Corticopontine Cells Research Articles

Differential involvement of mitochondria in post-tetanic potentiation at intracortical excitatory synapses of the medial prefrontal cortex.

Post-tetanic Ca2+ release from mitochondria produces presynaptic residual calcium, which contributes to post-tetanic potentiation. The loss of mitochondria-dependent post-tetanic potentiation is one of the earliest signs of Alzheimer's model mice. Post-tetanic potentiation at intracortical synapses of medial prefrontal cortex has been implicated in working memory. Although mitochondrial contribution to post-tetanic potentiation differs depending on synapse types, it is unknown which synapse types express mitochondria-dependent post-tetanic potentiation in the medial prefrontal cortex. We studied expression of mitochondria-dependent post-tetanic potentiation at different intracortical synapses of the rat medial prefrontal cortex. Post-tetanic potentiation occurred only at intracortical synapses onto layer 5 corticopontine cells from commissural cells and L2/3 pyramidal neurons. Among post-tetanic potentiation-expressing synapses, L2/3-corticopontine synapses in the prelimbic cortex were unique in that post-tetanic potentiation depends on mitochondria because post-tetanic potentiation at corresponding synapse types in other cortical areas was independent of mitochondria. Supporting mitochondria-dependent post-tetanic potentiation at L2/3-to-corticopontine synapses, mitochondria-dependent residual calcium at the axon terminals of L2/3 pyramidal neurons was significantly larger than that at commissural and corticopontine cells. Moreover, post-tetanic potentiation at L2/3-corticopontine synapses, but not at commissural-corticopontine synapses, was impaired in the young adult Alzheimer's model mice. These results would provide a knowledge base for comprehending synaptic mechanisms that underlies the initial clinical signs of neurodegenerative disorders.

Control of excitatory hierarchical circuits by parvalbumin-FS basket cells in layer 5 of the frontal cortex: insights for cortical oscillations.

The cortex contains multiple neuron types with specific connectivity and functions. Recent progress has provided a better understanding of the interactions of these neuron types as well as their output organization particularly for the frontal cortex, with implications for the circuit mechanisms underlying cortical oscillations that have cognitive functions. Layer 5 pyramidal cells (PCs) in the frontal cortex comprise two major subtypes: crossed-corticostriatal (CCS) and corticopontine (CPn) cells. Functionally, CCS and CPn cells exhibit similar phase-dependent firing during gamma waves but participate in two distinct subnetworks that are linked unidirectionally from CCS to CPn cells. GABAergic parvalbumin-expressing fast-spiking (PV-FS) cells, necessary for gamma oscillation, innervate PCs, with stronger and global inhibition to somata and weaker and localized inhibitions to dendritic shafts/spines. While PV-FS cells form reciprocal connections with both CCS and CPn cells, the excitation from CPn to PV-FS cells exhibits short-term synaptic dynamics conducive for oscillation induction. The electrical coupling between PV-FS cells facilitates spike synchronization among PV-FS cells receiving common excitatory inputs from local PCs and inhibits other PV-FS cells via electrically communicated spike afterhyperpolarizations. These connectivity characteristics can promote synchronous firing in the local networks of CPn cells and firing of some CCS cells by anode-break excitation. Thus subsets of L5 CCS and CPn cells within different levels of connection hierarchy exhibit coordinated activity via their common connections with PV-FS cells, and the resulting PC output drives diverse neuronal targets in cortical layer 1 and the striatum with specific temporal precision, expanding the computational power of the cortical network.

Segregated Excitatory-Inhibitory Recurrent Subnetworks in Layer 5 of the Rat Frontal Cortex.

A prominent feature of neocortical pyramidal cells (PCs) is their numerous projections to diverse brain areas. In layer 5 (L5) of the rat frontal cortex, there are 2 major subtypes of PCs that differ in their long-range axonal projections, corticopontine (CPn) cells and crossed corticostriatal (CCS) cells. The outputs of these L5 PCs can be regulated by feedback inhibition from neighboring cortical GABAergic cells. Two major subtypes of GABAergic cells are parvalbumin (PV)-positive and somatostatin (SOM)-positive cells. PV cells have a fast-spiking (FS) firing pattern, while SOM cells have a low threshold spike (LTS) and regular spiking. In this study, we found that the 2 PC subtypes in L5 selectively make recurrent connections with LTS cells. The connection patterns correlated with the morphological and physiological diversity of LTS cells. LTS cells with high input resistance (Ri) exhibited more compact dendrites and more rebound spikes than LTS cells with low Ri, which had vertically elongated dendrites. LTS subgroups differently inhibited the PC subtypes, although FS cells made nonselective connections with both projection subtypes. These results demonstrate a novel recurrent network of inhibitory and projection-specific excitatory neurons within the neocortex.

Pyramidal Cell Subtypes and Their Synaptic Connections in Layer 5 of Rat Frontal Cortex.

The frontal cortical areas make a coordinated response that generates appropriate behavior commands, using individual local circuits with corticostriatal and corticocortical connections in longer time scales than sensory areas. In secondary motor cortex (M2), situated between the prefrontal and primary motor areas, major subtypes of layer 5 corticostriatal cells are crossed-corticostriatal (CCS) cells innervating both sides of striatum, and corticopontine (CPn) cells projecting to the ipsilateral striatum and pontine nuclei. CCS cells innervate CPn cells unidirectionally: the former are therefore hierarchically higher than the latter among L5 corticostriatal cells. CCS cells project directly to both frontal and nonfrontal areas. On the other hand, CPn cells innervate the thalamus and layer 1a of frontal areas, where thalamic fibers relaying basal ganglia outputs are distributed. Thus, CCS cells can make activities of frontal areas in concert with those of nonfrontal area using corticocortical loops, whereas CPn cells are more involved in closed corticostriatal loops than CCS cells. Since reciprocal connections between CPn cells with facilitatory synapses may be related to persistent activity, CPn cells play a key role of longer time constant processes in corticostriatal as well as in corticocortical loops between the frontal areas.

Multiple Layer 5 Pyramidal Cell Subtypes Relay Cortical Feedback from Secondary to Primary Motor Areas in Rats

Higher-order motor cortices, such as the secondary motor area (M2) in rodents, select future action patterns and transmit them to the primary motor cortex (M1). To better understand motor processing, we characterized "top-down" and "bottom-up" connectivities between M1 and M2 in the rat cortex. Somata of pyramidal cells (PCs) in M2 projecting to M1 were distributed in lower layer 2/3 (L2/3) and upper layer 5 (L5), whereas PCs projecting from M1 to M2 had somata distributed throughout L2/3 and L5. M2 afferents terminated preferentially in upper layer 1 of M1, which also receives indirect basal ganglia output through afferents from the ventral anterior and ventromedial thalamic nuclei. On the other hand, M1 afferents terminated preferentially in L2/3 of M2, a zone receiving indirect cerebellar output through thalamic afferents from the ventrolateral nucleus. While L5 corticopontine (CPn) cells with collaterals to the spinal cord did not participate in corticocortical projections, CPn cells with collaterals to the thalamus contributed preferentially to connections from M2 to M1. L5 callosal projection (commissural) cells participated in connectivity between M1 and M2 bidirectionally. We conclude that the connectivity between M1 and M2 is directionally specialized, involving specific PC subtypes that selectively target lamina receiving distinct thalamocortical inputs.

Highly Differentiated Projection-Specific Cortical Subnetworks

Pyramidal cells in the neocortex are differentiated into several subgroups based on their extracortical projection targets. However, little is known regarding the relative intracortical connectivity of pyramidal neurons specialized for these specific output channels. We used paired recordings and quantitative morphological analysis to reveal distinct synaptic transmission properties, connection patterns, and morphological differentiation correlated with heterogeneous thalamic input to two different groups of pyramidal cells residing in layer 5 (L5) of rat frontal cortex. Retrograde tracers were used to label two projection subtypes in L5: crossed-corticostriatal (CCS) cells projecting to both sides of the striatum, and corticopontine (CPn) cells projecting to the ipsilateral pons. Although CPn/CPn and CCS/CCS pairs had similar connection probabilities, CPn/CPn pairs exhibited greater reciprocal connectivity, stronger unitary synaptic transmission, and more facilitation of paired-pulse responses. These synaptic characteristics were strongly correlated to the projection subtype of the presynaptic neuron. CPn and CCS cells were further differentiated according to their somatic position (L5a and L5b, the latter denser thalamic afferent fibers) and their dendritic/axonal arborizations. Together, our data demonstrate that the pyramidal projection system is segregated into different output channels according to subcortical target and thalamic input, and that information flow within and between these channels is selectively organized.

Cell Diversity and Connection Specificity between Callosal Projection Neurons in the Frontal Cortex

Recent advances have established that intralaminar and interlaminar excitatory networks between neocortical pyramidal cells are specialized into subnetworks. Here, we have investigated how the commissural system organizes the intracortical excitatory subnetworks to communicate between cortical hemispheres. Whole-cell recordings were obtained from callosal projection neurons [commissural (COM) cells], identified by in vivo injection of retrograde fluorescent tracer into one hemisphere, in rat frontal cortical slices. We found that layer V (L5) COM cells were heterogeneous in physiological and morphological properties that correlated with projection patterns to contralateral and ipsilateral cortical areas. The probability of synaptically connected pairs of L5 COM cells was higher in cell pairs of the same firing subtypes than that in different cell subtype pairs. In interlaminar connections, layer II/III (L2/3) COM cells preferentially innervated L5 COM cells. Moreover, pairs of the same L5 COM subtypes were more likely to share inputs from L2/3 COM cells than were different COM subtype cell pairs. In addition, common inputs from L2/3 COM cells were frequently observed in L5 pairs of corticopontine cells and given firing subtypes of COM cells. Our results suggest that callosal communications are achieved via several distinct COM cell subnetworks differentiated according to the ipsilateral corticocortical and subcortical projection patterns.

Cell diversity and connection specificity between callosal projection neurons in the frontal cortex

Recent advances have established that intralaminar and interlaminar excitatory networks between neocortical pyramidal cells are specialized into subnetworks. Here, we have investigated how the commissural system organizes the intracortical excitatory subnetworks to communicate between cortical hemispheres. Whole-cell recordings were obtained from callosal projection neurons [commissural (COM) cells], identified by in vivo injection of retrograde fluorescent tracer into one hemisphere, in rat frontal cortical slices. We found that layer V (L5) COM cells were heterogeneous in physiological and morphological properties that correlated with projection patterns to contralateral and ipsilateral cortical areas. The probability of synaptically connected pairs of L5 COM cells was higher in cell pairs of the same firing subtypes than that in different cell subtype pairs. In interlaminar connections, layer II/III (L2/3) COM cells preferentially innervated L5 COM cells. Moreover, pairs of the same L5 COM subtypes were more likely to share inputs from L2/3 COM cells than were different COM subtype cell pairs. In addition, common inputs from L2/3 COM cells were frequently observed in L5 pairs of corticopontine cells and given firing subtypes of COM cells. Our results suggest that callosal communications are achieved via several distinct COM cell subnetworks differentiated according to the ipsilateral corticocortical and subcortical projection patterns.

Recurrent Connection Patterns of Corticostriatal Pyramidal Cells in Frontal Cortex

Corticostriatal pyramidal cells are heterogeneous in the frontal cortex. Here, we show that subpopulations of corticostriatal neurons in the rat frontal cortex are selectively connected with each other based on their subcortical targets. Using paired recordings of retrogradely labeled cells, we investigated the synaptic connectivity between two projection cell types: those projecting to the pons [corticopontine (CPn) cell], often with collaterals to the striatum, and those projecting to both sides of the striatum but not to the pons [crossed corticostriatal (CCS) cell]. The two types were morphologically differentiated in regard to their apical tufts. The dendritic morphologies of CCS cells were correlated with their somatic depth within the cortex. CCS cells had reciprocal synaptic connections with each other and also provided synaptic input to CPn cells. However, connections from CPn to CCS cells were rarely found, even in pairs showing CCS to CPn connectivity. Additionally, CCS cells preferentially innervated the basal dendrites of other CCS cells but made contacts onto both the basal and apical dendrites of CPn cells. The amplitude of synaptic responses was to some extent correlated with the contact site number. Ratios of the EPSC amplitude to the contact number tended to be larger in the CCS to CCS connection. Therefore, our data demonstrate that these two types of corticostriatal cells distinct in their dendritic morphologies show directional and domain-dependent preferences in their synaptic connectivity.

Morphology of visual cortical neurons projecting to the pons. A study with intracellular injection of lucifer yellow in the cat.

The aim of this study is to describe the morphological features of the origin cells of the visual corticopontine projection in the cat. To this end, the pontine grey matter of adult cats was injected with Rhodamine Latex Microspheres. The resulting, retrogradely marked cells of the visual cortex, were subsequently injected intracellularly with Lucifer Yellow. The majority of these cells (68%) were layer V pyramidal cells. However, a significant proportion of these retrogradely-labeled cells were atypical (26.5%) and non-pyramidal (5.5%) in their morphology. The majority of the labeled cells were found at the interhemispheric surface. The somatic dimensions of the visual corticopontine cells were on average 22.8 x 17.0 microns. The mean number of basal dendrites of pyramidal cells was 5.3 and their average tangential spread was 296.1 microns. For non-pyramidal cells, the values were 3.2 and 195.6 microns respectively. The main bifurcation of apical dendrites was located on average at 95.0 microns from the soma and their mean tangential spread was 193.9 microns. The depth of the soma was on average 1086.9 microns from the pial surface.

Thermal threshold studies enhance the diagnosis of carpal tunnel syndrome

Subcortical projection structures and thalamic inputs of single neurons in cortical areas 5 and 7 of cat were extracellularly investigated in locally anesthetized, encéphale isolé preparations and chronically implanted, behaving animals. Testing stimulations were applied to the lateralis intermedius (LI), lateralis posterior (LP), ventralis anterior (VA), and centrum medianum (CM) thalamic nuclei, the mesencephalic reticular formation, and the pontine nuclei. Output cells (N = 101) were identified by antidromic invasion, selectively elicited in 91 cells from PN, CM, LI, or LP; the remaining 10 neurons had branching axons to two of the above targets. Two cortical-subcortical circuits involve areas 5 and 7. In one of them, afferents of LI and/or LP origin mainly reach cortico-CM cells and, less often, neurons giving rise to a reciprocal cortico-LI or cortico-LP projection, Within the other circuit, corticopontine cells are synaptically activated from the VA nucleus or convergently modulated from both the VA and LI nuclei. The unexpectedly high proportion of antidromically identified cells suggest that many functions of the parietal “association” cortex depend on downstream projections to the thalamic integrative nuclei and to the pontocerebellar system. Forty-two nonoutput cells were modulated synaptically, especially from the LI nucleus, with high-frequency spike barrages similar to activities described in putative interneurons recorded from other cerebral structures. The thalamically elicited excitation of nonoutput cells occurred immediately after the early discharges induced in output cells, in close time relation with secondary depth-negative waves and with the onset of a long-lasting slow positive shift associated with silenced firing in output cells.

The organisation of fibres within the rat basis pedunculi.

The organisation of corticofugal fibres within the basis pedunculi of rats was studied using wheat germ agglutinin-horseradish peroxidase as an orthograde tracer. Following cortical injections, labelled fibres were distributed within the cerebral peduncle in an orderly way. Fibres which originate from cells in the frontal cortex maintain a position in the ventromedial part of the basis pedunculi. Fibres from the occipital and temporal cortex travel in the most dorsolateral part. Somatosensory fibres travel between these two. The extent of labelled fibres within the peduncles is correlated with the relative density of corticopontine cells arising from different areas of the cerebral cortex.

Organization of cingulo-ponto-cerebellar connections in the cat

This study deals with three different aspects of the organization of connections from the cingulate gyrus to the cerebellum. (1) With the use of wheat germ agglutinin-horseradish peroxidase as a retrograde tracer, the distribution of cingulate neurons projecting to the pontine nuclei was studied. Retrogradely labeled cells were found in layer 5 in all parts of the cingulate gyrus. Average densities of cingulo-pontine cells were similar in the different cytoarchitectonic subdivisions, although some density gradients were observed. The projection was found to be remarkably strong. Average densities of corticopontine cells in the cingulate gyrus ranged from 500-700 cells per mm2 cortical surface, and the total number of neurons was in the range of 75,000-105,000 (n = 4). (2) A topographical organization of terminal fields of fibers originating in different parts of the cingulate gyrus was demonstrated with the combined use of anterograde degeneration and anterograde transport of wheat germ agglutinin-horseradish peroxidase. Terminal fibers originating in different zones of the cingulate gyrus were distributed in a patchy mosaic within a narrow band along the ventromedial aspect of the pontine nuclei. (3) We confirm, with the combined use of lesions in the cingulate gyrus and injections of wheat germ agglutinin-horseradish peroxidase in the ventral paraflocculus, that there is considerable overlap between terminal fibers originating in the cingulate gyrus, and cells retrogradely labeled from the ventral paraflocculus. The role of the ventral paraflocculus as a receiver of "limbic" input is discussed.

Basal ganglia and cerebellum receive different somatosensory information in rats.

There are two great subcortical circuits that relay sensory information to motor structures in the mammalian brain. One pathway relays via the pontine nuclei and cerebellum, and the other relays by way of the basal ganglia. We studied the cells of origin of these two major pathways from the posteromedial barrel subfield of rats, a distinct region of the somatosensory cortex that contains the sensory representation of the large whiskers. We injected tracer substances into the caudate putamen or the pontine nuclei and charted the location of retrogradely filled cortical cells. In preliminary studies, we used double-labeling techniques to determine whether the cells of origin of these two pathways send axon collaterals to other subcortical targets. Lamina V of the rat posteromedial barrel subfield contains two distinct populations of subcortically projecting neurons, which are organized into distinct sublamina. . Corticopontine cells are located exclusively in sublamina Vb, the deeper of two sublamina revealed by cytochrome oxidase staining. Corticostriate cells are located almost exclusively in the more superficial sublamina Va. Experiments using double-labeling fluorescent tracers demonstrate that about one-quarter of the corticopontine cells send a collateral branch to the superior colliculus. Other studies have shown that cells in Vb are activated at very short latency after vibrissal stimulation; hence, they would seem to be an appropriate relay for the rapid transmission of sensory information to the cerebellum for use in sensory guidance of movement.

Statistical analysis of corticopontine neuron distribution in visual areas 17, 18, and 19 of the cat

The spatial organization of visual corticopontine neurons was studied both at a "large scale" (in relation to cortical visual field maps) and at a "small scale" (in relation to cortical modular organization). Large injections of horse-radish peroxidase-wheat germ agglutinin were made in the pontine nuclei. In complete series of sections from parts of areas 17, 18, and 19, the position of each retrogradely labeled neuron was recorded with an x-y plotter connected to the microscope stage. Each cell was thus given a set of x, y, and z coordinates. After alignment of the sections, three-dimensional computer reconstructions of the distribution of the labeled cells were made. With program RPOP (developed by Blackstad and Bjaalie, '88), the reconstructions were studied with different rotations, scaling, etc. In addition, section-independent parts of reconstructions were isolated ("windows") and further analyzed. Curved parts were automatically unfolded for inspection of distribution patterns and determination of cell densities. The spatial distribution of the labeled cells was analyzed within small windows, where density gradients are negligible. We confirm and extend previous demonstrations of a large-scale aggregation of visual corticopontine cells due to density gradients by showing that densities of corticopontine neurons increase linearly as a function of distance from paracentral to lower visual field representations in area 17 (and partly in areas 18 and 19). We demonstrate that density gradients are steeper in area 17 than in area 18. For example, clear-cut differences between the areas in mediolateral density gradients are found. These findings are discussed in relation to the different visual field maps of the areas and the existence of a similar visual field representation in corticopontine projections from different visual areas. The type of small-scale distribution (randomness or non-randomness, aggregation into clusters, bands, etc.) was studied with statistical methods. Such analysis shows that the labeled cells within small zones are non-randomly distributed in all three areas. In most cases, the analysis indicates an aggregated spatial distribution. A possible relationship to the cortical map of direction selectivity is discussed. To our knowledge, this study is the first to combine the use of three-dimensional computer reconstructions of a population of labeled neurons, with subsequent statistical analysis of spatial point (cell distribution) patterns.

Uneven densities of corticopontine neurons in the somatosensory cortex: a quantitative experimental study in the cat

By use of large injections in the pontine nuclei of wheat germ agglutinin-horseradish peroxidase conjugate, the distribution of corticopontine cells in the primary somatosensory cortex (SI) was mapped quantitatively. The borders of the cytoarchitectonic areas 3a, 3b, 1 and 2, together constituting SI, were determined cyto- and myeloarchitectonically. Flat maps showing the distribution of labelled cells in SI were constructed. All labelled cells were confined to lamina V, and present in deep as well as superficial parts of this lamina. The size spectrum of the corticopontine cells seemed to encompass the whole range, from the smallest to the largest seen in lamina V of SI in thionin stained sections. Average densities of labelled corticopontine cells in SI varied from 114 to 248 per mm2 in different cases. No significant differences appear to exist between average densities in areas 3a, 3b, 1 and 2. However, densities vary consistently from medial to lateral within SI. When comparing this pattern with the physiological maps showing the somatotopical organization of SI, it appears that regions representing the trunk and proximal parts of the extremities have higher densities of corticopontine neurons than regions representing distal parts of the extremities and the face. The latter parts of SI have in common a much larger magnification factor than the former, that is, the volume of cortex devoted to a certain area in the periphery is much larger in e.g. the SI face region than in the trunk region. It thus would appear that the over-representation of the face and distal extremities in terms of cortical volume devoted to them, is not upheld in terms of number of corticopontine neurons. Although apparently not paralleled in other connections of SI, the uneven densities of corticopontine projections from SI are very similar to what has been described previously in the corticopontine projections from visual areas 17, 18, and 19, where regions with the largest magnification factors have the lowest densities of corticopontine neurons. On the basis of these findings we suggest that sensory information from proximal body parts and peripheral parts of the visual field is relatively more important for the cerebellum, with its main function in movement control, than it is for parts of the brain engaged in more direct analysis of sensory messages.

Corticopontine projection in the rat: the distribution of labelled cortical cells after large injections of horseradish peroxidase in the pontine nuclei.

The distribution of cortical cells projecting to the pontine nuclei in rats was studied by making large injections of horseradish peroxidase that filled the basilar pons and measuring the density of labelled cells in each cortical area. All retrogradely labelled cells were layer V pyramidal cells. The highest densities of labelled cells were observed in the motor areas. The lowest densities were in temporal association cortex and perirhinal cortex. Visual cortical areas, including the primary visual cortex, provided a major source of pontine projections. The distribution of corticopontine cells within the primary visual cortex was studied in more detail. In all cases the highest density of labelled cells was observed in the region of cortex that represents the nasal visual field. Control injections into brainstem regions adjacent to the pontine nuclei produced a much lower absolute density of retrogradely labelled cortical cells and the distribution of those cells was different from that observed following pontine injections. We conclude that every area of the rat's cerebral cortex projects to the pontine nuclei and that there are consistent variations in the density of the projections both between and within areas.

The corticopontine projection from area 20 and surrounding areas in the cat: Terminal fields and distribution of cells of origin as compared to other visual cortical areas

Visual area 20 in the cat projects to the pontine nuclei and thereby gives input to the cerebellum. The termination of the fibres and the distribution of the cells of origin were studied with anterograde and retrograde transport of horseradish peroxidase-wheat germ agglutinin. Fibres from area 20 terminate ipsilaterally in the medial half of the rostral three-quarters of the pontine nuclei. The labelled fibres occur in multiple well-restricted patches usually some distance away from the peduncle. The retrogradely labelled cells in areas 20a, 20b and the adjacent posterior suprasylvian visual area were quantitatively mapped. Areal borders were placed according to the maps of Tusa and Palmer ( J. comp. Neurol. 193, 147–164) and Updyke ( J. comp. Neurol. 246, 265–280). In terms of cortical densities (cells per mm 2 of cortex) area 20a is among the visual areas with the highest densities of corticopontine neurons (data from other visual areas from the author's previous works). Densities are 30–60% lower in area 20b than in 20a. The posterior suprasylvian visual area was found to be among the visual areas with the lowest densities of corticopontine neurons. Due to the small size of the areas investigated, the total number of corticopontine cells within them is small compared to many other areas. Within areas 20a and 20b, cortical densities are higher in the representation of visual space below than above the horizontal meridian. Furthermore, cortical densities are generally somewhat higher in the region devoted to central vision compared to regions devoted to the visual periphery. Since the part of cortex in areas 20a and 20b devoted to central vision is weakly over-represented also in terms of cortical volume, it follows that the “visual field density” (cells per degree 2 of visual field) of corticopontine cells is highest in the central visual field representation. The finding of an over-representation of central vision compared to peripheral vision in the corticopontine projection from areas 20a and 20b is particularly interesting in conjunction with previous findings in other visual areas. In the cortical representations of central vision, areas with high magnification factors (i.e. areas that greatly emphasize central vision in terms of cortical volume, such as areas 17, 18 and 19) have relatively low cortical densities of pontine projecting cells, whereas areas with low magnification factors (such as areas 20a, 20b and some of the lateral suprasylvian visual areas) have relatively high cortical densities. The “visual field density” of corticopontine neurons appears therefore to be fairly constant from area to area as concerns central vision.

Distribution of corticopontine neurons in visual areas of the middle suprasylvian sulcus: Quantitative studies in the cat

This study deals with the distribution of corticopontine neurons in four of the lateral suprasylvian visual areas, the anteromedial, anterolateral, posteromedial and posterolateral areas. Following large injections of horseradish peroxidase-wheat germ agglutinin in the pontine nuclei, labelled cells were quantitatively mapped. The borders of the four areas were determined cyto- and myeloarchitectonically or from standard retinotopic maps presented in frontal sections (Tusa et al., 1981). 50 Flat maps were constructed of each area showing the distribution of retrogradely labelled neurons. Maps of the retinotopic organization in the lateral suprasylvian visual areas (Palmer et al., 1978; Tusa et al., 1981) 29,50 were transferred to the flat maps. Thus, the density and number of labelled cells within each area and within smaller zones representing different visual field blocks could be determined. The average cell density (average number of labelled cells per mm 2 of flattened cortex) is highest in the anteromedial area. The average density in this area is higher than in areas 17,18 and 19. The total number of labelled cells is highest in the posteromedial area. The lateral syprasylvian visual areas together have a stronger pontine projection in terms of actual number of cells than each of areas 17, 18 and 19. [Data on areas 17, 18 and 19 from Bjaalie and Brodal (1983) and Bjaalie (1985).] 6,8 The four areas investigated have different internal distribution of corticopontine cells. The posteromedial and posterolateral areas have lower densities of labelled cells in the representations of the retinal central area than in the representations of the visual periphery. However, due to the enlarged cortical representations of central vision, the actual number of corticopontine cells per visual field block is higher in the cortex representing the central area; i.e. the central visual field region is moderately over-represented in the corticopontine projection from these areas. In the anteromedial and anterolateral areas, unlike in all other areas so far investigated, the cortical representations of the central visual field contain higher densities of corticopontine cells than the representations of the lower visual periphery. However, since the central area is only very weakly over-represented compared to the lower visual periphery in terms of cortical volume in these areas, the over-representation in the corticopontine projection of central vision compared to the lower visual periphery is fairly moderate. Thus, the over-representation of central vision in terms of number of corticopontine neurons seems to be of a fairly constant magnitude in all visual areas so far investigated (including areas 17, 18 and 19), even if the over-representation in terms of cortical volume varies dramatically between the various cortical visual areas.

Corticopontine projection in the macaque: the distribution of labelled cortical cells after large injections of horseradish peroxidase in the pontine nuclei.

We studied the distribution of corticopontine cells in the monkey cerebral cortex. Horseradish peroxidase (HRP) was injected into the brainstem of monkeys in an attempt to fill the pontine nuclei on one or both sides. In control animals we injected the medullary pyramids or varied the route, size, or location of pontine injections. All retrograde filled corticopontine neurons were layer V pyramidal cells. Corticopontine cells were distributed within a largely continuous area of cortex which extended from the cingulate cortex medially to the sylvian fissure laterally; from the superior temporal fissure caudally to the medial part of the frontal granular cortex rostrally. Areas 4 and 6 of Brodmann (1905) contained the highest density of filled cells. In the primary visual cortex, area 17, there were a few labelled cells restricted to the rostral portion of the upper bank of the calcarine fissure, in a region representing the lower periphery of the visual field. The results are discussed in relation to the possible functions of the corticopontine system, especially the role of the extrastriate visual areas in visually guided movement.