Processes of magnocellular neurosecretory cells (MNCs) are easily identifiable on the basis of their content in neurosecretory granules in the neuropil of the rostral division of the paraventricular nucleus (PVN) of the domestic fowl. In specimens sacrificed during the winter the synaptic organization of the neuropil and the pattern of synapses ending on neurosecretory processes were studied at the ultrastructural level. Synapses in the rostral part of the PVN neuropil may be divided into three main categories on the basis of their morphology and their content of clear and dense-core synaptic vesicles. These different types of terminals can be attributed to aminergic, peptidergic or other types of synapses. The percent distribution of synapses within these categories differs when all synapses observed in the neuropil or only those ending on MNC processes are compared. Present ultrastructural data obtained in birds support two physiological hypotheses already suggested for mammals, i.e., the probable existence of a recurrent pathway to MNCs via an interneuron, and the importance of aminergic and peptidergic input in regulating the electrical activity ov MNCs.
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