Endocrine System: Pituitary Gland and Hypothalamus

Pituitary Gland and Hypothalamus

• The endocrine system works with the nervous system to maintain homeostasis.
• The pituitary gland (hypophysis) secretes 9 major hormones, regulating body functions and other endocrine glands.
• The hypothalamus regulates the pituitary gland in response to hormones, sensory information, and emotions.

Pituitary Gland Structure

• Located at the base of the brain, just below the hypothalamus, sitting in the sella turcica of the sphenoid bone.
• 1 cm in diameter, 0.5–1.0 g (pea-sized)
• Connected to the hypothalamus by the infundibulum (stalk of tissue)
• Divided into the posterior pituitary (neurohypophysis) and the anterior pituitary (adenohypophysis)

Posterior Pituitary (Neurohypophysis)

• Continuous with the hypothalamus, forms from brain outgrowth.
• Stores and releases hormones produced by the hypothalamus.
• Hormones released are neuropeptides/neurohormones, transported through the hypothalamohypophysial tract.

Anterior Pituitary (Adenohypophysis)

• Derived from glandular epithelium (oral cavity)
• Not neural tissue, develops from the pituitary diverticulum (Rathke pouch) in the embryonic oral cavity
• Thin band of tissue, pars intermedia, at the border with the posterior pituitary (non-functional in adults).
• Secretes traditional hormones (not neuropeptides) regulated by the hypothalamus

Pituitary Gland and Hypothalamus Relationship

• Posterior pituitary regulation:
  • Via hypothalamohypophysial tract (neural pathway).
  • Hormones are produced by neurons in the hypothalamus, stored in the posterior pituitary, released into the circulatory system upon stimulation.
• Anterior pituitary regulation:
  • Via hypothalamohypophysial portal system (blood vessel pathway).
  • The hypothalamus secretes releasing/inhibiting hormones into a capillary bed that travels to the anterior pituitary, regulating its hormone secretion.

Hypothalamic Control of the Posterior Pituitary

• Neurons in the hypothalamus stimulate the secretion of posterior pituitary hormones.
• Action potentials conducted along axons of hypothalamic neurons through the hypothalamohypophysial tract to the posterior pituitary.
• Hormones stored in the posterior pituitary are released into circulation.
• Hormones influence their target tissues via the circulatory system.

Hypothalamic Control of the Anterior Pituitary

• Neurons in the hypothalamus produce neuropeptides (releasing/inhibiting hormones) and secrete them into a capillary bed.
• These hormones travel through the hypothalamohypophysial portal system to the anterior pituitary.
• The neuropeptides bind to anterior pituitary cells, regulating hormone secretion.
• Hormones secreted from the anterior pituitary enter the bloodstream and travel to target tissues.

Hypothalamic Hormones

• Releasing hormones: Stimulate secretion of anterior pituitary hormones.
• Inhibiting hormones: Decrease secretion of anterior pituitary hormones.

Pituitary Gland Hormones

• Posterior Pituitary (Neurohypophysis) Hormones:
  • Antidiuretic hormone (ADH):
    • Target: Kidneys.
    • Effect: Increases water reabsorption (reduces urine output).
  • Oxytocin:
    • Targets: Uterus, mammary glands.
    • Effects: Increases uterine contractions during birth, milk ejection from mammary glands.
• Anterior Pituitary (Adenohypophysis) Hormones:
  • Growth hormone (GH):
    • Target: Most tissues.
    • Effects: Promotes tissue growth, protein synthesis, fat breakdown, increases blood glucose levels, and IGF production.
  • Thyroid-stimulating hormone (TSH):
    • Target: Thyroid gland.
    • Effect: Increases thyroid hormone secretion.
  • Adrenocorticotropic hormone (ACTH):
    • Target: Adrenal cortex.
    • Effect: Increases glucocorticoid hormone secretion.
  • Lipotropins:
    • Target: Adipose tissue.
    • Effect: Increases lipid breakdown.
  • β-Endorphins:
    • Target: Brain.
    • Effects: Analgesia in the brain, inhibits GnRH secretion.
  • Melanocyte-stimulating hormone (MSH):
    • Target: Melanocytes in skin.
    • Effect: Increases melanin production (darkens skin), memory functions in the CNS.
  • Luteinizing hormone (LH):
    • Targets: Ovaries (females), testes (males).
    • Effects: Ovulation and progesterone production in females, testosterone synthesis and sperm production in males.
  • Follicle-stimulating hormone (FSH):
    • Targets: Follicles in ovaries (females), seminiferous tubules (males).
    • Effects: Follicle maturation, estrogen secretion (females); sperm cell production (males).
  • Prolactin:
    • Targets: Ovaries, mammary glands.
    • Regulation:
      • Stretch of the uterus, stimulation of the cervix, and nipple stimulation trigger oxytocin release.
      • Action potentials travel from sensory neurons to the hypothalamus and posterior pituitary, where oxytocin is released into the blood.
      • Oxytocin increases uterine contractions and milk letdown.

Anterior Pituitary Hormones Continued

• Anterior pituitary hormones are synthesized by cells within the gland.
• Secretion is regulated by hypothalamic releasing and inhibiting hormones.
• Hormones secreted from the anterior pituitary are proteins, glycoproteins, or polypeptides.

Growth Hormone (GH)

• Function: Stimulates growth in most tissues and regulates metabolism.
• Actions:
  • Increases amino acid uptake into cells, promotes protein synthesis, and decreases protein breakdown.
  • Increases lipolysis (breakdown of lipids) and release of fatty acids for energy use.
  • Increases glucose synthesis by the liver and reduces glucose usage in other tissues.
  • Stimulates production of insulin-like growth factors (IGFs), which promote bone and muscle growth.
• Regulation:
  • Growth hormone–releasing hormone (GHRH) stimulates GH secretion.
  • Growth hormone–inhibiting hormone (GHIH) (somatostatin) inhibits GH secretion.
  • GH secretion increases with stress or low blood glucose.
  • GH levels peak during deep sleep.

Prolactin (PRL)

• Function:
  • Promotes milk production in lactating females.
  • May enhance progesterone secretion after ovulation.
• Regulation:
  • Primarily controlled by prolactin-inhibiting hormone (PIH) (dopamine), which inhibits PRL secretion.
  • Dopamine levels decrease to release PRL.

Thyroid-Stimulating Hormone (TSH)

• Function: Stimulates the synthesis and secretion of thyroid hormones.
• Regulation:
  • Thyrotropin-releasing hormone (TRH) from the hypothalamus stimulates TSH secretion.
  • Negative feedback from thyroid hormones inhibits TSH and TRH release.

Adrenocorticotropic Hormone (ACTH)

• Function: Stimulates the secretion of cortisol from the adrenal cortex.
• Derived from: Proopiomelanocortin (POMC), a large precursor protein, which also produces lipotropins, β-endorphins, and melanocyte-stimulating hormone (MSH).
• Regulation:
  • Stress increases ACTH secretion.
  • ACTH secretion increases cortisol release, regulating chronic stress.

Prolonged Sustained Activity

• Glucose from the liver becomes insufficient to maintain blood glucose levels for brain function.
• Insulin decreases:
  • Prevents glucose uptake by most tissues, conserving it for the brain.
• Epinephrine, glucagon, cortisol, GH:
  • Increase fatty acids, triglycerides, and ketones in the blood.
  • GH increases protein synthesis and slows protein breakdown.
  • Glucose metabolism decreases.
  • Skeletal muscles shift to lipid metabolism for energy.
• End of long exercise: Muscles rely more on lipid metabolism for energy.

Pineal Gland

• Location: Pineal gland is located in the epithalamus of the brain.
• Primary function: Secretes hormones that act on the hypothalamus and gonads to inhibit reproductive functions.
• Key secretory products:
  • Melatonin:
    • Can decrease hypothalamic GnRH secretion.
    • May inhibit reproductive functions.
    • Helps regulate sleep cycles by increasing the tendency to sleep.
  • Arginine vasotocin:
    • Works with melatonin to regulate the reproductive system in some animals.
    • Inhibits GnRH secretion.

Regulation of Pineal Secretions

• Light and dark cycles regulate melatonin secretion:
  • Light enters the eye, stimulating neurons in the retina.
  • Action potentials are transmitted to the hypothalamus.
  • Action potentials from the hypothalamus are transmitted through the sympathetic division to the pineal gland.
  • Darkness: Increases sympathetic stimulation to the pineal gland and increases melatonin secretion.
  • Light: Decreases sympathetic stimulation and melatonin secretion.
• Melatonin’s effects:
  • Inhibits GnRH secretion from the hypothalamus.
  • May regulate sleep cycles.
  • Secretion increases at night, decreases during the day.
• Effect on the reproductive system:
  • Animals that breed in spring experience reduced pineal secretion with longer daylight, leading to reproductive hypertrophy in summer.
  • In humans, melatonin's role in reproductive function is unclear, but supplemental melatonin may affect reproductive systems.
• Pineal gland dysfunction:
  • Tumors destroying the pineal gland correlate with early sexual development.
  • Tumors increasing pineal hormone secretion correlate with delayed reproductive system development.
  • It is unclear if the pineal gland controls puberty onset.

Other Hormones and Chemical Messengers

• Thymus Hormones:
  • Thymus location: Neck and superior to the heart.
  • Hormone: Thymosin
    • Role in development and maturation of the immune system.
• Digestive Tract Hormones:
  • Several hormones regulate digestive functions by influencing the activity of the stomach, intestines, liver, and pancreas.
• Adipose Tissue Hormones:
  • Hormone: Leptin
    • Secreted by adipocytes.
    • Regulates satiety signals in the hypothalamus
    • Disabling leptin synthesis leads to excessive eating and obesity.

Hormonelike Chemicals

• Autocrine chemical messengers:
  • Released from cells and influence the same cell.
• Paracrine chemical messengers:
  • Released from one cell type, diffuse short distances, and influence target cells nearby.
• Examples of autocrine/paracrine messengers:
  • Eicosanoids:
    • Derived from arachidonic acid, includes:
      • Prostaglandins
      • Thromboxanes
      • Prostacyclins
      • Leukotrienes
    • Released from injured cells, involved in inflammation and other processes (e.g., uterine contractions, coagulation).
  • Prostaglandins:
    • Involved in pain sensation, vasodilation (linked to headaches).
    • Inhibited by anti-inflammatory drugs (e.g., aspirin).
  • Endogenous analgesics:
    • Include enkephalins, endorphins, and dynorphins.
    • Act on the same receptors as morphine.
    • Produced in the brain, pituitary gland, spinal cord, intestines.
    • Moderate pain sensation and increase during exercise and stress.
  • Growth factors:
    • Epidermal growth factor: Stimulates cell division and important for embryonic development.
    • Interleukin-2: Stimulates T lymphocyte proliferation, crucial for immune responses.

Summary

• Hormonelike chemicals and messengers play complex roles in regulating various bodily functions, often through autocrine and paracrine actions.
• Understanding these chemical messengers aids in the development of treatments for various diseases.

Explore the key functions and structures of the pituitary gland and hypothalamus in this comprehensive quiz. Understand the hormones secreted by the pituitary, its relationship with the hypothalamus, and how these glands contribute to homeostasis. Test your knowledge on the anatomy and physiological roles of this essential endocrine system.