14.1 An Overview of the Endocrine System

Communication is a process in which a sender transmits signals to one or more receivers to control and coordinate actions. In the human body, two major organ systems participate in relatively “long distance” communication: the nervous system and the endocrine system. Together, these two systems are primarily responsible for maintaining homeostasis in the body.

Neural and Endocrine Signalling

The nervous system uses two types of intercellular communication—electrical and chemical signalling—either by the direct action of an electrical potential, or in the latter case, through the action of chemical neurotransmitters such as serotonin or noradrenaline. Neurotransmitters act locally and rapidly. When an electrical signal in the form of an action potential arrives at the synaptic terminal, they diffuse across the synaptic cleft (the gap between a sending neuron and a receiving neuron or muscle cell). Once the neurotransmitters interact (bind) with receptors on the receiving (post-synaptic) cell, the receptor stimulation is transduced into a response such as continued electrical signalling or modification of cellular response. The target cell responds within milliseconds of receiving the chemical “message”; this response then ceases very quickly once the neural signalling ends. In this way, neural communication enables body functions that involve quick, brief actions, such as movement, sensation, and cognition. In contrast, the endocrine system uses just one method of communication: chemical signalling. These signals are sent by the endocrine organs, which secrete chemicals—the hormone—into the extracellular fluid. Hormones are transported primarily via the bloodstream throughout the body, where they bind to receptors on target cells, inducing a characteristic response. As a result, endocrine signalling requires more time than neural signalling to prompt a response in target cells, though the precise amount of time varies with different hormones. For example, the hormones released when you are confronted with a dangerous or frightening situation, called the fight-or-flight response, occur by the release of adrenal hormones—adrenaline and noradrenaline—within seconds. In contrast, it may take up to 48 hours for target cells to respond to certain reproductive hormones.

In addition, endocrine signalling is typically less specific than neural signalling. The same hormone may play a role in a variety of different physiological processes depending on the target cells involved, for example, the hormone oxytocin promotes uterine contractions in women in labour and it is also important in breastfeeding and may be involved in the sexual response and in feelings of emotional attachment in both males and females.

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 (Table 14.1.1). So how does the fight-or-flight response that was mentioned earlier happen so quickly if hormones are usually slower acting? It is because the two systems are connected. It is the fast action of the nervous system in response to the danger in the environment that stimulates the adrenal glands to secrete their hormones. As a result, the nervous system can cause rapid endocrine responses to keep up with sudden changes in both the external and internal environments when necessary.

Table 14.1.1. Endocrine and nervous systems

Structures of the Endocrine System

The endocrine system consists of cells, tissues and organs that secrete hormones as a primary or secondary function. The endocrine gland is the major player in this system. The primary function of these ductless glands is to secrete their hormones directly into the surrounding fluid. The interstitial fluid and the blood vessels then transport the hormones throughout the body. The endocrine system includes the pituitary, thyroid, parathyroid, adrenal and pineal glands (Figure 14.1.1). Some of these glands have both endocrine and non-endocrine (exocrine) functions, for example, the pancreas contains cells that function in digestion as well as cells that secrete the hormones insulin and glucagon, which regulate blood glucose levels. The hypothalamus, thymus, heart, kidneys, stomach, small intestine, liver, skin, female ovaries and male testes are other organs that contain cells with endocrine function. Additionally, adipose tissue has long been known to produce hormones and recent research has shown that even bone tissue has endocrine functions.

The ductless endocrine glands are not to be confused with the body’s exocrine system, whose glands release their secretions through ducts. Examples of exocrine glands include the sebaceous and sweat glands of the skin. As just noted, the pancreas also has an exocrine function: most of its cells secrete pancreatic secretion through the pancreatic and accessory ducts to the lumen of the small intestine.

Other Types of Chemical Signalling

In endocrine signalling, hormones secreted into the extracellular fluid diffuse into the blood or lymph and can then travel great distances throughout the body. In contrast, autocrine signalling takes place within the same cell. An autocrine (auto- = “self”) is a chemical that elicits a response in the same cell that secreted it. Interleukin-1, or IL-1, is a signalling molecule that plays an important role in inflammatory response. The cells that secrete IL-1 have receptors on their cell surface that bind these molecules, resulting in autocrine signalling.

Local intercellular communication is the province of the paracrine, also called a paracrine factor, which is a chemical that induces a response in neighbouring cells. Although paracrines may enter the bloodstream, their concentration is generally too low to elicit a response from distant tissues. A familiar example to those with asthma is histamine, a paracrine that is released by immune cells in the bronchial tree. Histamine causes the smooth muscle cells of the bronchi to constrict, narrowing the airways. Another example is the neurotransmitters of the nervous system, which act only locally within the synaptic cleft.

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