Hormones and the Endocrine System: Study Notes

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) but can be long-lasting.

• 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.

  • 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 or cellular function.

• 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.

  • Comparison: Water-soluble hormones act via membrane receptors and second messengers; lipid-soluble hormones directly regulate gene transcription.

Example: Epinephrine (a water-soluble amine) binds to receptors on liver cells, activating a cascade that leads to glucose release into the blood.

Concept 45.2: Feedback Regulation and Coordination with the Nervous System 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 the hypothalamus to release oxytocin, which causes milk ejection. Continued suckling increases oxytocin release (positive feedback).

• 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:

  • Location: The pituitary gland is located at the base of the brain, below the hypothalamus.

  • Posterior Pituitary: Releases neurohormones made in the hypothalamus (oxytocin, antidiuretic hormone/ADH).

  • Anterior Pituitary: Produces and releases hormones such as 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 homeostasis, development, and behavior. The same hormone can elicit different responses in different target cells due to variations in receptor type or signal transduction pathways.

• Differential Target Cell Responses:

  • Cells with different receptors or signal transduction components respond differently to the same hormone.

  • Example: Epinephrine causes vasodilation in skeletal muscle blood vessels but vasoconstriction in intestinal blood vessels.

• Adrenal Medulla Hormones:

  • Epinephrine and Norepinephrine: Mediate "fight-or-flight" responses (increase heart rate, blood pressure, blood glucose).

  • Control: Secretion is regulated by sympathetic nervous system signals.

• Adrenal Cortex Hormones:

  • Glucocorticoids (e.g., cortisol): Affect glucose metabolism, suppress immune response.

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

  • Sex hormones (small amounts): Androgens, estrogens, progestins.

• 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.

  • Production Variations: Males produce more androgens; females produce more estrogens and progestins.

Example: Cortisol secretion increases during stress, mobilizing energy reserves and suppressing non-essential functions.

Table: Major Hormones of the Adrenal Glands and Gonads

Gland	Hormone	Main Function	Control Mechanism
Adrenal Medulla	Epinephrine, Norepinephrine	"Fight-or-flight" responses (increase heart rate, blood glucose, etc.)	Sympathetic nervous system
Adrenal Cortex	Glucocorticoids (cortisol)	Glucose metabolism, stress response	ACTH from anterior pituitary
Adrenal Cortex	Mineralocorticoids (aldosterone)	Salt and water balance	Renin-angiotensin system
Gonads	Androgens (testosterone)	Male reproductive development	FSH, LH from anterior pituitary
Gonads	Estrogens (estradiol)	Female reproductive development	FSH, LH from anterior pituitary
Gonads	Progestins (progesterone)	Uterine preparation and maintenance	FSH, LH from anterior pituitary

Additional info: The endocrine system integrates with the nervous system to regulate physiological processes, and feedback mechanisms ensure stability and adaptability in response to environmental and internal changes.