ECU Lecture 10

MHSI102D Endocrine System

Learning Outcomes

By the end of this lecture, you should be able to:

• Define hormone and endocrine system clearly, recognizing their components and functions.

• Name the organs of the endocrine system, their locations, and the specific hormones they produce.

• Describe the intricate relationships between the hypothalamus and pituitary gland, highlighting their role as a regulatory axis and their effects on various bodily functions.

• List the hormones produced by the endocrine system and outline the main targets and functions of each hormone, understanding their physiological significance.

• Identify the chemical classes to which various hormones belong, exploring examples and their distinct properties.

• Describe how hormones stimulate their target cells, detailing the mechanisms of receptor binding and subsequent cellular responses.

• Outline the stages of the stress response, connecting physiological and psychological aspects of stress adaptation.

Homeostasis

Homeostasis is a critical concept in physiology, referring to the maintenance of a stable internal environment, including optimal temperature, pH levels, and electrolyte balance. It is essential for the proper functioning of organs, tissues, and cells within the body. Each of these biological structures operates within specific physiological limits, and when these limits are disrupted, disease can result. The endocrine system is the primary mechanism of homeostatic regulation, maintaining physiological variations within normal limits compatible with survival.

• Minor variations in body parameters such as temperature, blood pressure, and glucose levels are often ignored, allowing those values to oscillate within a normal range without triggering disease.

• When homeostasis is disrupted, the body activates compensatory mechanisms to restore balance, demonstrating the interconnectedness of various body systems. For example, an increase in blood glucose levels triggers insulin secretion to lower glucose levels, illustrating negative feedback regulation.

Overview of the Endocrine System

The endocrine system utilizes four principal mechanisms for communication among cells:

1. Gap Junctions: Specialized intercellular connections that allow direct signaling between adjacent cells through pores in their membranes, facilitating the movement of ions and small molecules.

2. Neurotransmitters: These are chemical messengers released from neurons that travel across synaptic clefts to target cells, enabling rapid communication and immediate responses.

3. Paracrine Chemicals: Substances secreted into tissue fluids that affect nearby cells; this local signaling plays a role in coordinating functions among neighboring cells and tissues in the same organ.

4. Hormones: These are chemical messengers synthesized and secreted by endocrine glands, traveling through the bloodstream to distant target tissues and organs. Hormones elicit a variety of physiological responses, including growth, metabolism, and reproduction regulation.

The endocrine system comprises a network of glands, tissues, and cells, each responsible for producing and secreting hormones essential for maintaining homeostasis and managing various bodily functions.

Endocrine vs. Exocrine Glands

• Exocrine Glands: These have ducts that carry secretions to epithelial surfaces or mucosa of the gastrointestinal tract. Examples include sweat glands, salivary glands, and gastric glands, which produce substances like enzymes and sweat that exert extracellular effects, such as aiding in digestion or thermoregulation.

• Endocrine Glands: These glands lack ducts and contain a rich network of capillaries. Hormones released by endocrine glands enter the bloodstream directly and induce intracellular changes by targeting receptor molecules on or within their specific target cells. Examples include the thyroid gland, adrenal glands, and pituitary gland, all of which play crucial roles in metabolic regulation and homeostasis.

Comparison of Nervous and Endocrine Systems

Both the nervous system (NS) and the endocrine system (ES) are essential for internal communication within the body, yet they exhibit notable differences:

• Communication: Both systems serve internal communication but do so through fundamentally different modalities. The NS uses electrical impulses for rapid communication, while the ES relies on hormonal signals for more widespread and sustained messages.

• Speed and Persistence of Response: The NS reacts quickly, often in milliseconds, to stimuli and stops almost immediately when the stimulus is removed. Conversely, the ES reacts more slowly (taking seconds to days for effects to manifest) and may have prolonged effects, lasting from minutes to days post-stimulation.

• Adaptation to Long-Term Stimuli: For prolonged stimuli, the NS response diminishes rapidly as it adapts to the stimulus, whereas the ES’s response persists and adapts at a more gradual pace, allowing for sustained physiological changes when needed.

• Area of Effect: The NS primarily targets specific organs for precise action (e.g., muscle contraction in response to nerve impulses), while the ES has more general effects that can influence multiple organs or systems simultaneously (e.g., adrenaline affecting heart rate and energy mobilization).

Communication by Nervous and Endocrine Systems

Figure 17.2 from the original material illustrates the significant differences in communication, signaling speed, pathway, and effect area between the nervous and endocrine systems, underlining their unique contributions to physiological regulation.

Anatomy of the Endocrine System

Key structures of the endocrine system include:

• Hypothalamus: Strategically located in the brain, the hypothalamus is an integral component of the diencephalon, overseeing various autonomic functions like temperature regulation, hunger, thirst, sleep-wake cycles, and stress responses. As a neuroendocrine organ, it plays a critical role in linking the nervous system to the endocrine system through the control of the pituitary gland.

Anatomy of the Pituitary Gland

The pituitary gland (hypophysis) is a pea-sized structure suspended from the hypothalamus by the infundibulum. It resides in the sella turcica of the sphenoid bone and comprises two distinct lobes:

• Adenohypophysis (anterior pituitary): This lobe develops from the oral ectoderm and produces hormones including growth hormone (GH), prolactin (PRL), adrenocorticotropic hormone (ACTH), thyroid-stimulating hormone (TSH), luteinizing hormone (LH), and follicle-stimulating hormone (FSH). These hormones regulate growth, metabolic processes, and reproductive functions.

• Neurohypophysis (posterior pituitary): This lobe is an extension of the hypothalamus and stores hormones synthesized by the hypothalamic neurons, including oxytocin (OT) and vasopressin (also known as antidiuretic hormone, ADH). The release of these hormones is governed by nerve impulses from the hypothalamus as necessary.

Embryonic Development of the Pituitary Gland

The development of the pituitary gland involves complex interactions among various embryonic structures including the telencephalon, neurohypophyseal bud, and the pharynx. These structures contribute to the formation of both lobes of the pituitary gland from weeks 4 to 16 of fetal development, highlighting the organ's intricate developmental pathway.

Control of Pituitary Secretion

The pituitary gland's secretion rates are dynamic rather than constant and are intricately regulated by:

• Negative Feedback: Hormonal control through increased levels of target organ hormones serves to inhibit hormone release from the hypothalamus and/or pituitary gland, thus maintaining a homeostatic balance. An example of this is the feedback loop between the hypothalamus, pituitary, and adrenal gland's production of cortisol.

• Positive Feedback: This mechanism is less common, with a notable example being the physiological process of childbirth. During labor, the stretching of the uterus signals the hypothalamus to release oxytocin, enhancing uterine contractions until delivery is accomplished.

Hormonal Chemistry

Hormones can be categorized based on their chemical structures, with three primary classes:

1. Steroids: These lipid-soluble molecules are derived from cholesterol. They include sex steroids produced by the gonads (estrogens, progesterone, testosterone) and corticosteroids produced by the adrenal glands (cortisol, aldosterone). Their lipophilic nature allows them to easily pass through cell membranes and bind to intracellular receptors, influencing gene expression directly.

2. Monoamines: These hormones are synthesized from amino acids. Examples include catecholamines (like dopamine and epinephrine) which play roles in the fight-or-flight response, and thyroid hormones which regulate metabolic processes. The structure of monoamines varies but they typically share hydrophilic characteristics, restricting their interaction to cell surface receptors.

3. Peptides: Composed of chains of amino acids, these hormones include insulin, glucagon, and various pituitary hormones (like GH and PRL). Peptide hormones are generally hydrophilic, meaning they cannot pass through cell membranes and must bind to surface receptors on target cells, triggering intracellular signaling pathways.

Hormone Secretion

Hormonal levels within the body are not static; they fluctuate throughout the day and can be influenced by multiple stimuli:

• Neural Stimuli: The release of certain hormones occurs in response to direct nerve impulses. For example, adrenal medulla releases epinephrine in response to sympathetic nervous system activation during stress.

• Hormonal Stimuli: Changes in hormone levels signal other endocrine glands to release their respective hormones. An example is the hypothalamic hormones that stimulate the anterior pituitary to secrete its hormones.

• Humoral Stimuli: Variations in blood-borne signals, such as changes in electrolyte or nutrient levels (e.g., increased blood glucose levels leading to insulin release), can influence hormone release.

Hormone-Receptor Interactions

Hormones exert their effects only on target cells possessing specific receptors for them, showcasing specificity and saturation characteristics.

• Second Messengers: Non-lipid-soluble hormones (Peptide or monoamine hormones) require second messengers to transmit their signals within the cell. A common second messenger is cyclic AMP (cAMP), which activates various intracellular processes in response to hormone binding.

• Steroid Hormones: These hormones can penetrate the plasma membrane to bind with intracellular receptors, leading to alterations in gene expression and ultimately affecting protein synthesis.

Stress and Adaptation

Stress is a condition that can disturb homeostasis, threatening both physical and mental well-being. The concept of General Adaptation Syndrome (GAS) outlines the body's physiological response to stressors in three stages:

1. Alarm Reaction: This initial response is triggered by the recognition of a stressor, leading to activation of the hypothalamic-pituitary-adrenal (HPA) axis. The result is the secretion of corticosteroids and catecholamines, such as norepinephrine and epinephrine, preparing the body for immediate action via the fight-or-flight response.

2. Stage of Resistance: In this stage, the body attempts to adapt to the stressor through sustained physiological changes, primarily mediated by cortisol. This hormone provides alternative energy sources and aids in regulating metabolism to cope with ongoing stress.

3. Stage of Exhaustion: Prolonged stress leads to the depletion of the body’s resources, rendering it unable to maintain homeostasis. This stage is characterized by significant physiological deterioration, muscle wasting, and, in severe cases, can culminate in death if multiple physiological systems fail due to unaddressed stressors.