The Endocrine and Nervous Systems Are Integrated in Their Control of Physiological Processes
The endocrine system interacts with the other main regulatory system, the nervous system, which coordinates activities that require rapid control. An example of the close interaction of the two systems is the reflex in which suckling causes the release of milk.
Suckling initiates the transmission of nerve impulses from the mammary gland to the hypothalamus (by way of the spinal tract). Neurosecretory neurons within the supraoptic and paraventricular nuclei are stimulated to synthesize oxytocin. Oxytocin is transported along axons of these nerves and is released from nerve endings in the posterior pituitary into the blood vascular system. Oxytocin is then carried to the mammary gland, where it causes contraction of myoepithelial cells. These cells surround the smallest unit of milk-secreting cells, called an alveolus. This results in the movement of milk into the large cisternae adjacent to the teat and subsequently into the teat.The interaction between the nervous and endocrine systems can be even more direct. For example, endocrine cells of the adrenal medulla are directly controlled by preganglionic neurons of the adrenal medulla, and the medullary hormones are released immediately in response to stressful stimuli. The endocrine and nervous systems also share transmitters; substances such as epinephrine, dopamine, histamine, and somatostatin are found in both endocrine and neural tissues.
The endocrine system is involved in control of physiological functions, including metabolism, growth, and reproduction. Metabolism can be divided into two forms: energy and mineral. The hormones that control energy metabolism include insulin, glucagon, cortisol, epinephrine, thyroid hormone, and growth hormone. The hormones that control mineral metabolism include parathyroid hormone, calcitonin, angiotensin, and renin. The hormones that control growth include growth hormone, thyroid hormone, insulin, estrogen and androgen (both reproductive hormones), and a large number of growth factors.
The hormones that control reproduction include estrogen, androgen, progesterone, luteinizing hormone (LH), follicle-stimulating hormone (FSH), prolactin (PRL), and oxytocin.One of the important characteristics of the endocrine system is the amplification of the signal. The action of one steroid molecule to activate a gene can result in the formation of many messenger ribonucleic acid (mRNA) molecules, and each of these can induce the formation of many enzyme molecules. Also, one protein molecule can influence the formation of many cyclic adenosine 3',5,-monophosphate (cAMP) molecules, and each of these can activate many enzymes. Amplification is the basis for the sensitivity of the endocrine system, which allows small amounts of hormones in plasma (10^,1 to IO'12 mol) to produce significant biological effects. Hormone action also influences rates of existing enzyme reactions, but not the initiation of new reactions. This implies that there are certain basal levels of enzyme activities even in the absence of hormones. Hormone action is relatively slow and prolonged, with the effects of hormones lasting minutes to days. This contrasts with the nervous system, in which the response is rapid and short (milliseconds to seconds).