Endocrine System Notes

The Endocrine System

Overview

• The endocrine system is a body control system where regulation requires duration rather than speed.
• It consists of glands that secrete chemical messengers (hormones) into the circulatory system (blood).

Hormone Characteristics

• Produced in small quantities.
• Transported some distance in the circulatory system.
• Acts on target tissues elsewhere in the body.
• Hormone secretion can be:
  • Acute: Sudden release due to stimulus, e.g., adrenaline in response to stress.
  • Chronic: Small variations over long periods, e.g., thyroid hormones.
  • Episodic: E.g., estrogen & progesterone during the menstrual cycle.
• Target cells respond to a hormone because they have the correct receptor.

Functions of the Endocrine System

1. Metabolism.
2. Control of food intake and digestion.
3. Tissue maturation.
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 (gametogenesis and pregnancy).
9. Uterine contractions and milk release.
10. Immune system regulation.

Basics of Chemical Communication

• Autocrine: Released by cells and have a local effect on the same cell type from which chemical signals are released
• Paracrine: Released by cells and affect other cell types locally without being transported in the blood
• Neurotransmitter: Produced by neurons and secreted into extracellular spaces by presynaptic nerve terminals; travels short distances; influences postsynaptic cells.
• Endocrine: Produced by cells of endocrine glands, enters the circulatory system, and affects distant cells.

Nervous vs. Endocrine Systems

Similarities

• Both systems are associated with the brain (hypothalamus).
• May use the same chemical messenger as neurotransmitter and hormone. E.g., epinephrine.
• The two systems are cooperative, e.g., some parts of the endocrine system are innervated directly by the nervous system (adrenal medulla).

Differences

Pituitary Gland and Hypothalamus

• Where nervous and endocrine systems interact.
• Hypothalamus regulates secretions of the anterior pituitary.
• The posterior pituitary is an extension of the hypothalamus.
• The pituitary gland produces nine major hormones that:
  • Regulate body functions.
  • Regulate the secretions of other endocrine glands.

Structure of the Pituitary Gland

• Posterior pituitary (neurohypophysis): Extension of the nervous system via the infundibulum.
  • Secretes neuropeptides.
• Anterior pituitary (adenohypophysis):
  • Develops from embryonic oral cavity; secretes traditional hormones.

Hypothalamic Control

• Posterior Pituitary:
  • Hormones are produced in neurons in the hypothalamus and stored in the posterior pituitary.
  • Axons form the hypothalamohypophysial tract.
  • Action potentials in these neurons cause hormone release.
• Anterior Pituitary:
  • Blood vessels make up the hypothalamohypophysial portal system, connecting the areas.
  • Hypothalamic releasing and inhibiting hormones stimulate or inhibit anterior pituitary hormone release.

Hypothalamus, Anterior Pituitary, Target Tissues

• Stimuli within the nervous system regulate the secretion of releasing hormones from neurons in the hypothalamus.
• Releasing hormones pass to the anterior pituitary.
• Releasing hormones stimulate the release of hormones from the anterior pituitary.
• Anterior pituitary hormones travel in the bloodstream to target tissue, which may be another endocrine gland.

Hypothalamic Hormones

• Growth hormone-releasing hormone (GHRH):
  • Causes increased secretion of GH.
• Growth hormone-inhibiting hormone (GHIH):
  • Causes decreased secretion of GH.
• Thyrotropin-releasing hormone (TRH):
  • Causes TSH secretion.
• Melanocyte releasing hormone (MRH):
  • Causes MSH secretion.
• Corticotropin-releasing hormone (CRH):
  • Causes ACTH secretion.
• Gonadotropin-releasing hormone (GnRH):
  • Causes secretion of gonadotropins LH and FSH.
• Prolactin-releasing hormone (PRH):
  • Causes increased prolactin secretion.
• Dopamine (prolactin-inhibiting hormone, PIH):
  • Causes decreased prolactin secretion.

Anterior Pituitary Hormones

1. Growth hormone (GH):
   • Acts on most cells of the body overall metabolism and growth.
2. Thyroid stimulating hormone (TSH):
   • Stimulates the thyroid to secrete T3 and T4.
3. Adrenocorticotropic hormone (ACTH):
   • Stimulates the adrenal cortex to secrete cortisol and aldosterone.
4. Melanocyte-stimulating hormone (MSH):
   • Causes melanocytes to produce more melanin.
5. Luteinizing hormone (LH):
6. Follicle-stimulating hormone (FSH):
   • Both hormones regulate the production of gametes and reproductive hormones.
   • Testes: To make testosterone and spermatogenesis (sperm).
   • Ovaries: To make estrogen and progesterone, and oogenesis (oocytes).
7. Prolactin:
   • Role in milk production (lactation).

Tropic vs. Non-tropic Hormones

• Tropic hormones: Stimulate the secretion of other hormones from target tissues.
• Non-tropic hormones: Initiate an effect.

Hypothalamus, Posterior Pituitary, Target Tissues

• Stimuli within the nervous system cause neurons in the hypothalamus to increase or decrease action potential frequency.
• Action potentials are conducted along neurons from the hypothalamus to the posterior pituitary.
• Axon terminals of these neurons store neurohormones.
• Action potentials cause the release of neurohormones into the circulatory system.
• Posterior pituitary hormones travel in the bloodstream to target tissue.

Posterior Pituitary Hormones

1. Antidiuretic hormone (ADH):
   • Stimulates increased reabsorption of sodium and water from nephrons, so less (more concentrated) urine is produced (water is conserved by the body).
   • If BP decreases, then ADH secretion is stimulated.
   • Also called vasopressin (vasoconstrictor, increases BP).
2. Oxytocin:
   • Uterine contractions during birth.
   • Ejection of milk from lactating breast (let-down reflex).

Control of Hormone Secretion

Negative Feedback

• The anterior pituitary secretes a tropic hormone that travels in the blood to the target endocrine cell.
• The hormone from the target endocrine cell travels to its target.
• The hormone from the target endocrine cell has a negative feedback (opposite) effect on the hypothalamus and anterior pituitary to decrease the secretion of the tropic hormone.

Positive Feedback

• The anterior pituitary secretes a tropic hormone that travels in the blood to the target endocrine cell.
• The hormone from the target endocrine cell travels to its target.
• The hormone from the target endocrine cell has a positive feedback effect on the hypothalamus and anterior pituitary to increase the secretion of the tropic hormone.

Key Hormones and Their Functions

Growth Hormone (GH)

• Stimulates uptake of amino acids; protein synthesis.
• Stimulates breakdown of fats to be used as an energy source.
• Promotes bone and cartilage growth.
• Regulates blood levels of nutrients after a meal.
• GH stimulates the liver and skeletal muscle to make IGF-1.
• Peak GH levels occur during deep sleep.

Thyroid Stimulating Hormone (TSH)

• Leads to the production of T3 and T4.

Thyroid Gland

• One of the largest endocrine glands; highly vascular.
• The only gland that stores hormone.
• Composed of follicles: follicular cells surrounding thyroglobulin/thyroid hormones.
• Iodine and tyrosine are necessary for the production of T3 and T4.
• Increases the rate of glucose, fat, and protein metabolism in many tissues, thus increasing body temperature.
• Normal growth of many tissues.
• Triiodothyronine or T3.
• Tetraiodothyronine or T4 or thyroxine.

Adrenocorticotropic Hormone (ACTH)

• Near the superior poles of the kidneys: inner medulla; outer cortex.
• CRH from the hypothalamus causes the release of ACTH from the anterior pituitary, which:
  • Causes cortisol secretion from the adrenal cortex (a glucocorticoid).
  • Causes aldosterone secretion from the adrenal cortex (a mineralocorticoid).
  • Causes androgen (weak sex hormone).

Adrenal Medulla

• Epinephrine = adrenaline.
• Norepinephrine = noradrenaline.
• Stress, physical activity, and low blood glucose levels act as stimuli to the hypothalamus, resulting in increased sympathetic nervous system activity.
• An increased frequency of action potentials conducted through the sympathetic division of the autonomic nervous system stimulates the adrenal medulla to secrete epinephrine and norepinephrine into the CVS.
• The secretion of hormones prepares the body for physical activity.
• Short-lived responses.
• Epinephrine and norepinephrine increase heart rate and force of contraction; cause blood vessels to constrict in the skin, kidneys, gastrointestinal tract, and other viscera.

Melanocyte-Stimulating Hormone (MSH)

• Acts on receptors in skin cells (melanocytes) and stimulates melanin (color pigment) in the skin.
• MSH also has a role in regulating appetite and sexual behavior.
• Poorly understood.
• MRH (sunlight).

Luteinizing Hormone (LH) & Follicle Stimulating Hormone (FSH)

• GnRH from the hypothalamus stimulates LH and FSH secretion.
• Gonadotropins: glycoprotein hormones that promote growth and function of the gonads.
• Both hormones regulate the production of gametes and reproductive hormones
• From testes - Testosterone: spermatogenesis, secondary sex characteristics.
• From ovaries - Estrogen and progesterone: sex organ development and characteristics, menstrual cycle, pregnancy.

Prolactin

• Non-tropic hormone.
• Breast milk production.
• Supply and demand.
• PIF (Prolactin-inhibiting factor)
• PRF (Prolactin-releasing factor)

Oxytocin

• Posterior pituitary.
• Non-tropic hormone.
• Positive feedback.
• Breast milk release (let-down).
• Supply and demand.

Antidiuretic Hormone (ADH) / Vasopressin

• Reduces urine formation.
• Keeps water in the body.
• Increases blood volume and thus blood pressure.
• Hot day / dehydrated = lots of ADH.
• Diuretics – tea, coffee, alcohol.

Pancreas

• Located along near intestine and stomach; retroperitoneal

• Exocrine gland: Produces pancreatic digestive juices

• Endocrine gland: Consists of pancreatic islets.

  • Alpha cells - secrete glucagon
  • Beta cells - secrete insulin

• Regulation of insulin secretion after a meal or during fasting.