Axonal flow

After inhalation exposure of mice to [ 4C]-/)ora-xylene, methylhippurate accumulated in nasal mucosa and the olfactory bulb, possibly due to axonal flow-mediated transport of the methylhippurate from the mucosa, where it is formed, to the olfactory lobe of the brain (Ghantous et al., 1990). Inhalation exposure of rats to /weto-xylene with or without ethyl acetate showed that ethyl acetate caused a decrease in the blood concentration of zweto-xylene (Freundt et al., 1989). 

Hohmann AG, Herkenham M (1999a) Cannabinoid receptors undergo axonal flow in sensory nerves. Neuroscience 92 1171-1175... 

Young WSd, Wamsley JK, Zarbin MA, Kuhar MJ (1980) Opioid receptors undergo axonal flow. Science 210 76-78... 

Kunzle H, Wiklund L (1982) Identification and distribution of neurons presumed to give rise to cerebellar climbing fibers in turtle. A retrograde axonal flow study using radioactive D-asparate as a marker. Brain Res., 252, 146-150. 

The hypothalamus controls the function of the anterior pituitary gland by the production of a series of chemical messengers. These are synthesized in areas of the hypothalamus not yet well defined in man. They are transported to the m an eminence, probably by a process of axonal flow similar to that involved in the transport of the posterior lobe hormones. They are stored in the median eminence, from which they are released in response to certain stimuli, passing in ibe portal venous system down the pituitary stalk to the anterior lobe, whore they affect die synthesis and release of the pituitary hormones. 

LH/FSH-RH is formed in the hypothalamus, particularly in the basal-ventral area, but the precise location of its origin remains to be elucidated. By the use of immunohistochemical techniques, LH/FSH-RH has been found in the arcuate nucleus and in the median eminence (PI). Electron microscopy studies revealed that some neural axons of the median eminence contain immunopositive LH/FSH-RH (P4). Norepinephrine appears to be the most important neurotransmitter regulating LH/FSH-RH biosynthesis. From this origin, LH/FSH-RH is transported, presumably by axonal flow, to the median eminence, where it is stored. It is secreted into the hypothalamic-pituitary portal system to reach the anterior pituitary gland, where it binds to membrane receptors on the LH- and FSH-producing cells. It has been shown that LH/FSH-RH induces ultrastructural changes in the gonadotropin-pro-... 

One frequently suggested theory is based on the fact that the Ap protein deposit is the earlier phenomenon which would be followed, in certain subjects and in certain specific regions, by neurofibrillar degeneration. These lesions would perturb axonal flow, disturb neurotransmission and synaptic activity, then finally lead to neurone loss. However, for other authors, senile plaques and neurofibrillar degeneration are only markers of a certain type of degeneration and not the cause of the clinical signs which would be related to synaptic loss and neurone death. 

The nervous system has proven useful for the study of another timely problem that of intracellular distribution of enzymes and substrates of structural lipid formation and degradation in eukaryotic cells and the mechanisms of their assembly into membranes. The neuron constitutes a particularly useful model, by virtue of its peculiar shape. Because its axon may be larger than the perikaryon by several magnitudes, we can take advantage of the process of axonal flow to study the rate of appearance of labeled lipid at some distance from sites of synthesis. 

The use of labeled cholesterol or its precursor, mevalonate, has the appeal that a limited number of products are presumably formed and that the lipid is believed to turn over very slowly within the nervous system. It should be noted, however, that the observed slow cholesterol turnover reflects primarily the major brain pool of this lipid, myelin. Axonal flow studies are however directed at neurons, not at the glial cells that synthesize myelin. MacGregor et al., (1973) noted that following injection of cholesterol into the lumbar region of the chick, aproximodistal gradient of cholesterol was found in the sciatic nerve. The rate was thought to be about that observed for protein. Both cholesterol and cholesterol ester were detected, but the relative proportions were variable. A slow and fast rate of axonal flow were... 

Coenzyme A is synthesized in the mitochondria. ChAc is probably synthesized in the body of the nerve cell and is thought to be transported down the axon towards the nerve endings with the axonal flow of cytoplasm. The nerve endings are richer in ChAc than any other part of the neuron, but the exact localization of the enzyme in the nerve endings is not yet certain it may be present in the synaptic vesicles and/or in the cytoplasm of the nerve endings. 

Microtubules, an integral component of the cellular cy-toskeleton, consist of cytoplasmic tubes 25 nm in diameter and often of extreme length. Microtubules are necessary for the formation and function of the mitotic spindle and thus are present in all eukaryotic cells. They are also involved in the intracellular movement of endocytic and exocytic vesicles and form the major structural components of cilia and flagella. Microtubules are a major component of axons and dendrites, in which they maintain structure and participate in the axoplasmic flow of material along these neuronal processes. 

The idea that signals are transmitted along the nerve channels as an electric current had arisen as early as the middle of the nineteenth century. Yet even the first measurements performed by H. Helmholtz showed that the transmission speed is about lOm/s (i.e., much slower than electric current flow in conductors). It is known today that the propagation of nerve impulses along the axons of nerve cells (which in humans are as long as 1.5m) is associated with an excitation of the axon s outer membrane. 

Saltatory conduction results in a significant increase in the velocity of conduction of the nerve impulse down the axon compared to that of local current flow in an unmyelinated axon (see Table 4.2). The speed of conduction is... 

A second factor that influences the velocity of action potential conduction is the diameter of the axon. The greater the diameter is then, the lower the resistance to current flow along the axon. Therefore, the impulse is... 

Myelin facilitates conduction. Myelin is an electrical insulator, although its function of facilitating conduction in axons has no exact analogy in electrical circuitry [3], In unmyelinated fibers, impulse conduction is propagated by local circuits of ion current that flow into the active region of the axonal membrane, through the axon, and... 

Nilsson OG, Kalen P, Rosengren E, Bjorklund A. 1990. Acetylcholine release in the rat hippocampus as studied by microdialysis is dependent on axonal impulse flow and increases during behavioural activation. Neuroscience 36(2) 325-338. 

Above we noted that the fusion of synaptic vesicles or secretory granules with the plasma membrane of the synaptic terminal was caused by the arrival of an electrical signal flowing from the cell body down the axon. What is the nature of this electrical signal ... 

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