Introduction and overview
As we consider regulation of homeostasis, two closely linked interacting physiological systems, the nervous and endocrine system, are critical. As a general rule, actions mediated by the nervous system are typically acute and relatively short-lived, whereas endocrine
effects are often slow to develop but frequently generate responses that continue for hours or even weeks.
Some simple examples illustrate these points. Consider what happens with overheating. As the core body temperature rises, warmer blood flowing to the hypothalamus and other brain areas initiates nerve impulses relayed by efferent spinal nerve tracts to the smooth muscle sphincters of the arterioles controlling blood flow to the dermis. This produces relaxation and thereby increased blood flow so that heat can be lost. Nerve fibers also stimulate the secretion of dermal sweat glands. These events increase the flow of warm blood near the body surface, and the release and evaporation of sweat serve to lower temperature and thereby return body temperature to its usual limits. These reactions occur very quickly.Anatomy and Physiology of Domestic Animals, Second Edition. R. Michael Akers and D. Michael Denbow. © 2013 John Wiley & Sons, Inc. Published 2013 by John Wiley & Sons, Inc.
Let's now consider how changes in the two hormones calcitonin (CT) and parathyroid hormone (PTH) act to regulate circulating concentrations of calcium. Decreases in calcium stimulate the release of PTH. PTH, in turn, directly impacts bone and kidney and indirectly impacts the GI tract. In the bone, PTH stimulates the secretion of calcium into extracellular fluids and ultimately blood by promoting the resorption of inorganic bone matrix by activated osteocytes and osteoblasts. In the kidney, PTH acts on the cells of the distal convoluted tubules of the nephrons to increase the recovery of calcium from the filtrate at the expense of phosphate secretion.
PTH also promotes the absorption of calcium from the lumen of the gut because it promotes activation of vitamin D, which is needed for maximal activity of calcium transport proteins by gut enterocytes. CT is secreted in situations when circulating calcium concentrations are too high. Cellular targets of CT are primarily in osseous tissue where the stimulated cells promote the deposition of calcium. However, it seems clear that the capacity to maintain calcium concentrations within a relatively narrow range is increased by having regulators that act specifically when concentrations are either too low or too high. As you might predict, responses that require synthesis of enzymes, that is, bone resorption or stimulation of transporter proteins (GI tract), are likely to occur over more prolonged periods compared with neural effects.However, it is not accurate to carry this generalization too far. For example, when dairy animals are prepared for milking, auditory cues (clanging of the milking equipment) and/or tactile stimulation to clean the udder and teats for attachment of the milking machine stimulate the secretion of oxytocin from the posterior pituitary. In the mammary tissue, oxytocin binds to receptors on the myoepithelial cells that surround the mammary alveoli. This induces the contraction of the myoepithelial cells. Since they are arranged in a network around the alveoli, this reduces volume, increases internal pressure, and forces milk into larger ducts and to the teat or nipple end for harvesting. In reality this is a neuroendocrine reflex; that is, neural input to the hypothalamus stimulates secretion of oxytocin that promotes milk ejection. Thus, a close relationship between the nervous system and endocrine system is necessary for this event to occur. Moreover, this endocrine-media ted effect is very rapid. The moral is simple: homeostasis is possible only because of the functional coordination between the nervous and endocrine systems.