4.6: Communication and the Endocrine System

Learning Objectives

1. Explain the difference between chemical signals in the nervous system and chemical signals in the endocrine system
2. Understand the reciprocal interactions between the influence of hormones on behavior and behavior on hormones
3. Identify at least three endocrine glands and describe their primary functions

Overview

Throughout the nervous system, neurons communicate via electrical and chemical signals. Another form of chemical communication is the secretion of hormones into the bloodstream, which is accomplished via endocrine glands located in the endocrine system. This chapter introduces the endocrine system and the major endocrine glands found in the brain and the body.

Communication in the Nervous System versus the Endocrine System

Nerve impulses are covered in depth in the nervous system communication chapter. Briefly, neurons in the nervous system communicate via electrical and chemical signals. When a neuron transmits an electrical signal, called a nerve impulse (or action potential), it travels down the axon and causes neurotransmitters to be released into the synapse. This chemical signal then influences the receiving cell in either an excitatory or inhibitory manner. A cell that receives nerve impulses from a neuron may be excited to perform a function, inhibited from carrying out an action, or otherwise controlled. In this way, the information transmitted by the nervous system is specific to particular cells and is transmitted very rapidly. In fact, the fastest nerve impulses travel at speeds greater than 100 meters per second! Compare this to the chemical messages carried by the hormones that are secreted into the blood by endocrine glands (below). These hormonal messages are “broadcast” to all the cells of the body, and they can travel only as quickly as the blood flows through the cardiovascular system.

Neural Transmission versus Hormonal Communication

Hormones are similar in function to neurotransmitters, the chemicals used by the nervous system in coordinating animals’ activities (as mentioned above). However, hormones can operate over a greater distance and over a much greater temporal range than neurotransmitters. Neural and hormonal communication both rely on chemical signals, but several important differences exist. Communication in the nervous system is analogous to traveling on a train. You can use the train in your travel plans as long as tracks exist between your intended origin and destination. Likewise, neural messages can travel only to destinations along existing nerve tracts. Hormonal communication, on the other hand, is like traveling in a car. You can drive to many more destinations than train travel allows because there are many more roads than railroad tracks. Similarly, hormonal messages can travel anywhere in the body via the circulatory system; any cell receiving blood is potentially able to receive a hormonal message.

Not all cells are influenced by each and every hormone. Rather, any given hormone can directly influence only cells that have specific hormone receptors for that particular hormone. Cells that have these specific receptors are called target cells for the hormone. When a hormone engages its receptor, a series of subsequent events either activates enzymes or turns genes on or off to regulate protein synthesis. The newly synthesized proteins may then activate or deactivate other genes, causing additional effects. Importantly, sufficient numbers of appropriate hormone receptors must be available for a specific hormone to produce any effects. For example, testosterone is important for male sexual behavior. If men have too little testosterone, their sexual motivation may be low, which can be restored by testosterone treatment. However, if men have normal or even elevated levels of testosterone yet display low sex drive, then it might be possible for a lack of receptors to be the cause, in which case treatment with additional hormones will not be effective.

Another difference is that neural messages are all-or-none events that have rapid onset and offset: neural signals can take place in milliseconds. For example, the nervous system regulates immediate food intake and directs body movement, changes in the body that are relatively rapid. In contrast, hormonal messages are graded events that may take seconds, minutes, or even hours to occur. Hormones can mediate long-term processes, such as growth, development, reproduction, and metabolism.

Interactions Between Hormones and Behavior

The scientific study of the interaction between hormones and behavior is called behavioral endocrinology. This interaction is bidirectional: hormones can influence behavior, and behavior can sometimes influence hormone concentrations. Hormones are chemical messengers produced and released by specialized glands called endocrine glands (see below). Hormones are released from these glands into the blood, where they travel to act on target structures anywhere in the body, influencing the nervous system to regulate an individual's behaviors (such as aggression, mating, and parenting). Examples of hormones that influence behavior include steroid hormones such as testosterone (a common type of androgen), estradiol (a common type of estrogen), progesterone (a common type of progestin), and cortisol (a common type of glucocorticoid). Several types of protein or peptide (small protein) hormones also influence behavior, including oxytocin, vasopressin, prolactin, and leptin.

There are many ways that hormones influence behavior and behavior feeds back to influence hormone secretion. Some examples include hormones in the mediation of food and fluid intake, social interactions, salt balance, learning and memory, stress coping, as well as psychopathology including depression, anxiety disorders, eating disorders, postpartum depression, and seasonal depression. Some other hormone-behavior interactions that are related to reproductive behaviors are described in Chapter 13.1.

Introduction to the Endocrine System

As the endocrine system is not a part of the nervous system, this section may surprise you. However, as textbook author James W. Kalat states, “Hormonal influences resemble synaptic transmission in many ways, including the fact that many chemicals serve both as neurotransmitters and hormones” (Kalat, 2019, page 59). Thus, to understand many of the topics that will be covered in upcoming chapters, you will need a basic understanding of the endocrine system.

The endocrine system is a system of glands called endocrine glands that release chemical messenger molecules called hormones into the bloodstream. Other glands of the body, including sweat glands and salivary glands, also secrete substances but not into the bloodstream. Instead, they secrete them through ducts that carry them to nearby body surfaces. These other glands are called exocrine glands.

Endocrine hormones must travel through the bloodstream to the cells they affect, and this takes time. Because endocrine hormones are released into the bloodstream, they travel throughout the body wherever blood flows. As a result, endocrine hormones may affect many cells and have body-wide effects. Endocrine hormones may cause effects that last for days, weeks, or even months.

Major Glands of the Endocrine System

Figure \(\PageIndex{1}\): Endocrine Glands. All of the glands in this figure are part of the endocrine system. Note that the ovary and testis are the only endocrine glands that differ in males and females.

The major glands of the endocrine system are shown in Figure \(\PageIndex{1}\). The glands in the figure are described briefly in the rest of this section. Refer to the figure as you read about the glands in the following text.

Endocrine Glands in the Brain

The pituitary gland is located at the base of the brain. It is controlled by the nervous system via the brain structure called the hypothalamus, to which it is connected by a thin stalk (called the infundibulum). The pituitary gland consists of two lobes, called the anterior (front) lobe and posterior (back) lobe. The posterior lobe, composed of nervous tissue, stores and secretes hormones synthesized by the hypothalamus, specifically the hormones oxytocin and vasopressin (also called antidiuretic hormone).

The anterior lobe, composed of glandular tissue, synthesizes and secretes many of its own endocrine hormones, also under the influence of the hypothalamus. One endocrine hormone secreted by the anterior pituitary gland is growth hormone, which stimulates cells throughout the body to synthesize proteins and divide. Most of the other endocrine hormones secreted by the anterior pituitary gland are called tropic hormones because they control other endocrine glands. Generally, tropic hormones direct other endocrine glands to secrete either more or less of their own hormones, such as adrenocorticotrophic hormone (ACTH) stimulating the adrenal gland to produce the "stress" hormone cortisol. This is why the pituitary gland is often referred to as the “master gland” of the endocrine system. Since the hypothalamus is the structure that controls both the anterior and posterior parts of the pituitary, you can think of it as the "master" of the "master gland".

The pineal gland (also called the epithalamus) is a tiny gland located near the center of the brain. It secretes the hormone melatonin, which controls the sleep-wake cycle and several other processes. The production of melatonin is stimulated by darkness and inhibited by light. Cells in the retina of the eye detect light and send signals to a structure in the hypothalamus named the suprachiasmatic nucleus (SCN). Nerve fibers carry the signals from the SCN to the pineal gland via the autonomic nervous system.

Endocrine Glands in the Body

Each of the other major glands of the endocrine system is summarized briefly below. Several of these endocrine glands are also discussed in greater detail as they relate to other topics in separate chapters.

• The thyroid gland is a large gland in the neck. Thyroid hormones such as thyroxine increase the rate of metabolism in cells throughout the body. They control how quickly cells use energy and make proteins.
• The thymus gland is located in front of the heart. It is the site where immune system cells called T cells mature. T cells are critical to the adaptive immune system, facilitating the body's adaption to specific pathogens.
• The pancreas is located near the stomach. Its endocrine hormones include insulin and glucagon, which work together to control the level of glucose in the blood. The pancreas also secretes digestive enzymes into the small intestine.
• The two adrenal glands are located above the kidneys. Adrenal glands secrete several different endocrine hormones, including the hormone epinephrine (also known as adrenaline), which is involved in the fight-or-flight response. Other endocrine hormones secreted by the adrenal glands have a variety of functions. For example, the hormone aldosterone helps to regulate the balance of minerals in the body. The "stress" hormone cortisol is also an adrenal gland hormone.
• The paired gonads include the ovaries in females and testes in males. They secrete sex hormones, such as testosterone (in males) and estrogen (in females). These hormones control sexual maturation during puberty and the production of gametes (sperm or egg cells) by the gonads after sexual maturation.

Summary

Neurons communicate via electrical and chemical signals. The endocrine system also communicates via chemical signals, using hormones traveling through the bloodstream. Hormones are secreted by endocrine glands, and have similar features to neurotransmitters (which are released at synapses between neurons in the nervous system). Both neural and hormonal communication rely on chemical signals. However, hormones can operate over a greater distance (anywhere in the body via the circulatory system) and over a much greater time length, and can thus mediate long-term processes. A particular hormone can only influence cells that have receptors for that specific hormone. The interaction between hormones and behavior is bidirectional: hormones can influence behavior, and behavior can sometimes influence hormone concentrations.

A basic understanding of the endocrine system is necessary to understand many of the topics covered in other chapters. Endocrine glands in the brain include the pituitary gland (located at the base of the brain), the hypothalamus (immediately above the pituitary gland), and the pineal gland (near the center of the brain). The pituitary gland is controlled by the hypothalamus, to which it is connected by a thin stalk (the infundibulum). The pituitary gland consists of an anterior (front) lobe and a posterior (back) lobe. The posterior lobe (composed of nervous tissue) stores and secretes hormones synthesized by the hypothalamus, specifically oxytocin and vasopressin (also called antidiuretic hormone). The anterior lobe (composed of glandular tissue) synthesizes and secretes many of its own endocrine hormones, one of which is growth hormone, which stimulates cells throughout the body to synthesize proteins and divide. Most of the anterior pituitary gland hormones control other endocrine glands, such as adrenocorticotrophic hormone (ACTH), which stimulates the adrenal gland to produce cortisol. The pineal gland secretes the hormone melatonin, which controls the sleep-wake cycle.

Some major endocrine glands in the body include the thyroid gland, the thymus gland, the pancreas, the adrenal glands, and the gonads. The thyroid gland is in the neck and secretes hormones that control how quickly cells use energy and make proteins. The thymus gland is located in front of the heart, and is the site where T cells (of the immune system) mature. T cells facilitate the body's adaption to specific pathogens. The pancreas is located near the stomach. Its endocrine hormones (insulin and glucagon) work together to control the level of glucose in the blood. The two adrenal glands (located above the kidneys) secrete several different endocrine hormones, including the hormone epinephrine (adrenaline), involved in the fight-or-flight response, and the "stress" hormone cortisol. The paired gonads include the ovaries in females and testes in males. The gonads secrete sex hormones (such as testosterone in males and estrogen in females), which control sexual maturation during puberty and the production of gametes (sperm or egg cells).

References

Kalat, J.W. (2019). Biological Psychology (13th ed.). Cengage.

Additional Resources

Most people want to live a long, healthy life. Geneticist Cynthia Kenyon’s research suggests that endocrine hormones may be a key to human longevity. Watch this fascinating TED talk to learn how.

Attributions

1. Endocrine glands by Mariana Ruiz Villarreal CC BY-NC 3.0 via CK-12 Foundation
2. Text adapted from:
   1. " Introduction to the Nervous System" by Suzanne Wakim & Mandeep Grewal, LibreTexts is licensed under CC BY-NC .
   2. " Introduction to the Endocrine System" by Suzanne Wakim & Mandeep Grewal, LibreTexts is licensed under CC BY-NC .
   3. Hormones & Behavior by Randy J. Nelson, licensed CC BY-NC-SA 4.0 via Noba Project.
3. Changes: Text (and image) from above three sources with some minor modifications and additional content added by Naomi I. Gribneau Bahm, PhD., Psychology Professor at Cosumnes River College, Sacramento, CA.