Hormones and the Endocrine System: Study Guide

Chapter 45: Hormones and the Endocrine System

Concept 45.1: Hormones and Other Signaling Molecules Bind to Target Receptors, Triggering Specific Response Pathways

The endocrine and nervous systems are the two major internal communication systems in animals. Hormones are chemical messengers that regulate physiology and behavior by binding to specific receptors on or in target cells, initiating signal transduction pathways.

• Comparison of Response Times:

  • Nervous System: Transmits signals rapidly via neurons; responses occur within milliseconds to seconds.

  • Endocrine System: Releases hormones into the bloodstream; responses are slower (seconds to hours or longer) but can be more prolonged.

• Mechanisms of Intercellular Information Flow:

  • Endocrine signaling: Hormones travel through the bloodstream to distant target cells.

  • Paracrine signaling: Local regulators act on neighboring cells.

  • Autocrine signaling: Cells respond to signals they produce themselves.

  • Synaptic signaling: Neurotransmitters diffuse across synapses to target cells (nervous system).

  • Neuroendocrine signaling: Neurohormones are released from neurons into the blood.

• Three Major Classes of Hormones in Vertebrates:

  • Polypeptides: Chains of amino acids (e.g., insulin).

  • Amines: Derived from amino acids (e.g., epinephrine, thyroxine).

  • Steroids: Derived from cholesterol (e.g., cortisol, sex hormones).

• Three Key Events in Hormone Signaling:

  1. Reception: Hormone binds to its specific receptor.

  2. Signal Transduction: The receptor triggers a cascade of biochemical events inside the cell.

  3. Response: The cell changes its activity (e.g., gene expression, enzyme activity).

• Signal Transduction Pathways for Water-Soluble Hormones:

  • Water-soluble hormones (e.g., polypeptides, some amines) cannot cross the plasma membrane.

  • They bind to cell-surface receptors, activating second messenger pathways (e.g., cAMP).

  • This leads to rapid changes in enzyme activity, ion channel opening, or gene expression.

• Intracellular Receptors for Lipid-Soluble Hormones:

  • Lipid-soluble hormones (e.g., steroids, some amines) pass through cell membranes.

  • They bind to receptors in the cytoplasm or nucleus.

  • The hormone-receptor complex acts as a transcription factor, altering gene expression directly.

  • Comparison: Water-soluble hormones act via membrane receptors and second messengers; lipid-soluble hormones act via intracellular receptors and direct gene regulation.

Example: Epinephrine (a water-soluble amine) binds to receptors on liver cells, activating cAMP pathways that promote glucose release.

Concept 45.2: Feedback Regulation and Coordination with the Nervous System Are Common in Hormone Pathways

Hormone pathways are regulated by feedback mechanisms and often interact with the nervous system to maintain homeostasis and coordinate complex processes.

• Negative Feedback Loop Example:

  • Regulation of blood glucose by insulin: High blood glucose stimulates insulin release, which lowers glucose levels, reducing the stimulus for further insulin secretion.

• Positive Feedback Example:

  • Milk release in nursing mothers: Suckling stimulates oxytocin release, which increases milk ejection, leading to more suckling and further oxytocin release.

• Hypothalamus and Pituitary Gland Interaction:

  • The hypothalamus receives neural signals and secretes releasing or inhibiting hormones.

  • These hormones regulate the pituitary gland, which in turn controls other endocrine glands.

• Pituitary Gland Location and Hormones:

  • Located at the base of the brain, below the hypothalamus.

  • Posterior pituitary: Releases oxytocin and antidiuretic hormone (ADH).

  • Anterior pituitary: Releases growth hormone (GH), prolactin, follicle-stimulating hormone (FSH), luteinizing hormone (LH), thyroid-stimulating hormone (TSH), and adrenocorticotropic hormone (ACTH).

• Tropic Hormones:

  • Hormones that regulate the function of other endocrine glands (e.g., TSH, ACTH, FSH, LH).

  • Releasing hormones: Stimulate the anterior pituitary to secrete specific hormones.

  • Inhibiting hormones: Suppress hormone secretion from the anterior pituitary.

Example: The hypothalamic-pituitary-thyroid axis regulates metabolism via a cascade of releasing and tropic hormones.

Concept 45.3: Endocrine Glands Respond to Diverse Stimuli in Regulating Homeostasis, Development, and Behavior

Endocrine glands secrete hormones in response to various internal and external stimuli, affecting physiological processes, development, and behavior.

• Differential Responses to the Same Hormone:

  • Target cells may have different receptors, signal transduction pathways, or gene expression profiles, leading to varied responses to the same hormone.

  • Example: Epinephrine causes vasodilation in skeletal muscle but vasoconstriction in the intestines due to different receptor types.

• Adrenal Medulla Hormones:

  • Epinephrine and norepinephrine (catecholamines) are secreted in response to stress (fight-or-flight response).

  • Functions: Increase heart rate, blood pressure, blood glucose, and divert blood to essential organs.

  • Secretion is controlled by sympathetic nervous system signals.

• Adrenal Cortex Hormones:

  • Glucocorticoids (e.g., cortisol): Regulate metabolism and stress responses.

  • Mineralocorticoids (e.g., aldosterone): Regulate salt and water balance.

  • Sex hormones (small amounts): Influence sexual development.

• Gonadal Steroid Hormones:

  • Androgens (e.g., testosterone): Higher in males; stimulate development and maintenance of male characteristics.

  • Estrogens (e.g., estradiol): Higher in females; stimulate development and maintenance of female characteristics.

  • Progestins (e.g., progesterone): Involved in preparing and maintaining the uterus for pregnancy.

  • Both sexes produce all three categories, but in different proportions.

Example: During puberty, increased secretion of gonadal steroids leads to the development of secondary sexual characteristics.

Additional info:

• Hormone action is often regulated by complex feedback loops involving multiple glands and tissues.

• Disorders of the endocrine system can result from hormone excess, deficiency, or insensitivity.