The Endocrine System

• The endocrine system works with the nervous system to regulate body functions for homeostasis.
• It is a unique system because the organs are not anatomically connected.

Major Endocrine Glands

• Pituitary
• Thyroid
• Parathyroid
• Adrenal
• Pancreas
• Pineal
• Thymus
• Ovaries
• Testes

Endocrine Glands

• Cells, tissues, and organs make up the system.
• They are ductless and secrete hormones directly into body fluids.
• Endocrine refers to internal secretion.
• Hormones act on target cells with specific receptors.

Exocrine Glands

• Glands secrete into ducts or tubes leading to a body surface.
• They secrete externally and deliver products to specific sites.

Local Hormones

• Some cells secrete chemical messengers internally, called local hormones.
• Paracrine secretions affect nearby cells.
• Autocrine secretions affect the cells that secrete them.

Comparison Between Nervous and Endocrine System

• Both communicate via chemicals binding to receptor molecules.
• The nervous system releases neurotransmitters into synapses.
• The endocrine system secretes hormones into the bloodstream.
• The nervous system responds faster, but the endocrine system's effects last longer.

Hormones

• Hormones are released into extracellular fluid and then diffuse into the blood.
• Transport through the blood depends on whether the hormone is lipid-soluble or water-soluble.
• Hormones are very powerful in low concentrations.

Chemistry of Hormones

• Hormones are organic compounds, either steroid or nonsteroid.

Steroid Hormones

• Steroids are lipids containing complex rings of carbon and hydrogen atoms.
• All steroid hormones are produced from cholesterol.
• Examples include sex hormones (testosterone, estrogens) and adrenal cortex hormones (cortisol, aldosterone).

Nonsteroid hormones

• Amines: Derived from tyrosine (epinephrine, norepinephrine, thyroxine)
• Proteins: Composed of long chains of amino acids (growth hormone)
• Peptides: Short chains of amino acids (ADH, oxytocin)
• Glycoproteins: Carbohydrates joined to proteins (TSH)

Hormone Actions

• Hormones exert effects by altering metabolic processes.
• They may alter enzyme activity or change the rate of membrane transport of a substance.
• They deliver messages by binding to receptors on/in target cells.
• Even low concentrations can cause changes.
• Receptor number determines the strength of response and can be altered.
• Upregulation: Increases the number of receptors on the target cell in response to a decrease in hormone level.
• Downregulation: Decreases the number of receptors on the target cell due to an increase in hormone level.

Steroid and Thyroid Hormones

• They have poor water solubility and are transported through the blood bound to plasma proteins.
• Steroid hormones diffuse through the lipid bilayer of cell membranes.
• Thyroid hormones enter the cell by specific transport methods.
• Both bind to receptors inside the cell, usually in the nucleus.
• This binding causes transcription of specific genes in DNA.
• Protein synthesis leads to the action of the hormone.

Steroid Hormone Action

• Endocrine glands secrete steroid hormones.
• Blood carries the hormone molecules throughout the body.
• Unbound steroid hormones diffuse through the target cell membrane and enter the cytoplasm or nucleus.
• The hormone combines with a receptor molecule in the cytoplasm or nucleus.
• The steroid hormone-receptor complex binds to DNA in the nucleus and promotes messenger RNA transcription.
• Messenger RNA enters the cytoplasm and directs protein synthesis.
• Newly synthesized proteins produce the steroid hormone's specific effects.

Nonsteroid Hormones

• They cannot penetrate the lipid bilayer of cell membranes.
• They bind to receptors on the target cell membranes.
• The hormone is considered a first messenger.
• Chemicals that induce changes leading to a hormone's effect are considered second messengers.
• Many hormones use cyclic adenosine monophosphate (cAMP) as a second messenger.
• The entire process of chemical communication, from outside cells to inside, is called signal transduction.

Nonsteroid Hormones Steps

• Endocrine glands secrete nonsteroid hormones.
• Blood carries hormone molecules throughout the body.
• Hormones combine with receptor sites on the membrane of the target cell, activating G protein.
• Adenylate cyclase molecules are activated in the target cell's membrane.
• Adenylate cyclase converts ATP into cyclic AMP.
• Cyclic AMP activates protein kinases.
• Protein kinases activate protein substrates in the cell that change metabolic processes.
• Cellular changes produce the hormone's effects.

Prostaglandins

• They are paracrine substances.
• They are potent in small amounts, like hormones.
• They are synthesized just before release and rapidly inactivated after use.
• They regulate cellular responses to hormones.
• Prostaglandins can activate or inhibit adenylate cyclase, controlling cAMP production.
• They have a wide variety of effects, such as contracting or relaxing smooth muscle, stimulating or inhibiting secretion, regulating blood pressure, controlling movement of H2O and Na+ in the kidneys, and promoting inflammation.

Hormone Secretion and Effects

• Secretion is precisely regulated.
• Secretion is primarily controlled by negative feedback mechanisms.
• Effects can be short-lived (a few minutes) or may last for days.
• Some are excreted in the urine after exerting their effects.
• They can be broken down by enzymes, mainly from the liver, to stop their effects.

Control Sources Negative feedback

• Rising hormone levels lead to a decrease in hormone secretion.
• As the hormone is used up, inhibition stops, and secretion begins again.

3 Methods of Negative Feedback

• The hypothalamus controls the release of anterior pituitary hormones; then, the pituitary hormones secrete hormones that control the activity of other glands.
• (Tropic hormones: Hormones that act on other glands)
• The nervous system directly stimulates some glands to secrete their hormones (via nerve impulses).
• Changes in the composition of the internal environment stimulate or inhibit the secretion of certain hormones.

Positive feedback

• rising levels of a hormone lead to an increase in secretion; mainly in the reproductive system.

Pituitary Gland

• Lies at the base of the brain, in the sella turcica of the sphenoid bone.
• Controlled by the brain and considered part of the nervous system.
• Attached to the hypothalamus by the pituitary stalk (infundibulum).
• Consists of 2 distinct portions: the anterior lobe and the posterior lobe.
• Secretion from 2 lobes is controlled by different methods by the hypothalamus.
• A small intermediate lobe (pars intermedia) develops in the fetus and produces melanocyte-stimulating hormone (MSH), which starts melanin production.

Hypothalamic Control of Pituitary Gland Anterior Lobe Regulation

• Hypothalamic releasing hormones (or release-inhibiting hormones) are transported to the anterior lobe through the Hypophyseal Portal Veins.
• They stimulate cells of the anterior lobe to release hormones.
• Each releasing hormone acts on a specific group of cells in the anterior pituitary and can stimulate or inhibit pituitary secretion.

Posterior Lobe Regulation

• Nerve impulses from the hypothalamus travel to the posterior lobe through the infundibulum
• These impulses stimulate nerve endings in the posterior lobe to release hormones.

Hypothalamic Control of Peripheral Endocrine Glands

• Hypothalamus controls hormone secretion from peripheral endocrine glands via 3-step pathways:
• The releasing (or release-inhibiting) hormone from the hypothalamus acts on specific hormone-secreting cells in the anterior pituitary gland.
• The anterior pituitary hormone acts on cells in a peripheral endocrine gland to stimulate its secretion.
• The peripheral endocrine gland secretes its hormone, which exerts effects on target cells.

Negative feedback

• Final hormone in the pathway inhibits the secretion of both the releasing hormone and the anterior pituitary hormone.
• Anterior pituitary hormone inhibits the secretion of the releasing hormone.

Anterior Pituitary Hormones

• The anterior lobe of the pituitary gland consists of glandular epithelial tissue.
• Anterior pituitary hormones are produced in the anterior lobe by 1 of 5 types of secretory cells.
• Each anterior lobe hormone is released in response to a releasing hormone from the hypothalamus; some are inhibited by release-inhibiting hormones from the hypothalamus.
• Major anterior pituitary hormones: Growth hormone (GH), Prolactin (PRL), Thyroid-stimulating hormone (TSH), Adrenocorticotropic hormone (ACTH), Follicle-stimulating hormone (FSH), and Luteinizing hormone (LH).

Releasing Hormones of the Hypothalamus

• Growth hormone (GH): Released by Growth hormone-releasing hormone (GHRH) and inhibited by Growth hormone-inhibiting hormone (GHIH, or somatostatin, SS)
• Prolactin (PRL): Inhibited by Prolactin-inhibiting hormone (PIH) and possibly released by Prolactin-releasing factor (PRF)
• Thyroid-stimulating hormone (TSH or Thyrotropin): Released by Thyrotropin-releasing hormone (TRH)
• Adrenocorticotropic hormone (ACTH or Corticotropin): Released by Corticotropin-releasing hormone (CRH)
• Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH): Both released by Gonadotropin-releasing hormone (GnRH)

Functions of Anterior Pituitary Hormones

• Growth Hormone (somatotropin): Stimulates cells to enlarge and divide rapidly, increases amino acid uptake and protein synthesis, decreases the rate of carbohydrate usage, and increases the rate of fat usage.
• Prolactin: Promotes milk production in females, uncertain function in males.
• Thyroid-stimulating Hormone (TSH): Stimulates the secretion of thyroid hormones (T3 and T4) from the thyroid gland.
• Adrenocorticotropic Hormone (ACTH): Stimulates the secretion of cortisol and other glucocorticoids from the adrenal cortex.
• Follicle-stimulating Hormone (FSH): Causes the growth and development of ovarian follicles in females and sperm production in males.
• Luteinizing Hormone (LH): Causes ovulation in females and sex hormone production in both genders.

Pathway for Thyroid Hormone Secretion

• TRH is secreted by the hypothalamus.
• TRH causes the anterior pituitary to secrete TSH.
• TSH travels to the thyroid gland and causes it to secrete Thyroid Hormones.
• Thyroid hormones exert effects on target cells.
• Thyroid hormones also exert negative feedback on the secretion of TRH and TSH.
• TSH secretion is controlled by the level of TRH and negative feedback by thyroid hormones.