Endocrine System

Dr. Michael P. Gillespie

Mediator Molecules in Nervous & Endocrine Systems

The nervous system utilizes neurotransmitters to control body functions.

The endocrine system utilizes hormones to control body functions.

Site Of Mediator Action In Nervous & Endocrine Systems

The neurotransmitters perform their action close to the site of release.

The hormones usually perform their action far from their site of release.

Types Of Target Cells In Nervous & Endocrine Systems

The nervous system acts upon muscle cells (smooth, cardiac, and skeletal), glands, and other neurons.

The endocrine system acts upon virtually all cells of the body.

Time To Onset Of Action In Nervous & Endocrine Systems

In the nervous system, action typically occurs within milliseconds of neurotransmitter release.

In the endocrine system, action can take seconds to days to occur after release of the hormone.

Duration Of Action In Nervous & Endocrine Systems

The actions tend to be briefer in duration in the nervous system and longer in duration in the endocrine system.

Comparison Of Control By The Nervous & Endocrine Systems

Refer to table 18.1 on page 587

Hormones

A hormone is a mediator molecule that is released in one part of the body but regulates activity of cells in other parts of the body.

Most hormones enter the interstitial fluid and then the bloodstream.

Hormones travel through the bloodstream to cells throughout the body.

Several neurotransmitters are also hormones (i.E. norepinephrine).

Functions Of Hormones

Help regulate: Chemical composition and volume of the

internal environment (interstitial fluid). Metabolism and energy balance. Contraction of smooth and cardiac muscle fibers. Glandular secretions. Some immune system activities.

Control growth and development.

Functions Of Hormones

Regulate operation of reproductive systems. Help establish circadian rhythms.

“Supersystem”

The nervous and endocrine systems function together.

Parts of the nervous system stimulate or inhibit the release of hormones.

Hormones can promote or inhibit the release of nerve impulses.

Exocrine Glands Versus Endocrine Glands

Exocrine glands (Exo = outside) – secrete their products into ducts that carry secretions into body cavities, into the lumen of an organ, or to the outer surface of the body.

Endocrine glands – secrete their hormones into the interstitial fluid surrounding the secretory cells.

Exocrine Glands Versus Endocrine Glands

Exocrine glands. Sudoriferous (sweat). Sebaceous (oil). Mucous. Digestive.

Exocrine Glands Versus Endocrine Glands

Endocrine glands. Pituitary. Thyroid. Parathyroid. Adrenal. Pineal. Other organs that secrete hormones:

Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, SI, skin, heart, adipose tissue, & placenta.

Endocrine System

All endocrine glands and hormone secreting cells comprise the endocrine system.

Endocrinology (-logy = study of) is the study of the science of endocrine glands, function of endocrine glands, diagnosis of endocrine disorders and treatment of endocrine disorders.

Hormone Receptors

Hormones bind with specific receptors. Only target cells for a given hormone have specific

receptors that bind and recognize that hormone. A target cell can have anywhere between 2000 and

100,000 receptors for a particular hormone. Receptors are constantly being synthesized and

broken down to meet the needs of the body.

Down-regulation

If a hormone is present in excess, the number of target cell receptors may decrease.

Down-regulation decreases the responsiveness of the target cell to the hormone.

Up-regulation

When a hormone (or neurotransmitter) is deficient, the number of receptors may increase.

Up-regulation makes a target cell more receptive to a specific hormone.

Synthetic Hormones

Synthetic hormones in the form of drugs can block the receptors from naturally occurring hormones.

Circulating & Local Hormones

Circulating hormones – pass from secretory cells that make them into the interstitial fluid and then into the blood. Most hormones are of this type.

Local hormones – act locally on neighboring cells or on the same cells that secreted them without first entering the bloodstream.

Local Hormones

Paracrines – (para = beside or near) act on neighboring cells.

Autocrines – (auto – self) act on the same cell that secreted them.

Example Of A Local Hormone

Interleukin 2 (IL-2) is an example of a local hormone. It is released by helper T cells during immune

responses. It acts on nearby immune cells (paracrine) and

on itself (autocrine). This generates more helper T cells and boosts the

immune response.

Duration of Local Versus Circulating Hormones

Local hormones usually are inactivated quickly.

Circulating hormones linger longer. The liver eventually deactivates circulating

hormones and the kidneys excrete them.

Chemical Classes Of Hormones

Lipid-soluble hormones. Steroid hormones. Thyroid hormones. Nitric oxide (NO).

Chemical Classes Of Hormones

Water-soluble hormones. Amine hormones. Peptide hormones and protein hormones. Eicosanoid hormones.

Prostaglandins.Leukotrienes.

Hormone Transport In Blood

Most water-soluble hormones are transported in their “free” form (not attached to plasma proteins).

Most lipid-soluble hormones are bound to transport proteins.

Hormone Receptors

Lipid-soluble hormones – the receptors are located inside the target cells.

Water-soluble hormones – the receptors are located within the plasma membrane of the target cells.

Action Of Lipid-soluble Hormones

Lipid soluble hormones turn specific genes of the nuclear DNA on or off.

This directs the synthesis of a new protein (often an enzyme).

These new proteins alter the cells activity.

Action Of Water-soluble Hormones

Water soluble hormones are the first messenger. They activate the second messenger i.e. cyclic AMP (cAMP).

This initiates a cascade of events within the cell that produces millions of enzymes to catalyze reactions.

Phosphodiesterase inactivates cAMP.

Responsiveness Of The Target Cell

The responsiveness of the target cell depends upon the following: The hormone’s concentration. The number of the hormone receptors on the

target cell. Influences exerted by other hormones.

Influences Of Other Hormones

Permissive effect – the action of a 2nd hormone is required for the 1st hormone to take effect. Thyroid hormones (2nd) allow epinephrine to

stimulate lipolysis.

Influences Of Other Hormones

Synergistic effect – the sum of the actions of the 2 hormones is greater than either hormone individually. Estrogens and FSH promote development of oocytes.

Antagonistic effect – one hormone opposes the actions of another. Insulin promotes synthesis of glycogen and glucagon

stimulates breakdown of glycogen.

Control Of Hormonal Secretion

Hormone secretion is regulated by: Signals from the nervous system. Chemical changes in the blood. Other hormones.

Hypothalamus

Serves as a major integrating link between the nervous system and the endocrine system.

Painful, stressful, and emotional experiences cause changes in hypothalamic activity.

Synthesizes at least 9 different hormones. Regulates the pituitary gland.

Pituitary Gland (hypophysis)

Synthesizes at least 7 different hormones. Release of anterior pituitary hormones is

stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus.

Types Of Anterior Pituitary Cells & Their Hormones

Somatotrophs – secretes human growth hormone (hGH) or somatotropin, which stimulates tissues to secrete insulinlike growth factors (IGFs).

Thyrotrophs – secrete thyroid-stimulating hormone (TSH) or thyroptropin.

Types Of Anterior Pituitary Cells & Their Hormones

Gonadotrophs – secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) which act on the gonads. They stimulate the secretion of estrogen and

progesterone and the maturation of oocytes in the ovaries.

They stimulate the secretion of testosterone and sperm production in the testes.

Types Of Anterior Pituitary Cells & Their Hormones

Lactotrophs – secrete prolactin (PRL), which initiates milk production.

Corticotrophs – secrete adrenocorticotropic hormone (ACTH) or corticotropin, which stimulates the adrenal cortex to secrete glucocorticoids.

Tropic Hormones (tropins)

Hormones that influence another gland are called tropic hormones or tropins.

Control Of Secretion By The Anterior Pituitary

(adenohypophysis) The hypothalamus secretes five releasing

hormones and two inhibiting hormones. Negative feedback loops from hormones

released from target glands decrease the release from the anterior pituitary gland.

Human Growth Hormone & Insulinlike Growth Factors

The main function of hGH is to promote synthesis of IGFs.

IGFs cause cells to grow and multiply. They help to maintain the mass of muscles and

bones. They promote healing of injuries and tissue repair. They enhance lypolysis in adipose tissue.

Release Of hGH

Two hypothalamic hormones control the release of hGH: Growth hormone releasing hormone (GHRH).

Stimulated by hypoglycemia. Inhibited by hyperglycemia.

Growth hormone inhibiting hormone (GHIH).Stimulated by hyperglycemia. Inhibited by hypoglycemia.

Thyroid-stimulating Hormone

Thyroid-stimulating hormone (TSH) stimulates the synthesis and secretion of two thyroid hormones: Triiodothyronine (T3). Thyroxine (T4).

Thyrotropin-releasing hormone (TRH) from the hypothalamus controls TSH secretion.

Negative feedback from T3 and T4 inhibits the release of TRH.

Follicle-stimulating Hormone

FSH initiates the development of ovarian follicles and stimulates follicular cells to secrete estrogens in females.

FSH stimulates sperm production in the testes in males.

Follicle-stimulating Hormone

Gonadotropin-releasing hormone (GnRH) from the hypothalamus stimulates FSH release.

Estrogens in females and testosterone in males suppresses release of GnRH and FSH through negative feedback systems.

Luteinizing Hormone (LH)

In females LH triggers ovulation. FSH and LH work together to stimulate the

release of estrogen. In males, LH stimulates the release of

testosterone from the testes.

Prolactin (PRL)

Initiates and maintains secretion of milk by the mammary glands.

By itself, prolactin has only a weak effect.

Prolactin (PRL)

Only with the effects of estrogens, progesterone, glucocorticoids, hGH, thyroxine and insulin does PRL bring about milk.

The hypothalamus secretes both inhibitory and excitatory hormones that regulate PRL secretion: Prolactin-inhibiting hormone (PIH). Prolactin-releasing hormone (PRH).

Hypersecretion Of Prolactin

In males – erectile dysfunction. In females – galactorrhea (inappropriate

lactation) and amenorrhea (absence of menstrual cycles).

Adrenocorticotropic Hormone (ACTH)

ACTH controls secretion of cortisol and other glucocorticoids by the cortex of the adrenal gland.

Corticotropin-releasing hormone (CRH) from the hypothalamus stimulates secretion of ACTH.

Glucocorticoids cause inhibition of CRH and ACTH through negative feedback systems.

Principle Actions Of Anterior Pituitary Hormones

Table 18.4 page 600.

Posterior Pituitary (neurohypophysis)

The posteror pituitary does not synthesize any hormones; however, it does store and release two hormones from the hypothalamus: Oxytocin (OT). Antidiuretic hormone (ADH) a.k.a. vasopressin.

Oxytocin

During delivery, oxytocin enhances contraction of smooth muscle cells in the wall of the uterus.

After delivery, oxytocin stimulates milk ejection (“letdown”) from the mammary glands in response to the suckling infant.

Antidiuretic Hormone (ADH)

ADH is a substance that decreases urine production.

ADH causes the kidneys to return more water to the blood.

Summary Of Posterior Pituitary Hormones

Table 18.5 page 602.

Thyroid Gland

Located inferior to the larynx (voice box). Right and left lateral lobes connected by an

isthmus. Anterior to the trachea. Highly vascular. Consists of thyroid follicles (spherical sacs). The walls of each follicle contain follicular cells,

which extend into the lumen of the follicle.

Follicular Cells

Produce two thyroid hormones (both are lipid soluble). Thyroxine (tetraiodothyronine or T4). Triiodothyrronine (T3).

Parafollicular Cells

Also known as C cells. Produce the hormone calcitonin (CT).

Regulates calcium homeostasis. CT inhibits the action of osteoclasts. CT accelerates the uptake of calcium and

phosphates into the bone matrix.

Actions Of Thyroid Hormones

Increase basal metabolic rate (BMR). Calorigenic effect.

Helps to regulate body temperature. Regulate metabolism.

Protein synthesis. Increase the use of glucose and fatty acids for ATP. Increase lypolysis.

Accelerate body growth, especially of the nervous system.

Parathyroid Glands

Embedded into the posterior surface of the lateral lobes of the thyroid gland.

Superior and inferior parathyroid glands. Two kinds of epithelial cells.

Chief (principal) cells – produce parathyroid hormone (PTH) or parathormone.

Oxyphil cell – function unknown.

Parathyroid Hormone

Regulates levels of calcium, magnesium, and phosphate ions in the blood.

PTH increases the activity and # of osteoclasts.

PTH stimulates the kidneys to synthesize calcitrol (active form of vitamin D). Calcitrol stimulates increased absorption of Ca2+

from the GI tract.

Adrenal Glands

The paired adrenal (suprarenal) glands lie superior to each kidney.

Consists of the adrenal cortex (80-90% of the gland) and the adrenal medulla.

Highly vascularized.

Adrenal Gland Hormones

Adrenal Cortex – steroid hormones. Adrenal Medulla – 3 catecholamine

hormones. Norepinephrine. Epinephrine. Dopamine.

Aldosterone (Cortex)

Regulates homeostasis of sodium ions and postassium ions.

Promotes the secretion of H+ in the urine to regulate acid-base balance. Prevents acidosis (pH below 7.35).

Renin-angiotensin-aldosterone (RAA) pathway controls the secretion of aldosterone (Controls blood pressure).

Glucocorticoids (Cortex)

Regulate metabolism and resistance to stress. Cortisol (hydrocortisone). Corticostrerone. Cortisone.

Low levels of glucocorticoids, mainly cortisol, stimulate the hypothalamus to secrete corticotropin-releasing hormone (CRH), which promotes the release of ACTH from the anterior pituitary, which in turn stimulates glucocorticoid secretion.

Effects Of Glucocorticoids

Increase the rate of protein breakdown. Stimulates glucose formation by breaking

down glycogen stores and through gluconeogenesis.

Stimulates lypolysis.

Effects Of Glucocorticoids

Resistance to stress by supplying ATP and raising BP in cases of severe blood loss.

Anti-inflammatory effects – inhibit WBCs (also slows wound healing).

Depression of immune responses (utilized with organ transplant recipients).

Androgens (Cortex)

The adrenal cortex secretes small amounts of androgens (primarily dehydroepiandrosterone DHEA).

Assists in early growth of axillary and pubic hair in both sexes.

Contributes to libido and provides a source of estrogens after menopause in females.

ACTH stimulates its secretion.

Adrenal Medulla Hormones

Epinephrine (adrenaline). Norepinephrine (noradrenaline). These two hormones augment the fight or flight

response. Increase the heart rate and force of contraction. Dilates the airways in the lungs. Shunts blood to heart, liver, skeletal muscles, and

adipose tissue. Increases blood levels of glucose and fatty acids.

Pancreatic Islets

Both and endocrine and exocrine gland. Pancreatic islets (a.k.a. islets of Langerhans). 4 major cell types:

Alpha (A) cells – secrete glucagon. Beta (B) cells – secrete insulin. Delta (D) cells – secretes somatostatin (identical

to growth hormone inhibiting hormone). F cells – secrete pancreatic polypeptide.

Pancreatic Hormones

Glucagon raises blood glucose levels. Insulin lowers blood glucose levels. Somatostatin inhibits both glocagon and insulin

release. Pancreatic polypeptide inhibits somatostatin

secretion, gallbladder contraction and secretion of digestive enzymes by the pancreas.

Regulation Of Glucagon & Insulin Secretion

Hypoglycemia stimulates release of glucagon.

Glucagon causes hepatocytes to convert glucagon to glucose (gkycogenolysis).

Hyperglycemia inhibit release of glucagon.

Regulation Of Glucagon & Insulin Secretion

Insulin allows glucose to diffuse into cells, increases amino acid uptake by cells, and increaes fatty acid uptake by cells.

This facilitates glucose conversion into glycogen (glycogenesis), synthesis of proteins, and synthesis of fatty acids (lipogenesis).

Gonads

Ovaries (female gonads). Produce steroid hormones.

Estrogens. Progesterone.

Produce inhibin. Produce relaxin.

Testes (male gonads). Produce testosterone (an androgen). Produce inhibin.

Females Sex Hormones

Estrogen and progesterone along with FSH and LH (from the anterior pituitary), regulate the menstrual cycle, maintain pregnancy, and prepare the mammary glands for lactation.

Maintain the feminine secondary sex characteristics (larger breasts and hips).

Inhibin & Relaxin

Inhibin inhibits secretion of FSH. Relaxin increases the flexibility of the pubic

symphisis during pregnancy and helps dilate the cervix during labor and delivery.

Male Sex Hormones (Androgens)

Testosterone regulates the production of sperm.

Stimulates the production of male secondary sex characteristics (beard growth and deepening of the voice).

Pineal Gland

A small endocrine gland attached to the roof of the third ventrical of the brain.

Secretes melatonin. More melatonin is released in darkness. Melatonin contributes to the body’s

biological clock.

Seasonal Affective Disorder (SAD)

Thought to be due to overproduction of melatonin during the winter months.

Full spectrum bright-light therapy can assist with SAD and jet lag.

Thymus

Located behind the sternum between the lungs.

Hormones produced – thymosin, thymic humoral factor (THF), thymic factor (TF), amd thymopoeietin.

Promotes the maturation of T cells and may retard the aging process.