Cutaneous representations of the hand and other body parts in the cuneate nucleus of a primate, and some relationships to previously described cortical representations

Comparative Functional Anatomy of Marmoset Brains

Marmosets and closely related tamarins have become popular models for understanding aspects of human brain organization and function because they are small, reproduce and mature rapidly, and have few cortical fissures so that more cortex is visible and accessible on the surface. They are well suited for studies of development and aging. Because marmosets are highly social primates with extensive vocal communication, marmoset studies can inform theories of the evolution of language in humans. Most importantly, marmosets share basic features of major sensory and motor systems with other primates, including those of macaque monkeys and humans with larger and more complex brains. The early stages of sensory processing, including subcortical nuclei and several cortical levels for the visual, auditory, somatosensory, and motor systems, are highly similar across primates, and thus results from marmosets are relevant for making inferences about how these systems are organized and function in humans. Nevertheless, the structures in these systems are not identical across primate species, and homologous structures are much bigger and therefore function somewhat differently in human brains. In particular, the large human brain has more cortical areas that add to the complexity of information processing and storage, as well as decision-making, while making new abilities possible, such as language. Thus, inferences about human brains based on studies on marmoset brains alone should be made with a bit of caution.

Functional organization and connectivity of the dorsal column nuclei complex reveals a sensorimotor integration and distribution hub

The dorsal column nuclei complex (DCN-complex) includes the dorsal column nuclei (DCN, referring to the gracile and cuneate nuclei collectively), external cuneate, X, and Z nuclei, and the median accessory nucleus. The DCN are organized by both somatotopy and modality, and have a diverse range of afferent inputs and projection targets. The functional organization and connectivity of the DCN implicate them in a variety of sensorimotor functions, beyond their commonly accepted role in processing and transmitting somatosensory information to the thalamus, yet this is largely underappreciated in the literature. To consolidate insights into their sensorimotor functions, this review examines the morphology, organization, and connectivity of the DCN and their associated nuclei. First, we briefly discuss the receptors, afferent fibers, and pathways involved in conveying tactile and proprioceptive information to the DCN. Next, we review the modality and somatotopic arrangements of the remaining constituents of the DCN-complex. Finally, we examine and discuss the functional implications of the myriad of DCN-complex projection targets throughout the diencephalon, midbrain, and hindbrain, in addition to their modulatory inputs from the cortex. The organization and connectivity of the DCN-complex suggest that these nuclei should be considered a complex integration and distribution hub for sensorimotor information.

Altered Expression of Reorganized Inputs as They Ascend From the Cuneate Nucleus to Cortical Area 3b in Monkeys With Long-Term Spinal Cord Injuries

Chronic deafferentations in adult mammals result in reorganization of the brain. Lesions of the dorsal columns of the spinal cord at cervical levels in monkeys result in expansion of the intact chin inputs into the deafferented hand representation in area 3b, second somatosensory (S2) and parietal ventral (PV) areas of the somatosensory cortex, ventroposterior lateral nucleus (VPL) of the thalamus, and cuneate nucleus of the brainstem. Here, we describe the extent and nature of reorganization of the cuneate and gracile nuclei of adult macaque monkeys with chronic unilateral lesions of the dorsal columns, and compare it with the reorganization of area 3b in the same monkeys. In both, area 3b and the cuneate nucleus chin inputs expand to reactivate the deafferented neurons. However, unlike area 3b, neurons in the cuneate nucleus also acquire receptive fields on the shoulder, neck, and occiput. A comparison with the previously published results shows that reorganization in the cuneate nucleus is similar to that in VPL. Thus, the emergent topography following deafferentations by spinal cord injuries undergoes transformation as the reorganized inputs ascend from subcortical nuclei to area 3b. The results help us understand mechanisms of the brain plasticity following spinal cord injuries.

Neural Basis of Touch and Proprioception in Primate Cortex

The sense of proprioception allows us to keep track of our limb posture and movements and the sense of touch provides us with information about objects with which we come into contact. In both senses, mechanoreceptors convert the deformation of tissues-skin, muscles, tendons, ligaments, or joints-into neural signals. Tactile and proprioceptive signals are then relayed by the peripheral nerves to the central nervous system, where they are processed to give rise to percepts of objects and of the state of our body. In this review, we first examine briefly the receptors that mediate touch and proprioception, their associated nerve fibers, and pathways they follow to the cerebral cortex. We then provide an overview of the different cortical areas that process tactile and proprioceptive information. Next, we discuss how various features of objects-their shape, motion, and texture, for example-are encoded in the various cortical fields, and the susceptibility of these neural codes to attention and other forms of higher-order modulation. Finally, we summarize recent efforts to restore the senses of touch and proprioception by electrically stimulating somatosensory cortex. © 2018 American Physiological Society. Compr Physiol 8:1575-1602, 2018.

Retrograde analysis of the cerebellar projections to the posteroventral part of the ventral lateral thalamic nucleus in the macaque monkey

The organization of cerebellothalamic projections was investigated in macaque monkeys using injections of retrograde tracers (cholera toxin B and fluorescent dextrans) in the posteroventral part of the ventrolateral thalamic nucleus (VLpv), the main source of thalamic inputs to the primary motor cortex. Injections that filled all of VLpv labeled abundant neurons that were inhomogeneously distributed among many unlabeled cells in the deep cerebellar nuclei (DCbN). Single large pressure injections made in face-, forelimb-, or hindlimb-related portions of VLpv using physiological guidance labeled numerous neurons that were broadly dispersed within a coarse somatotopographic anteroposterior (foot to face) gradient in the dentate and interposed nuclei. Small iontophoretic injections labeled fewer neurons with the same somatotopographic gradient, but strikingly, the labeled neurons in these cases were as broadly dispersed as in cases with large injections. Simultaneous injections of multiple tracers in VLpv (one tracer per somatic region with no overlap between injections) confirmed the general somatotopography but also demonstrated clearly the overlapping distributions and the close intermingling of neurons labeled with different tracers. Significantly, very few neurons (<2%) were double-labeled. This organizational pattern contrasts with the concept of a segregated "point-to-point" somatotopy and instead resembles the complex patterns that have been observed throughout the motor pathway. These data support the idea that muscle synergies are represented anatomically in the DCbN by a general somatotopography in which intermingled neurons and dispersed but selective connections provide the basis for plastic, adaptable movement coordination of different parts of the body. Indexing terms:

Organization of primary afferent projections to the gracile nucleus of the dorsal column system of primates

In order to reveal the somatotopic organization of the gracile nucleus of the dorsal column-trigeminal complex, neuroanatomical tracers were injected subcutaneously into various parts of the hindlimb and tail of prosimian galagos, New World monkeys, and Old World monkeys. In most cases, tracers were injected bilaterally, and into more than one body part. In six cases, two different, distinguishable tracers were injected into the same hindlimb. Brainstem and spinal cord sections were processed for tracers transported by cutaneous afferents to terminations in the gracile nuclei. Foci of terminations were related to the cell-cluster architecture of the gracile nuclei in sections processed for cytochrome oxidase or stained for cell bodies (Nissl stain). In all taxa, terminations labeled by the injections were distributed in a patchy fashion along the rostrocaudal length of the ipsilateral gracile nucleus. Terminations were largely but not completely focused within the cytochrome oxidase dense cell clusters. Across taxa, afferents from the tail, foot, lower leg, and upper leg terminated in a mediolateral sequence within the gracile nucleus. Afferents from the glabrous skin of toes 1-5 terminated in a ventromedial to dorsolateral sequence in owl, squirrel, and macaque monkeys, but an altered arrangement was seen in the galagos, with a ventrolateral location for toe 1. The use of two tracers in squirrel monkeys indicated that terminations from adjacent toes formed adjacent and largely segregated patches. Terminations of afferents from the plantar pad (sole) of the foot tended to surround those from the glabrous toes.

Chondroitinase ABC digestion of the perineuronal net promotes functional collateral sprouting in the cuneate nucleus after cervical spinal cord injury

Upregulation of extracellular chondroitin sulfate proteoglycans (CSPGs) after CNS injuries contributes to the impediment of functional recovery by restricting both axonal regeneration and synaptic plasticity. In the present study, the effect of degrading CSPGs with the application of the bacterial enzyme chondroitinase ABC (chABC) into the cuneate nucleus of rats partially denervated of forepaw dorsal column axons was examined. A dorsal column transection between the C6-C7 dorsal root entry zones was followed immediately by an ipsilateral brainstem injection of either chABC or a bacterial-derived control enzyme [penicillinase (P-ase)] and then subsequently (1 week later) followed with a second brainstem enzyme injection and cholera toxin B subunit (CTB) tracer injection into the ipsilateral forepaw digits and pads. After 1 additional week, the rats underwent electrophysiological receptive field mapping of the cuneate nucleus and/or anatomical evaluation. Examination of the brainstems of rats from each group revealed that CSPGs had been reduced after chABC treatment. Importantly, in the chABC-treated rats (but not in the P-ase controls), a significantly greater area of the cuneate nucleus was occupied by physiologically active CTB traced forepaw afferents that had been spared by the initial cord lesion. These results demonstrate, for the first time, a functional change directly linked to anatomical evidence of sprouting by spinal cord afferents after chABC treatment.

Neurons in the dorsal column nuclei of the rat emit a moderate projection to the ipsilateral ventrobasal thalamus

The dorsal column nuclei (DCN; gracile and cuneate nuclei) give rise to the medial lemniscus, the fibre system that provides an organised somatosensory input to the thalamus. Unlike the spinothalamic and trigeminothalamic tracts that project, also to the ipsilateral thalamus, the medial lemniscus system is believed to be entirely crossed. We demonstrate that DCN emit a small number of axons that reach the ipsilateral thalamus. As retrograde fluorescent neuronal tracer Fluoro-gold was stereotaxically injected in the ventrobasal thalamus of nine young adult Wistar rats. The injection foci were voluminous and encroached upon adjacent nuclei, but the periphery of the injection halo never spilled over to the contralateral thalamus. All sections of the contralateral gracile and cuneate nuclei and the midline nucleus of Bischoff contained abundant retrogradely labelled neurons. The comparison with the Nissl-stained parallel sections suggests that approximately 70-80% of the DCN neurons project to the contralateral thalamus. Counting of retrogradely labelled neurons in two cases revealed 4,809 and 4,222 neurons in the contralateral and 265 and 214 in the ipsilateral DCN, respectively. Thus, although less prominent than the ipsilateral spinothalamic tract, the lemniscal system also emits an ipsilateral projection that accounts for about 5% of the neuronal population in DCN that innervates the ventrobasal thalamus.

Quantitative stereological evaluation of the gracile and cuneate nuclei and their projection neurons in the rat

Stereological methods were employed to estimate the volume and neuron numbers of the rat dorsal column nuclei (DCN). These methods were applied to Nissl-stained sections from control animals and cases that received injections of horseradish peroxidase in the thalamus, the cerebellum, or the spinal cord. Additional cases received combinations of fluorescent tracers in the same structures, to examine whether some of the retrogradely labeled neurons sent collaterals to different targets. The mean volume of the DCN is 0.81 mm(3) (range 0.65-1.10 mm(3)), of which 3%, 39%, and 59% correspond, respectively, to the nucleus of Bischoff (Bi), the gracile (Gr), and the cuneate (Cu) nuclei. Within Cu, the middle division (CuM) is the largest (42%), followed by the rostral (CuR; 36%) and caudal (CuC; 22%) divisions. The mean total number of neurons in the DCN is 16,000 (range 12,400-19,500), of which 2.4%, 34.0% and 63.6% correspond, respectively, to Bi, Gr, and Cu. Within Cu, CuM contains 48% of all neurons, and 27% correspond to CuR and 25% to CuC. Interanimal variability is moderate for the whole DCN and Cu but increases when individual nuclei are considered. About 80% of DCN neurons project to the thalamus, 3% to the spinal cord, and 7% to the cerebellum. Thalamic-projecting cells are more numerous in CuM and Gr (83%), and relatively less common in Bi and CuC (72-74%). Most of the DCN neurons projecting to the spinal cord appear in CuC and CuM. Two-thirds of the neurons projecting to the cerebellum are located in CuR, 20% in CuM, and 15% in Gr. A small fraction of neurons projects simultaneously to spinal cord and thalamus.

The chemo- and somatotopic architecture of the Galago cuneate and gracile nuclei

The pattern of peripheral nerve inputs into the dorsal column nuclei, cuneate and gracile, was investigated in the prosimian Galago garnetti. The major findings were, that there is a greater segregation of the inputs from the fingers/hand within the cuneate compared with input form the toes/foot within the gracile. In both nuclei, cell clusters can be identified as cytochrome oxidase dense blotches, reactive also for the activity-dependent enzyme nitric oxide synthase. In the cuneate, cell clusters were apparent as six main cytochrome oxidase/nitric oxide synthase-reactive ovals arranged in a medial to lateral sequence. In contrast in the gracile, a higher degree of parcellation was noted and several cytochrome oxidase/nitric oxide synthase blotches were distributed along the rostrocaudal axis of the nucleus. This different architecture parallels differences in the organization of the inputs from the hand and from the foot. In the cuneate, cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals from the glabrous and hairy skin of digits d1 to d5 segregated in each of the five most lateral cytochrome oxidase/nitric oxide synthase blotches. Afferents from the thenar, palmar pads and hypothenar overlapped with those from digit 1, digit 2 to digit 4 and digit 5, respectively. Inputs from wrist arm and shoulder were segregated in the most medial blotch. In the gracile, multiple foci of cholera toxin B subunit conjugated to horseradish peroxydase labeled terminals were observed upon injections of single sites in the toes or plantar pads. Although in multiple foci, inputs from different toes segregated from one another as well. Terminals from the plantar pads appeared to converge on the same cytochrome oxidase/nitric oxide synthase blotches targeted by inputs from the toes. In both the cuneate and the gracile, cytochrome oxidase/nitric oxide synthase blotches also presented intense immunoreactivity for GABA, calbindin, parvalbumin, and brain derived neurotrophic factor. Finally, in the cuneate the cell cluster region presented similarities in prosimian galagos and four species of New World monkeys, whereas it appeared more differentiated and complex in the Old Word macaque monkeys. In conclusion, the different pattern of segregation of the inputs from the hand and from the foot can be related to the different metabolic organization of the cuneate and of the gracile, respectively.

Adaptive responses of monkey somatosensory cortex to peripheral and central deafferentation

This study deals with two kinds of activity-dependent phenomena in the somatosensory cortex of adult monkeys, both of which may be related: (1) mutability of representational maps, as defined electrophysiologically; (2) alterations in expression of genes important in the inhibitory and excitatory neurotransmitter systems. Area 3b of the cerebral cortex was mapped physiologically and mRNA levels or numbers of immunocytochemically stained neurons quantified after disrupting afferent input peripherally by section of peripheral nerves, or centrally by making lesions of increasing size in the somatosensory thalamus. Survival times ranged from a few weeks to many months. Mapping studies after peripheral nerve lesions replicated results of previous studies in showing the contraction of representations deprived of sensory input and expansion of adjacent representations. However, these changes in representational maps were in most cases unaccompanied by significant alterations in gene expression for calcium calmodulin-dependent protein kinase isoforms, for glutamic acid decarboxylase, GABA(A) receptor subunits, GABA(B) receptors, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) or N-methyl-D-aspartate (NMDA) receptor subunits. Mapping studies after lesions in the ventral posterior lateral nucleus (VPL) of the thalamus revealed no changes in cortical representations of the hand or fingers until >15% of the thalamic representation was destroyed, and only slight changes until approximately 45% of the representation was destroyed, at which point the cortical representation of the finger at the center of a lesion began to shrink. Lesions destroying >60% of VPL resulted in silencing of the hand representation. Although all lesions were associated with a loss of parvalbumin-immunoreactive thalamocortical fiber terminations, and of cytochrome oxidase staining in a focal zone of area 3b, no changes in gene expression could be detected in the affected zone until >40-50% of VPL was destroyed, and even after that changes in mRNA levels or in numbers of GABA-immunoreactive neurons in the affected zone were remarkably small. The results of these studies differ markedly from the robust changes in gene expression detectable in the visual cortex of monkeys deprived of vision in one eye. The results confirm the view that divergence of the afferent somatosensory pathways from periphery to cerebral cortex is sufficiently great that many fibers can be lost before neuronal activity is totally silenced in area 3b. This divergence is capable of maintaining a high degree of cortical function in the face of diminishing inputs from the periphery and is probably an important element in promoting representational plasticity in response to altered patterns of afferent input.

Progressive transneuronal changes in the brainstem and thalamus after long-term dorsal rhizotomies in adult macaque monkeys

This study deals with a potential brainstem and thalamic substrate for the extensive reorganization of somatosensory cortical maps that occurs after chronic, large-scale loss of peripheral input. Transneuronal atrophy occurred in neurons of the dorsal column (DCN) and ventral posterior lateral thalamic (VPL) nuclei in monkeys subjected to cervical and upper thoracic dorsal rhizotomies for 13-21 years and that had shown extensive representational plasticity in somatosensory cortex and thalamus in other experiments. Volumes of DCN and VPL, number and sizes of neurons, and neuronal packing density were measured by unbiased stereological techniques. When compared with the opposite, unaffected, side, the ipsilateral cuneate nucleus (CN), external cuneate nucleus (ECN), and contralateral VPL showed reductions in volume: 44-51% in CN, 37-48% in ECN, and 32-38% in VPL. In the affected nuclei, neurons were progressively shrunken with increasing survival time, and their packing density increased, but there was relatively little loss of neurons (10-16%). There was evidence for loss of axons of atrophic CN cells in the medial lemniscus and in the thalamus, with accompanying severe disorganization of the parts of the ventral posterior nuclei representing the normally innervated face and the deafferented upper limb. Secondary transneuronal atrophy in VPL, associated with retraction of axons of CN neurons undergoing primary transneuronal atrophy, is likely to be associated with similar withdrawal of axons from the cerebral cortex and should be a powerful influence on reorganization of somatotopic maps in the somatosensory cortex.

Evidence for brainstem and supra-brainstem contributions to rapid cortical plasticity in adult monkeys

Cortical maps can undergo amazingly rapid changes after injury of the body. These changes involve functional alterations in normal substrates, but the cortical and/or subcortical location(s) of these alterations, and the relationships of alterations in different substrates, remain controversial. The present study used neurophysiological approaches in adult monkeys to evaluate how brainstem organization of tactile inputs in the cuneate nucleus (CN) changes after acute injury of hand nerves. These data were then compared with analogous data from our earlier cortical area 3b studies, which used the same approaches and acute injury, to assess relationships of cuneate and cortical changes. The results indicate that cuneate tactile responsiveness, receptive field locations, somatotopic organization, and spatial properties of representations (i.e., location, continuity, size) change during the first minutes to hours after injury. The comparisons of cuneate and area 3b organization further show that some cuneate changes are preserved in area 3b, whereas other cuneate changes are transformed before being expressed in area 3b. The findings provide evidence that rapid reorganization in area 3b, in part, reflects mechanisms that operate from a distance in the cuneate nucleus and, in part, reflects supracuneate mechanisms that modify brainstem changes.

Rapid changes in brainstem maps of adult primates after peripheral injury

Cerebral cortical maps in adult primates reorganize within minutes-hours after peripheral injuries, but subcortical versus intracortical contributions to this rapid reorganization remain controversial. The present results show that injury of nerves to the hands of adult monkeys triggers rapid (minutes-hours) changes in maps of the hand in the brainstem main cuneate nucleus. These findings suggest that peripheral injury causes an initial concurrent reorganization of brainstem and cortical substrates and that early sensory changes emerge from reorganization involving multiple central levels.