Neurosecretion(redirected from neurosecretory)
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The synthesis and release of hormones by neurons. Such neurons are called neurosecretory cells, and their products are often called neurohormones. Like conventional (that is, nonglandular or ordinary) neurons, neurosecretory cells are able to receive signals from other neurons. But unlike ordinary neurons that have cell-to-cell communication over short distances at synapses, neurosecretory cells release their product into an extracellular space that may be at some distance from the target cells. In an organism with a circulatory system, the neurohormones are typically sent by the vascular route to their target, whereas in lower invertebrates that lack an organized circulatory system the neurohormones apparently simply diffuse from the release site to the target. It is now clear that the nervous and endocrine systems interact in many ways, as in the suckling reflex of mammals (where the hormone oxytocin, a neurohormone, elicits milk ejection and is reflexly released in response to nerve impulses generated by stimulation of the nipples), and neurosecretory cells form a major link between them. See Endocrine system (invertebrate), Endocrine system (vertebrate)
It has been shown that peptides or low-molecular-weight proteins as well as amines, such as octopamine and dopamine, are released from neurosecretory cells into the circulatory systems of various animals, where they function as neurohormones. In classical neurosecretory cells, the secreted material is synthesized in the cell body by the rough-surfaced endoplasmic reticulum and subsequently packaged in the form of membrane-bounded granules by the Golgi apparatus, and is then typically transported along the axon to the axonal terminals, where it is stored until released. The release of neurohormones from axonal terminals into an extracellular space is triggered when the electrical activity (action potential) that is propagated by the axon enters the neurosecretory terminals. Calcium ions are essential for neurohormone release. See Biopotentials and ionic currents, Endoplasmic reticulum, Golgi apparatus
Neurohormones have a wide variety of functions. The role of the vertebrate hypothalamo-neurohypophysial system has been especially well elucidated. The pars nervosa is the site of release of vasopressin (also called the antidiuretic hormone) and oxytocin, and the median eminence is the release site for several hypothalamic neurohormones that regulate the adenohypophysis, the nonneural portion of the pituitary gland. See Adenohypophysis hormone, Nervous system (vertebrate), Neurohypophysis hormone, Pituitary gland
the ability of certain neurons, called neurosecretory cells, to elaborate and secrete specifically active substances—neurosecretions (also called neurohormones).
All neurons are able to synthesize and secrete physiologically active substances. The typical neuron synthesizes a mediator substance (chemical transmitter) that has a local effect at the site of secretion in the synapses. However, the neurosecretions produced by the neurosecretory cells have a remote action, spreading like the endocrine hormones through the bloodstream to all parts of the body and affecting the activity of the target organs and systems.
Neurosecretory cells are most developed in arthropods and vertebrates, although even the primitive nervous system of flat-worms contains such cells. In insects, neurosecretory cells are found in the suprapharyngeal ganglion; in crustaceans, within the ventral nerve cord; and in vertebrates, in the hypothalamus (in fish, neurosecretory cells are also found in the urohypophysis —the caudal section of the spinal cord).
A neurosecretory cell, unlike other neurons, contains secretory granules formed in the perikaryon around the cell nucleus. The synthesis of neurosecretions is initiated in the endoplasmatic reticulum of the perikaryon and is completed in the Golgi apparatus, where the neurosecretory granules are finally formed and stored. The granules are subsequently distributed along the axons (the longest neuronal outgrowths) and accumulate in the axons’ terminals. As a rule, the axons of neurosecretory cells come into contact with capillary blood vessels. This neurovascular connection is the point at which the neurosecretions are released into the bloodstream. In the most primitive invertebrates, which do not have a well-developed vascular system, neurosecretions can be transported by diffusion.
The principal neurosecretions in mammals and man include vasopressin, oxytocin, and several adenohypophysiotropic hormones, or releasing factors. The last are produced and secreted in the hypothalamus. They then penetrate the anterior lobe of the hypophysis by way of the hypophyseal portal system of blood vessels. Thus, it can be said that the hypophysis is under hypothalamic control. Within the hypophysis the releasing factors stimulate or inhibit the secretion of the adenohypophyseal hormones—chemical substances each of which is directed at a specific target gland. In this manner, the effect of the initial impulse that passed through the neurosecretory cell in the hypothalamus can be felt in very remote glands or in effectors, for example, in the thyroid gland. The hypophysis and the hypothalamus form an integrated complex—the hypothalamicohypophyseal system. In insects, an equivalent complex is formed between the protocerebrum and the corpus cardiacum; in crustaceans, between the X organ and the sinus gland.
Like other neurons, a neurosecretory cell perceives afferent signals that reach it from other divisions of the nervous system. However, the response to this input is transmitted by a neurohumoral mechanism and not by a propagative nerve impulse. By combining the properties of neurons and endocrine cells, neurosecretory cells thus unite neural and endocrine regulatory mechanisms into a single neuroendocrine system, thereby precisely coordinating an organism’s bodily functions and its adaptation to a changing environment.
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