA or above; one variety of pyramidal neuron with basal dentritic field in lamina VI makes a dentritic link with these same regions, its apical dendrite arborising first within lamina VB and then in lamina IIIA and above. Axons of spiny stellate neurons of lamina IVCα (which receives the projection of the magnocellular layers of the lateral geniculate nucleus) as well as distributing widely within lamina IVCα also contribute to laminae IVB and VA; a link is again made by a specific variety of pyramidal neuron, with basal dendtritic field in lamina VI, which shows branching to its apical dendtrite only in laminae VA and as a terminal arborisation in IVCα. Another variety of pyramidal neuron with basal dendtric field in lamina VI has apical dendritic arborisation only in lamina IVB. The pyramidal neurons with basal dendritic field in lamina IVB and apical dendrite arborising in lamina IIIB and above, also contribute axonal collatetrals to lamina IIIA and above; their horizontal axon collaterals, together with the axons of spiny stellate neurons of laminae IVCα and IVB, form the horizontal fiber band of lamina IVB (to which the axons of laminae III and II pyramidal neurons do not contribute. The descending axon projection of the spiny stellate and pyramidal neurons of lamina IVB appears to be principally to lamina VI. The pattern of branching of pyramidal neuron apical dendrites is therefore neither random nor a continuum of one basic pattern; instead it is a series of separate patterns, each spatially distributed in a highly specific and unique fashion relating to the patterns of projection of afferent information through the cortex.     

Morphologic characteristics of the population of neurons in the cat auditory cortex, projecting into the medial geniculate body

Neurons from the auditory cortex projecting into the medial geniculate body were studied in cats using the horseradish peroxidase. Such neurons were located in deep layers of the auditory cortex--predominantly in layer VI, and to a lesser extent in layer V. Dimensions of the pericarions of the labelled neurons were measured and types of neurons were determined. The overwhelming majority of cortico-geniculate neurons was pyramidal, and quantity of such neurons in layer VI of the first auditory cortex may reach 60% of the total number of cells in this layer. On the basis of the anterograde transport of HRP deep layer III and layer IV of the auditory cortex were determined as main targets of geniculocortical fibres.     

Projection neurons of the basolateral amygdala: A correlative Golgi and retrograde tract tracing study

This study analyzed the projection neurons of the anterior subdivision of the rat basolateral amygdaloid nucleus (BLa) by correlating the morphology of Golgi-stained neurons with the morphology of neurons that were retrogradely labeled by injections into the main terminal fields of BLa. In each animal multiple injections of horseradish peroxidase (HRP) and wheat germ agglutinin-conjugated HRP were made into the prefrontal cortex and rostral striatum. These injections labeled approximately 85% of BLa neurons. The great majority of labeled neurons were the same shape and relative size as the pyramidal (class I) neurons described in previous Golgi studies. The unlabeled neurons appeared to correspond to the nonpyramidal (class II and class III) neurons described in Golgi studies. Thus this investigation provides experimental evidence that the pyramidal neurons are the main projection neurons of BL, whereas most of the nonpyramidal cells are local circuit neurons.     

Prefrontal cortex in the rat: projections to subcortical autonomic, motor, and limbic centers

This paper describes the quantitative areal and laminar distribution of identified neuron populations projecting from areas of prefrontal cortex (PFC) to subcortical autonomic, motor, and limbic sites in the rat. Injections of the retrograde pathway tracer wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) were made into dorsal/ventral striatum (DS/VS), basolateral amygdala (BLA), mediodorsal thalamus (MD), lateral hypothalamus (LH), mediolateral septum, dorsolateral periaqueductal gray, dorsal raphe, ventral tegmental area, parabrachial nucleus, nucleus tractus solitarius, rostral/caudal ventrolateral medulla, or thoracic spinal cord (SC). High-resolution flat-map density distributions of retrogradely labelled neurons indicated that specific PFC regions were differentially involved in the projections studied, with medial (m)PFC divided into dorsal and ventral sectors. The percentages that WGA-HRP retrogradely labelled neurons composed of the projection neurons in individual layers of infralimbic (IL; area 25) prelimbic (PL; area 32), and dorsal anterior cingulate (ACd; area 24b) cortices were calculated. Among layer 5 pyramidal cells, approximately 27.4% in IL/PL/ACd cortices projected to LH, 22.9% in IL/ventral PL to VS, 18.3% in ACd/dorsal PL to DS, and 8.1% in areas IL/PL to BLA; and 37% of layer 6 pyramidal cells in IL/PL/ACd projected to MD. Data for other projection pathways are given. Multiple dual retrograde fluorescent tracing studies indicated that moderate populations (<9%) of layer 5 mPFC neurons projected to LH/VS, LH/SC, or VS/BLA. The data provide new quantitative information concerning the density and distribution of neurons involved in identified projection pathways from defined areas of the rat PFC to specific subcortical targets involved in dynamic goal-directed behavior.     

Characterization of transient cortical projections from auditory, somatosensory, and motor cortices to visual areas 17, 18, and 19 in the kitten

We have examined the anatomical features of ipsilateral transient cortical projections to areas 17, 18, and 19 in the kitten with the use of axonal tracers Fast Blue and WGA-HRP. Injections of tracers in any of the three primary visual areas led to retrograde labeling in frontal, parietal, and temporal cortices. Retrogradely labeled cells were not randomly distributed, but instead occurred preferentially at certain loci. The pattern of retrograde labeling was not influenced by the area injected. The main locus of transiently projecting neurons was an isolated region in the ectosylvian gyrus, probably corresponding to auditory area A1. Other groups of transiently projecting neurons had more variable locations in the frontoparietal cortex. The laminar distribution of neurons sending a transient projection to the visual cortex is characteristic and different from that of parent neurons of other cortical pathways at the same age. In the frontoparietal cortex, transiently projecting neurons were located mainly in layer 1 and the upper part of layers 2 and 3. In the ectosylvian gyrus, nearly all the neurons are located in layers 2 and 3. In addition, a few transiently projecting neurons are found in layer 6 and in the white matter. Transiently projecting neurons have a pyramidal morphology except for the occasional spindle-shaped cell of layer 1 and multipolar cells observed in the white matter. Anterograde studies were used to investigate the location of transient fibers in the visual cortex. Injections of WGA-HRP at the site of origin of transient projections gave rise to few retrogradely labeled cells in areas 17, 18, and 19, demonstrating that transient projections to these areas are not reciprocal. Although labeled axons were found over a wide area of the posterior cortex, they were more numerous over certain regions, including areas 17, 18, and 19, and absent from other more lateral cortical regions. Transient projecting fibers were present in all cortical layers at birth. Plotting the location of transient fibers in numerous sections and at all ages showed that these fibers are not more plentiful in the white matter than they are in the gray matter. We found no evidence that the white/gray matter border constituted a physical barrier to the growth of transient axons. Comparison of the organization of this transient pathway to that of other transient connections is discussed with respect to the development of the cortex.     

Spinocerebellar projections to lobules I and II of the anterior lobe in the cat, as studied by retrograde transport of horseradish peroxidase

Spinocerebellar tract (SCT) neurons projecting to lobules I and II of the cerebellar anterior lobe were identified by the retrograde horseradish peroxidase technique in the cat. Instead of a conventional stereotaxic approach, we removed ventral parts of the vermis of the posterior lobe and approached the posterior aspect of lobule I through the fourth ventricle. Under direct visual guidance, discrete injections were made into lobule I or II with a glass micropipette. Neurons projecting to lobule I were located mainly in the central cervical nucleus (CCN), the medial part of lamina VII of L6 to the causal segments, and in lamina VIII of S2 to the caudal segments (with crossed ascending axons). The latter two groups correspond to medial lamina VII group of the lumbar to the caudal segments and the ventral horn group of the sacral-caudal segments of our previous studies. A small number of Clarke column neurons (with uncrossed ascending axons) also projected to lobule I. All of these neuronal groups projected to lobule II. In addition, large neurons in lamina V and the border between laminae IV and V from S2 to the caudal segments projected to sublobule IIA, and more numerously to sublobule IIB (with crossed ascending axons). They belong to the dorsal horn group of the sacral-caudal segments of our previous studies. Spinal border cells (with crossed ascending axons) projected to sublobule IIB, and a small number, to sublobule IIA. It was suggested that the CCN neurons project more densely to the median region whereas Clark column neurons project to the lateral part of these lobules.     

Organization of ventral tegmental area cells projecting to the occipital cortex and forebrain in the rat

Our horseradish peroxidase retrograde tracing study revealed a specific subpopulation of ventral tegmental area (VTA) neurons that send axons to the occipital cortex in the rat. A fluorescent retrograde tracing study demonstrated that neuronal populations in the VTA projecting to the occipital cortex are distributed in a manner separate from those projecting to forebrain structures such as the frontal/anterior cingulate cortices and nucleus accumbens. The scarcity of collateral projections from the VTA contrasts with the extensive collateralization of projection neurons in the substantia nigra pars compacta. Projections to the occipital cortex may define the distribution of cells comprising the VTA and thus the clear hodological separation of the A9 and A10 dopamine cell groups.     

Spinocerebellar projections to lobules III to V of the anterior lobe in the cat,as studied by retrograde transport of horseradish peroxidase

Spinocerebellar tract (SCT) neurons projecting to lobules III to V of the cerebellar anterior lobe were identified by the retrograde horseradish peroxidase technique. SCT neurons projecting to lobule III with crossed ascending axons were located mainly in the central cervical nucleus (CCN), the medial part of lamina VII of L6 to the caudal segments, and the dorsal horn (lamina V) and ventral horn (lamina VIII) of the sacral-caudal segments. Spinal border cells with crossed ascending axons also projected to lobule III. SCT neurons projecting to this lobule with uncrossed ascending axons were located in the medial part of lamina VI of the cervical segments and the middle part of lamina VII of C6 to T1, lamina V of the lower cervical, thoracic and the lumbar segments, Clarke's column including marginal neurons, and the medial part of lamina VI of L5 and L6. These neuronal groups also projected to lobule IV, except for those present caudal to L6 (in the medial part of lamina VII, and laminae V and VIII of the sacral-caudal segments). A far smaller number of similar neurons projected to lobule V. Injections of HRP restricted to the vermal region labeled mainly neurons in the CCN and Clarke's column while restricted injections to the intermediate-lateral regions labeled ipsilaterally spinal border cells, lamina V neurons, and Clarke column neurons, especially of the lumbar segments as well as marginal neurons of this column.     

The postnatal development of the association projection from visual cortical area 17 to area 18 in the cat

The postnatal development of the association projection from area 17 to area 18 was studied in normal and binocularly deprived kittens between 1 and 28 days of age, using retrograde transport of horseradish peroxidase conjugated with wheat germ agglutinin. The positions of injection sites in the visual cortex, defined in relation to the borders of visual areas 17, 18, and 19 located in Nissl- and cytochrome oxidase-stained sections, were confirmed by observing the patterns of labeling of cells in the lateral geniculate nucleus. The association projection is present and is arranged at least roughly topographically from birth onward; at all ages it arises from cells in both the superficial layers (II, III, and the upper part of IV) and the deep layers (V and VI). In older kittens (20 days or more), however, the origin of the pathway is principally from the upper layers, as in adult cats, whereas in younger animals the projection arises roughly equally from cells in superficial and deep laminae. Initially, the association neurons in area 17 are distributed uniformly along each lamina. Periodic clustering of labeled cells in the upper layers can just be discerned at 10 days, and this patchiness has reached its adult clarity by 20 days, at which stage the projection from the lower layers is greatly diminished. Binocular deprivation until the age of 28 days did not prevent these developmental changes in the projection. Various controls established that the patterns of labeling seen in this study were not due to direct spread of tracer into area 17, to uptake of tracers by fibers-of-passage, or to transcellular transport via the thalamus.     

Bihemispheric Collateralization of the Cortical and Subcortical Afferents to the Rat's Visual Cortex

A fluorescent dye (usually fast blue or rhodamine tagged latex microspheres) was injected into cortical area 17 (or area 17 and the lateral part of area 18b) of adult and juvenile (15 - 22 day old) Sprague-Dawley albino rats. Another fluorescent dye (usually diamidino yellow) was injected into cortical areas 17, 18a and 18b of the opposite hemisphere. The injections involved only the cortical grey matter. After postinjection survival of 2 - 14 days the distribution of retrogradely labelled mesencephalic and prosencephalic cells was analysed. Both small and large injections labelled retrogradely a substantial number of cells in specific and nonspecific dorsal thalamic nuclei (lateral geniculate, lateral posterior, ventromedial, several intralaminar nuclei and nucleus Reuniens) as well as a small number of cells in the preoptic area of the hypothalamus and the mesencephalic ventral tagmental area (VTA). While labelled thalamic cells contained only the dye injected into the ipsilateral cortex, a small proportion of hypothalamic and VTA cells was labelled with the dye injected into the contralateral cortex. Virtually none of the cells in these areas were double labelled with both dyes. Both small and large injections labelled cells in the ipsilateral telencephalic magnocellular nuclei of the basal forebrain and the caudal claustrum. A substantial minority of labelled cells in these structures was labelled by the dye injected into the contralateral cortex. Furthermore, a small proportion (about 1%) of claustral cells projecting to the ipsilateral cortex were double labelled with both dyes. In several cortical areas ipsilateral to the injected area 17, associational neurons were intermingled with commissural neurons projecting to the contralateral visual cortex. A substantial proportion of associational neurons projecting to ipsilateral area 17 also projected to the contralateral visual cortex (associational-commissural neurons). Thus, in visual area 18a, the associational-commissural neurons were located in all laminae, with the exception of lamina 1 and the bottom of lamina 6, and constituted about 30% of the neurons projecting to ipsilateral area 17. In paralimbic association area 35/13, associational-commissural neurons were located in lamina 5 and constituted about 20% of neurons projecting to ipsilateral area 17. In the limbic area 29d, the associational-commissural neurons were located in laminae 4, 5 and the upper part of lamina 6 and constituted about 10% of the associational-commissural neurons projecting to ipsilateral area 17. In oculomotor area 8, double-labelled neurons were located in lamina 5 and constituted about 10% of the neurons projecting to ipsilateral area 17. Thus, it appears that the axons of mesencephalic and diencephalic neurons projecting to the visual cortex do not send collaterals into both hemispheres. The bihemispheric projection to the rat's visual cortex originates almost exclusively in the retinotopically organized cortical area 18a and in integrative cortical areas 35/13, 29d and 8.     

Spinocerebellar projections to the vermis of the posterior lobe and the paramedian lobule in the cat,as studied by retrograde transport of horseradish peroxidase

Spinal neurons projecting to the posterior lobe of the cerebellum were identified with the retrograde horseradish peroxidase technique in the cat. In four cases with the injections, which were preceded by hemisections at cervical or thoracic levels, it was determined whether in the spinal cord the identified neurons give rise to crossed ascending axons or uncrossed ascending ones. The main groups of neurons projecting to sublobule VIIIB were located in the central cervical nucleus (with crossed ascending axons), Clarke's column (with uncrossed ascending axons), and the medial part of lamina VII of L6 to the caudal segments (with crossed ascending axons). Additional labeled neurons were found in the medial part of lamina VI between C2 and T1 and of L5 and L6 (with uncrossed ascending axons), and in the ventral as well as dorsal horns of the sacral-caudal segments (with crossed ascending axons). On the other hand, neurons projecting to sublobule C (the copular part) of the paramedian lobule, which appeared always ipsilaterally to the side of the injections, were located in lamina V of C8 to L4 (with uncrossed ascending axons). Marginal neurons of Clarke's column (with uncrossed ascending axons) and spinal border cells (with crossed ascending axons that recross in the cerebellum) projected specifically to this part. At L1 and L2 or L2 and L3 labeled large and medium-sized neurons were also found within Clarke's column. The present study suggests that there are segregated projections of spinal neurons to the cortex of the cerebellar posterior lobe.