Fig. 1.1 
Neurosecretory peptidergic neurones involved in pituitary regulation and conventional neurotransmitter synapses. 
(a) The classical neurohypophysial system. Nerves project from cells of origin in the supraopticohypophysial and paraventriculohypophysial nuclei and end in the neural lobe. Oxytocin and ADH (vasopressin) are synthesised on the endoplasmic reticulum as part of a larger prohormone (which includes the specific oxytocin- and ADH-related neurophysins) and are then packaged into granules where they are stored in nerve endings and subsequently released into the peripheral blood. Since they affect tissues at a remote site they are classified as neurohormones. (b) The hypophysiotrophic neurones that arise in the hypothalamus and terminate in the median eminence in  contiguity to the specialised capillary blood vessels of the hypophysial-portal circulation. Their secretions are also considered to be neurohormones since they are secreted into blood and act remotely. (c) Conventional neurones may secrete the same substances as those that are released into the blood by the classical neurosecretory neurones. The main difference however, is that their products are secreted into a synaptic cleft. At receptors on neurones neuropeptides can act as classical neurotransmitters or as neuromodulators. A neuromodulator is a substance that modifies neuronal responses to neurotransmitters. Modified from Reichlin (1977); by courtesy of Raven Press.