A neurosecretory neuron residing mainly in the hypothalamic supraoptic and paraventricular nuclei and in a number of smaller accessory cell groups between these two nuclei, that is capable of secreting the hormones oxytocin or vasopressin, and sometimes both, into the systemic circulation. [ MP:0009467 http://www.ncbi.nlm.nih.gov/pubmed/10711808 GOC:nv ]