Untitled Flashcards Set

Lecture 1: Endocrine System: Coordinating Body Functions

The endocrine system is a network of glands and organs that regulate and control various bodily functions through the secretion of hormones. Unlike the nervous system, which transmits signals rapidly through nerve impulses, the endocrine system uses hormones as chemical messengers that travel through the bloodstream to specific target cells or organs. This hormonal signaling is generally slower but plays a crucial role in maintaining long-term homeostasis and regulating processes such as growth, metabolism, and reproduction

Endocrine Glands:

·       Pituitary Gland: Often referred to as the "master gland," it controls the function of many other endocrine glands and secretes hormones that regulate growth, reproduction, and other endocrine activities.

·       Thyroid Gland: Produces hormones (T3 and T4) that regulate metabolism, growth, and energy expenditure.

·       Parathyroid Glands: Regulate calcium and phosphorus levels in the blood and bones.

·       Adrenal Glands: Secrete hormones like cortisol and adrenaline, crucial for the body's stress response and regulation of metabolism.

·       Pancreas: Produces insulin and glucagon, which regulate blood sugar levels.

·       Gonads (Ovaries and Testes): Responsible for the production of sex hormones and control of reproductive functions.

Hormonal Regulation:

·       Hormones are chemical messengers secreted by glands into the bloodstream, exerting effects on specific target cells to regulate physiological processes such as growth, metabolism, reproduction, and stress responses. They are produced in response to changes in the body's internal environment, conveying messages to target cells to restore balance. For example, when blood glucose levels are elevated, the pancreas releases the hormone insulin. Insulin travels through the bloodstream to target cells, particularly in the liver and muscles, signaling them to increase glucose uptake from the blood. This process reduces blood glucose levels, restoring them to normal.  Conversely, when blood glucose levels are low, the pancreas secretes glucagon, another hormone that signals the liver to release stored glucose, thereby increasing blood glucose levels.  This dynamic interplay between insulin and glucagon ensures the maintenance of blood glucose homeostasis.

·       Feedback Mechanisms: The endocrine system is regulated by feedback loops that maintain hormone levels within a narrow range. Negative feedback mechanisms ensure that hormone production decreases when levels become too high and increases when levels drop below normal.

Chemical Classifications of Hormones

Hormones are classified/categorized based on their chemical structures into three primary classes:

1. Lipid-Derived (Steroid) Hormones: These are fat-soluble, allowing them to easily pass through cell membranes, enter the nucleus of the cell and bind to receptors in the nucleus of their target. Examples include:

   • Testosterone: A male sex hormone produced primarily in the testes.

   • Estrogen: A female sex hormone produced mainly in the ovaries.

   • Progesterone: Another female sex hormone involved in the menstrual cycle and pregnancy.

2. Peptide and Protein Hormones: Composed of chains of amino acids, these hormones are water-soluble and cannot cross cell membranes, so they bind to the receptor. Examples include:

   • Insulin: A hormone produced by the pancreas that regulates blood glucose levels.

   • Glucagon: Also produced by the pancreas, it works to raise blood glucose levels.

3. Amino Acid-Derived Hormones: These are synthesized from single amino acids, such as tyrosine or tryptophan. Examples include:

   • Thyroid Hormones (e.g., Thyroxine): Produced by the thyroid gland, they regulate metabolism.

   • Epinephrine (Adrenaline): Produced by the adrenal glands, it plays a role in the body's fight-or-flight response.

   • Norepinephrine: Also produced by the adrenal glands, it functions similarly to epinephrine.

   • Melatonin: Produced by the pineal gland, it regulates sleep-wake cycles.

Hormone Action:

Hormones influence cellular functions through various mechanisms:

1.     Enzyme Synthesis: Hormones can signal target cells to alter the production of enzymes by affecting gene expression, leading to changes in enzyme synthesis.

2.     Enzyme Quantity: Hormones can regulate the degradation or stabilization of enzymes.

3.     Enzyme Activity: Hormones can modify the functional state of enzymes, either activating or inhibiting them, which influences the rate of biochemical reactions.

4.     Ion Channel Gating: Certain hormones affect the opening and closing (gating) of ion channels, altering the flow of ions across cell membranes and impacting cellular excitability and signaling.

These actions enable hormones to regulate a wide array of physiological processes, ensuring the body's maintenance of internal balance and adaptability.

Major Hormones and Functions:

·       Insulin: Regulates blood glucose levels by promoting the uptake of glucose into cells.

·       Thyroid Hormones (T3 and T4): Control metabolism and influence growth and development.

·       Adrenaline (Epinephrine) and Cortisol: Regulate the body's stress response, affecting heart rate, blood pressure, and energy metabolism.

·       Growth Hormone (GH): Promotes growth, cell reproduction, and regeneration.

·       Human chorionic gonadotropin (hCG) – Prevents disintegration of the corpus luteum.  At the ovary’s corpus luteum produces progesterone

·       Estrogens – in addition to having a role in secondary sexual characteristics it is involved in the thickening of the endometrium

·       Progesterone - enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the growing fetus

·       Human Placental Lactogen (hPL) - decreases maternal insulin sensitivity, and therefore raises maternal blood glucose levels, whilst decreasing maternal glucose utilization, which helps ensure adequate fetal nutrition

·       Placental prolactin- stimulates the mammary glands to produce milk.  Increased serum concentrations of prolactin during pregnancy cause enlargement of the mammary glands of the breasts and increase the production of milk. However, the high levels of progesterone during pregnancy stop the ejection of milk. After childbirth progesterone decreases and milk ejection initiates.

·       Gonadotropin releasing hormone (GnRH) promotes the secretion of FSH and LH

·       ADH (Anti Diuretic Hormone) is produced in response to changes in blood osmotic pressure. When blood pressure is low, the posterior pituitary gland produces ADH. The primary function of antidiuretic hormone is to increase water conservation/retention by the kidneys.

Anterior Pituitary Hormones:

·    The anterior pituitary gland secretes several hormones, each with specific functions:

1.     Adrenocorticotropic Hormone (ACTH): Stimulates the adrenal glands to produce cortisol and other hormones

2.     Follicle-Stimulating Hormone (FSH): In females, stimulates the ovaries to prepare eggs for ovulation and release estrogen; in males, stimulates the testes to produce sperm

3.     Luteinizing Hormone (LH): In females, triggers the ovaries to produce progesterone; in males, stimulates testosterone production.

4.     Human Growth Hormone (HGH): It stimulates cell growth and replication by accelerating protein synthesis.  In children, it stimulates growth; in adults, helps maintain healthy muscles and bones and impacts body fat distribution.

5.     Prolactin: Stimulates breast milk production after childbirth and can affect menstrual periods, fertility, and sexual function.

6.     Thyroid-Stimulating Hormone (TSH): Triggers the thyroid to produce and release thyroid hormones, which are essential for maintaining the body's metabolic rate.

Posterior Pituitary Gland:

·  The posterior pituitary gland, also known as the neurohypophysis, is a vital component of the endocrine system. Unlike the anterior pituitary, it does not produce hormones but stores and releases two key hormones synthesized in the hypothalamus:

·       Oxytocin: This hormone plays a crucial role in childbirth by stimulating uterine contractions and facilitating milk ejection during breastfeeding.

·       ADH (Antidiuretic Hormone): It is produced in response to changes in blood osmotic pressure. When blood pressure is low, the posterior pituitary gland produces ADH. The primary function of antidiuretic hormone is to increase water conservation/retention by the kidneys.  In the absence of ADH urine production will increase and the urine will be very diluted.

·  These hormones are produced in the hypothalamus and transported to the posterior pituitary for storage and release into the bloodstream. The posterior pituitary is connected to the hypothalamus via a stalk-like structure containing nerve fibers and blood vessels, allowing for the direct release of these hormones into the circulation

Disorders of the Endocrine System:

·       In medical terminology, "hypo-" denotes a deficiency or underactivity, while "hyper-" signifies an excess or overactivity. These prefixes are commonly used to describe conditions related to hormone production.

·       Example: Hypothyroidism and Hyperthyroidism

·       The thyroid gland plays a crucial role in regulating metabolism through hormone production.

·       Hypothyroidism: This condition arises when the thyroid gland is underactive and doesn't produce sufficient thyroid hormones. Symptoms may include weight gain, fatigue, and sensitivity to cold.

·       Hyperthyroidism: Conversely, hyperthyroidism occurs when the thyroid gland is overactive, leading to excessive hormone production. This can result in weight loss, rapid heartbeat, and nervousness

·       Diabetes Mellitus: Characterized by abnormal blood glucose regulation due to insufficient insulin production (Type 1) or resistance to insulin's effects (Type 2).

Anterior Pituitary Gland:

·       The anterior pituitary gland, or adenohypophysis, plays a pivotal role in regulating various physiological processes by secreting hormones such as thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), and prolactin (PRL). These secretions are primarily regulated by releasing and inhibiting hormones from the hypothalamus, which reach the anterior pituitary via the hypophyseal portal system—a specialized network of blood vessels connecting the two regions.

·       If the hypophyseal portal system is destroyed, the hypothalamus would no longer be able to control the secretion of TSH, ACTH, and PRL.

Hormonal Mechanism of Action:

·       Steroid Hormones: Steroid hormones, being lipid-soluble, can easily cross the cell membrane and bind to specific receptors located in the cytoplasm or nucleus of target cells. Once bound, the hormone-receptor complex moves into the nucleus and directly influences gene expression by binding to DNA, thus altering mRNA production. This leads to changes in protein synthesis, affecting the quantity and type of proteins produced in the cell.

·       Peptide Hormones: Peptide hormones, due to their inability to pass through the cell membrane, bind to specific receptors found on the surface of the cell membrane. This binding triggers a cascade of events known as the secondary messenger system. For instance, the peptide hormone binds to its receptor, activating a series of molecules within the cell, such as cyclic adenosine monophosphate (cAMP), calcium ions, or other secondary messengers. These messengers transmit the hormonal signal to the cell's interior, initiating various cellular responses.

G Proteins: The Bridge Between First and Second Messengers:

·       When a peptide hormone binds to its receptor on a cell's surface, it doesn't enter the cell. Instead, this binding activates a G protein located inside the cell membrane. The G protein then triggers the production of a second messenger inside the cell, such as cyclic AMP (cAMP), which carries the signal to the cell's interior, leading to the desired response. Think of the G protein as a relay runner, passing the baton (the signal) from the outside to the inside of the cell.

Second Messenger:

·       Cyclic adenosine monophosphate (cAMP) is a crucial second messenger in cellular communication. When a signaling molecule, like a hormone, binds to a receptor on a cell's surface, it activates an enzyme called adenylyl cyclase. This enzyme converts ATP into cAMP, which then relays the signal inside the cell, leading to specific responses.

·       Example: When the hormone epinephrine binds to receptors on liver cells, it triggers the production of cAMP. This cAMP then signals the cells to break down glycogen into glucose, providing energy for the body.

·       Analogy: Think of cAMP as an internal memo in a company. A manager (the signaling molecule) sends a memo to a department head (the receptor), who then creates copies (cAMP) to distribute to employees (the cell's internal machinery), instructing them to perform specific tasks.

Impact on Cellular Operations:

·       Enzyme Quantities, Activities, and Identities: Hormones have the ability to regulate cellular operations by influencing various aspects of enzyme function. They can alter the quantities of enzymes present within a cell, modulate the activities of existing enzymes, and even change the identities or properties of critical enzymes involved in cellular processes. For instance, they can induce the synthesis of new enzymes or inhibit/enhance the activity of existing enzymes, thereby regulating metabolic pathways or cellular functions.

·       Regulation of Cellular Functions: By modulating enzyme quantities, activities, or identities, hormones play a crucial role in regulating cellular functions. This regulation extends across various physiological processes, including metabolism, growth, development, reproduction, and responses to stress or external stimuli.

Understanding the endocrine system's intricate network of glands, hormones, and their regulatory mechanisms is crucial for comprehending how the body maintains homeostasis and responds to internal and external stimuli.  The diverse mechanisms of action employed by steroid and peptide hormones and their influence on enzyme function within cells help elucidate the intricate ways in which hormones regulate cellular operations and physiological responses.