The nervous system in planarians: Peripheral and gastrodermal plexuses,pharynx innervation,and the relationship between central nervous system structure and the acoelomate organization

The Champy-Maillet osmium tetroxide-zinc iodide technique and a new method using azur B-sodium thioglycolate were used to study the general nervous tissue structure in planarians. A subepidermal and a submuscular nerve plexus, partially reported by earlier authors, are described, and a gastrodermal plexus is reported for the first time in triclads. The possible functions for each one of these plexuses are discussed. By the Champy-Maillet method, the innervation within the parenchyma appears as an array of numerous single nerve fibers that course between the parenchyma cells making apparent synaptic contacts. The pharynx has outer and inner nerve nets similar in structure to the submuscular nerve plexus. Both nerve nets are connected to each other by radial nerves. The central nervous system has a sponge-like structure with many lacunae filled with cell bodies, dorso-ventral muscle fibers, parenchymal cell processes and excretory ducts. The existence of this sponge-like nervous tissue structure is discussed in relation to the still incomplete centralization of the nervous tissue in these organisms, to the lack of a true vascular system and to the acoelomate level of organization. A comparison with the nervous tissue structure of more advanced groups like polyclads and nemertines is suggested.     

The organization of prolactin-like-immunoreactive neurons in the rat central nervous system

Summary The localization and distribution of prolactinlike-immunoreactive perikarya and nerve fibers in the rat central nervous system have been studied by a preembedding immunoperoxidase method using well-characterized specific immunsera to rat prolactin. Although the localization of labeled neuronal structures in a number of brain areas correlates with the data of previous immunocytochemical studies, we found prolactin-immunoreactive neurons in various regions not previously reported. In untreated animals, the highest concentrations of prolactinfibers were observed: (i) in the external layers of the median eminence where they exhibited close contact with blood vessels, and (ii) in the bed nucleus of the stria terminalis and in the central nucleus of the amygdala where they closely surrounded unlabeled perikarya. Dense networks of finely varicose prolactin fibers were also observed in the organum vasculosum of the lamina terminalis, in the subfornical organ, and in the dorsolateral regions of the medulla oblongata and the spinal cord. Lastly, a number of large, varicose, intensely immunoreactive fibers were found in the olfactory bulb, the cingulum, and the periventricular regions of the hypothalamus and central gray, whereas isolated fibers could be detected in the caudate nucleus and in the cerebral cortex. In animals treated with colchicine, prolactin-immunoreactive perikarya were essentially located within the periventricular and perifornical regions of the hypothalamus, and within the bed nucleus of the stria terminalis. Although corticotropin (ACTH 17-39)-immunoreactive fibers could be detected in several regions found to contain prolactin fibers, the distribution and organization of both fiber types clearly differed in numerous brain regions, and the regions containing the corresponding perikarya did not overlap. The ultrastructural organization of the prolactin-immunoreactive fibers revealed by electronmicroscopic immunocytochernistry in various brain regions, allowed the characterization of two main types of prolactinergic neurons including: (i) endocrine neurons, whose axons terminated in close vicinity to portal blood vessels in the external median eminence, and (ii) neurons projecting to extrahypothalamic regions, whose axons formed typical synaptic connections with unidentified neuronal units.     

Programming and reprogramming neuronal subtypes in the central nervous system

Recent discoveries in nuclear reprogramming have challenged the dogma that the identity of terminally differentiated cells cannot be changed. The identification of molecular mechanisms that reprogram differentiated cells to a new identity carries profound implications for regenerative medicine across organ systems. The central nervous system (CNS) has historically been considered to be largely immutable. However, recent studies indicate that even the adult CNS is imparted with the potential to change under the appropriate stimuli. Here, we review current knowledge regarding the capability of distinct cells within the CNS to reprogram their identity and consider the role of developmental signals in directing these cell fate decisions. Finally, we discuss the progress and current challenges of using developmental signals to precisely direct the generation of individual neuronal subtypes in the postnatal CNS and in the dish.     

Modular organization of finger movements by the human central nervous system

The motor system may generate automated movements, such as walking, by combining modular spinal motor synergies. However, it remains unknown whether a modular neuronal architecture is sufficient to generate the unique flexibility of human finger movements, which rely on cortical structures. Here we show that finger movements evoked by transcranial magnetic stimulation (TMS) of the primary motor cortex reproduced distinctive features of the spatial representation of voluntary movements as identified in previous neuroimaging studies, consistent with naturalistic activation of neuronal elements. Principal component analysis revealed that the dimensionality of TMS-evoked movements was low. Principal components extracted from TMS-induced finger movements resembled those derived from end-postures of voluntary movements performed to grasp imagined objects, and a small subset of them was sufficient to reconstruct these movements with remarkable fidelity. The motor system may coordinate even the most dexterous movements by using a modular architecture involving cortical components.     

Muscle fibers in the central nervous system of nemerteans: Spatial organization and functional role

The system of muscle fibers associated with the brain and lateral nerve cords is present in all major groups of enoplan nemerteans. Unfortunately, very little is known about the functional role and spatial arrangement of these muscles of the central nervous system. This article examines the architecture of the musculature of the central nervous system in two species of monostiliferous nemerteans (Emplectonema gracile and Tetrastemma cf. candidum) using phalloidin staining and confocal microscopy. The article also briefly discusses the body‐wall musculature and the muscles of the cephalic region. In both species, the lateral nerve cords possess two pairs of cardinal muscles that run the length of the nerve cords and pass through the ventral cerebral ganglia. A system of peripheral muscles forms a meshwork around the lateral nerve cords in E. gracile. The actin‐rich processes that ramify within the nerve cords in E. gracile (transverse fibers) might represent a separate population of glia‐like cells or sarcoplasmic projections of the peripheral muscles of the central nervous system. The lateral nerve cords in T. cf. candidum lack peripheral muscles but have muscles similar in their position and orientation to the transverse fibers. The musculature of the central nervous system is hypothesized to function as a support system for the lateral nerve cords and brain, preventing rupturing and herniation of the nervous tissue during locomotion. The occurrence of muscles of the central nervous system in nemerteans and other groups and their possible relevance in taxonomy are discussed. J. Morphol. 2012. © 2012 Wiley Periodicals, Inc.     

Regulation of neuronal plasticity in the central nervous system by phosphorylation and dephosphorylation

Neuronal plasticity can be defined as adaptive changes in structure and function of the nervous system, an obvious example of which is the capacity to remember and learn. Long-term potentiation and long-term depression are the experimental models of memory in the central nervous system (CNS), and have been frequently utilized for the analysis of the molecular mechanisms of memory formation. Extensive studies have demonstrated that various kinases and phosphatases regulate neuronal plasticity by phosphorylating and dephosphorylating proteins essential to the basic processes of adaptive changes in the CNS. These proteins include receptors, ion channels, synaptic vesicle proteins, and nuclear proteins. Multifunctional kinases (cAMP-dependent protein kinase, Ca²⁺/phospholipid-dependent protein kinase, and Ca²⁺/calmodulin-dependent protein kinases) and phosphatases (calcineurin, protein phosphatases 1, and 2A) that specifically modulate the phosphorylation status of neuronal-signaling proteins have been shown to be required for neuronal plasticity. In general, kinases are involved in upregulation of the activity of target substrates, and phosphatases downregulate them. Although this rule is applicable in most of the cases studied, there are also a number of exceptions. A variety of regulation mechanisms via phosphorylation and dephosphorylation mediated by multiple kinases and phosphatases are discussed.     

Non-synaptic integration of neuronal cell bodies in the snail central nervous system

Approach to regions occupied by perikarya of the snail 5-HT-ergic neurons produced excitation. The strongest action was recorded on the PedA cluster surface, and it was further enhanced by adding the 5-HT precursor. The findings suggest that the mechanisms underlying 5-HT-dependent behaviour include a mutual excitatory co-operation between somata of neighbouring 5-HT-ergic neurons.     

The organization of cytoplasm at the presynaptic active zone of a central nervous system synapse

The axoplasm at the presynaptic active zone of excitatory synapses between parallel fibers and Purkinje cell spines contains a meshwork of distinct filaments intermingled with synaptic vesicles, seen most clearly after the rapid freezing, freeze-etch technique of tissue preparation. One set of filaments extends radially from synaptic vesicles and intersects similar filaments associated with vesicles as well as larger filaments arising from the presynaptic membrane. The small, vesicle-associated filaments appear to link synaptic vesicles to one another and to enmesh them in the vicinity of the synaptic junction. The vesicle-associated filaments could be synapsin I because they have the same molecular dimensions and are distributed in the same pattern as synapsin I immunoreactivity.     

The central nucleus of the amygdala: cytoarchitechtonics, neuronal organization, and histophysiology

Neurologists' attention is attracted by amygdala's central nucleus which takes part in formation of stress response and adaptive behaviour in animals. The data on cytoarchitectonics, neuronal organization and histophysiology of this important centre are based on current knowledge of its subnuclear organization.     

The effect of local application of homocysteine on neuronal activity in the central nervous system of the rat

Homocysteine, a monocarboxylic, sulfur-containing amino acid, produces convulsions in rats and mice when administered systemically. Convulsions and high serum concentrations of homocysteine are among the symptoms that characterize patients with homocystinuria, a hereditary disorder of amino acid metabolism. In order to evaluate the effects of homocysteine on the central nervous system directly, extracellular recordings were made from neurons in rat cerebral cortex, cerebellum and midbrain during local application of homocysteine by pressure ejection or iontophoresis. Both methods of drug delivery produced dose-dependent increases in the activity of neurons in every area tested. Activity was increased by D, L-homocysteine and L-glutamate in 67 percent of cells tested with both drugs. The doses required to produce equivalent excitations in this group of cells were similar, suggesting that homocysteine is at least as potent as glutamate. The excitatory effects of both homocysteine and glutamate were antagonized by local application of betaine, a biological methyl donor which blocks convulsions produced by systematic administration of pentylenetetrazol and electroshock as well as homocysteine. The effects of local application of homocysteine were also blocked by local application of the glutamate antagonist glutamate diethylester (GDEE). In 6 of 7 cells tested, GDEE appeared to preferentially affect homocysteine-induced excitations. These data indicate that homocysteine has an excitatory action on neurons, a finding which may account for some of the symptoms associated with certain disorders of amino acid metabolism.     

New aspects of cholesterol in the central nervous system

Cholesterol is a multifacetted molecule, which serves as essential membrane component, as cofactor for signaling molecules and as precursor for steroid hormones. Despite intense research on the diverse aspects of cholesterol, the role of cholesterol in the nervous system is still little understood. Our recent studies on primary cultures of highly purified neurons from the rodent central nervous system (CNS) suggest that during development, neurons reduce or even abandon cholesterol synthesis and import cholesterol from astrocytes via lipoproteins. Neurons use the glia-derived cholesterol to form numerous and efficient synapses. This provokes new ideas about the role of astrocytes as cholesterol producers and about the function of cholesterol in the CNS and its involvement in neurodegenerative diseases.     

The renin-angiotensin system and the central nervous system.

One of several factors affecting the secretion of renin by the kidneys is the sympathetic nervous system. The sympathetic input is excitatory and is mediated by beta-adrenergic receptors, which are probably located on the membranes of the juxtaglomerular cells. Stimulation of sympathetic areas in the medulla, midbrain and hypothalamus raises blood pressure and increases renin secretion, whereas stimulation of other parts of the hypothalamus decreases blood pressure and renin output. The centrally active alpha-adrenergic agonist clonidine decreases renin secretion, lowers blood pressure, inhibits ACTH and vasopressin secretion, and increases growth hormone secretion in dogs. The effects on ACTH and growth hormone are abolished by administration of phenoxybenzamine into the third ventricle, whereas the effect on blood pressure is abolished by administration of phenoxybenzamine in the fourth ventricle without any effect on the ACTH and growth hormone responses. Fourth ventricular phenoxybenzamine decreases but does not abolish the inhibitory effect of clonidine on renin secretion. Circulating angiotensin II acts on the brain via the area postrema to raise blood pressure and via the subfornical organ to increase water intake. Its effect on vasopressin secretion is debated. The brain contains a renin-like enzyme, converting enzyme, renin substrate, and angiotensin. There is debate about the nature and physiological significance of the angiotensin II-generating enzyme in the brain, and about the nature of the angiotensin I and angiotensin II that have been reported to be present in the central nervous system. However, injection of angiotensin II into the cerebral ventricles produces drinking, increased secretion of vasopressin and ACTH, and increased blood pressure. The same responses are produced by intraventricular renin. Angiotensin II also facilitates sympathetic discharge in the periphery, and the possibility that it exerts a similar action on the adrenergic neurons in the brain merits investigation.     

The fine structure of neuroglia in the central nervous system of nereid polychaetes

Summary The principal supportive elements of the nereid central nervous system are non-neuronal cells that are referred to as supportive glia. Supportive glial cells form a conspicuous cortex in the nerve cord. The inner region of this cortex consists of closely packed processes and cell bodies of fibrous supportive glial cells that are arranged in concentric layers around the perimeter of the neuropile. The fibrous appearance of the glial cells results from dense bundles of cytoplasmic filaments. Many fibrous glial processes penetrate the neuropile and ramify among the neuronal elements. Larger, irregularly shaped cells are the chief supportive glial elements of the peripheral region of the cortex where they line the stromal sheath (neural lamella) and invest the neuronal perikarya with extensive concentric systems of lamellate processes. These glial cells usually possess a relatively undifferentiated cytoplasm with scattered glycogen granules, but occasionally have a well developed Golgi apparatus, endoplasmic reticulum and densely packed particulate glycogen. The supportive glia exhibits numerous desmosomes as well as 5-layered (tight) and 7-layered (gap) junctions. Interspersed among the supportive glial cells are non-neuronal cells referred to as granulocytes. These cells have abundant large, granular inclusions, electron lucent vesicles, plasmalemmal infoldings and microtubules. The granulocytes may be derived from undifferentiated glial cells or may represent coelomocytes that have invaded the nervous tissue.Supported by USPHS Grants No. NIH 5P01 NS-07512, NIH 2T01 GM-00102, and NB-00840.The author acknowledges the excellent technical assistance of Sarah Wurzelmann and Stanley Brown, and thanks Dr. Berta Scharrer for many stimulating discussions.