Overview
This lecture introduces the endocrine system, focusing on hormone types (protein vs. steroid), hormone regulation, and the hypothalamus-pituitary relationship.
Endocrine System Basics
- The endocrine system maintains homeostasis and communicates using hormones released into the bloodstream.
- Endocrine glands secrete hormones internally; exocrine glands release substances onto body surfaces.
- Only target cells with specific receptors respond to a particular hormone.
- Hormone effects can be long-lasting and widespread, unlike the short-term, localized effects of the nervous system.
Types of Hormones
- Protein hormones are made of amino acids, water-soluble, cannot cross cell membranes, and bind to cell surface receptors.
- Protein hormones trigger "second messenger" pathways inside cells to produce effects without entering the cell.
- Steroid hormones are made from cholesterol, lipid-soluble, can cross cell and nuclear membranes, and bind to internal receptors.
- Steroid hormones directly influence gene expression, leading to synthesis of new proteins.
Hormone Response & Regulation
- Each hormone can have different effects on various target cells depending on receptors and signaling pathways.
- Epinephrine (adrenaline) is a protein hormone that prepares the body for stress (increases blood glucose, redirects blood flow).
- Most hormone pathways are regulated by negative feedback to prevent over- or under-response.
- Antagonistic hormones have opposite effects (e.g., insulin lowers blood glucose, glucagon raises it).
- Positive feedback (rare) amplifies the response, as in oxytocin release during childbirth.
Hypothalamus and Pituitary Gland
- The hypothalamus (in the brain) produces neurohormones that control the pituitary gland.
- The pituitary gland consists of anterior and posterior lobes with different hormone release mechanisms.
- Posterior pituitary releases hormones (ADH, oxytocin) made in the hypothalamus, stored in axon terminals.
- Anterior pituitary releases hormones in response to hypothalamic releasing or inhibiting hormones through local blood vessels.
- Hormone cascades (often three glands/hormones) allow amplification and fine control of responses.
Key Terms & Definitions
- Homeostasis — maintenance of stable internal conditions.
- Hormone — chemical messenger released into the blood affecting distant cells.
- Endocrine gland — gland that secretes hormones into the bloodstream.
- Exocrine gland — gland that releases substances onto body surfaces/cavities.
- Protein hormone — water-soluble hormone made of amino acids.
- Steroid hormone — lipid-soluble hormone derived from cholesterol.
- Tropic hormone — hormone that stimulates another gland to release hormones.
- Negative feedback — response mechanism that inhibits further hormone release.
- Antagonistic hormones — pairs of hormones with opposing effects.
- Neurohormone — hormone produced by neurons (e.g., hypothalamus).
- Anterior pituitary — front pituitary lobe, releases hormones in response to hypothalamic signals.
- Posterior pituitary — rear pituitary lobe, releases hypothalamic hormones.
Action Items / Next Steps
- Review the major endocrine glands and their basic hormone functions.
- Memorize required hormone details: producing gland, target, stimulus, response, and feedback type.
- Prepare for detailed study of 11 main hormones covered in this unit.