The Endocrine System

The Endocrine System

The endocrine and nervous systems work together to act as a communication system for the human body.

The endocrine system acts as a communication tool within the human body, working in tandem with the nervous system to communicate with the body's other internal systems. Both the nervous and endocrine systems send messages everywhere inside the human body. These messages allow your heart to beat, your lungs to breathe in air, and your mind to make decisions. In the nervous system, signals travel very quickly, leading to instantaneous responses. However, within the endocrine system, signals move slowly but last longer.

Hormones

Hormones are chemicals within the endocrine system that affect physiological activity. They are secreted by one tissue and conveyed by the bloodstream to another tissue. Hormones have high levels of specificity, which means they only react with certain receptor sites in the body. The best way to describe hormones is to think of a lock and a key: only a certain hormone (lock) can create a certain response within your body's receptive tissue (key).

Hormones can be divided into four separate groups: amino acids, polypeptides and proteins, steroids, and eicosanoids. The first three groups have hormones that can have a large impact psychologically; the eicosanoids primarily regulate blood movement.

Amino Acids

• Epinephrine: also known as adrenaline; comes from the adrenal gland; affects blood pressure and other stress responses.
• Melatonin: comes from the pineal gland; affects circadian rhythm and sleep cycles.
• T₃ and T₄ (thyroxine): come from thyroid gland; increase metabolism.

Peptides

There are a huge number of hormones that can be categorized as peptides. Some of the ones that are most important to psychology are:

• Oxytocin: the "cuddle" hormone; secreted by the pituitary gland; affects breast-feeding, trust between people;
• Growth hormone (HGH): secreted by the pituitary gland; affects growth.

Steroids

• Steroids are frequently secreted from sexual organs. Some of the most important steroids are:
• Testosterone: produced in sex organs (ovaries, testes) and adrenal glands; sometimes called the "male hormone" (though it is present in both men and women); affects libido, muscle growth.
• Estrogen: produced in sex organs; sometimes called "the female hormone" (though like testosterone it is found in both sexes); has an entire host of homeostatic and regulatory functions.
• Progesterone: produced in sex organs, or the placenta when pregnant; can support pregnancy and has other regulatory functions.

Endocrine Glands

There are eight major endocrine glands, each with a different function.

• Pituitary gland: the "brain" of the endocrine system; regulates all seven of the other glands and secretes growth hormone.
• Thyroid: regulates a person's metabolic rate, which is the amount of energy expended daily by a person at rest.
• Thymus: assists in the development of a person's immune system.
• Adrenal gland: regulates fluid and sodium balance within the body, and secretes epinephrine ("adrenaline") when the body is under stress, producing the fight-or-flight response.
• Ovaries (in females) and testes (in males): control the development of secondary sex characteristics.
• Pancreatic islets: regulate blood sugar.
• Pineal gland: regulates biorhythms and mood, and stimulates the onset of puberty.

The HPA axis

The hypothalamic-pituitary-adrenal axis (HPA or HTPA axis) is a complex set of direct influences and feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands.

The interactions among these glands constitute the HPA axis, a major part of the neuroendocrine system that controls reactions to stress and regulates many body processes, including digestion, the immune system, mood and emotions, sexuality, and energy storage and expenditure. While steroid hormones are produced mainly in vertebrates, the physiological role of the HPA axis and corticosteroids in stress response is so fundamental that analogous systems can be found in invertebrates and monocellular organisms as well.

The Endocrine System and Stress

The hypothalamic-pituitary-adrenal axis regulates stress in vertebrates.

Stress is the simple name for what happens when the body's emergency response is activated; a stressful event is one that activates the sympathetic (fight-or-flight) nervous system. Because it elevates arousal, heart rate, and breathing, stress is useful for helping animals and humans escape dangerous situations; however, it can damage the body to be under stressful conditions for too long.

The HPA and Stress

Stressors can come in many forms, from immediate physical threats like an angry bear, to social threats like an angry friend. In experimental studies in rats, a distinction is often made between social stress and physical stress, but both types activate the HPA axis, albeit through different pathways. The hypothalamic-pituitary-adrenal (HPA or HTPA) axis is a complex set of direct influences and steroid-producing feedback interactions among the hypothalamus, the pituitary gland, and the adrenal glands. All vertebrates have an HPA, but the steroid-producing stress response is so important that even invertebrates and monocellular organisms have analogous systems.

The HPA is important to psychology because it is intimately involved with many mood disorders involving stress, including anxiety disorder, bipolar disorder, insomnia, PTSD, borderline personality disorder, ADHD, depression, and many others. Antidepressants work by reglulating the HPA axis.

The Function of the HPA Axis

The hypothalamus contains neurons that synthesize and secrete vasopressin and corticotropin-releasing hormone (CRH). These two hormones travel through blood to the anterior pituitary, where they cause the secretion of stored adrenocorticotropic hormone (ACTH). The ACTH acts on the adrenal cortex, which produces steroids—in humans, primarily the steroid cortisol. This causes a negative feedback cycle in which the steroids inhibit the hypothalamus and the pituitary gland, and it also causes the adrenal gland to produce the hormones epinephrine (also known as adrenaline) and norepinephrine.

Cortisol, Stress, and Health

In the process described above, the HPA axis ultimately produces cortisol. Studies on people show that the HPA axis is activated in different ways during chronic stress—depending on the type of stressor, the person's response to the stressor, and other factors. Stressors that are uncontrollable, threaten physical integrity, or involve trauma tend to have a high, flat profile of cortisol release (with lower-than-normal levels of cortisol in the morning and higher-than-normal levels in the evening) resulting in a high overall level of daily cortisol release. On the other hand, controllable stressors tend to produce higher-than-normal morning cortisol. Stress hormone release tends to decline gradually after a stressor occurs. In post-traumatic stress disorder there appears to be lower-than-normal cortisol release, and it is thought that a blunted hormonal response to stress may predispose a person to develop PTSD.

There is growing evidence that prenatal stress can affect HPA regulation in humans. Children who were stressed prenatally may show altered cortisol rhythms. For example, several studies have found an association between maternal depression during pregnancy and childhood cortisol levels. Prenatal stress has also been implicated in a tendency toward depression and short attention span in childhood. However, there is no clear indication that HPA disregulation caused by prenatal stress can alter adult behavior.

The Endocrine System and Hunger

Hunger is divided into long-term and short-term regulation, each stimulating different hormone responses from the hypothalamus.

Hunger is the set of physical and psychological sensations that arise when food is needed by the body. It appears to increase activity and movement in many animals; this response may increase an animal's chances of finding food. Food consumption (particularly overconsumption) can result in weight gain, whereas insufficient consumption, or malnutrition, will cause significant weight and motivational energy loss. Hunger is controlled by the hypothalamus and hormones. It is regulated over both the long term and the short term.

Hormones

The physical sensation of hunger comes from contractions of the stomach muscles. These contractions are believed to be triggered by high concentrations of the hormone ghrelin. Two other hormones, peptide YY and leptin, cause the physical sensations of being full. Ghrelin is released if blood sugar levels get low, a condition that can result from going long periods without eating.

Hypothalamus

The hypothalamus regulates the body's physiological homeostasis. When you are dehydrated, freezing, or exhausted, the appropriate biological responses are activated automatically: body fat reserves are utilized, urine production is inhibited, and blood is shunted away from the surface of the body. The drive to eat, or drink water, or seek warmth is activated.

In the 1940s, the "dual-center" model, which divided the hypothalamus into hunger (lateral hypothalamus) and satiety (ventromedial hypothalamus) centers, was popular. This theory developed from the findings that bilateral lesions of the lateral hypothalamus can cause anorexia, a severely diminished appetite for food, while bilateral lesions on the ventromedial hypothalamus can cause overeating and obesity. Recently, further study has called the dual-center model into question, but the hypothalamus certainly does play a role in hunger.

Long-Term Hunger Regulation

The long-term regulation of hunger prevents energy shortfalls and is concerned with the regulation of body fat. Leptin, a hormone secreted exclusively by adipose cells in response to an increase in body-fat mass, helps regulate long-term hunger and food intake. Leptin serves as the brain's indicator of the body's total energy stores. The function of leptin is to suppress the release of neuropeptide Y (NPY), which in turn prevents the release of appetite-enhancing orexins from the lateral hypothalamus. This decreases appetite and food intake, promoting weight loss. Though rising blood levels of leptin do promote weight loss to some extent, its main role is to protect the body against weight loss in times of nutritional deprivation.

Short-Term Hunger Regulation

The short-term regulation of hunger deals with appetite and satiety. It involves neural signals from the GI tract, blood levels of nutrients, and GI-tract hormones.

Neural Signals from the GI Tract

The brain can evaluate the contents of the gut through vagal nerve fibers that carry signals between the brain and the gastrointestinal (GI) tract. Studies have shown that the brain can sense differences between macronutrients through these vagal nerve fibers. Stretch receptors (mechanoreceptors that respond to an organ being stretched or distended) work to inhibit appetite when the GI tract becomes distended. They send signals along the vagus nerve afferent pathway and ultimately inhibit the hunger centers of the hypothalamus.

Nutrient Signals

Blood levels of glucose, amino acids, and fatty acids provide a constant flow of information to the brain that may be linked to regulating hunger and energy intake. Nutrient signals indicate fullness. They inhibit hunger by raising blood glucose levels, elevating blood levels of amino acids, and affecting blood concentrations of fatty acids.

Hormonal Signals

Hormones can have a wide range of effects on hunger. The hormones insulin and cholecystokinin (CCK) are released from the GI tract during food absorption and act to suppress feelings of hunger. However, during fasting, glucagon and epinephrin levels rise and stimulate hunger. When blood sugar levels fall, the hypothalamus is stimulated. Ghrelin, a hormone produced by the stomach, triggers the release of orexin from the hypothalamus, signaling to the body that it is hungry.

Starvation

Starvation is a severe deficiency in caloric energy, nutrient, and vitamin intake. It is the most extreme form of malnutrition. Prolonged starvation can cause permanent organ damage and, untreated, leads to death. Individuals experiencing starvation lose substantial fat and muscle mass, called catabolysis, when the body breaks down its own fat and muscle for energy. Vitamin deficiency, diarrhea, skin rashes, edema, and heart failure are also common results of starvation. In a state of starvation, other motivators—such as the desire for sleep, sex, and social activities— decrease. Individuals suffering from starvation may experience irritability, lethargy, impulsivity, hyperactivity, and more apathy over time.