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Chapter 18
Intercellular Communication
▪Direct communication▪Paracrine communication▪Endocrine communication▪Synaptic communication▪Hormone▪Target CellOverview of the Endocrine System
▪Endocrine system includes all the endocrine cells and tissues of the body which produce
hormones or paracrine factors that have effects beyond their tissues of origin.
▪Classes of Hormones- hormones can be divided into three groups based on their chemical
structure; amino acid derivatives, peptide hormones, lipid derivatives.
▫Amino Acid Derivatives▫Peptide Hormones▫Lipid Derivatives- two classes
-steroid hormones-eicosanoids▪Secretion and Distribution of Hormones- Hormone release typically occurs where there are
abundant capillaries and can enter the bloodstream quickly.
▪Mechanisms of Hormone Action- a target cell must possess the correct receptor for the hormone
to have effect. Receptors can be located in two places; on the cell membrane, or inside the cell.
▫Receptors on the cell membrane (E, NE, dopamine, peptide hormones, and eicosanoids).
-hormones act as first messengers, causing the appearance of a…
-second messenger in the cytoplasm. The second messenger functions inside
the cell as an enzyme activator, inhibitor, or cofactor, which can change the
metabolism within the cell.
-cAMP, cGMP, and calcium ions are the most important of the second
messengers.
-amplification-receptor cascade-down-regulation-up-regulation-a G-protein is the link between the first messenger and the second messenger.
-activation of the G-protein usually increases the concentration of the second
messenger (cAMP, cGMP, calcium).
▫Receptors inside the cell (steroids, thyroid hormones)
-hormones diffuse inside the cell through the phospholipid membrane.
-once inside the cell, they bind to receptors in the cytoplasm or in the nucleus.
-the hormone-receptor complex then can alter the cell’s metabolism by affecting
the transcription of certain genes (or produce more ATP, as with thyroid
hormones).
▪Control of Endocrine Activity- Uses endocrine reflexes to control activity.
▫Endocrine Reflex- functional counterparts of neural reflexes, can be triggered by 3
possible ways.
▫Simple endocrine reflexes involve only one hormone; controls hormone secretion by the
heart, pancreas, parathyroid gland, and digestive tract.
▫Complex endocrine reflexes involve one or more steps and two or more hormones.
-The hypothalamus is the highest level of endocrine control; it integrates the
activities of the nervous and endocrine systems, using 3 mechanisms:
1.
2.
3.
The Pituitary Gland- also known as the hypophysis. Can be divided into the anterior and posterior
lobes.
▪Anterior Lobe (adenohypophysis)- Can be subdivided into three regions; pars distalis, pars
tuberalis, and pars intermedia.
▫Hypophyseal Portal System- vascular arrangement that carries blood from one capillary
bed in and around the median eminence to another capillary bed in the anterior lobe
-allows for the hypothalamus to release regulatory hormones directly to the
target cells in the pituitary gland.
▫Hypothalamic Control of the Anterior Lobe
-Releasing hormone (RH)-Inhibiting hormone (IH)-These regulatory hormones are delivered to the anterior lobe by the
hypophyseal portal system.
▫Hormones of the Anterior Lobe- There are 7 hormones secreted by the anterior lobe of
the pituitary gland.
-Thyroid-Stimulating Hormone (TSH)
Target(s):
Hormonal Effect(s):
Regulatory Control:
-AdrenoCorticoTropic Hormone (ACTH)
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Follicle-Stimulating Hormone (FSH)- a gonadotropin
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Lutenizing Hormone (LH)- a gonadotropin
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Prolactin (PRL)
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Growth Hormone GH
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Melanocyte-Stimulating Hormone (MSH)
Target(s):
Hormonal Effect(s):
Regulatory Control:
▪Posterior Lobe (neurohypophysis)
▫Contains the axons of hypothalamic neurons
▫Neurons of the supraoptic nucleus produces ADH
▫Neurons of the paraventricular nucleus produces oxytocin
-AntiDiuretic Hormone (ADH)
Target(s):
Hormonal Effect(s):
Regulatory Control:
-Oxytocin (OXT)
Target(s):
Hormonal Effect(s):
Regulatory Control:
Thyroid Gland
▪Left and right lobe connected by the isthmus,
▪Thyroid Follicles▪Follicle Cavity▪Follicle Cells produce a globular protein called thyroglobulin, it is secreted into the colloid.
▪Formation of thyroid hormones involves three basic steps
1. iodide ions (I-) are transported into the follicle cells
2. iodide ions (I-) are converted to iodine atoms and attached to the tyrosine molecule
of the thyroglobulin.
3. tyrosine molecules that have the iodine attached are paired, forming molecules of
thyroid hormone (T3 and T4), still incorporated into thyroglobulin
▪TSH controls the rate of thyroid hormone release. In the presence of TSH, the following steps
occur:
4. Follicle cells remove thyroglobulin from the follicles.
5. Thyroglobulin is broken down, and the thyroid hormones enter the cytoplasm
6. T3 and T4 diffuse across the basement membrane into the bloodstream
7. Most of the hormones are attached to transport proteins called thyroid-binding
globulins and transthyretin, or albumin. Very small percentage remain free circulating.
▪An equilibrium is reached between bound and unbound thyroid hormones.
▪In the absence of TSH, the thyroid follicles become inactive.
▪Functions of Thyroid Hormones
1.
2.
▪C cells of the thyroid gland
▫C cells▫Calcitonin (CT)-net effect of calcitonin release is a drop in Ca2+ concentration in body fluids,
accomplished by:
1.
2.
Parathyroid Glands- Two pair are embedded in the posterior surfaces of the thyroid gland.
▪Two cell populations can be found within the parathyroid glands
▫parathyroid cells▫oxyphils▪The chief cells monitor the circulating concentration of calcium ions. When the Ca 2+
concentration falls below normal, the chief cells secrete PTH, parathyroid hormone. Has four
major functions:
1.
2.
3.
Adrenal Glands- sit on the superior border of each kidney.
▪The adrenal gland is divided into two parts: the adrenal cortex and the adrenal medulla.
▫Adrenal Cortex- outer layer of the gland, yellowish in color due to stored lipids.
▫Produces more than two dozen steroid hormones, collectively called corticosteroids.
▫Corticosteroids exert their effects by determining which genes are transcribed in the
nuclei of the target cells, and at what rate.
▫There are three distinct regions, or zones, of the adrenal cortex; zona glomerulosa,
zona fasciculate, and zona reticularis. Each zone synthesizes specific steroid
hormones.
-Zona Glomerulosa- produces mineralocorticoids (MC’s), mainly aldosterone.
-Mineralcorticoids, mainly aldosterone
Target:
Hormonal Effect:
Regulatory Control:
-Zona Fasciculata- produces glucocorticoids (GC’s) mainly cortisol and
corticosterone.
-Glucocorticoids, mainly cortisol and corticosterone.
Target:
Hormonal Effects:
Regulatory Control:
-Zona Reticularis- produces small quantities of androgens
-Androgens(released from the adrenal glands)
Target:
Hormonal Effects:
Regulatory Control:
▫Adrenal Medulla- internal part of adrenal glands, reddish brown, due to many blood
vessels
-Contains two populations of secretory cells, one produces epinephrine, the other
norepinephrine
-Epinephrine and Norepinephrine
Target:
Hormonal Effects:
Regulatory Control:
Pineal Gland- part of the epithalamus, lies in the posterior portion of the roof of the third ventricle.
▪Contains special secretory cells called pinealocytes that secrete melatonin.
▪Melatonin is synthesized from the neurotransmitter serotonin.
▪Collaterals from the visual pathways enter the pineal gland and affect the rate of melatonin
production; lowest during the daylight hours, highest at night.
Several functions have been suggested for melatonin in humans:
1.
2.
3.
Pancreas- lies within the abdominopelvic cavity in the J-shaped loop between the stomach and the small
intestine.
▪99% of pancreatic volume consists of exocrine gland cells and ducts, the remaining is the
endocrine pancreas.
▪The endocrine pancreas consists of clusters of cells called pancreatic islets, or the islets of
Langerhans. Each islet contains four types of cells
▫Alpha cells- produce glucagon.
-Glucagon
Target:
Hormonal Effect:
Regulatory Control:
▫Beta cells- produce insulin
-Insulin
Target:
Hormonal Effect:
Regulatory Control:
▫Delta cells- produce a peptide hormone identical to GH-IH
-GH-IH
Target:
Hormonal Effect:
Regulatory Control:
▫F cells- produce pancreatic polypeptide (PP)
-PP
Target:
Hormonal Effect:
Regulatory Control:
The Endocrine Tissues of other Systems
▪The intestines- release a variety of hormones that coordinate the activities of the digestive
system.
▪The kidneys- release two hormones and one enzyme, calcitriol, erythropoietin, and renin,
respectively.
▫Calcitriol- a steroid hormone released in response to the presence of PTH.
-Calcitriol is synthesized indirectly from cholecalciferol
-Best-known function is the stimulation of calcium and phosphate ion absorption
along the digestive tract
-Other effects on calcium metabolism include:
1. stimulating the formation and differentiation of osteoprogenitor cells
and osteoclasts
2. stimulating bone resorption by osteoclasts
3. stimulating Ca2+ reabsorption at the kidneys
4. suppressing PTH production
▫Erythropoietin (EPO)- a peptide hormone released in response to low oxygen levels in
kidney tissues
-Stimulates the production of red blood cells in the bone marrow
▫Renin- an enzyme released in response to sympathetic stimulation or a decline in renal
blood flow.
-Starts an enzymatic chain reaction known as the renin-angiotensin system.
1st- renin converts angiotensinogen into angiotensin I
2nd- angiotensin I is modified to the hormone angiotensin II in the
capillaries of the lungs.
3rd- angiotensin II stimulates the secretion of aldosterone and ADH,
which restricts salt and water loss at the kidneys. Also stimulates thirst
and elevates blood pressure.
▪The heart- secretes two hormones in response to high blood volumes.
▫Both are natriuretic peptides, work in opposition of the angiotensin II.
▫Natriuretic peptides promote the loss of Na+ and water at the kidneys,
and inhibit
renin release and the secretion of ADH and aldosterone.
▫The two types of natriuretic peptides are ANP (atrial natriuretic peptide) and BNP (brain
natriuretic peptide).
▪The thymus- produces several hormones that are important to the development and
maintenance of immune defenses. Collectively called thymosins.
▪The gonads- produce various hormones in both males and females.
▫Males- interstitial cells of the testes produce androgens, testosterone is the most
important. Nurse cells in the testes secrete inhibin
▫Females- cells surrounding the developing oocyte produce estrogens, the principle
estrogen is estradiol. The corpus luteum releases progestins, progesterone being the
principle one.
▪Adipose tissue- produces two hormones, leptin and resistin.
▫Leptin- best known function of feedback control of the appetite.
▫Resistin- reduces insulin sensitivity throughout the body.
Patterns of Hormonal Interaction
▪Cells generally respond to multiple hormones simultaneously. There can be four possible
outcomes
1.
2.
3.
4.
▪Hormones and growth- six hormones are required for normal growth and the cooperation of
several endocrine organs; GH, thyroid hormones, PTH, calcitriol, insulin, and reproductive
hormones.
▪Hormones and stress- stress is any condition that threatens homeostasis.
▫The body has a generalized stress response that produces the same pattern of hormonal
and physiological adjustments. These responses are called general adaptation
syndrome. GAS has three main phases:
-The Alarm Phase-
-The Resistance Phase-
-The Exhaustion Phase-
▪Hormones and behavior- changes in the mixture of hormones that reach the CNS can have
significant effects on intellectual capabilities, memory, learning, and emotional states.
Aging and hormones- relatively few functional changes occur to the endocrine system with age.
▪The most dramatic change is that of reduced reproductive hormones.
▪Some endocrine tissues become less responsive to stimulation.
Name
Thyroid
Stimulating
Hormone
Adrenocorticotropic
Hormone
FollicleStimulating
Hormone
Luteinizing
Hormone
Prolactin
Growth
Hormone
MelanocyteStimulating
Hormone
Antidiuretic
Hormone
Oxytocin
Thyroxine/
Triiodothyronine
Calcitonin
Parathyroid
Hormone
Abbreviation
Target
Function
Regulation
Mineralcorticoids
(Aldosterone)
Glucocorticoids
(cortisol, hydrocortisone, corticosterone)
Epinephrine/
Norepinephrine
Melatonin
Insulin
Glucagon
Calcitriol
Erythropoietin
Renin/Angiotensin I+II
Natriuretic
Peptides
Thymosins
Androgens
(testosterone)
Estrogens/
Progesterone