A&P Endocrine Chap 10

The Endocrine SystemThe Endocrine SystemChapter 10Chapter 10

“Control system” along with nervous system

Helps maintain homeostasis

Allows for communication among cells

Endocrine System

Comparison of Nervous & EndocrineNervous system

quick response (milliseconds) relatively short duration of effect works via electrical impulses (action potentials) and direct

release of chemicals (neurotransmitters) across interstitial fluid to adjacent cells affects specific sets of cells (neuron, muscle, gland) resulting in

specific responses (eg. muscle contraction or glandular secretion)

Endocrine system slower response (minutes to hours) potentially long duration of effects (hours to days) works via chemical signals (“hormones”) which are released

through interstitial fluid into blood capillaries affects multiple cells throughout the body that have specific

hormone receptors (“target cells”) resulting in change of metabolic activities of cells (eg. affects protein synthesis)

Endocrine system includes all endocrine cells, tissues, and endocrine glands (organs) of the body

Many different organs have some endocrine cells/tissues within their structure, but the organ’s primary function is not endocrine (ie: heart, kidney, digestive organs, pancreas hypothalamus, gonads, thymus)

Some organs are primarily endocrine organs (endocrine glands) (ie. pituitary, thyroid, parathyroid, adrenal, pineal)

Overview of Endocrine System

Structure of HormonesHormones classified based on their chemical structure:

Amino acid derivatives – eg. epinephrine, norepinephrine, thyroid hormones, melatonin

Peptide/protein hormones – eg. ADH, oxytocin, growth hormone, hypothalamic hormones, pancreatic hormones, thymic hormones

Lipid derivatives – include steroid hormones (eg. estrogens, testosterone, adrenal hormones) & eicosanoids (aka cytokines/local hormones) (eg. prostaglandins)

Secretion, Distribution & Effect of Hormones Hormones are released on demand – only when needed

Hormone release is primarily regulated by negative feedback mechanisms

changes in ECF (ISF/plasma) concentration/composition

changes in circulating levels of hormone itself or regulating hormones

neural stimuli

Hormones secreted by endocrine cells into interstitial fluid, and distributed through bloodstream

Only cells that posses receptors for that particular hormone can be affected (“target cell/target organ”)

Effect of hormones is on metabolic activity of cells, via activation/inactivation of specific genes, affecting protein synthesis

Secretion, Distribution & Effect of Hormones

Mechanism of Hormonal Action

Depends on whether hormone can cross cell membrane & enter cell:

Epi, norepi, peptide hormones are not lipid soluble & therefore will bind to receptors on cell membrane (membrane receptors). Eicosanoids also bind to membrane receptors

Steroid hormones & thyroid hormones can enter cell & therefore will bind to receptors within the cell (intracellular receptors)

Mechanism of Hormonal Action

Hormones that bind to membrane receptors work indirectly, via “second messengers”

Most common second messenger is cyclic AMP (cAMP)

Mechanism of Hormonal Action

Hormones that can enter the cell and bind to intracellular receptors can directly affect the structure or function of the cell, via affect on protein synthesis &/or energy production

Pituitary Gland (Hypophysis)

“master gland”

Sits within sella turcica

Attached to hypothalamus via infundibulum

Two regions – anterior pituitary & posterior pituitary

Regulated by hypothalamus

Secretes 9 hormones

Anterior Pituitary Gland Comprised of endocrine cells that produce & secrete 7 hormones: Adrenocorticotropic hormone (ACTH)

Thyroid stimulating hormone (TSH)

Growth Hormone (GH)

Prolactin (PRL)

Follicle stimulating hormone (FSH)

Luteinizing hormone (LH)

Melanocyte-stimulating hormone (MSH)

Controlled by regulatory hormones (releasing hormones &/or inhibiting hormones) from hypothalamus

Hypothalamic regulatory hormones reach anterior pituitary via “hypophyseal portal system”

Effects of Anterior Pituitary HormonesAdrenocorticotropic hormone (ACTH) -

Thyroid stimulating hormone (TSH) -

Growth Hormone (GH) -

Prolactin (PRL) - Follicle stimulating hormone (FSH) -

Luteinizing hormone (LH) -

Melanocyte-stimulating hormone (MSH) -

Targets adrenal cortex to secrete glucocorticoids (cortisol, corticosterone), which affect glucose metabolism

Targets thyroid gland to secrete thyroid hormones (thyroxine (T4), triiodothyronine (T3)), which affect metabolism, growth and development

Targets most body tissues, but especially liver, bone, muscle & cartilage, affecting growth, repair & mobilization of energy reserves. Works indirectly through release of “somatomedins” (hormones) from liver.

Targets mammary gland for development & lactation. May affect prostate gland in males.

Targets gonads (ovaries/testes) for gamete (egg/sperm) production & development. Also causes secretion of estrogens by ovaries.

Targets ovaries causing ovulation & release of estrogens and progesterone, and testes causing secretion of testosterone

Produced primarily during fetal development; targets melanocytes. No secretion in heathy adults

ACTHAdrenal cortex

Glucocorticoids(cortisol, corticosterone)

TSH

Thyroid gland

Thyroid hormones (T3, T4)

GH

Liver

Somatomedins

Bone, muscle, cartilage, other

PRL

Mammary glands (& prostate)

MSH

Melanocytes (uncertain significance)

Overview of Anterior Pituitary Hormones

Testosterone

LH

Testes

FSH

Ovaries

Estrogen ProgesteroneSperm Egg

Posterior Pituitary Gland

Comprised of axons, telodendria and terminals of neurons that originate in the hypothalamus

2 hormones (antidiuretic hormone (ADH) & oxytocin (OT) are produced by the hypothalamic neurons & transported through the axons running within infundibulum to terminals

Hormone release controlled via AP stimulation

Effects of Posterior Pituitary Hormones Antidiuretic hormone (aka vasopressin) – targets kidneys to decrease amount of urine produced (increased water reabsorption), & blood vessels causing vasoconstriction to increase BP

Oxytocin – targets uterus during childbirth (positive feedback), mammary glands for milk release (“let-down”); In non-pregnant females & in males, may contribute to orgasm

Thyroid Gland Located anterior to trachea, just below thyroid cartilage of larynx

Two lobes connected by isthmus

Lobes contain thyroid follicles

Follicular cells secrete 2 hormones under the influence of TSH – Thyroxine (T4) & Triiodothyronine (T3)

Thyroid Gland Thyroid hormones target almost every body cell

Can enter cells & bind to intracellular receptors on mitochondria & in nucleus

Effects include:

increased ATP production

increased cellular metabolism, energy utilization & oxygen consumption

increased body temperature

growth & development of skeletal, muscular & nervous system in fetus & children

Thyroid DisordersHyperthyroidism – different causes but resulting increased production of thyroid hormones may lead to: increased metabolic rate, increased heart rate & blood pressure, increased body temperature, excitability & nervousness, weight loss, possible goiter, possible exopthalmos

Hypothyroidism – decreased secretion of thyroid hormones may cause: lower metabolic rate, sluggishness, fatigue, decreased heart rate & BP, decreased body temperature, weight gain, possible goiter, subcutaneous swelling (myxedema), hair loss, dry skin

Hypothyroidism during pregnancy or in early childhood may lead to “cretinism” – abnormal skeletal & nervous development

Adrenal (Suprarenal) Glands Lie superior to each kidney

Comprised of two parts – outer adrenal cortex & inner adrenal medulla

Adrenal Cortex Divided into 3 functional zones with each zone producing different types of “corticosteroids”

Middle zone secretes steroid hormones known as “Glucocorticoids” (cortisol (hydrocortisone), corticosterone, cortisone), under the control of ACTH

Glucocorticoids target most body cells, especially liver, muscle, adipose & white blood cells

Effects include: increased glucose synthesis, “gluconeogenesis” & glycogen formation mainly in liver release of fatty acids from adipose tissue & protein breakdown in muscle cells for energy use helps provide resistance to stress due to increase in available energy inhibits activity of WBCs & immune responses – anti-inflammatory effects but slow wound healing & resistance to disease

Adrenal Medulla Comprised of cells that are post-ganglionic sympathetic neurons

Sympathetic stimulation results in release of epinephrine (adrenaline) and norepinephrine (noradrenaline) into bloodstream

Effects are sympathomimetic

Pancreas Endocrine & exocrine (digestive) gland

Endocrine function by cells within pancreatic islets (islets of Langerhans)

Alpha (α) cells secrete glucagon

Beta (β) cells secrete insulin

Both regulate blood glucose levels

Regulation of Blood Glucose

Diabetes Mellitus

Most common endocrine disorder of the pancreas

2 major types:

Type 1 (insulin dependent/ ”juvenile”) – actual deficiency of beta cells; sudden onset; may be autoimmune

Type 2 (non-insulin dependent/ “adult”) – insulin resistance due to decreased sensitivity (or number) of insulin receptors

Initial symptoms include: polydipsia, polyphagia, polyurea, glycosuria, weight loss, fatigue

Long term symptoms: neuropathy, retinopathy, gangrene, kidney failure…

Hormones & Stress – General Adaptation Syndrome (GAS)