SOME EFFERENT CONNECTIONS OF THE MOTOR AND SOMATOSENSORY CORTEX OF SIMIAN PRIMATES AND FELID, CANID AND PROCYONID CARNIVORES

A comparative anatomical network analysis of the human and chimpanzee brains

Spatial interactions among anatomical elements help to identify topological factors behind morphological variation and can be investigated through network analysis. Here, a whole-brain network model of the chimpanzee (Pan troglodytes, Blumenbach 1776) is presented, based on macroanatomical divisions, and compared with a previous equivalent model of the human brain. The goal was to contrast which regions are essential in the geometric balance of the brains of the two species, to compare underlying phenotypic patterns of spatial variation, and to understand how these patterns might have influenced the evolution of human brain morphology. The human and chimpanzee brains share morphologically complex inferior-medial regions and a topological organization that matches the spatial constraints exerted by the surrounding braincase. These shared topological features are interesting because they can be traced back to the Chimpanzee-Human Last Common Ancestor, 7-10 million years ago. Nevertheless, some key differences are found in the human and chimpanzee brains. In humans, the temporal lobe, particularly its deep and medial limbic aspect (the parahippocampal gyrus), is a crucial node for topological complexity. Meanwhile, in chimpanzees, the cerebellum is, in this sense, more embedded in an intricate spatial position. This information helps to interpret brain macroanatomical change in fossil hominids.

The tracts, cytoarchitecture, and neurochemistry of the spinal cord

The human spinal cord can be described using a range of nomenclatures with each providing insight into its structure and function. Here we have comprehensively reviewed the key literature detailing the general structure, configuration of tracts, the cytoarchitecture of Rexed's laminae, and the neurochemistry at the spinal segmental level. The purpose of this review is to detail current anatomical understanding of how the spinal cord is structured and to aid researchers in identifying gaps in the literature that need to be studied to improve our knowledge of the spinal cord which in turn will improve the potential of therapeutic intervention for disorders of the spinal cord.

Evolving characterization of the human hyperdirect pathway

The hyperdirect pathway (HDP) represents the main glutamatergic input to the subthalamic nucleus (STN), through which the motor and prefrontal cerebral cortex can modulate basal ganglia activity. Further, direct activation of the motor HDP is thought to be an important component of therapeutic deep brain stimulation (DBS), mediating the disruption of pathological oscillations. Alternatively, unintended recruitment of the prefrontal HDP may partly explain some cognitive side effects of DBS therapy. Previous work describing the HDP has focused on non-human primate (NHP) histological pathway tracings, diffusion-weighted MRI analysis of human white matter, and electrophysiology studies involving paired cortical recordings with DBS. However, none of these approaches alone yields a complete understanding of the complexities of the HDP. As such, we propose that generative modeling methods hold promise to bridge anatomy and physiology results, from both NHPs and humans, into a more detailed representation of the human HDP. Nonetheless, numerous features of the HDP remain to be experimentally described before model-based methods can simulate corticosubthalamic activity with a high degree of scientific detail. Therefore, the goals of this review are to examine the experimental evidence for HDP projections from across the primate neocortex and discuss new data which are required to improve the utility of anatomical and biophysical models of the human corticosubthalamic system.

The neural mechanisms of manual dexterity

The hand endows us with unparalleled precision and versatility in our interactions with objects, from mundane activities such as grasping to extraordinary ones such as virtuoso pianism. The complex anatomy of the human hand combined with expansive and specialized neuronal control circuits allows a wide range of precise manual behaviours. To support these behaviours, an exquisite sensory apparatus, spanning the modalities of touch and proprioception, conveys detailed and timely information about our interactions with objects and about the objects themselves. The study of manual dexterity provides a unique lens into the sensorimotor mechanisms that endow the nervous system with the ability to flexibly generate complex behaviour.

How the parcellation theory of comparative forebrain specialization emerged from the Division of Neuropsychiatry at the Walter Reed Army Institute of Research

The Golgi method gave birth to modern neuroscience. The Nauta method, developed in a novel Army think tank at the Walter Reed Army Medical Center, was the next major breakthrough before neuroscience emerged as a separate discipline. Dr. Walle Nauta's (1916-1994) method allowed for the first time the ability to trace interneuronal connections accurately to their termination. The think tank, created by Dr. David Rioch (1900-1985), provided a unique intellectual environment for interdisciplinary neuroscience research, the first of its kind. Rioch hired exceptional senior faculty and recruited outstanding young investigators who were drafted into the Army, typically after finishing their M.D.s or Ph.D.s, and were interested in brain research. Many of these young investigators went on to illustrious careers in neuroscience. I worked with Walle Nauta at a time when the technique was first being applied to nonmammalian vertebrate brains. Along with other Army draftees, I was encouraged to pursue my own research interests. This led me on a quest to understand interspecific variability of connections in relation to evolution and ontogeny of the brain. By 1980, I had found that the variability of all known connections could be explained by a theory to the effect that new structures such as the neocortex were not formed by one system invading another and mingling, as Clarence Luther Herrick (1858-1904) had proposed, but by selective proliferation and differentiation sometimes involving the select loss of connections to reduce cross-modality interference as in the case of the parcellation and differentiation of cortical areas. The resulting parcellation theory predicts that elements of a primordial neocortex existed from the beginning of vertebrate evolution and did not originate by an invasion of nonolfactory modalities into the olfactory lobe, as commonly believed before the introduction of the Nauta method. This theory would not have been created if it were not for the brilliant environment that was Walter Reed in the 1960s.

The Connectivity Fingerprint of the Human Frontal Cortex, Subthalamic Nucleus, and Striatum

Within the cortico basal ganglia (BG)-thalamic network, the direct and indirect pathways comprise of projections from the cortex to the striatum (STR), whereas the hyperdirect pathway(s) consist of cortical projections toward the subthalamic nucleus (STN). Each pathway possesses a functionally distinct role for action selection. The current study quantified and compared the structural connectivity between 17 distinct cortical areas with the STN and STR using 7 Tesla diffusion weighted magnetic resonance imaging (dMRI) and resting-state functional MRI (rs-fMRI) in healthy young subjects. The selection of these cortical areas was based on a literature search focusing on animal tracer studies. The results indicate that, relative to other cortical areas, both the STN and STR showed markedly weaker structural connections to areas assumed to be essential for action inhibition such as the inferior frontal cortex pars opercularis. Additionally, the cortical connectivity fingerprint of the STN and STR indicated relatively strong connections to areas related to voluntary motor initiation such as the cingulate motor area and supplementary motor area. Overall the results indicated that the cortical-STN connections were sparser compared to the STR. There were two notable exceptions, namely for the orbitofrontal cortex and ventral medial prefrontal cortex, where a higher tract strength was found for the STN. These two areas are thought to be involved in reward processing and action bias.

Comparing the function of the corticospinal system in different species: Organizational differences for motor specialization?

An appreciation of the comparative functions of the corticospinal tract is of direct relevance to the understanding of how results from animal models can advance knowledge of the human motor system and its disorders. Two critical functions of the corticospinal tract are discussed: first, the role of descending projections to the dorsal horn in the control of sensory afferent input, and second, the capacity of direct cortico-motoneuronal projections to support voluntary execution of skilled hand and finger movements. We stress that there are some important differences in corticospinal projections from different cortical regions within a particular species and that these projections support different functions. Therefore, any differences in the organization of corticospinal projections across species may well reflect differences in their functional roles. Such differences most likely reflect features of the sensorimotor behavior that are characteristic of that species. Insights into corticospinal function in different animal models are of direct relevance to understanding the human motor system, providing they are interpreted in relation to the functions they underpin in a given model. Studies in non-human primates will continue to be needed for understanding special features of the human motor system, including feed-forward control of skilled hand movements. These movements are often particularly vulnerable to neurological disease, including stroke, cerebral palsy, movement disorders, spinal injury, and motor neuron disease.

Is digital dexterity really related to corticospinal projections?: a re-analysis of the Heffner and Masterton data set using modern comparative statistics

Using a data set of 69 different mammalian species, Heffner and Masterton propose that the longer and deeper the fibres of the corticospinal tract, the greater an animal's digital dexterity. Because of the effects that phylogeny may have upon the extant phenotype of a given species, however, data from a wide range of species can rarely be considered to represent fully independent data points. Using modern comparative statistics, which incorporate phylogenetic information, we reanalysed their data set such that the assumption of independence was not violated. If Heffner and Masterton's hypothesis is correct, then one would expect evidence of strong correlated evolution between corticospinal tract anatomy and digital dexterity once the effects of the phylogenetic relationships between the species in the data set have been removed. The results show that a distinct bias in the number of primate species sampled by Heffner and Masterton significantly affected their findings. Furthermore, once phylogeny has been taken into account, only the length of the corticospinal tract fibres showed a significant relationship with two out of the four behaviours analysed, digital dexterity and hand-eye coordination. Based upon our results we recommend the use of modern comparative statistics for synthesising neural structure and behaviour, rather than examining structure function relationships in an ahistorical context. It is also evident that there is a need for data on the length and depth of the corticospinal fibres for a greater range of species so that the relationship between the corticospinal tract structure and motor behaviour for mammals as a whole can be more readily interpreted.

Upper motor neuron lesions in stroke patients do not induce anterograde transneuronal degeneration in spinal anterior horn cells

BACKGROUND AND PURPOSE

To determine whether upper motor neuron lesions in stroke can cause transneuronal degeneration of lower motor neurons, we assessed spinal anterior horn cells in patients dying with poststroke hemiplegia.

METHODS

Subjects were four stroke patients with severe left hemiplegia and four age-matched control subjects who died of nonneurological disease. After histological processing and staining, cytoarchitectonic assessment was made of all neurons in the ventral horns of the 4th lumbar segment of the spinal cord according to cell diameter and topography.

RESULTS

In the four stroke patients, no differences were seen in anterior horn cell populations or diameter and size distribution patterns between affected and unaffected sides or between these patients and the control subjects.

CONCLUSIONS

The present quantitative analysis provides no evidence of anterograde transneuronal degeneration of lower motor neurons after upper motor neuron damage in stroke patients.

Primary motor cortex influences on the descending and ascending systems

The motor cortex plays a crucial role in the co-ordination of movement and posture. This is possible because the pyramidal tract fibres have access both directly and through collateral branches to structures governing eye, head, neck trunk and limb musculature. Pyramidal tract axons also directly reach the dorsal laminae of the spinal cord and the dorsal column nuclei, thus aiding in the selection of the sensory ascendant transmission. No other neurones in the brain besides pyramidal tract cells have such a wide access to different structures within the central nervous system. The majority of the pyramidal tract fibres that originate in the motor cortex and that send collateral branches to multiple supraspinal structures do not reach the spinal cord. Also, the great majority of the corticospinal neurones that emit multiple intracraneal collateral branches terminate at the cervical spinal cord level. The pyramidal tract fibres directed to the dorsal column nuclei that send collateral branches to supraspinal structures also show a clear tendency to terminate at supraspinal and cervical cord levels. These facts suggest that a substantial co-ordination between descending and ascending pathways might be produced by the same motor cortex axons at both supraspinal and cervical spinal cord sites. This may imply that the motor cortex co-ordination will be mostly directed to motor responses involving eye-neck-forelimb muscle synergies. The review makes special emphasis in the available evidence pointing to the role of the motor cortex in co-ordinating the activities of both descending and ascending pathways related to somatomotor integration and control. The motor cortex may function to co-operatively select a unique motor command by selectively filter sensory information and by co-ordinating the activities of the descending systems related to the control of distal and proximal muscles.

Disease-specific patterns of neuronal loss in the spinal ventral horn in amyotrophic lateral sclerosis, multiple system atrophy and X-linked recessive bulbospinal neuronopathy, with special reference to the loss of small neurons in the intermediate zone

The ventral horn cells of the fourth lumbar segment were morphometrically analysed in six cases of amyotrophic lateral sclerosis (ALS; there common forms and three pseudopolyneuritic forms), six of multiple system atrophy (MSA) with autonomic failure, four of X-linked recessive bulbospinal neuronopathy (X-BSNP), and seven age-matched autopsy cases of non-neurological disorders. In the common form of ALS, large and medium-sized neurons of the medial and lateral nuclei were markedly lost; small neurons in the intermediate zone were slightly diminished but fairly well preserved. In the pseudopolyneuritic form of ALS, marked loss was present in the large and medium-sized neurons, and in the small neurons located in the intermediate zone as well. In the MSA, in contrast to ALS, there was a marked reduction in small neurons in the intermediate zone, and large and medium-sized neurons of the medial and lateral nuclei tended to be preserved. In X-BSNP, large and medium-sized neurons were almost completely lost and small neurons were also markedly depopulated. These findings indicated that the pattern of neuron loss in the ventral horn is distinct among these diseases depending on size, location and function of the ventral horn cell population. These disease-specific patterns of neuron loss suggest a difference in the process of neuronal degeneration of ventral horn cells among the disease examined.

The evolution of the dorsal thalamus of jawed vertebrates, including mammals: cladistic analysis and a new hypothesis

The evolution of the dorsal thalamus in various vertebrate lineages of jawed vertebrates has been an enigma, partly due to two prevalent misconceptions: the belief that the multitude of nuclei in the dorsal thalamus of mammals could be meaningfully compared neither with the relatively few nuclei in the dorsal thalamus of anamniotes nor with the intermediate number of dorsal thalamic nuclei of other amniotes and a definition of the dorsal thalamus that too narrowly focused on the features of the dorsal thalamus of mammals. The cladistic analysis carried out here allows us to recognize which features are plesiomorphic and which apomorphic for the dorsal thalamus of jawed vertebrates and to then reconstruct the major changes that have occurred in the dorsal thalamus over evolution. Embryological data examined in the context of Von Baerian theory (embryos of later-descendant species resemble the embryos of earlier-descendant species to the point of their divergence) supports a new 'Dual Elaboration Hypothesis' of dorsal thalamic evolution generated from this cladistic analysis. From the morphotype for an early stage in the embryological development of the dorsal thalamus of jawed vertebrates, the divergent, sequential stages of the development of the dorsal thalamus are derived for each major radiation and compared. The new hypothesis holds that the dorsal thalamus comprises two basic divisions--the collothalamus and the lemnothalamus--that receive their predominant input from the midbrain roof and (plesiomorphically) from lemniscal pathways, including the optic tract, respectively. Where present, the collothalamic, midbrain-sensory relay nuclei are homologous to each other in all vertebrate radiations as discrete nuclei. Within the lemnothalamus, the dorsal lateral geniculate nucleus of mammals and the dorsal lateral optic nucleus of non-synapsid amniotes (diapsid reptiles, birds and turtles) are homologous as discrete nuclei; most or all of the ventral nuclear group of mammals is homologous as a field to the lemniscal somatosensory relay and motor feedback nuclei of non-synapsid amniotes; the anterior, intralaminar and medial nuclear groups of mammals are collectively homologous as a field to both the dorsomedial and dorsolateral (including perirotundal) nuclei of non-synapsid amniotes; the anterior, intralaminar, medial and ventral nuclear groups and the dorsal lateral geniculate nucleus of mammals are collectively homologous as a field to the nucleus anterior of anamniotes, as are their homologues in non-synapsid amniotes. In the captorhinomorph ancestors of extant land vertebrates, both divisions of the dorsal thalamus were elaborated to some extent due to an increase in proliferation and lateral migration of neurons during development.(ABSTRACT TRUNCATED AT 400 WORDS)

Cortical influences on cervical motoneurons in the rat: recordings of synaptic responses from motoneurons and compound action potential from corticospinal axons

The synaptic responses of cervical motoneurons to intracortical stimulation (ICS) of the motor cortex were studied in the rat by means of intracellular recordings. Motoneurons (n = 80) were identified either by their antidromic response to peripheral nerve electrical stimulation and/or by intracellular staining with biocytin. As a result of ICS (0.6-1.5 mA) of the contralateral motor cortex, the vast majority of motoneurons responded with EPSPs (77 out of 80), while only three motoneurons exhibited IPSPs. For increasing ICS intensities, the amplitude of the EPSPs in a given motoneuron increased, whereas their latency was not substantially affected. For the whole population of motoneurons, identified mainly by their antidromic response, the latency of the EPSPs was on average 8.45 ms (SD 1.6 ms), ranging from 4.7 to 12.6 ms. A very comparable latency distribution was obtained from the subpopulation of biocytin stained motoneurons (n = 23). In 7 of 19 tested motoneurons EPSPs could follow high frequencies (50-100 Hz) of stimulation without change of latency. The compound action potential (descending volley) travelling along corticospinal fibers reached the level of intracellular recording with a minimal latency estimated to be about 3 ms after ICS. The conduction velocity of corticospinal axons contributing to the descending volley was calculated to range from 9 to 19.7 m/s, based on morphometric measurements of conduction distance from the motor cortex and duration of the compound action potential. The time delay between the latency of descending volley and the latency of early EPSPs on the one hand, and frequency following properties of EPSPs on the other hand, suggest that some cervical motoneurons receive secure, most likely, indirect (presumably disynaptic) inputs from fast conducting corticospinal axons or direct contacts from slower conducting corticospinal fibers. The biocytin labeled cervical motoneurons exhibited extraordinary long dendritic trees, extending both laterally in the white matter near the edge of the spinal cord and medially in the gray matter as far as the midline of the spinal cord. The motoneurons were also characterized by the presence of one or several recurrent axon collaterals, ramifying profusely in the neuropil, with numerous boutons en passant and terminaux contacting most likely neighboring cervical neurons.

Corticomotoneuronal connections in the rat: evidence from double-labeling of motoneurons and corticospinal axon arborizations

In order to investigate the possibility of direct corticomotoneuronal (CM) connections in the rat, an anterograde-retrograde double-labeling method was developed. Phaseolus vulgaris-leucoagglutinin (PHA-L) anterograde tracing of corticospinal axons was combined with retrograde labeling of spinal motoneurons either by a conjugate of choleragen subunit B with horseradish peroxidase (CB-HRP) or by wheat germ agglutinin (WGA). The location of PHA-L injection unilaterally in the forelimb area of sensorimotor cortex and the CB-HRP or WGA injections in corresponding contralateral wrist or digit extensors or flexors were determined and matched on the basis of movement responses elicited by intracortical microstimulation. Light microscopic observation showed, in addition to the main contralateral dorsal corticospinal tract (CST), the presence of four other CST minor components in the contralateral lateral, ipsilateral ventral, and ipsilateral dorsal funiculi of the cervical spinal white matter and at the base of contralateral dorsal horn of the gray matter, respectively. PHA-L-labeled CST axonal arbors were observed from Rexed's lamina I through lamina X of contralateral spinal gray matter, most extensively in laminae VI and VII; some CST axons reached the zone of motoneuronal somata in lamina IX and a few of them also entered the lateral and occasionally the ventral funiculi, ramifying in the white matter. Between the zones of PHA-L-labeled CST axonal arbors on the one hand and CB-HRP/WGA labeled spinal motoneuronal somata with their extensive dendritic trees on the other, there was a large overlap, covering partly both the gray and the white matter. PHA-L-labeled axonal boutons (en passant or terminaux) were seen to contact the dendrites or even the somata of motoneurons in the gray matter, according to light-microscopic criteria for identification of synaptic contacts. Axodendritic CM contacts were occasionally observed in the lateral funiculus of the white matter as well. In general, only a single contact was observed between an individual PHA-L-labeled CST axon and a given retrogradely labeled motoneuron. In contrast to the common notion that direct CM connections are a specialty of primates, the present morphological data support the presence of direct CM connections also in some other mammals, such as the rat.

Motor outflow to cervical motoneurons from raccoon motorsensory cortex

Motor outflow from forelimb motorsensory cortex (MsI) to forelimb muscle motoneurons in raccoon has been investigated using three approaches: 1) determination of latencies for cortically evoked efferent discharge in forelimb nerves; 2) determination of latencies for cortical facilitation of forelimb monosynaptic reflexes; and 3) intracellular recording of cortically evoked synaptic potentials. All three approaches indicated a major polysynaptic pathway (minimally disynaptic) for corticofugal facilitation or inhibition of cervical motoneurons. Suggestive evidence for a monosynaptic connection between forelimb MsI and cervical motoneurons was found for only one motoneurons. Nevertheless, the motor pathway between MsI and cervical motoneurons was shown to be more efficacious (defined on the basis of central delays) than in the cat under similar experimental conditions. The results are discussed in terms of organization changes in forelimb MsI which appear to be related to the extent to which certain mammals use their forelimbs for manipulating and exploring objects.

Raccoon forelimb motorsensory cortex: I. Somatic afferent inputs to different cytoarchitectonic areas

The distribution of potentials evoked in and around forelimb MsI by graded electrical stimulation of forelimb nerves has been studied in the raccoon (Procyon lotor). These data have been correlated with cytoarchitectonic characteristics of pericruciate cortical tissue. Potentials evoked by cutaneous nerve stimulation were widely distributed in MsI and SmI, but were smaller in amplitude and of longer latency in MsI than in SmI. Stimulation of ulnar, median or deep radial nerve at 1-1.4T, a strength considered to activate only Group I muscle afferent fibers, caused evoked potentials in a localized region mostly confined to posterior sigmoid gyrus. On the basis of cytoarchitectonic features it is concluded that: a) Anterior sigmoid gyrus, to near the level of the tip of the cruciate sulcus, is area 6 cortex; b) The lateral portion of the posterior sigmoid gyrus, cortex comprising the caudal bank of the cruciate sulcus and cortex surrounding the lateral tip of the cruciate sulcus is area 4 cortex; c) The middle portion of the posterior sigmoid gyrus, almost to the lip of the cruciate sulcus rostrally and extending onto the rostral bank of the ascending coronal and postcruciate sulci caudally, is area 3a cortex. The cortical focus for Group I afferent-evoked potentials coincides with area 3a cortex. It is concluded that forelimb MsI of raccoon is organized in a fashion similar to MsI of cats and monkeys.

Afferent connections of the subthalamic nucleus: a combined retrograde and anterograde horseradish peroxidase study in the rat

A comprehensive characterization of the afferent connections of the subthalamic nucleus of Luys (STN) is a necessary step in the unraveling of extrapyramidal mechanisms. In the present study, the STN afferents in the rat were systematically investigated with the aid of retrograde and anterograde horseradish peroxidase tracer techniques. The results indicate that, besides a massive input from the dorsal pallidum, the STN receives substantial projections from several districts of the cerebral cortex (the medial division of the prefrontal cortex, the first motor and primary somatosensory areas, and the granular insular territory), the ventral pallidum, the parafascicular nucleus of the thalamus and the pedunculopontine tegmental nucleus, as well as a modest innervation from the dorsal raphe nucleus. In spite of the fact that many additional structures were found to contain retrogradely labeled neurons after tracer injections in the STN, no other projection to the latter nucleus could be effectively established in our anterograde experimental series.

A comparative review of the primate motor system

Primates have evolved separately from other mammals since the late Cretaceous, and during this time the two major extant primate groups, prosimians (lorises, lemurs, and tarsiers) and anthropoids (monkeys, apes, and humans) arose. Concurrently, structures within the central nervous system acquired primate characteristics. Not all of the uniquely primate features have been identified in the brain, but several are well known. The pyramidal system, the best studied motor system, shows a distinct primate pattern in its terminal connections in the spinal cord. Other descending systems are less well known, but primate specializations in the vestibular system and red nucleus have been observed. The primary and secondary motor cortices are topographically separated in primates, suggesting one basis for increased complexity. Given the size of the brain, structures in the basal ganglia are relatively enlarged in primates as compared with other mammals, whereas the cerebellum has the same relative size.

Descending pathways to the spinal cord: a comparative study of 22 mammals

In order to estimate the qualitative commonalities and range of variation among major descending spinal pathways relevant to mankind's ancestral lineage, the supraspinal cell groups originating fibers descending directly to the spinal cord were examined in 22 mammalian species. In a standardized retrograde tract-tracing procedure, flakes of raw HRP were applied directly to the freshly cut fibers of the spinal cord after it had been hemisected at the C1-C2 junction. After a 72-hour survival period, brain and spinal cord tissues were processed by conventional HRP-processing techniques. This procedure was performed on 94 individual animals. Of this total, 41 individual cases were eliminated by a rigorous culling procedure. The results are based on 53 individuals representing 15 species selected for their successive kinship with mankind and seven species in two other lineages selected for the convergence of their visual or sensorimotor systems with anthropoids. The 22 species represent 19 genera, 14 families, eight orders, and two subclasses of Mammalia. The results show that at least 27 supraspinal cell groups, each containing intensely labeled cells, can be readily identified in each of the species. Despite vast quantitative differences in cell number and cell size, this qualitative uniformity among the relatively large number of diverse taxa suggests that the same pathways were probably present in the extinct ancestors throughout mankind's mammalian lineage and are probably still present in extant viviparous mammals as well. If so, these pathways are as old in phylogenetic history as the last common ancestor of marsupial and placental mammals--dating from the late Jurassic to early Cretaceous, perhaps 145-120 million years ago. Further comparison of the results with similar experimental findings in members of other vertebrate classes supports the notion that several of these same pathways can be traced to even more remote ancestry, with some possibly as old as the entire vertebrate subphylum--dating from the early Devonian or before, perhaps 430 million years ago. Within mankind's ancestral lineage, from the appearance of vertebrates to the appearance of mammals, there seems to have been an irregular stepwise augmentation of the set of descending pathways until the full mammalian complement was finally attained with the appearance of the corticospinal tract.

Somatosensory inputs to the subthalamic nucleus: a combined retrograde and anterograde horseradish peroxidase study in the rat

Previous physiological studies have shown that neurons in the subthalamic nucleus (STN) respond to peripheral somatosensory stimulation. In an attempt to identify anatomical pathways that could mediate such responses, the possible existence of direct projections from somatosensory central territories to the STN was investigated in the rat with the aid of retrograde and anterograde horseradish peroxidase tracer techniques. Our main findings indicate the existence of a hitherto undescribed and relatively substantial direct projection from the primary somatosensory cortex to the ipsilateral STN. The projection appears to originate chiefly from neurons in layer Vb of the rostral half of this cortical area and to terminate basically in the dorsolateral district of the STN. Moreover, our data are compatible with the existence of very sparse direct projections from the spinal trigeminal and dorsal column nuclei to the contralateral STN, but the evidence on this point is hardly conclusive.

Thalamic connectivity of rat somatic motor cortex

The purpose of this study was to investigate certain organizational features regarding the connectional relationships between the somatic motor cortex (SMI) and dorsal thalamus in the rat. This was accomplished by applying microiontophoretically horseradish peroxidase and tritiated amino acids into low threshold stimulation sites within hindlimb, forelimb and face representations of the SMI as defined by intracortical microstimulation. Injections into the SMI produced labeling in distinct sets of specific and non-specific thalamic nuclei. The former included the ventrolateral (VL), ventromedial (VM), posteromedial (Pom) and posterior (Po) nuclei, and the latter included the centrolateral (CL), paracentral (PC), centromedial (CeM) and parafascicular (PF) nuclei. The densest labeling, both retrograde and anterograde, was found in the VL, and to a slightly lesser extent, in the Pom nuclei. Labeling of specific nuclei was more extensive than in non-specific groups. Thalamic-SMI projections were reciprocal in nature with the exception of the reticular thalamic nucleus (R). The ventrobasal (VB) nucleus was labeled only after injections into the SMI representations of the hindlimb and forelimb. A topographic organization between the SMI and dorsal thalamic nuclei was indicated by the varied position of label following injections into different cortical representations. Injections into the hindlimb representation resulted in labeling in the rostrodorsolateral VL, ventrolateral VM, rostrodorsomedial Pom, dorsal Po, rostrodorsolateral R, dorsolateral PF, CL and PC. In contrast, injections into the face representation resulted in labeling in the caudoventromedial VL, dorsolateral VM, caudoventrolateral Pom, ventral Po, caudoventromedial R, ventrolateral PF, the PC, CeM and ventral CL. The position of thalamic labeling following injections into the forelimb representation was intermediate between that for hindlimb and face representations. In coronal section, distinct patterns of labeling were identified in all specific thalamic nuclei following injections into each topographic region of the SMI. In the VL, labeling appeared as an obliquely-oriented, longitudinal strip or band (more C-shaped following injections of the hindlimb representation); in the VM as a horizontal strip; in the Pom as an irregular V-shaped band; and in the Po as a circular-to-ovoid cluster. No distinct patterns of labeling were discerned in non-specific nuclei. Labeling in the anterior grouping of non-specific nuclei overlapped to a large extent, but this was not the case in either the specific thalamic nuclei or the PF nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological study of the cortical-induced excitation of subthalamic nucleus neurons in the rat: evidence for amino acids as putative neurotransmitters

Extracellular records were made from subthalamic nucleus neurons during microiontophoretic application of drugs and stimulation of the corticosubthalamic nucleus pathway. In 87% of the subthalamic nucleus cells, cortical stimulation induced a powerful excitation, consisting of a burst of 1-7 spikes. This projection must arise from a large area of the cortex since stimulation of nearly all the ipsilateral cortex and the rostral two-thirds of the contralateral cortex was found to influence the activity of subthalamic nucleus neurons. Experiments were undertaken in order to determine the identity of the neurotransmitter involved in the corticosubthalamic nucleus pathway. Glutamic acid diethyl ester reversibly suppressed subthalamic nucleus excitations induced by ipsi- or contralateral cortical stimulation or microiontophoretically applied glutamate. On the same cells, this compound had no effect on acetylcholine-evoked excitation and gamma-aminobutyric acid-evoked inhibition and subthalamic excitation induced by stimulation of the tegmenti pedunculopontine nucleus. Atropine at doses which antagonized the acetylcholine response, flupenthixol at dose which antagonized the dopamine response, and bicuculline at doses which antagonized the gamma-aminobutyric acid response failed to block excitations evoked by cortical stimulation and by glutamate. These experiments excluded a role for acetylcholine, dopamine and gamma-aminobutyric acid in the cortically evoked excitation of subthalamic nucleus cells. Since an amino acid seemed to play a role as neurotransmitter of the corticosubthalamic nucleus pathway, further experiments were designed to confirm these data and to determine the contribution of each amino acid receptor type in the cortical-induced excitation of subthalamic cells. All the subthalamic cells recorded were also excited by microiontophoretically applied N-methyl-D-aspartic, quisqualic and kainic acids. The cortical-evoked activation of subthalamic nucleus neurons was reversibly suppressed by kynurenic acid and cis-2,3-piperidine dicarboxylic acid, two broad-spectrum antagonists of excitatory amino acids, microiontophoretically applied at doses which also blocked excitations induced by N-methyl-D-aspartic, quisqualic and kainic acids. Application of 2-amino-5-phosphonovaleric acid inhibited excitation induced by N-methyl-D-aspartic acid but not those elicited by quisqualic or kainic acid, while glutamate excitation was only slightly affected. This compound had no effect on the cortically evoked excitation of subthalamic nucleus neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferent connections of the nuclei reticularis pontis oralis and caudalis: a horseradish peroxidase study in the rat

The afferent connections of the nuclei reticularis pontis oralis and caudalis were studied experimentally in the rat by the aid of either free horseradish peroxidase or horseradish peroxidase conjugated with wheat germ agglutinin used as retrograde tracers. The results suggest that the nucleus reticularis pontis oralis receives its main input from the zona incerta and field H1 of Forel, the superior colliculus, the central gray substance, and the mesencephalic and magnocellular pontomedullary districts of the reticular formation. Many other structures seem to represent modest additional sources of projections to the nucleus reticularis pontis oralis; these structures include numerous cortical territories, the nucleus basalis, the central amygdaloid nucleus, hypothalamic districts, the anterior pretectal nucleus, the substantia nigra, the cuneiform, the accessory oculomotor and the deep cerebellar nuclei, trigeminal, parabrachial and vestibular sensory cell groups, the nuclei raphe dorsalis and magnus, the locus coeruleus, the dorsolateral tegmental nucleus, and the spinal cord. While the afferentation of the rostral portion of the nucleus reticularis pontis caudalis appears to conform to the general pattern outlined above, some deviations from that pattern emerge when the innervation of the caudal district of the nucleus reticularis pontis caudalis is considered; the most striking of these differences is the fact that both spinal and cerebellar inputs seem to distribute much more heavily to the referred caudal district than to the remaining magnocellular pontine reticular formation. The present results may contribute to the elucidation of the anatomical substrate of the functionally demonstrated involvement of the nuclei reticularis pontis oralis and caudalis in several domains that include the regulation of the sleep-waking cycle and cortical arousal, somatic motor mechanisms and nociceptive behavior.

Increase in glutamate sensitivity of subthalamic nucleus neurons following bilateral decortication: a microiontophoretic study in the rat

Responses of subthalamic neurons (STN) to the iontophoretic application of glutamate (Glu), acetylcholine (ACh) and GABA were studied in rats 2 or 3 weeks following bilateral decortication, and were compared to those obtained in normal animals. All the cells recorded in lesioned rats showed a decrease in their spontaneous firing rate. They also proved to be significantly more sensitive to the excitatory action of Glu, although their responsiveness to ACh or GABA was not modified.

Afferent and efferent connections of the dorsolateral precentral gyrus (area 4, hand/arm region) in the macaque monkey, with comparisons to area 8

The afferent and efferent connections of the dorsolateral precentral gyrus, the primary motor cortex for control of the upper extremity, were studied by using the retrograde and anterograde capabilities of the horseradish peroxidase (HRP) technique in three adult macaque monkeys that had received HRP gel implants in this cortical region. Reciprocal corticocortical connections were observed primarily with the supplementary motor area (SMA) in medial premotor area 6 and dorsal bank of the cingulate sulcus, postarcuate area 6 cortex, dorsal cingulate cortex (area 24), superior parietal lobule (area 5, PE/PEa), and inferior parietal lobule (area 7b, PF/PFop, including the secondary somatosensory SII region). In these heavily labeled regions, the associational intrahemispheric afferents originated primarily from small and medium sized pyramidal cells in layer III, but also from layer V. The SMA projections were columnar in organization. Intrahemispheric afferents from contralateral homologous and nonhomologous frontal and cingulate cortices also originated predominantly from layer III, but the connections from contralateral area 4 were almost exclusively from layer III. The bilateral connections with premotor frontal area 6 and cingulate cortices were not observed with parietal regions; i.e., only ipsilateral intrahemispheric parietal corticocortical connections were observed. There were no significant connections with prearcuate area 8 or the granular frontal (prefrontal) cortex. Subcortical afferents originated primarily from the nucleus basalis of Meynert, dorsal claustrum, ventral lateral (VLo and VLc), area X, ventral posterolateral pars oralis (VPLo), central lateral and centromedian thalamic nuclei, lateral hypothalamus, pedunculopontine nucleus, locus ceruleus and subceruleus, and superior central and dorsal raphe nuclei. Lesser numbers of retrogradely labeled neurons were observed in the nucleus of the diagonal band, mediodorsal (MD), paracentral, and central superior lateral thalamic nuclei, nucleus limitans, nucleus annularis, and the mesencephalic and pontine reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Afferents to the zona incerta in the rat: a combined retrograde and anterograde study

In a first set of experiments, the retrograde transport of horseradish peroxidase (HRP) was utilized to investigate the afferent projections to the zona incerta (ZI) in the hooded rat. HRP was introduced in its crystalline form into various sectors of the ZI of seven subjects. The largest contingent of afferents arises from the following centers: the cingulate and somatosensory cortices, central amygdaloid nucleus, ventromedial hypothalamic nucleus, posterior thalamic nucleus, anterior pretectal nucleus, peripeduncular area, deep and intermediate layers of the superior colliculus, dorsal and ventral parabrachial nuclei, principal and interpolar trigeminal subnuclei, and cuneate nucleus. Other centers less systematically or more sparsely labeled were the lateral hypothalamic area, ventrobasal complex, lateral geniculate nucleus pars ventralis, medial geniculate nucleus, interstitial nucleus of Cajal, Darkschewitsch nucleus, perirubral fields, cuneiform, tegmental pedunculopontine, and deep mesencephalic reticular nuclei, pontine reticular nucleus pars oralis, lateral and interpositus cerebellar nuclei, and gracile nucleus. In a second set of experiments, an anterograde tracer (WGA-HRP) was injected in several centers projecting to the ZI in order to localize their terminal fields within this structure. It has been thus possible to distinguish a ventral zone (ventral sector of pars caudalis and pars ventralis) in which the somesthetic (somatosensory cortex, trigeminal complex, and dorsal column nuclei (DCN), collicular, and cerebellar projections terminate, from a dorsal zone (pars dorsalis) to which a limbic input (cingulate cortex and ventromedial hypothalamic nucleus) is directed. In most cases, the labeled terminal fields consisted of well-delimited, narrow bands disposed obliquely, parallel to the cerebral peduncle or the internal capsule. The contingent of somatosensory afferents is relatively large and there is a high degree of overlapping between the different somatosensory terminal fields within the ventral ZI. This suggests a participation of this structure in the treatment of somesthetic information and/or in the transmission of noxious stimuli.

Light microscopic analysis of Golgi-impregnated rat subthalamic neurons

The neuronal morphology of the rat subthalamic nucleus (STH) was studied using Golgi techniques and Nissl stain. The results show that the somatic shapes of STH neurons vary from fusiform to oval or polygonal. Somatic cross-sectional areas vary between 140 microns2 and 440 microns2. Some of the cells have a few somatic spines. Two to six primary dendrites gave rise to tapering daughter dendrites which extend up to 500 microns. These dendrites are sparsely covered with spines. Some distal dendrites and primary dendrites of the STH also bear filiform appendages. Neurons located in the deep portion of the STH have oval dendritic fields whose long axis is parallel to the long axis of the nucleus in frontal or sagittal planes. Some of these neurons have one or two dendrites which cross the borders of the STH into the zona incerta, the lateral hypothalamus, or the cerebral peduncle. Generally, neurons located at the borders of the STH have their dendritic fields extending parallel to the borders and are confined to the nucleus. However, some neurons adjacent to the ventrolateral border of the nucleus have some dendrites extending into the cerebral peduncle. Quantitative analysis of the STH neurons showed a unimodal distribution of somatic sizes as well as the number of primary dendrites. No neurons with obvious Golgi type II characteristics were found. Two types of afferent fibers were observed entering the STH. One type consists of axon collaterals arising from the cerebral peduncle ventrolaterally, or the internal capsule rostrally, while the other enters the nucleus after crossing the internal capsule rostrally. These results suggest that the rat STH is an open nucleus in contrast to other species such as man, monkey, and cat, where it is closed, and that the rat STH may contain only one type of neuron.

Topographical projections of the cerebral cortex to the subthalamic nucleus

Corticosubthalamic projections in the rat were investigated using the autoradiographic anterograde axonal tracing technique. After unilateral injections of tritiated amino acids in the cerebral cortex, projections to the ipsilateral subthalamic nucleus (STH) could be found arising only from the frontal agranular cortex and the zone of MI-SI overlap. Injections into granular areas of the cortex (e.g., somatosensory and visual areas) did not result in labeling in STH. Following injections in the frontal agranular cortex, labeling was present in the ipsilateral but not the contralateral STH. In general, injections that involved the lateral agranular field of frontal cortex, as defined by Donoghue and Wise ('82), resulted in a greater amount of labeling in STH than injections within the medial agranular area or the zone of MI-SI overlap. The projection from the frontal agranular areas to STH is topographically organized. The rostral part of the lateral agranular cortex projects to the lateral portion of the rostral two-thirds of STH, and the caudal part of this field projects to the ventral aspect of the middle third of STH. Injections in the rostral part of the medial agranular cortex resulted in labeling throughout the ventral two-thirds of the medial half of STH. The caudal part of the medial agranular cortex projects to the dorsolateral part of the caudal two-thirds of STH. The present results reveal projections from only the frontal agranular cortex and the zone of MI-SI overlap to STH in the rat. The cortico-STH projection is ipsilateral and terminates in a topographical manner in all parts of STH.

Efferent connections of the centromedian and parafascicular thalamic nuclei: an autoradiographic investigation in the cat

The efferent projections of the centromedian and parafascicular (CM-Pf) thalamic nuclear complex were analyzed by the autoradiographic method. Our findings show that the CM-Pf complex projects in a topographic manner to specific regions of the rostral cortex. These fibers distribute primarily to cortical layers I and III; however, the projection to layer I is more extensive. Following an injection into the rostral portion of the CM-Pf complex, label is found within the lateral rostral cortex, particularly within the presylvian, anterior ectosylvian, and anterior lateral sulci, and within the rostral medial cortex where label is present within the cruciate and anterior splenial sulci and anterior cingulate gyrus. An injection into the caudal dorsal portion of the CM-Pf complex results in label within the more ventral portions of the rostral lateral cortex where it is present within the anterior sylvian gyrus, presylvian regions, and gyrus proreus; and within the rostral medial cortex, where it is present within the rostral cingulate gyrus, and within the cruciate sulcus, and an extensive region ventral to the cruciate sulcus which includes the anterior limbic area. Injections into the caudal ventral portion of the CM-Pf complex result in virtually no cortical label, although a few labeled fibers are found in the subcortical white matter. The subcortical projection from the CM-Pf complex terminates within the caudate nucleus, putamen, globus pallidus, subthalamic nucleus, zona incerta, fields of Forel, hypothalamus, thalamic reticular nucleus, and rostral intralaminar nuclei. Prominent silver grain aggregates are also present within the ventral lateral, ventral anterior, ventral medial, and lateral posterior nuclei, and ventrobasal complex. The aggregates in the thalamus appear to be fibers of passage, but whether these are also terminals cannot be determined with the techniques used in the present study.

Bilateral corticosubthalamic nucleus projections: an electrophysiological study in rats with chronic cerebral lesions

The present study sought to determine the distribution of the cortical areas giving rise to the corticosubthalamic nucleus projections, using extracellular stimulating and recording techniques in rats with and without chronic lesions. In acute rats, cortical stimulation induced a powerful excitation in 87% of the subthalamic nucleus cells recorded. This response was obtained from stimulation over a large extent of the cortex since nearly all the ipsilateral cortex and the rostral two-thirds of the contralateral side was found to influence the activity of the subthalamic nucleus neurones. An excitatory response quite similar to that induced by cortical stimulation was recorded in the subthalamic nucleus after striatal or internal capsule stimulations. Therefore in order to eliminate the possibility of recording a polysynaptic excitation, similar experiments were performed in rats bearing various chronic lesions. With either ipsilateral or contralateral cortical stimulations, there was no major consequence of these lesions on the type or characteristics of the response recorded or on the percentage of responding cells. The cortical origin of the excitation of the subthalamic neurones was further supported by the results of experiments performed in chronically decorticated rats. It is concluded that (1) the subthalamic nucleus receives an excitatory cortical input, (2) this control comes from ipsilateral and contralateral cortical areas and (3) it only involves direct corticosubthalamic nucleus fibres. The subthalamic nucleus is, together with the striatum, the only basal ganglia nucleus known to receive afferents from extensive regions of the cortex. By its two main afferents (cortex and external segment of the pallidum), the subthalamic nucleus is in a position to compare direct cortical informations with cortical informations processed at the striatopallidal complex level.

Afferent connections of the zona incerta: a horseradish peroxidase study in the rat

Restricted microelectrophoretic injections either of free horseradish peroxidase or of horseradish peroxidase conjugated with wheat germ agglutinin were given to albino rats in order to study the afferent connections of structures of the subthalamic region. The results suggest that the zona incerta receives its main input from several territories of the cerebral cortex, the mesencephalic reticular formation, deep cerebellar nuclei, regions of the sensory trigeminal nuclear complex and the dorsal column nuclei. Substantial input to the zona incerta appears to come from the superior colliculus, the anterior pretectal nucleus and the periaqueductal gray substance, whereas many other structures, among which hypothalamic nuclei, the locus coeruleus, the raphe complex, the parabrachial area and medial districts of the pontomedullary reticular formation, seem to represent relatively modest but consistent additional input sources. The afferentation of neurons in Forel's fields H1 and H2 appears to conform to the general pattern outlined above. As pointed out in the Discussion, the present results provide hodological support for the classic concept according to which the zona incerta can be regarded as a rostral extent of the midbrain reticular core. Some of the possible physiological correlates of the fiber connections of the zona incerta in the context of the sleep-waking cycle, ingestive behaviors, somatic motor mechanisms, visual functions and nociceptive behavior are briefly discussed.

Influences of pyramidal tract on the subthalamic nucleus in the cat

In adult cats, with mesencephalic decerebration sparing the cerebral peduncles and ablation of the sensorimotor cortex, changes in firing of single cells of subthalamic nucleus (STN) were analyzed upon stimulation of ipsilateral medullary pyramidal tract (PT). Twenty-two out of 44 of the STN cells exhibited, following PT stimulation, discharge changes that in the greatest part of cases (91%) were excitatory in nature. Excitations, always followed by inhibitory rebound, appeared with latency values compatible with a monosynaptic linkage.

Collateral innervation of the medial and lateral prefrontal cortex by amygdaloid, thalamic, and brain-stem neurons

The distribution of the afferents to the rat's prefrontal cortex originating in the thalamic mediodorsal nucleus and the amygdala was investigated with two fluorescent tracers. Special emphasis was laid on detecting the loci of neurons which project via axonal collaterals into both lateral and medial portions of the prefrontal cortex. It was found that a high number of neurons of the anterior portion of the basolateral amygdaloid nucleus terminate via collaterals in both the medial and lateral subfields of the prefrontal cortex. On the other hand, only a small number of mediodorsal thalamic cells were found to project to both sides of the prefrontal hemisphere via bifurcating axonal collaterals. These cells were situated exclusively in the lateral part of the medial segment of the mediodorsal nucleus. The majority of both thalamic and amygdaloid neurons with bifurcating axons originate from subregions whose cells innervate primarily the medial prefrontal cortex. In brain-stem, neurons of the nucleus raphé dorsalis also project via collaterals to the medial and lateral prefrontal regions. Furthermore, neurons of the dorsal and ventral premamillary nuclei, the lateral mamillary nucleus, the ventral tegmental area of Tsai, and the ventral tegmental nucleus of Gudden were found to project to the medial prefrontal cortex. Our results indicate a differential collateral organization of thalamic and amygdaloid afferents to prefrontal cortical fields. The anterior basolateral amygdala (which innervates via collaterals both the medial and lateral prefrontal subfields) may add a common input to either subfield, such as information on the significance of incoming stimuli to the animal's behavior, while the mediodorsal nucleus (whose segments are principally connected to only one prefrontal subfield) may add segment-specific information, for example, of a spatial-cognitive nature for the lateral segment and of an emotional nature for the central and medial segments. The existence of a basolateral limbic circuit, composed of the amygdala, the thalamic mediodorsal nucleus, and the prefrontal cortex, is confirmed and knowledge on its interconnectivity is extended. From an anatomical point of view these data provide arguments for both unitary and diverging functions of the prefrontal cortex.

The morphology of intracellularly labeled rat subthalamic neurons: a light microscopic analysis

Light microscopic analysis of rat subthalamic (STH) neurons which were intracellularly labeled with horseradish peroxidase, following the acquisition of electrophysiological data, revealed the following: (1) The somata of STH neurons were polygonal or oval with occasionally a few somatic spines. Usually three or four primary dendrites arose from the soma. Dendritic trunks tapered slightly and divided into long, thin, sparsely spined branches. Dendrites of some STH neurons extended into the cerebral peduncle. (2) Reconstruction of the dendritic field was made in three different planes. In either sagittal or frontal planes, the dendritic field was usually oval and the long axis was parallel to the main axis of STH. In the horizontal plane, the dendritic field of all neurons was polygonal. (3) The axons of all the neurons analyzed originated from the soma and were traced beyond the borders of STH, thus indicating that they were projection neurons. All the parent axons bifurcated at least once. After bifurcation, one axon branch coursed dorsolaterally within the cerebral peduncle and terminated in the globus pallidus. The other branch coursed caudally or mediocaudally and arborized in the substantia nigra. Frequently, the axon branches projecting toward the globus pallidus emitted fine axon collaterals within the entopeduncular nucleus. (4) About one-half of the analyzed STH neurons had intranuclear axon collaterals. The neurons with intranuclear collaterals had a higher dendritic tips/stems ratio than neurons without intranuclear collaterals. This observation indicated that STH neurons could be divided into two groups according to their axonal morphology. (5) The axonal terminal arborization observed in all the target sites (i.e., globus pallidus, entopeduncular nucleus, STH, and substantia nigra) were formed with varicose collateral branches which also gave rise to short filaments with beaded endings. Some of these projection neurons could therefore communicate with the target neurons in the globus pallidus, substantia nigra, entopeduncular nucleus, as well as STH through their collateral system.

Pallidal inputs to subthalamus: intracellular analysis

Neuronal responses of the subthalamic nucleus (STH) to stimulation of the globus pallidus (GP) and the substantia nigra (SN) were studied by intracellular recording in the decorticated rat. (1) GP and SN stimulation evoked antidromic spikes in STH neurons with a mean latency of 1.2 ms and 1.1 ms, respectively. Based on the above latencies, the mean conduction velocity of the STH neurons projecting toward GP was estimated to be 2.5 m/s, and that toward SN was 1.4 m/s. Many STH neurons could be activated following stimulation of both GP and SN, indicating that single STH neurons project to two diversely distant areas. In spite of differences in conduction distance of GP and SN from STH, differences in the conduction velocities of bifurcating axons make it possible for a simultaneous arrival of impulses in the target areas to which these STH neurons project. (2) GP stimulation evoked short duration (5-24 ms) hyperpolarizing potentials which were usually followed by depolarizing potentials with durations of 10-20 ms. These potentials were tested by intracellular current applications and intracellular injections of chloride ions. The results indicated that the hyper- and depolarizing potentials were IPSPs and EPSPs respectively. These IPSPs were considered to be monosynaptic in nature since changes in the stimulus intensities of GP did not alter the latency of IPSPs. The mean latency of the IPSPs was 1.3 ms. Based on the above mean latency the mean conduction velocity of GP axons projecting to STH was estimated to be 3.8 m/s. (3) Analysis of electrical properties of STH neurons indicated that: (i) input resistance estimated by a current-voltage relationship ranged from 9 to 28 M omega; (ii) the membrane showed rectification in the hyperpolarizing direction; (iii) direct stimulation of neurons by depolarizing current pulses produced repetitive firings with frequencies up to 500 Hz. (4) Morphology of the recorded STH neurons was identified by intracellular labeling of neurons with horseradish peroxidase. Light microscopic analysis indicated that the recorded neurons were Golgi type I neurons with bifurcating axons projecting toward GP and SN.

Afferent and efferent pathways of the vibrissal region of primary motor cortex in the mouse

The afferent and efferent connections of the vibrissal representation within the mouse primary motor cortex (MsI) were identified by using the retrograde transport of horseradish peroxidase (HRP) and the anterograde transport of tritiated amino acids injected into MsI. Following aldehyde perfusion brains were frozen-sectioned at 40 microns and reacted for HRP using the 3-3' diaminobenzidine-cobalt chloride technique of Adams ('77). Alternate HRP reacted sections were processed for autoradiography. HRP-filled pyramidal cell somata and concentrations of developed silver grains above background levels were observed in both the vibrissal area of primary somatosensory cortex (SmI) cortex (i.e., the posteromedial barrel subfield; PMBSF cortex) and in the face region of SmII (area 40). In both regions labeled somata occurred predominantly in cortical layers II-III and V. Autoradiographic label was superimposed over the regions containing labeled somata but exhibited a less distinct laminar organization. A dense reciprocal projection connected the injection site with the homotopic area in contralateral MsI; somata occurred for the most part in layers III and V. Developed silver grains were uniformly dispersed over the area containing labeled cell bodies. HRP-labeled pyramidal somata were noted in contralateral PMBSF cortex, but no silver grains occurred in this region. Reciprocal projections linked MsI cortex with the ipsilateral thalamic nuclei: ventralis pars lateralis (VL) and centralis pars lateralis (CL) and with the zona incerta (ZI). Labeled cell bodies and developed silver grains were more dense in VL than in CL. The ipsilateral striatum and thalamic reticular nucleus (NRT) received afferents from the motor cortex but did not project to it. Thus, the vibrissal area of primary motor cortex is connected with a number of cortical and subcortical structures, each of which has been shown to play a role in motor performance. Identification of the afferent and efferent pathways of MsI cortex will now enable further investigation of the ultrastructural and synaptic organization of the vibrissal area of MsI.

Corticothalamic projections from postcruciate area 4 in the dog

Corticothalamic projections from postcruciate area 4, located on the rostral part of the posterior sigmoid gyrus, were traced with the autoradiographic technique in the dog. Injections of tritiated amino acids were made into the lateral and medial parts of area 4 in regions corresponding to the forelimb and hindlimb areas of the primary motor cortex, respectively. In cases with injections placed in the lateral part of area 4, dense accumulations of label were present in the lateral part of the ventral anterior nucleus (VA), the central part of the ventral lateral nucleus (VL), the ventral half of the ventral posterior inferior nucleus (VPI), the caudal part of the central lateral nucleus (CL), and the centrum medianum (CM). Lighter label was also present in the lateral part of the cytoarchitectonically distinct VL region bordering the ventrobasal complex (VB), as well as in the ventrolateral part of the mediodorsal nucleus (MD), and in the lateral posterior nucleus (LP). In one case in which the injection site involved an adjacent part of area 3a, label was also seen ventrally in the medial division of the posterior nuclear group (POm). However, no detectable differences in VL, MD, or intralaminar labeling patterns were noted between this case and the four other cases with injections confined to the lateral part of area 4. In two cases with injections restricted to the medial part of area 4, dense label was present in the lateralmost part of VL, the ventral part of VPI, the caudal part of CL, and CM. Lighter label was also present in the VL region bordering the dorsolateral edge of VB and in LP. An additional case in which the injection also involved the rostral border of area 3a showed a similar pattern of thalamic labeling. Projections from both the lateral and medial parts of area 4 were also noted in the subthalamic nucleus, zona incerta, and nucleus of Darkschewitsch. These results suggest that corticothalamic projections from postcruciate area 4 to VL are organized topographically such that projections from the lateral part of area 4 project centrally within VL while those from the medial part of area 4 project more laterally. Both parts of area 4 also project topographically to a cytoarchitectonically distinct region of VL located immediately adjacent to VB. In contrast, the projections to the intralaminar nuclei do not appear to be topographically organized. The data from cases involving spread of the injection into area 3a suggest that projection patterns from area 3a to ventral, intralaminar, and medial thalamic nuclei are similar to those from area 4. However, it appears that at least the lateral part of area 3a also projects to POm.