Axons of the hypothalamic magnocellular and parvicellular neurosecretory nuclei form special nerve endings in the neurohormonal release areas of the neurohypophysis and median eminence. Similar terminals are formed by neurons of the organon vasculosum of the terminal lamina (OVLT), vascular sac and urophysis and by the receptor-like medullospinal cerebrospinal fluid (CSF)-contacting neurons of various vertebrates and humans. Axon-type processes of pinealocytes also form neurohormonal endings on the vascular surface of the pineal organ. All these axons have a similar ultrastructure: their terminal enlargements tentatively called neurohormonal or neurosecretory nerve endings are characterized by several synaptic and granular vesicles. In contrast to the usual viewpoint they do not terminate directly on vessels. Rather, these axons are attached by half-desmosomes among glial endfeet on the basal lamina of the external and/or vascular surface of the brain tissue. The terminals are separated from the basal lamina of vessels by perivasal spaces. These spaces are continuous with the subarachnoidal CSF space, therefore, the bioactive materials released from the terminals by exocytosis primarily enter the perivasal fluid and subarachnoideal external CSF and, secondarily, by diffusion into the vessels. The actual composition and flow of the external CSF, may have a modulatory effect on the action of the bioactive substances released by these terminals. Some neurons send their axons into the internal CSF and form there free endings containing granular vesicles. Exocytosis of vesicles indicates a release of molecules in the internal CSF. Ependymal cells in the spinal cord are connected by tight junctions, not regularly found in the ventricular ependyma, permitting an exchange between intercellular fluid of the brain tissue and ventricular CSF. This morphological organization suggests a role of bioactive materials released into the CSF in the nonsynaptic signal transmission of the brain tissue. Receptory dendrites of CSF-contacting neurons protruding into the internal and external CSF may serve for regulatory loops between neurohormonal, nonsynaptic and synaptic signals. The nerosecretory nerve endings may represent a phylogenetically old form of signal transduction in vertebrates. Similarly to the somatomotor nerve terminals of the procordate lancelet they all have a similar ultrastructure and terminate on the external surface of the brain among glial endfeet. As the estimated age of lancelets is about 600 million years, the axon terminals of neurohormonal-type may be considered as an old and basic structure for essential functions of the central nervous system.
|Title of host publication||Neural Synapse Research Trends|
|Publisher||Nova Science Publishers, Inc.|
|Number of pages||23|
|Publication status||Published - Jan 1 2007|
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