The nervous system is primarily responsible for rapid communication throughout the body.  As discussed in previous chapters, the nervous system utilizes two types of signals – electrical and chemical (Table 17.1).  Electrical signals are sent via the generation and propagation of action potentials which move along the membrane of a cell.  Once the action potential reaches the synaptic terminal, the electrical signal is converted to a chemical signal as neurotransmitters are released into the synaptic cleft.  When the neurotransmitters binds with receptors on the receiving (post-synaptic) cell, a new electrical signal is generated and quickly continues on to its destination.  In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition.

In contrast, the endocrine system relies on only a single method of communication: chemical signaling (Table X.1).  Hormones are the chemicals released by endocrine cells that regulate other cells in the body.   Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, triggering a response.  Because of this dependence on the cardiovascular system for transport, this type of communication is much slower than that observed for neural signaling.  As such, hormonal communication is usually associated with activities that go on for relatively long periods of time.

In general, the nervous system involves quick responses to rapid changes in the external environment, and the endocrine system is usually slower acting—taking care of the internal environment of the body, maintaining homeostasis, and controlling reproduction. This does not mean, however, that the two systems are completely independent of one another.  Take for example the release of adrenaline from the adrenal medulla as part of the ‘fight-or-flight’ response.  Although adrenaline uses blood for transportation throughout the body, the effects are evident within seconds after the event has occurred;  how does the response happen so quickly if hormones are usually slower acting?   It occurs so rapidly because the nervous and endocrine system are both involved in the process: it is the fast action of the nervous system responding to the danger in the environment that stimulates the adrenal glands to quickly secrete their hormones.  In such a situation, the nervous system causes a rapid endocrine response to deal with sudden changes in both the external and internal environments when necessary.

Stimuli Controlling Hormone Production

The production and secretion of hormones by endocrine glands are tightly regulated by various stimuli, including changes in the internal and external environment, physiological needs, and feedback mechanisms. These stimuli serve as signals that prompt endocrine glands to release specific hormones in response to specific physiological requirements.

One example of a stimulus controlling hormone production is the regulation of insulin secretion by blood glucose levels. When blood glucose levels rise, such as after a meal, pancreatic beta cells sense this increase and respond by secreting insulin into the bloodstream. Insulin acts to facilitate the uptake of glucose by cells, thereby reducing blood glucose levels and restoring homeostasis. Conversely, when blood glucose levels decrease, insulin secretion decreases, allowing for the release of glucose from storage sites to maintain glucose homeostasis.

Similarly, the hypothalamus-pituitary-adrenal (HPA) axis responds to stress-related stimuli by releasing adrenocorticotropic hormone (ACTH), which stimulates the adrenal glands to produce cortisol. During periods of stress, such as physical injury or psychological distress, the hypothalamus detects the stressor and releases corticotropin-releasing hormone (CRH), which signals the pituitary gland to release ACTH. In turn, ACTH stimulates the adrenal cortex to secrete cortisol, initiating the body’s stress response and helping to mobilize energy reserves to cope with the stressor.

Other stimuli controlling hormone production include changes in blood levels of ions or nutrients, hormonal signals from other endocrine glands, and neural signals from the autonomic nervous system. For example, the release of parathyroid hormone (PTH) by the parathyroid glands is stimulated by low blood calcium levels, triggering PTH secretion to increase calcium levels by promoting its release from bone and enhancing its reabsorption from the kidneys.

Moreover, hormonal signals from other endocrine glands, such as thyroid-stimulating hormone (TSH) from the anterior pituitary, regulate the production and secretion of hormones from target glands like the thyroid gland. TSH stimulates the thyroid gland to produce thyroid hormones (T3 and T4), which regulate metabolism and energy balance.

Endocrine Organs

Hormones are released by secretory cells that are derived from epithelial tissue.  Often, these cells are clustered together, forming endocrine glands.  Unlike exocrine glands, which have a duct for conveying secretions to the outside of the body (e.g., sweat gland), endocrine glands secrete substances directly into the surrounding interstitial fluid.  From there, hormones then enter the bloodstream for distribution throughout the body.

The major endocrine glands found in the human body include the pituitary gland, thyroid gland, parathyroid glands, thymus gland, adrenal glands, pineal gland, testes, and ovaries (Figure X.1). While some of the glands are pure endocrine (e.g., thyroid gland), others serve both endocrine and exocrine function. For example, the pancreas contains cells that secrete digestive enzymes and juices into the small intestine (exocrine function) and cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels.

In addition to the endocrine glands, major organs of the body show endocrine function including the hypothalamus, heart, kidneys, stomach, small intestine, and liver.  Moreover, adipose tissue has long been known to produce hormones, and recent research has revealed a role for bone tissue in hormone production and secretion.