The Endocrine System

By Joseph Hernandez

Objectives Know the general mechanisms by which molecules integrate

and control the various metabolic activities in both vertebrate and invertebrate animals.

Understand how the neuroendocrine center controls secretion rates of other endocrine glands and responses in nerves and muscles.

Know the major endocrine glands and their secretions

Be familiar with other endocrine glands and their role in the body

Know how sugar levels are regulated by hormones

Differentiate the modes of action of steroid and nonsteroid hormones

Basic Terms:Glands are organs that release substances onto

the skin, or into a body cavity or the interstitial fluid.Two Types of Glands

Endocrine Glands are glands that do not have ducts. These glands secrete their product in the interstitial fluid; where then it diffuses it the blood through capillaries.

Exocrine Glands are glands that have ducts. They deliver their secretions onto external or internal surfaces.

Target Cells are specific cells that respond to different hormones.

Sebaceous and Sweat Gland (Exocrine Glands)

Intercellular Signaling in Animals

In all animals, cells constantly signal one another in response to changes in the internal and external environments. Receiving such signals can influence a cell’s metabolic activity, division, or gene expression.

Gap Junctions

Local Signaling Molecules are chemical signaling molecules that are secreted into the interstitial fluid.

Animal Hormones are longer-range communication molecules, which are secreted to the blood through the capillaries.

Pheromones diffuse through water or air and bind to target cells in individuals.

Nervous-Endocrine Interactions

Glands and other hormone sources are collectively referred to as an animal’s Endocrine System.Both neurons and endocrine cells are derived from the

ectodermal (outer) layer. Both respond to the hypothalamus (the command center). Also, most organs respond to both nervous signals and hormones.

Hormones influence: Development of Brain. (Before and After birth) Sleep/Wake Cycles Emotion Memory

Lastly, the Nervous system influences the endocrine system by controlling the amount secreted of a hormone.

Categories of Hormones

Cholesterol makes Steroid Hormones.

Modified amino acids make Amine Hormones.

Short Chains of Amino Acids make Peptide Hormones.

Longer Chains make Protein Hormones.

**Note: Refer to figure 35.1 on page.600, for specific hormone group.**

The Nature of Hormone Action

There are three steps involved in cellular communication:

Signal Reception- a signaling molecule binds to the receptor.

Signal Transduction- the signal is changed into a form that affects target cell behavior.

Cellular Response- the cell makes a response.

Intracellular Reception

Since a Steroid hormone is a lipid, it can easily diffuse across the cells membrane. Once inside the cell, the steroid hormone forms a hormone-receptor complex by binding to a receptor in the cytoplasm or nucleus. This triggers the activation of the promoter (region of the DNA that allows for transcription to occur) where genes are transcribed to form mRNA and thus be shipped out of the nucleus to the cytosol. Lastly, the mRNA translates into a protein that carries out the target cells response to the signal.

Plasma Membrane Receptors

Most Amine hormones, and all peptide or protein hormones, are to polar so they can’t diffuse across a membrane.They bind to receptors on a target cell’s plasma

membrane. This binding activates an enzyme that converts ATP to cAMP (cyclic adenosine monophosphate) then functions as a second messenger.

Second Messenger: is a molecule that forms inside a cell in response to an external signal and affects that cells activity.

Receptor Function and Diversity

A cell can only respond to a hormone for which it has appropriate receptors.

Since all hormone receptors are proteins, gene mutations can make them less efficient. (Could reduce their ability to function)

Ex. When there is a mutation of the functional receptors for testosterone or androgen (steroid hormones). Testes will not descend because the inability for the receptors to read steroid hormones.

Variations in receptor structure also affect responses to hormones. Different tissues often have receptor proteins that respond in different ways to bonding of the same hormone.

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The Hypothalamus

The Hypothalamus is the main center for control of the internal environment; it also connects to the pituitary gland.

Pituitary Gland

The Pituitary Gland is the vertebrae endocrine gland located inside the brain; it interacts with the hypothalamus to control physiological functions, including activity of many other glands. It posterior lobe stores and secretes hormones from the hypothalamus; the anterior lobe makes and secretes it’s own hormones.

Posterior Pituitary

Posterior Pituitary Function(ADH) and oxytocin are made in the hypothalamus

by secretory neurons (neurons that make hormones rather than neurotransmitters). These hormones are stored in the axon terminals of the secretory hormones and are released when neuron is excited (action potential).

Antidiuretic Hormone (ADH) affects certain kidney cells. The hormone causes these cells to reabsorb more water.

Oxytocin triggers muscle contractions during childbirth. Helps move milks into the mammary glands, and also effects social behavior in some species.

Anterior Pituitary

The anterior pituitary produces hormones on its own, but hormones from the hypothalamus control its secretion.

Adrenocorticotropic hormone (ACTH) stimulates the release of hormones by adrenal glands.

Thyroid-Stimulating hormone (TSH) regulates the secretion of thyroid hormone by the thyroid glands.

Follicle-stimulating hormone (FSH) and luteinizing hormone (LH) affect sex hormone secretion and production of gametes by gonads.

Prolactin (PRL) targets the mammary glands, by stimulating milk production for a nursing mother.

Growth Hormone (GH) has targets in most tissues. Promotes growth and metabolism.

Growth Hormone Functions and Disorders

Gigantism occurs when there is an overproduction of GH hormone during childhood. These people have proportional bodies but are large.

Acromegaly occurs when there is an overproduction of GH during adulthood. Since bones cannot grow any larger, they thicken; usually in hands, feet, and facial features.

Dwarfism occurs when the body produces too little GH or receptors are not properly responding to the GH. People are usually short in stature but proportional.

Feedback MechanismsMost hypothalamic hormones that encourage

secretion are called releasers (POSITIVE FEEDBACK); hormones that influence reduction of secretion of hormones are called inhibitors. (NEGATIVE FEEDBACK)

When inhibitors or releasers are secreted by the secretory neurons located in the hypothalamus, they enter the stalk that connects the pituitary to the hypothalamus. (The stalk is the point in which they meet). There the hormones move into the capillaries in the stalk and are transferred to the pituitary gland and diffuse out to the specific target cells. After these specific hormones influence cells there byproduct enters capillaries.

ThyroidThe Thyroid Gland is the endocrine gland

located at the base of the neck; its hormones influence growth, development, and metabolic rate.

The thyroid gland produces calcitonin, causes deposition of calcium in bones of growing children.

How the Thyroid WorksNegative Feedback Loop

The anterior pituitary gland and hypothalamus regulate thyroid hormone secretion. When iodine level in blood declines the hypothalamus secretes a releasing hormone (TRH or Thyroid Releasing Hormone) to the anterior pituitary. The releaser causes the pituitary to secrete thyroid-stimulating hormone (TSH). TSH thus goes to the thyroid gland and induces secretion of the Thyroid Hormone. When thyroid hormone reaches its normal amount the TSH and TRH molecules slow down.

ParathyroidsThere are four (4) parathyroid glands; they are

situated on the posterior side of the thyroid gland.These glands release parathyroid hormone (PTH)

in response to decline in calcium levels. (NOTE: Calcium ions have roles in neuron signaling, blood clotting, muscle contraction, and other physiological processes

Pancreas The pancreas is an organ that lies in the abdominal

cavity, behind the stomach. And has both exocrine and endocrine functions.

Alpha Cells (located in pancreatic islets) These cells secrete glucagon. Glucagon targets cells in the liver that causes the activation of

enzymes, which break down glycogen into glucose subunits.

Beta Cells (located in the pancreatic islets): These cells secrete insulin. Main target cells are the liver, fat, and skeletal muscles cells. Insulin stimulates the uptake of glucose by fat and muscle cells. Insulin activates enzymes that function in protein and fat

synthesis, and it inhibits the enzymes that catalyze protein and fat breakdown.

Type 1 Diabetes Known as Juvenile-onset diabetes.White blood cells recognize the beta cells as non-self

or foreign and destroy them.Individuals who have this type of diabetes must take

in injections of insulin so the sugar level in blood can lower.

Since insulin discourages metabolism of fats, when there is too little insulin in the blood excessive fat and protein breakdown occur. Results in weightless and accumulation of ketones on blood and urine. Ketones come from the breaking down of fats, and when to many are present, ketoacidosis occurs. The acidity can interfere with brain function; which may lead to death or a coma.

Type 2 DiabetesMost common type.Target cells don’t respond to insulin; while insulin

levels are relatively normal.Genetics can be factor.Need Insulin injection to lower sugar levels in blood.

AdrenalGlands

The Adrenal Glands There are two adrenal glands; they are both perched above the

kidneys; size of big grape. Its outer layer is the adrenal cortex and its inner portion is the

adrenal medulla. The two parts secrete different hormones, and are controlled by different mechanisms.

Hormonal Control of the Adrenal Cortex The adrenal Cortex secretes three steroid hormones:

Aldosterone-controls sodium and water reabsorption. Small amounts of male and female sex hormones. Cortisol- and adrenal hormone that affects metabolism and immunity.

Cortisol Control: A negative feedback loop governs the cortisol level in blood. A decrease in cortisol triggers secretion of CRH (corticotrophin-

releasing hormone) by the hypothalamus. CRH then stimulates secretion of ACTH (adrenocorticotropic hormone) in the anterior pituitary gland. Then CRH promotes secretion of cortisol in the adrenal cortex.

The GonadsThe Gonads

The gonads are the reproductive organs in male and female bodies.

They produce gametes and sex hormones.Male gonads are called the testes, and they produce

testosterone.Female gonads are called ovaries, and they produce

female sex hormones known as progesterone and estrogen.

These sex hormones are both produced when the hypothalamus produces GnRH (Gonadotropin-releasing hormone).GnRH causes the anterior pituitary to secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH). These end products lead to the secretion of sex hormones

The Pineal GlandThe pineal gland secretes melatonin.

Melatonin is a hormone that serves as a part of an internal timing mechanism, or biological clock.

Melatonin secretion declines with bright light, and increases with bright light due to neurons that lower body temperature; which make us drowsy.

The ThymusThe thymus lies beneath the breastbone.

It secretes thymosins, which are hormones that help WBCs called T-lymphocytes.

Grows until puberty, then it shrinks due to sex hormones.

Aids in immune function.