Endocrine System Overview and Hormone Pathways

Endocrine System Overview

• The endocrine system and nervous system are the two major communication pathways in the human body, responsible for maintaining homeostasis.
• Hormones are chemicals produced by tissues and organs, affecting the function of other tissues and organs.
• Hormones vary in composition from simple amino acids to complex combinations of lipids, proteins, and carbohydrates.
• Endocrine system functions include metabolism, growth and development, muscle and fat distribution, fluid and electrolyte balance, and sexual development and reproduction.
• Different types of hormones exist, including those synthesized and released by neurons (neurotransmitters) and hormones released as part of an inflammatory/immune response; these serve as chemical messengers.
• These hormones are often categorized as amino acid derivatives, peptides or steroids, each with distinct structures and functions.

Hormone Pathways

• Intracellular hormone receptors are located inside the cell. Hormones binding to these receptors must be able to cross the plasma membrane. These hormones can pass through the cell membrane due to their lipid solubility.
• Membrane hormone receptors are located at least in part, on the extracellular surface of the cell membrane. Water-soluble hormones bind to these receptors. These hormones cannot pass through the cell membrane because they are water-soluble.
• Five major pathways for cell-to-cell communication exist, allowing hormones to travel from production to secretion to response: paracrine, autocrine, endocrine, synaptic, and neuroendocrine pathways. Paracrine signals target neighboring cells, and autocrine signals target the cell that produced them.

Factors Affecting Target Cell Response

• Permissive effect: the presence of one hormone enables another hormone to act. Dietary iodine deficiency can affect reproductive system development.
• Synergistic effect: two hormones with similar effects produce an amplified response. Reproductive hormones (FSH from pituitary and estrogens from ovaries) are required for egg maturation in females.
• Antagonistic effect: two hormones have opposing effects. Insulin and glucagon in the pancreas regulate blood glucose levels.

Feedback Loops

• Negative feedback loops function as an internal thermometer, and involve the inhibition of further hormone secretion once adequate levels have been reached in response to hormone levels. (Glucocorticoid release from the adrenal glands.)
• Positive feedback loops stimulate increased hormone production until the cycle is interrupted. (oxytocin and uterine contractions)

Endocrine Gland Stimuli

• Humoral stimuli: changes in ion or nutrient levels in the blood. (e.g., blood calcium levels affecting the release of parathyroid hormone (PTH).
• Hormonal stimuli: changes in hormone levels initiate or inhibit the secretion of another hormone. (e.g., hypothalamus releasing hormones to stimulate the release of other hormones from the pituitary gland).
• Neural stimuli: Nerve impulses prompt the secretion or inhibition of a hormone. (e.g., the adrenal medulla responding to the sympathetic nervous system during stress or emergency).

Hypothalamic-Pituitary Complex

• The hypothalamus-pituitary complex is the "command center" of the endocrine system.
• It secretes several hormones that directly produce responses in target tissues.
• It also secretes hormones that regulate the synthesis and secretion of hormones in other glands.

Hypothalamic-Pituitary Axis

• The hypothalamus contains neurons that produce hormones which act on the anterior and posterior pituitary gland.
• Releasing hormones (e.g., Growth hormone releasing hormone (GhRH), Thyroid releasing hormone (TRH)) stimulate other glands.
• Inhibitory hormones (e.g., Dopamine, Somatostatin) inhibit the release of other hormones.

Thyroid Gland

• The thyroid gland is located in the neck, wrapping around the trachea.
• It is composed primarily of thyroid follicles.
• The thyroid hormones (T3 and T4) are metabolic hormones that regulate the body's basal metabolic rate and energy use at rest.
• The release of these hormones is regulated by Thyroid-stimulating hormone (TSH) from the anterior pituitary.

Parathyroid Glands

• The parathyroid glands, embedded in the posterior surface of the thyroid gland, secrete parathyroid hormone (PTH).
• PTH regulates blood calcium levels
• When calcium levels are low, the parathyroid glands release PTH, and when calcium levels are high, PTH release is inhibited.

Adrenal Glands

• Adrenal glands are located atop the kidneys and have an outer cortex and inner medulla.
• The adrenal cortex produces steroid hormones (glucocorticoids, mineralocorticoids, androgens) involved in the long-term response to stress.
• The adrenal medulla produces catecholamines (epinephrine and norepinephrine) These catecholamines regulate the body’s short term stress response.

Reproductive Hormones

• The male and female reproductive systems are regulated by follicle-stimulating hormone (FSH), luteinizing hormone (LH), and gonadotropin-releasing hormone (GnRH).
• Hormones like testosterone are crucial for male reproduction and development of secondary sex characteristics.
• Estrogens and progesterone are crucial for female reproduction, egg development and maintaining pregnancy
• The placenta produces hormones critical to maintaining a pregnancy.

Pancreas

• The pancreas produces insulin and glucagon, regulating blood glucose levels.

Other Organs with Endocrine Functions

• Other organs, like the heart, gastrointestinal tract, kidneys, skeleton, adipose tissue, skin, thymus, and liver, also have secondary endocrine functions. These organs secrete various hormones impacting a multitude of bodily functions.

Development and Aging

• The endocrine system originates from three embryonic layers (endoderm, ectoderm, mesoderm).
• Aging affects endocrine glands, decreasing hormone production like human growth hormone, cortisol, aldosterone, sex hormones, and thyroid hormones.

Regulation of Hormones

• Specific factors, like changes in ion levels and the levels of other hormones, impact hormone synthesis and release.
• Homeostatic mechanisms ensure blood glucose regulation, maintaining levels within a set range.
• Negative and positive feedback loops regulate levels of hormones in the blood and prevent additional production.

Important Note

• The slides provided often contain anatomical structures that are not essential. I have mainly focused on the details of endocrine function and related hormone pathways.

Explore the complex interactions within the human endocrine system and its functions. This quiz delves into hormone types, their compositions, and their roles in maintaining homeostasis and overall health. Test your knowledge on how these chemical messengers impact various bodily processes.