ENDOCRINE SYSTEM – COMPLETE FINAL NOTES (Biopsychology, UG Semester 2)
Introduction: What is the Endocrine System?
The endocrine system is a major regulatory system of the body. Its main function is to
release hormones – chemical substances secreted by endocrine glands directly into the
bloodstream. Hormones travel through the blood to target cells, where they bind to specific
receptors (like a key in a lock) to relay messages. This system continuously monitors and
adjusts body functions to maintain homeostasis.
Hormone Mechanisms of Action (Important for Biopsychology)
Hormones can be classified by their chemical structure, which determines how they act:
- Peptide/protein hormones (e.g., insulin, growth hormone, prolactin, oxytocin):
Water-soluble. Cannot cross cell membrane; bind to cell surface receptors → activate
second messenger systems (e.g., cAMP, IP3) → rapid effects.
- Steroid hormones (e.g., cortisol, aldosterone, testosterone, estrogen, progesterone):
Lipid-soluble. Cross cell membrane → bind to intracellular receptors (cytoplasm or nucleus)
→ directly affect gene transcription → slower but longer-lasting effects.
- Amine hormones (e.g., adrenaline, noradrenaline, thyroid hormones, melatonin): Mixed.
Adrenaline/noradrenaline act like peptides (cell surface receptors); thyroid hormones act like
steroids (intracellular receptors).
Why this matters in biopsychology: Steroid hormones (like cortisol and sex hormones)
can change brain structure and function over days/weeks; peptide hormones (like oxytocin)
have rapid, transient effects on behavior.
---
,Major Glands of the Endocrine System
1. Hypothalamus – The Master Regulator
The hypothalamus is a small region at the base of the brain, just above the pituitary gland. It
serves as the critical link between the nervous system and the endocrine system. It receives
input from almost every part of the brain and body, and responds by secreting releasing and
inhibiting hormones that control the pituitary gland.
Key Hypothalamic Hormones That Control the Anterior Pituitary:
- GHRH (growth hormone-releasing hormone) → stimulates GH release
- Somatostatin (GHIH) → inhibits GH release
- TRH (thyrotropin-releasing hormone) → stimulates TSH release
- CRH (corticotropin-releasing hormone) → stimulates ACTH release
- GnRH (gonadotropin-releasing hormone) → stimulates FSH and LH release
- PIH (dopamine / prolactin-inhibiting hormone) → inhibits prolactin release
Feedback Loops (Essential for Biopsychology)
Most endocrine systems operate via negative feedback – the final hormone signals the
hypothalamus and pituitary to stop releasing more.
- Example: Thyroid hormone negative feedback – Hypothalamus releases TRH → pituitary
releases TSH → thyroid releases T3/T4
. High T3/T4 levels feed back to inhibit both TRH and TSH release.
- Example: Cortisol negative feedback – Hypothalamus releases CRH → pituitary releases
ACTH → adrenal cortex releases cortisol. High cortisol inhibits CRH and ACTH.
Positive feedback (rare, but important): Oxytocin during childbirth – uterine contractions
stimulate more oxytocin release, which strengthens contractions until delivery.
—
, 2. Pituitary Gland (Hypophysis)
The pituitary gland (pea-sized, located just below the hypothalamus) is attached to the
hypothalamus by a single stalk called the infundibulum. Structurally, it is divided into two
distinct parts:
2.1 Anterior Pituitary (Adenohypophysis)
It accounts for most of the pituitary gland and releases six major peptide hormones. These
hormones are stored within the lobe (glandular tissue).
Hormones of the Anterior Pituitary:
a. Growth Hormone (GH) – Produced by somatotropic cells. Also called somatotropin.
- Actions: Anabolic, tissue-building hormone; mobilizes fat (increases fatty acids); promotes
growth via insulin-like growth factors (IGFs) produced by the liver.
- Regulation: GHRH (stimulates), somatostatin (inhibits).
b. Thyroid Stimulating Hormone (TSH) – Also called thyrotropin.
- Actions: Stimulates thyroid gland development and secretion of T3/T4.
- Regulation: TRH from hypothalamus.
c. Adrenocorticotropic Hormone (ACTH)
- Actions: Stimulates adrenal cortex to release glucocorticoids (mainly cortisol).
- Regulation: CRH from hypothalamus.
d. Prolactin (PRL)
- Actions: Stimulates mammary gland milk production. Also produced in males (function less
understood).
- Regulation: Dopamine (PIH) inhibits; TRH can stimulate.
Introduction: What is the Endocrine System?
The endocrine system is a major regulatory system of the body. Its main function is to
release hormones – chemical substances secreted by endocrine glands directly into the
bloodstream. Hormones travel through the blood to target cells, where they bind to specific
receptors (like a key in a lock) to relay messages. This system continuously monitors and
adjusts body functions to maintain homeostasis.
Hormone Mechanisms of Action (Important for Biopsychology)
Hormones can be classified by their chemical structure, which determines how they act:
- Peptide/protein hormones (e.g., insulin, growth hormone, prolactin, oxytocin):
Water-soluble. Cannot cross cell membrane; bind to cell surface receptors → activate
second messenger systems (e.g., cAMP, IP3) → rapid effects.
- Steroid hormones (e.g., cortisol, aldosterone, testosterone, estrogen, progesterone):
Lipid-soluble. Cross cell membrane → bind to intracellular receptors (cytoplasm or nucleus)
→ directly affect gene transcription → slower but longer-lasting effects.
- Amine hormones (e.g., adrenaline, noradrenaline, thyroid hormones, melatonin): Mixed.
Adrenaline/noradrenaline act like peptides (cell surface receptors); thyroid hormones act like
steroids (intracellular receptors).
Why this matters in biopsychology: Steroid hormones (like cortisol and sex hormones)
can change brain structure and function over days/weeks; peptide hormones (like oxytocin)
have rapid, transient effects on behavior.
---
,Major Glands of the Endocrine System
1. Hypothalamus – The Master Regulator
The hypothalamus is a small region at the base of the brain, just above the pituitary gland. It
serves as the critical link between the nervous system and the endocrine system. It receives
input from almost every part of the brain and body, and responds by secreting releasing and
inhibiting hormones that control the pituitary gland.
Key Hypothalamic Hormones That Control the Anterior Pituitary:
- GHRH (growth hormone-releasing hormone) → stimulates GH release
- Somatostatin (GHIH) → inhibits GH release
- TRH (thyrotropin-releasing hormone) → stimulates TSH release
- CRH (corticotropin-releasing hormone) → stimulates ACTH release
- GnRH (gonadotropin-releasing hormone) → stimulates FSH and LH release
- PIH (dopamine / prolactin-inhibiting hormone) → inhibits prolactin release
Feedback Loops (Essential for Biopsychology)
Most endocrine systems operate via negative feedback – the final hormone signals the
hypothalamus and pituitary to stop releasing more.
- Example: Thyroid hormone negative feedback – Hypothalamus releases TRH → pituitary
releases TSH → thyroid releases T3/T4
. High T3/T4 levels feed back to inhibit both TRH and TSH release.
- Example: Cortisol negative feedback – Hypothalamus releases CRH → pituitary releases
ACTH → adrenal cortex releases cortisol. High cortisol inhibits CRH and ACTH.
Positive feedback (rare, but important): Oxytocin during childbirth – uterine contractions
stimulate more oxytocin release, which strengthens contractions until delivery.
—
, 2. Pituitary Gland (Hypophysis)
The pituitary gland (pea-sized, located just below the hypothalamus) is attached to the
hypothalamus by a single stalk called the infundibulum. Structurally, it is divided into two
distinct parts:
2.1 Anterior Pituitary (Adenohypophysis)
It accounts for most of the pituitary gland and releases six major peptide hormones. These
hormones are stored within the lobe (glandular tissue).
Hormones of the Anterior Pituitary:
a. Growth Hormone (GH) – Produced by somatotropic cells. Also called somatotropin.
- Actions: Anabolic, tissue-building hormone; mobilizes fat (increases fatty acids); promotes
growth via insulin-like growth factors (IGFs) produced by the liver.
- Regulation: GHRH (stimulates), somatostatin (inhibits).
b. Thyroid Stimulating Hormone (TSH) – Also called thyrotropin.
- Actions: Stimulates thyroid gland development and secretion of T3/T4.
- Regulation: TRH from hypothalamus.
c. Adrenocorticotropic Hormone (ACTH)
- Actions: Stimulates adrenal cortex to release glucocorticoids (mainly cortisol).
- Regulation: CRH from hypothalamus.
d. Prolactin (PRL)
- Actions: Stimulates mammary gland milk production. Also produced in males (function less
understood).
- Regulation: Dopamine (PIH) inhibits; TRH can stimulate.
