Axons ultrastructure

Myelin facilitates conduction 51 Myelin has a characteristic ultrastructure 52 Myelin is an extension of a glial plasma membrane 55 Myelin affects axonal structure 56... 

Mutant Tbce mice. Progressive motor neuropathy (PMN), an autosomal recessive murine disease, manifests as weakness beginning within a few weeks of birth [14, 136]. These mice are homozygous for a Trp 524 Gly substitution of Tbce (tubulin-specific chaperone E), localized to mouse chromosome 13 [14]. Tbce mRNA is present in neurons in the spinal cord. Degenerative changes are conspicuous in motor axons, and ultrastructural studies of peripheral nerves of PMN mice disclose reduced numbers of microtubules in these axons. Mutations of the highly conserved Trp524 residue, which appears to influence... 

Spencer PS, Schaumburg HH. 1977a. Ultrastructural studies of the dying-back process. III. The evolution of experimental peripheral giant axonal degeneration. J Neuropathol Exp Neurol 36(2) 276-299. 

Politis MJ, Pellegrino RG, Spencer PS. 1980. Ultrastructural studies of the dying-back process. 5. Axonal neurofilaments accumulate at sites of 2,5-hexanedione application Evidence for nerve fibre dysfunction in experimental hexacarbon neuropathy. [Abstract] J Neurocytol 9 505-516. 

Since the first description of glial cells, Camillo Golgi (1843-1926) and Santiago Ramon y Cajal (1852-1934) recognized that astrocytes are located in strategic positions between neurons and capillaries to act as a conduit for signals between different cells types in the central nervous system (CNS Ramon y Cajal, 1899). Later on, examination of the nervous system at the ultrastructural level has shown that astrocytes can be intimately associated with synapses, literally enwrapping many pre- and postsynaptic terminals. For instance, in the hippocampus, 57% of the axon-spine interfaces are associated with astrocytes (Ventura and Harris, 1999). It is likely that this close physical relationship provides an opportunity for many functional interactions between astrocytes and neurons. 

Wenzel HJ, Buckmaster PS, Anderson NL, Wenzel ME, Schwartzkroin PA. 1997. Ultrastructural localization of neurotransmitter immunoreactivity in mossy cell axons and their synaptic targets in the rat dentate gyrus. Hippocampus 7 559-570. 

It is noteworthy that some attempts have been made to visualize synaptic vesicles at an ultrastructural level in the brains of individuals with schizophrenia. In one study, tissue obtained from a temporal lobe biopsy of an individual with schizophrenia (Ong and Garey, 1993) showed many unusual asymmetric synapses, including clumped but numerous, synaptic vesicles located at short synaptic active zones. In another study, electron microscopic examination of the postmortem caudate nucleus revealed swelling of some axon terminals, shrinkage of some axon boutons and fewer synaptic vesicles (Uranova et al., 1996). These observations are similar to a series of prior postmortem electron microscopic studies, where... 

The ultrastructural organization of dopaminergic boutons in the striatum has been extensively investigated (see, inter alia, the reviews of Smith and Bolam, 1990 Sesack, 2003). Extrastriatal inputs, including the dopaminergic one, terminate mainly on the more distal part of the dendritic tree of medium spiny neurons, while intrinsic inputs terminate mainly on the proximal parts of the dendritic shaft and on the cell body. A small proportion of dopaminergic axons also contact the cell body of striatal projection neurons (Fig. 18B). 

The investigation of DAT immunolabeling at the ultrastructural level (Nirenberg et al., 1997b) visualized the DAT in varicose and intervaricose segments of axons in both the NAc core and the shell, with an organization similar to that detected in the dorsal striatum. Symmetric synapses were seen in both the two main NAc subregions, but DAT-immunoreactive processes only rarely formed synaptic junctions. DAT was not detected over synaptic densities and was instead mostly distributed on the extrasynaptic portions of the plasma membranes, near appositions with somata, dendrites, dendritic spines and astrocytes. 

The ultrastructural observations of Sesack et al. (1998) indicated that considerable extracellular diffusion of DA in the prelimbic prefrontal cortex may result, at least in part, from a paucity of DAT content in dopaminergic mesocortical axons, as well as from a distribution of DAT protein at a distance from synaptic release sites. In this study, DAT-immunoreactive profiles in the striatum and in the cingulate cortex included both varicose and intervaricose segments of axons, but intervaricose axon segments predominated in the prelimbic cortex. 

At variance with the ultrastructural features observed in the NAc shell, dopaminergic axon terminals and boutons of thalamocortical axons deriving from the thalamic paraventricular nucleus were not found to converge on the same structures, such as dendritic shafts or spines (Pinto et al., 2003). Also the terminals of hippocampal fibers did not show obvious synaptic relationship with dopaminergic terminals in the prefrontal cortex (Carr and Sesack, 1996), indicating a segregation of different sets of cortical inputs. 

Studies at the ultrastructural level in the rat NAc shell showed that 65% of D4 receptors were contained in axons and axon terminals, in plasma and vesicular membranes, while only 22% were located in dendrites and dendritic spines which received input from TH-positive terminals (Svingos et al., 2000). The labeled terminals formed occasionally asymmetrical synapses, and only about 17% of them exhibited TH immunoreactivity. In the NAc, therefore, the D4 receptor appears to be involved in presynaptic rather than postsynaptic functions (Svingos et al., 2000). 

Nirenberg MJ, Chan J, Pohorille A, Vaughan RA, Uhl GR, Kuhar MJ, Pickel VM (1997b) The dopamine transporter comparative ultrastructure of dopaminergic axons in limbic and motor compartments of the nucleus accumbens. J Neurosci 77 6899-6907. 

Nakai Y, Shioda S, Ochiai H, Kudo J, Hashimoto A (1983) Ultrastructural relationship between monoamine-and TRH-containing axons in the rat median eminence as revealed by combined autoradiography and immunocytochemistry in the same tissue section. Cell Tiss. Res 230 1-14. 

Schimchowitsch S, Vuillez P, Tappaz ML, Klein MJ, Stoeckel ME (1991) Systemic presence of GABA-immunoreactivity in the tubero-infundibular and tubero-hypophyseal dopaminergic axonal systems an ultrastructural immunogold study on several mammals. Exp Brain Res 53 575-586. 

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