Supraoptic nucleus  The supraoptic nucleus (SON) is a nucleus of magnocellular neurosecretory cells in the hypothalamus of the mammalian brain. The nucleus is situated at the base of the brain, adjacent to the optic chiasm. In humans, the SON contains about 3,000 neurons. The cell bodies produce the peptide hormone vasopressin, which is also known as anti-diuretic hormone (ADH). This chemical messenger travels via the bloodstream to its target cells in the papillary ducts in the kidneys, enhancing water reabsorption. In the cell bodies, the hormones are packaged in large, membrane-bound vesicles that are transported down the axons to the nerve endings. The secretory granules are also stored in packets along the axon called Herring bodies. Similar magnocellular neurons are also found in the paraventricular nucleus. Each neuron in the nucleus has one long axon that projects to the posterior pituitary gland, where it gives rise to about 10,000 neurosecretory nerve terminals. The magnocellular neurons are electrically excitable: In response to afferent stimuli from other neurons, they generate action potentials, which propagate down the axons. When an action potential invades a neurosecretory terminal, the terminal is depolarised, and calcium enters the terminal through voltage-gated channels. The calcium entry triggers the secretion of some of the vesicles by a process known as exocytosis. The vesicle contents are released into the extracellular space, from where they diffuse into the bloodstream. Vasopressin (antidiuretic hormone, ADH) is released in response to solute concentration in the blood, decreased blood volume, or blood pressure. Some other inputs come from the brainstem, including from some of the noradrenergic neurons of the nucleus of the solitary tract and the ventrolateral medulla. However, many of the direct inputs to the supraoptic nucleus come from neurons just outside the nucleus (the "perinuclear zone"). Of the afferent inputs to the supraoptic nucleus, most contain either the inhibitory neurotransmitter GABA or the excitatory neurotransmitter glutamate, but these transmitters often co-exist with various peptides. Other afferent neurotransmitters include noradrenaline (from the brainstem), dopamine, serotonin, and acetylcholine. The supraoptic nucleus is an important "model system" in neuroscience.