Endocrine System: Hormones and Glands

Chemical Messengers of the Endocrine System

• Hormones are the chemical messengers of the endocrine system.

Exocrine vs Endocrine Glands

• Exocrine glands release secretions through ducts to the outside of the body or onto a surface within the body.
• Endocrine glands release hormones directly into the bloodstream without the use of ducts.

Types of Hormones

• Steroid hormones are derived from cholesterol (e.g., cortisol, testosterone).
• Peptide/protein hormones are composed of amino acids (e.g., insulin, growth hormone).
• Amine hormones are derived from single amino acids (e.g., epinephrine, thyroid hormones).
• Eicosanoids are derived from fatty acids (e.g., prostaglandins).

Hormone Action

• A target cell is a cell that has specific receptors for a hormone, allowing the hormone to exert its effects on that cell.
• Receptors are proteins located on the surface or inside of a target cell that bind to specific hormones, enabling the cell to respond to the hormone's signal.
• The second messenger system amplifies the signal and initiates a cascade of cellular processes, leading to the hormone's effects.

Steroid Hormones

• Steroid hormones pass through the cell membrane and bind to intracellular receptors.
• The hormone-receptor complex then enters the nucleus and binds to DNA, altering gene transcription and ultimately protein synthesis, affecting cellular function.

Gene Transcription

• Gene transcription is the process by which a segment of DNA is copied into RNA by the enzyme RNA polymerase, leading to protein synthesis.

Stimuli for Hormone Release

• Humoral stimuli: changes in blood levels of ions and nutrients trigger hormone release (e.g., calcium levels regulate parathyroid hormone).
• Hormonal stimuli: hormones from one gland stimulate or inhibit the release of hormones from another gland (e.g., hypothalamic hormones regulate pituitary hormones).
• Neural stimuli: nerve fibers stimulate hormone release (e.g., sympathetic nervous system stimulates adrenal medulla to release epinephrine).

Hypothalamus Control of the Pituitary Gland

• The hypothalamus releases releasing and inhibiting hormones into the hypophyseal portal system, which directly influences the anterior pituitary to release or inhibit its hormones.
• The hypothalamus produces hormones (oxytocin and ADH) that are transported down nerve fibers to the posterior pituitary, from where they are released into the bloodstream.

Releasing and Inhibiting Hormones

• Releasing hormones stimulate the anterior pituitary to secrete hormones (e.g., TRH, CRH).
• Inhibiting hormones prevent the anterior pituitary from secreting hormones (e.g., somatostatin, dopamine).

Hypophyseal Portal System

• A network of blood vessels that directly connects the hypothalamus with the anterior pituitary, allowing hypothalamic hormones to quickly reach the anterior pituitary.

Anterior Pituitary Gland

• The anterior pituitary gland (adenohypophysis) is true glandular tissue.

Posterior Pituitary Gland

• Stores and releases oxytocin and antidiuretic hormone (ADH) that are produced by the hypothalamus.

Six Hormones of the Anterior Pituitary

• Growth Hormone (GH): targets liver, muscle, bone, and other tissues; stimulates growth and metabolic functions.
• Thyroid-Stimulating Hormone (TSH): targets thyroid gland; stimulates the secretion of thyroid hormones.
• Adrenocorticotropic Hormone (ACTH): targets adrenal cortex; stimulates the release of cortisol.
• Follicle-Stimulating Hormone (FSH): targets ovaries and testes; stimulates gamete production.
• Luteinizing Hormone (LH): targets ovaries and testes; stimulates the formation of corpus luteum and testosterone production.
• Prolactin (PRL): targets breast tissue; stimulates milk production.