Bio163Ch10

Chapter 10: Endocrine System

Endocrine System Glands

• Glands: Major endocrine glands include:

  • Pituitary

  • Thyroid

  • Thymus

  • Pineal gland

  • Parathyroids (posterior part of thyroid)

  • Adrenals

  • Pancreas (islets)

  • Ovaries (female)

  • Testes (male)

Cell Secretion Types 1

• Autocrine:

  • Released by cells affecting the same cell type.

  • Example: Eicosanoids.

• Paracrine:

  • Released by cells that affect other nearby cell types.

  • Example: Somatostatin.

Cell Secretion Types 2

• Neurotransmitter and Neuromodulators:

  • Secreted by nerve cells affecting nervous system functions.

  • Example: Various neurotransmitters.

• Hormones and Neurohormones:

  • Secreted into the blood, bind to specific receptor sites.

  • Examples: Epinephrine and Insulin.

Endocrine System Functions

• 1. Metabolism

• 2. Control of food intake and digestion

• 3. Tissue development

• 4. Ion regulation

• 5. Water balance

• 6. Heart rate and blood pressure regulation

• 7. Control of blood glucose and other nutrients

• 8. Control of reproductive functions

• 9. Uterine contraction and milk release

• 10. Immune system regulation

Characteristics of the Endocrine System

• Composed of endocrine glands and specialized endocrine cells throughout the body.

• Secretes hormones into the bloodstream rather than ducts.

• Hormones travel to target tissues or effectors via blood circulation.

Target Tissues and Hormonal Response

• Target tissues possess specific receptors for particular hormones.

• Hormones induce specific responses in these target tissues.

Types of Hormones

• Water-soluble hormones:

  • Include proteins, peptides, amino acids.

  • Common examples: Growth hormone, Antidiuretic, Prolactin.

• Lipid-soluble hormones:

  • Include steroids and eicosanoids.

  • Common examples: LH, FSH, Androgens.

Control of Hormone Release 1

• Blood-borne chemicals can stimulate hormone release, termed humoral stimuli.

• Hormone release may also be under neural control via action potentials in neurons secreting neurotransmitters.

Control of Hormone Release 2

• Hormone release can be controlled by other hormones, where one hormone stimulates the secretion of another.

• Humoral, Neural, and Hormonal stimuli can influence hormone release positively or negatively.

Neural Regulation

• An action potential (AP) in a neuron stimulates endocrine cell secretion of a hormone into the blood, targeting its specific function.

Hormonal Regulation

• Hypothalamus releases stimulatory hormones called releasing hormones into the anterior pituitary.

• These hormones prompt the anterior pituitary to release its own hormones, which target specific endocrine cells.

Inhibition of Hormone Release 1

• Humoral inhibition involves companion hormones that typically have opposite functions.

• Example: Aldosterone raises blood pressure while Atrial Natriuretic Peptide lowers it.

Inhibition of Hormone Release 2

• Neurons can inhibit hormone release as well; for example, inhibitory neurotransmitters prevent endocrine glands from secreting their hormones.

• Thyroid hormones can inhibit their own tropic hormone's secretion, modulating levels accordingly.

Regulation of Blood Hormone Levels

• Hormone levels are maintained via negative feedback and positive feedback mechanisms.

• Most hormones use negative feedback to prevent excessive levels; others may utilize positive feedback during certain physiological processes, such as childbirth.

Hormone Receptors 1

• Hormones affect cells that have specific receptors for them; receptor sites show high specificity for binding.

• Example: Epinephrine can bind to various similar receptors.

Hormone Receptors 2

• Lipid-soluble hormones bind to nuclear receptors, diffusing through cell membranes easily.

• Water-soluble hormones bind to membrane-bound receptors due to polar characteristics preventing cell membrane passage.

Action of Nuclear Receptors

• Lipid-soluble hormones provoke protein synthesis by binding nuclear receptors that regulate specific gene transcription.

Membrane Receptor Actions 1

• Membrane receptors influence G proteins or directly activate intracellular enzymes, producing second messengers.

Membrane Receptor Actions 2

• Second messengers like cAMP are produced following ligand binding, signaling cellular processes in response to hormones.

G-Protein Activation 1

• G proteins, consisting of three subunits, initiate responses linked to adenylate cyclase, converting ATP to cAMP.

G-Protein Activation 2

• Protein kinases adjust enzyme activity, ebbed by phosphodiesterase, which diminishes cAMP level and represses further cell stimulation.

Signal Amplification 1

• Hormones initiating second messenger synthesis induce rapid and vast amplification of the hormonal response.

Signal Amplification 2

• Each hormone can activate many second messengers, culminating in extensive production of the final product due to exceptional amplification efficiency.

Pituitary Gland

• A small gland in the brain, directed by the hypothalamus, split into anterior and posterior regions, secreting at least six significant hormones.

Anterior Pituitary Gland Actions

• Synthesizes hormones under hypothalamic control, where releasing hormones encourage production, and inhibiting hormones reduce secretion of specific anterior pituitary hormones.

Anterior Pituitary Hormones 1

• Growth Hormone:

  • Target tissues: Most

  • Functions: Stimulates growth in bones, muscles, and organs.

  • Abnormalities: Excess GH leads to giantism; insufficient GH causes dwarfism.

Anterior Pituitary Hormones 2

• Thyroid-Stimulating Hormone (TSH):

  • Target tissues: Thyroid gland

  • Function: Regulates secretions of the thyroid gland.

  • Abnormalities: High TSH causes thyroid enlargement; low TSH results in shrinking.

Regulation of Thyroid Hormone Secretion

• Stress and hypothermia stimulate TRH release to anterior pituitary, leading to TSH secretion, prompting T3 and T4 synthesis; T3 and T4 can hinder TRH and TSH release.

Anterior Pituitary Hormones 3

• LH (Luteinizing Hormone):

  • Female Target: Ovaries (promotes ovulation, progesterone production).

  • Male Target: Testes (promotes sperm production, testosterone).

Anterior Pituitary Hormones 4

• FSH (Follicle-Stimulating Hormone):

  • Female Target: Ovaries (stimulates follicle maturation, estrogen secretion).

  • Male Target: Testes (promotes sperm production).

Anterior Pituitary Hormones 5

• Prolactin:

  • Target tissues: Mammary glands, ovaries.

  • Functions: Involved in milk production.

Anterior Pituitary Hormones 6

• Melanocyte Stimulating Hormone (MSH):

  • Target: Melanocytes in skin.

  • Function: Stimulates melanin production.

  • ACTH (Adrenocorticotrophic Hormone):

    • Target: Adrenal cortex cells.

    • Function: Stimulates production of corticosteroids.

Posterior Pituitary Actions

• Synthesizes and releases hormones: Antidiuretic hormone (ADH) and Oxytocin from neuroendocrine cells in the hypothalamus.

Posterior Gland Hormones 1

• Antidiuretic Hormone (ADH):

  • Target tissues: Kidneys.

  • Function: Conserves water.

  • Abnormality: Low ADH results in Diabetes Insipidus (excess dilute urine).

Posterior Gland Hormones 2

• Oxytocin:

  • Target: Uterus.

  • Function: Augments uterine contractions during labor.

Hypothalamus and Posterior Pituitary

• Neural stimuli raise or lower action potential frequency in hypothalamic neurons which modulate neurohormone release affecting target tissues through blood.

Thyroid Gland

• One of the largest glands requiring iodine; secretes thyroid hormone and Calcitonin.

• Thyroid hormones regulate metabolic rates and growth.

• Calcitonin helps reduce blood calcium levels when elevated.

Thyroid Hormone Disorders

• Hypothyroidism:

  • Symptoms: Weight gain, fatigue, lower body temperature, dry skin. Myxedema in adults, cretinism in infants.

• Hyperthyroidism:

  • Symptoms: Weight loss, nervousness, warm skin. Can lead to Graves' disease.

Parathyroid Gland

• Secretes Parathyroid hormone (PTH):

  • Targets: Bones and kidneys.

  • Functions: Regulates blood Ca2+ levels, being more potent than calcitonin.

  • Action: Low Ca2+ triggers osteoclasts; high Ca2+ inhibits bone breakdown, affecting urinary calcium excretion.

Adrenal Gland Actions

• Located atop each kidney, comprising an outer cortex and inner medulla, respectively controlling separate endocrine functions.

Adrenal Gland Hormones 1

• Medullary hormones (Epinephrine/Norepinephrine):

  • Target tissues: Heart, blood vessels, liver, fat cells.

  • Functions: Activate the body's fight or flight response.

Regulation of Adrenal Medullary Secretions

• Stimuli like stress, physical activity, or low glucose levels activate the sympathetic nervous system to prompt adrenal medulla secretion of epinephrine and norepinephrine, impacting various metabolic processes and cardiovascular functions.

Adrenal Gland Hormones 2

• Aldosterone:

  • Type: Mineralocorticoid.

  • Target: Kidneys.

  • Functions: Promotes sodium and water retention, regulating blood pressure and volume.

Adrenal Gland Hormones 3

• Cortisol:

  • Type: Glucocorticoid.

  • Target: Most tissues.

  • Functions: Elevates energy via fat and protein breakdown, reduces inflammatory responses.

Regulation of Cortisol Secretion

• Triggered by Corticotropin-Releasing Hormone (CRH) from the hypothalamus due to stress; stimulates Adrenocorticotropic Hormone (ACTH) in anterior pituitary to promote cortisol production from adrenal cortex.

• As cortisol levels rise, it inhibits CRH and ACTH secretion (negative feedback).

Adrenal Gland Hormones 4

• Androgens:

  • Target tissues: Most.

  • Functions: Influence secondary sexual characteristics; govern sex drive in females.

Pancreas Actions

• The pancreas functions as both an exocrine (digestive enzymes) and endocrine gland (islet of Langerhans).

• The endocrine portion contains:

  • Alpha cells (glucagon)

  • Beta cells (insulin)

  • Delta cells (somatostatin).

Pancreas Hormones 1

• Insulin:

  • Target tissues: Liver, skeletal muscle, adipose.

  • Functions: Regulates blood glucose, enhancing glucose uptake and storage after meals.

Insulin Abnormalities

• Diabetes Mellitus:

  • Causes: Inadequate insulin or receptor malfunction.

  • Symptoms: Increased appetite, excessive urination, dehydration, fatigue.

  • Type I: Insulin-dependent.

  • Type II: Can be managed with lifestyle changes but may evolve into Type I.

Pancreas Hormones 2

• Glucagon:

  • Target tissues: Liver.

  • Functions: Elevates blood glucose levels by promoting glycogen breakdown during fasting intervals.

Regulation of Blood Glucose Levels

-Insulin vs. Glucagon: Pancreatic islets detect glucose variations and respond appropriately.

• Insulin secretion: Stimulated by elevated blood glucose and digestive hormones; leads to glucose uptake and glycogen storage.

• Glucagon secretion: Activated by low blood glucose; promotes glycogen breakdown and glucose synthesis.

Testes Hormone

• Testosterone:

  • Target tissues: Most.

  • Functions: Aids in sperm and reproductive organ development/function.

Ovarian Hormones

• Estrogen/Progesterone:

  • Target tissues: Most.

  • Functions: Essential for uterine and mammary gland development and regulating the menstrual cycle.

Thymus Gland Hormone

• Thymosin:

  • Target tissues: Immune system tissues.

  • Functions: Promotes immune system development and function.

Pineal Gland Hormone

• Melatonin:

  • Target tissues: Hypothalamus.

  • Functions: Influences puberty onset and circadian rhythms, modulated by light.