Neural elements

Pethidine is predominantly a p agonist and it exerts its action on the CNS and the neural elements in the bowel. In equianalgesic doses, pethidine produces as much sedation, respiratory depression and euphoria as does morphine. A few patients may experience dysphoria. 

The occurrence of a variety of neuropeptides in the brain has been discussed in Section A. The first of these to be discovered811 was the 11-residue substance P (Table 30-4), which was isolated in 1931. Like other neuropeptides it may function either as a transmitter or neuromodulator or perhaps both. Substance P, as well as many other neuropeptides, has been localized to specific neurons. Along with somatostatin, CCK, and enkephalins, it is found in high concentrations in the basal ganglia. Enkephalin and substance P are also found in specific neural elements in the visual system of lobsters.812 In some cases a neuron contains both synaptic vesicles containing a major neurotransmitter and also vesicles containing a peptide or other cotrans-mittor. The peptide pituitary hormones ACTH, MSH, and vasopressin as well as the hypothalamic neurohormones may have effects on learning and behavior.813... 

In cestodes, most, perhaps all, of the neural elements are found in the ganglia of the scolex and the main nerve tracts. In these, the cell bodies are arranged peripherally around a core made up of a tangled mass of neuronal processes often referred to as neurites because it is impossible to distinguish between axons and dendrites. This mass constitutes the so-called neuropile . Like all differentiated cells in cestodes - and probably in all invertebrates - nerve cells are incapable of mitosis and hence new nerve cells must arise directly from the germinative ( = stem) cells (276). In D. dendriticum, the neuropile is composed of a dense network of unmyelinated nerve fibres with no extracellular stroma between the fibres (277). The fibres differ mainly in the content of different types of vesicles, which can be dense-core, small clear or large clear vesicles. The structure of synapses is discussed further on pp. 25—7. 

Fig. 2.12. Scolex of Diphyllobothrium dendriticum plerocercoid. The neural elements have been labelled with growth hormone releasing factor (GRF), a vertebrate neuropeptide. In the main nerve cord (n) one GRF-immunoreactive cell body can be seen (large arrow) in the peripheral nervous system (P) several GRF-immunoreactive cells bodies occur (large arrows). The small arrows point to nerve terminals beneath the basal lamina of the tegument along the inner border of the bothridia. Sections stained with Stemberger s immunoperoxidase-antiperoxidase (PAP) technique. (Courtesy Dr Margaretha K. S. GustafFson.)...

At present, the only brain manipulation that produces a clinical state behaviorally indistinguishable from schizophrenia is the blockade of brain NMDA receptors. The primary concept underlying glutamatergic models of schizophrenia, therefore, is that the dysfunction or dysregulation of NMDA receptor-mediated neurotransmission represents the common final mechanism underlying the symptom formation in schizophrenia. Although in some individuals, the NMDA dysfunction may be related to disturbances of the receptors themselves, for other individuals, these deficits may relate more to impairments of either upstream or downstream elements involving pre- and postsynaptic neural elements respectively. 

The clinical signs of EPM are the result of both direct damage to neurons, by protozoal proliferation within the neuronal cell bodies, and indirect damage to neural elements, produced by edema and inflammation in response to the merozoites and meronts in the CNS. Deposition of the parasite... 

Brain Surface of neural elements on Cerebrospinal fluid. 

Scherlag BJ, Nakagawa H, Jackman WM, et al. Electrical stimulation to identify neural elements on the heart their role in atrial fibrillation. J Interv Card Electrophysiol 2005 13 Suppl 1 37-42. 

Border BG, Mihailoff GA (1985) GAD-immunoreactive neural elements in the basilar pontine nuclei and nucleus reticularis tegmenti pontis of the rat. I. Light microscopic studies. Exp. Brain Res., 59, 600-614. 

Immediately deep to the glomeruli is a layer with a relatively low cell density but a very dense neuropil, the external plexiform layer (EPL). Golgi-stained sections reveal that the predominant neural elements in this layer are the dendrites of mitral/tufted and granule cells. The principal neuron types in EPL are external, middle (Fig. 12B) and deep tufted cells, named according to their relative depth in EPL, and the Van Ge-huchten cells. 

Hussong JW, Perkins SL, Huff V, et al. Familial Wilms tumor with neural elements characterization by histology, immunohistochemistry, and genetic analysis. Pediatr Dev Pathol. 2000 3 561-567. 

Self-organizing maps (SOMs) are one manifestation of neural network approaches to clustering. They have been extensively used in many fields of computational medicinal chemistry [51]. SOMs consist of a grid of neural elements, each containing a vector of a certain dimension. The map is trained by presenting a series of new data objects to the... 

Meperidine is predominantly a /i-receptor agonist and acts principally on the CNS and the neural elements in the bowel. Meperidine is no longer recommended for the treatment of chronic pain because of concerns over metabolite toxicity. It should not be used for longer than 48 hours or in doses >600 mg/day. 

There are three problems in particular that complicate interpretation of much of the data on structure-activity relations in olfaction. First, the different techniques used often yield data that are not strictly comparable. Recordings from a single or a few receptors, for example, are more reliable indicators of the odorant-receptor interaction than are recordings of the massed action of many neural elements in the olfactory bulb. 

FIGURE 30.6 Properties of central nervous system stimulation, (a) The strength-duration relationship describes the amplitude required for stimulation as a function of the stimulation pulse duration. Strength-duration curves for intracellular stimulation of different neural elements were constructed from data summarized in Ranck, 1976. (b) The current-distance relationship describes the threshold intensity required for stimulation as a function of the distance between the electrode and the neuron. Current-distance curves for axons and cells were constructed from data summarized in Ranck, 1976. 

The fundamental properties of excitation of CNS neurons were presented with a focus on what neural elements around the electrode are activated under different conditions. During CNS stimulation action potentials are initiated in the axons of local cells, even for electrodes positioned over the cell body. The threshold difference between cathodic and anodic stimuli arises due to differences in the mode of activation. Anodic stimuli cause depolarization of the axon and excitation via a virtual cathode, while cathodic stimuli cause hyperpolarization at the site of excitation and the action potential is initiated during repolarization. The threshold for activation of presynaptic terminals projecting into the region of stimulation is often less than or equal to the threshold for direct excitation of local cells, and indirect effects mediated by synaptic transmission may alter the direct effects of stimulation on the postsynaptic cell. The fundamental understanding provided by this analysis enables rational design and interpretation of studies and devices employing electrical stimulation of the brain or spinal cord. 

Alternatively, melanin formation and destruction may be seen as a physiologically precarious balance process with a metastable equilibrium in those genetically disposed towards it. Overstimulation of neural elements, trauma, sunburn etc., may upset this homeostasis in favour of melanin destruction, again with incontinence of antigenic material and resultant immunologic melanocyte destruction (35, 144, 189). 

O Hara P. T., Lieberman A. R., Hunt S P and Wu J.-Y (1983) Neural elements containing glutamic acid decarboxylase (GAD) in the dorsal lateral geniculate nucleus of the rat Immunohistochemical studies by light and electron microscopy Neuroscience 8, 189-211... 

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