Introduction
Cell-to-cell communication among the trillions of cells that make up multicellular animals relies primarily on the specialized tissues of the nervous and endocrine systems. These two systems are intricately connected, with the former having evolved from the latter during the past 500 million years of animal life. The endocrine system consists of specialized ductless glands located throughout the animal body that produce and secrete hormones directly into the bloodstream. Hormones are chemical messengers that usually are composed of protein or steroid subunits. The bloodstream transports the hormones to various target body tissues, where the hormones contact cell membranes and trigger a sequence of enzyme reactions that ultimately result in the activation or inactivation of genes located on chromosomes in the cell nucleus.
A gene is a segment of a chromosome that is composed of deoxyribonucleic acid (DNA). The DNA nucleotide sequence of the gene encodes a molecule of messenger ribonucleic acid (mRNA) which, in turn, encodes a specific protein for the given gene. If the control sequence of a gene is activated, then ribonucleic acid (RNA) and protein will be produced. If the control sequence of a gene is inactivated, then RNA and protein will not be produced. Hormones target the genes in specific cells to start or stop the manufacture of certain proteins. Within cells and the entire organism, proteins perform important functions. Therefore, hormones control the production of proteins by genes and, as a result, control many activities of the entire animal.
The nervous system, which in vertebrate animals has evolved to become more elaborate than the endocrine system, consists of billions of neurons (nerve cells) that conduct electrical impulses throughout the body. Neurons transmit information, contract and relax muscles, and detect pressures, temperature, and pain. Neuron networks are most dense in the brain (where there are 100 billion neurons) and spinal cord, where much of the electrical information is centralized, relayed, and analyzed. Neurons must communicate electrical information across the gaps, or synapses, that separate them. To accomplish this goal, the transmitting neuron releases hormones called neurotransmitters, which diffuse across the synapse to the receiving neuron, thereby instructing the receiving neuron to continue or stop the conduction of the electrical message. There are many different types of neurotransmitters, just as there are many different types of regular hormones.
Nervous System-Endocrine System Interactions
The link between the nervous and endocrine systems lies in two glands located between the cerebrum and the brain stem, the hypothalamus and the hypophysis (the pituitary gland). Electrical impulses from neurons in the cerebral cortex may activate the hypothalamus to release hormones that activate the hypophysis to release its hormones, which in turn activate or inactivate other endocrine glands throughout the body. These glands include the thyroid, parathyroids, thymus, pancreas, adrenals, and reproductive organs. This entire system operates by negative feedback homeostasis
so that, once information is transferred and specific bodily functions are achieved, nervous or hormonal signals travel back to the hypothalamus to terminate any further action.
Animal behavior occurs as a result of the actions of the nervous and endocrine systems. There is a complex interplay among these two body systems, the environment, and an individual’s genetic makeup in terms of the cause-and-effect, stimulus-response events that constitute behavior. An animal receives external information via its special senses (eyes, ears, nose, mouth) and somatic sense organs (touch, pain, temperature, pressure). This external information travels along sensory neurons toward the brain and spinal cord, where the information is analyzed and a motor response to the external stimulus is initiated. Some of these motor responses will be directed toward the sense organs, locomotory muscles, and organs such as the heart and intestines. Other impulses will be directed toward the hypothalamus, which controls body cycles such as all endocrine system hormones, heart rate, sleep-wake cycles, and hunger.
When the hypothalamus releases the hormone corticoliberin, the pituitary gland (the hypophysis) releases the hormones thyrotropin (which activates the thyroid gland), prolactin (which stimulates milk production in the female breast), and growth hormone (which triggers growth in children and metabolic changes in adults). When the thyroid gland is activated, hormones such as thyroxine and triiodothyronine are released to accelerate cellular metabolism, an event that may occur in certain situations such as stress or fight-or-flight encounters.
If the pituitary gland releasesadrenocorticotropic hormone (ACTH), the adrenal glands will be activated to release their hormones. The adrenal cortex produces and secretes a variety of hormones, such as aldosterone, which regulates the blood-salt balance directly and blood pressure indirectly; cortisol, which accelerates body metabolism; and androgens, or sex hormones. All of these are steroid hormones, which are involved in rapidly preparing the body for strenuous performance. Even more pronounced are the effects of the adrenal medulla, which produces and secretes the hormone neurotransmitters epinephrine and norepinephrine; these two hormones accelerate heart, muscle, and nerve action as well as stimulate the release of fat and sugar into the bloodstream for quick energy, all of which are extremely important for spontaneous activity such as fighting with or fleeing from enemies. The control of sugar storage and release from the liver by the pancreatic hormones insulin and glucagon also are important in this process.
The Effects of Hormones on Behavior
The study of hormones and their effects on individual and group behaviors is of immense interest to psychologists. Hormones represent the biochemical control signals for much of animal and human behaviors. Understanding precisely how hormones affect individuals, both psychologically and physiologically, could be of great value in comprehending many different human behaviors, in treating abnormal behaviors, and in helping individuals cope psychologically with disease and stress. The hormonal control of behavior in humans and in many other animal species has been extensively studied, although much research remains to be performed. Hormones have been clearly linked to reproductive behavior, sex-specific behavioral characteristics, territoriality and mating behaviors, physiological responses to certain external stimuli, and stress.
The pineal gland, located in the posterior cerebrum, releases the hormone melatonin, which regulates the body’s circadian rhythms
and possibly its sexual cycles as well. Melatonin is normally synthesized and secreted beginning shortly after dusk throughout the night and ending around dawn. It thus corresponds with the individual’s normal sleep-wake cycle. Melatonin may play an important role in humans adapting to shift work. It is promoted as a nutritional supplement to help people get a good night’s sleep.
Hormones and Reproduction
The most extensive research involving hormonal effects on behavior has been conducted on reproductive behavior. Among the most powerful behavior-influencing hormones are the pituitary gonadotropins luteinizing hormone (LH) and follicle-stimulating hormone (FSH). These two hormones target the reproductive organs of both male and female children and stimulate these organs to initiate sexual development and the production of sexual steroid hormones—estrogen and progesterone in women, testosterone in men. These sex hormones are responsible not only for the maturation of the reproductive organs but also for secondary sexual characteristics such as male aggression and female nesting behavior.
Reproductive patterns vary from species to species in occurrence, repetition of occurrence, and behaviors associated with courtship, mating, and caring for young. The achievement of reproductive maturity and reproductive readiness in a given species is subject to that species’ circadian rhythm, a phenomenon regulated by hormones released from the hypothalamus, hypophysis, and pineal gland. These three endocrine glands are influenced primarily by the earth’s twenty-four-hour rotation period and the twenty-eight-day lunar cycle. Furthermore, genetically programmed hormonal changes at specific times during one’s life cycle also play a major role in the occurrence of reproductive behaviors.
In female vertebrates, LH, FSH, and estrogen are responsible for the maturation of the ovaries, the completion of meiosis (chromosome halving) and the release of eggs for fertilization, and secondary sexual characteristics. The secondary sexual characteristics involve physiological and closely related behavioral changes. In bird species, these changes include the construction of a nest and receptivity to dominant males during courtship rituals. In mammals, these same hormones are involved in female receptivity to dominant males during courtship. Physiological changes in mammals include the deposition of fat in various body regions, such as the breasts and buttocks, and increased vascularization (more blood vessel growth) in the skin. Females of most mammal and bird species go into heat, or estrus, one or several times per year, based on hormonally regulated changes in reproductive organs. Female humans follow a lunar menstrual cycle in which LH, FSH, estrogen, and progesterone oscilla in production rates. These hormonal variations influence female body temperature and behavior accordingly.
Male sexual behavior is controlled predominantly by testosterone produced in the testicles and male androgens produced in the adrenal cortex. These steroid hormones cause muscle buildup, increased hair, and aggressive behavior. As a consequence, such steroids are often used (illegally) by athletes to improve their performance. In a number of mammal and bird species, elevation of sex steroids causes increased coloration, which serves both as an attractant for females and as an antagonistic signal to competitor males. The aggressive behavior that is stimulated by the male sex steroid hormones thus plays a dual role in courtship and mating rituals and in territorial behavior, phenomena that are tightly linked in determining the biological success of the individual.
Pheromones
are hormones released from the reproductive organs and skin glands. These hormones target the sense organs of other individuals and affect the behavior of these individuals. Sex pheromones, for example, attract males to females and vice versa. Other pheromones enable a male to mark his territory and to detect the intrusion of competitor males into his territory. Others enable an infant to imprint on its mother. Such hormones number in the hundreds, but only a few dozen have been studied in detail. Pheromones released by males serve as territorial markers, as is evidenced by most mammalian males spraying urine on objects in their own territory. Exchanges of pheromones between males and females are important stimulants for courtship and mating. In some species, the release of pheromones—or even the sight of a potential mate—will trigger hormonally controlled ovulation in the female. Furthermore, in several species, such as elephant seals and lions, the takeover of a harem by a new dominant male, a process that usually involves the murder of the previous male’s offspring, stimulates the harem females to ovulate. The diversity of reproductive behaviors that is regulated by hormones seems to be almost as great as the number of species.
Hormones and Stress
The fight-or-flight response is a hormone-controlled situation in which the body must pool all its available resources within a relatively short time span. The detection of danger by any of the special senses (sight, smell, hearing) triggers the hypothalamus to activate the pituitary gland to release adrenocorticotropic hormone, which causes the adrenal gland to release its highly motivating hormones and neurotransmitters. Many body systems are subsequently affected, especially the heart and circulatory system, the central nervous system, the digestive system, and even the immune system. One reason the fight-or-flight response is of major interest to psychologists is its link to stress.
Stress is overexcitation of the nervous and endocrine systems. It is caused by the body’s repeated exposure to danger, excessive physical exertion, or environmental pressures that affect the individual psychologically. Stress is a major problem for humans in a fast-paced technological society. The physiological and behavioral manifestations of stress are very evident. There is considerable evidence that stress is associated with heart disease, cancer, weakened immune systems, asthma, allergies, accelerated aging, susceptibility to infections, learning disorders, behavioral abnormalities, insanity, and violent crime. The demands that are placed on individuals in fast-paced, overpopulated societies are so great that many people exhibit a near-continuous fight-or-flight response. This response, in which the body prepares for maximum physical exertion in a short time span, is the physiological basis of stress. It is not intended to be maintained for long periods of time; if it is not relieved, irreparable effects begin to accumulate throughout the body, particularly within the nervous system. Medical psychologists seek to understand the hormonal basis of physiological stress to treat stress-prone individuals.
Hormones and Aging
Another hormone that greatly influences human behavior and development is human growth hormone (HGH). This hormone is produced by the anterior pituitary (adenohypophysis) gland under the control of the hypothalamus. HGH production peaks during adolescence, corresponding to the growth spurt. Although it is produced throughout life, it declines with age in all species studied to date. In humans, HGH production tends to drop quickly beginning in the thirties so that by age sixty, HGH production is only about 25 percent of what it was earlier in life, and it continues to decline until death. The decrease in HGH production with age has been tied to thinning of skin and wrinkle formation, muscle wasting, sleep problems, cognitive and mood changes, decreased cardiac and kidney function, lessening of sexual performance, and weakening of bones, contributing to osteoporosis. Nutritional supplements including the amino acids arginine, lysine, and glutamine are being investigated as growth hormone releasers, which may then decrease signs of aging.
Hormone Treatment of Health Problems
The ultimate goals of hormone studies are to arrive at an understanding of the physiological basis of behavior and to develop treatments for behavioral abnormalities. Synthetic hormones can be manufactured in the laboratory. Their mass production could provide solutions to many psychological problems such as stress, deviant behavior, and sexual dysfunction. Synthetic hormones already are being used as birth control mechanisms aimed at fooling the female body’s reproductive hormonal systems.
Ongoing research focuses on the importance of many hormones, especially on understanding their functions and how they might be used in the treatment of common disorders. Two hormones produced by the hypothalamus and released by the posterior pituitary (neurohypophysis) gland are vasopressin (antidiuretic hormone) and oxytocin. Vasopressin keeps the kidneys from losing too much water and helps maintain the body’s fluid balance. Variants of vasopressin that decrease blood pressure, identified by Maurice Manning, may lead to a new class of drugs to control high blood pressure. Oxytocin induces labor by causing uterine contractions and also promotes the production of milk for breastfeeding. Manning and Walter Chan are working to develop oxytocin receptor antagonists that may be used to prevent premature births.
The Past, Present, and Future of Hormones
The activities of all living organisms are functionally dependent on the biochemical reactions that make up life itself. Since the evolution of the first eukaryotic cells more than one billion years ago, hormones have been used in cell-to-cell communication. In vertebrate animals (fish, amphibians, reptiles, birds, and mammals), endocrine systems have evolved into highly complicated nervous systems. These nervous systems are even evident in the invertebrate arthropods (crustaceans, spiders, and so on), especially among the social insects, such as ants. The endocrine and nervous systems are intricately interconnected in the control of animal physiology and behavior.
Psychologists are interested in the chemical basis of human behavior and therefore are interested in human and mammalian hormones. Such hormones control a variety of behaviors, such as maternal imprinting (in which an infant and mother bond to each other), courtship and mating, territoriality, and physiological responses to stress and danger. Animal behaviorists and psychologists study the connection between hormones and behavior in humans, primates, and other closely related mammalian species. They identify similarities in behaviors and hormones among a variety of species. They also recognize the occurrence of abnormal behaviors, such as antisocial behavior and sexual deviance, and possible hormonal imbalances that contribute to these behavioral anomalies.
Although the biochemistry of hormones and their effects on various behaviors have been established in considerable detail, numerous behaviors that are probably under hormonal influence have yet to be critically analyzed. Among them are many subtle pheromones that affect a person’s interactions with other people, imprinting pheromones that trigger attraction and bonding between individuals, and hormones that link together a variety of bodily functions. These hormones may number in the hundreds, and they represent a challenging avenue for further research. Unraveling the relationships between hormones and behavior can enable researchers to gain a greater understanding of the human mind and its link to the rest of the body and to other individuals. These studies offer potential treatments for behavioral abnormalities and for mental disturbances created by the physiologically disruptive effects of drug use, a major problem in American society. They also offer great promise in the alleviation of stress, another major social and medical problem.
Bibliography
Borer, Katarina T. Exercise Endocrinology. Champaign, Ill.: Human Kinetics, 2003. Print.
Bronson, Phyllis J. Moods, Emotions, and Aging: Hormones and the Mind-Body Connection. Lanham: Rowman and Littlefield, 2013. Print.
Ellison, Peter T., and Peter B. Gray, eds. Endocrinology of Social Relationships. Cambridge, Mass.: Harvard University Press, 2009. Print.
Kraemer, W. J., and Alan D. Rogol, eds. The Endocrine System in Sports and Exercise. Malden, Mass.: Blackwell, 2005. Print.
Lovallo, William R. Stress and Health: Biological and Psychological Interactions. 2d ed. Thousand Oaks, Calif.: Sage, 2005. Print.
Neal, J. Matthew. How the Endocrine System Works. Malden, Mass.: Blackwell Science, 2001. Print.
Neave, Nick. Hormones and Behaviour: A Psychological Approach. New York: Cambridge University Press, 2008. Print.
Simonsen, Davis. Hormones and Behavior. New York: Nova Biomedical, 2013. Print.
Svare, Bruce B. Hormones and Aggressive Behavior. New York: Springer Verlag, 2013. Print.
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