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The Endocrine System


Intercellular Communication

Endocrine versus Nervous system

      Nervous system performs short term crisis management

      Endocrine system regulates long term ongoing metabolic

      Endocrine communication is carried out by endocrine cells releasing hormones

    Alter metabolic activities of tissues and organs

    Target cells

      Paracrine communication involves chemical messengers between cells within one tissue


An Overview of the Endocrine System

Endocrine system

      Includes all cells and endocrine tissues that produce hormones or paracrine factors

 

Hormone structure

      Amino acid derivatives

    Structurally similar to amino acids

      Peptide hormones

    Chains of amino acids

      Lipid derivatives

    Steroid hormones and eicosanoids

 

Hormones can be

      Freely circulating

    Rapidly removed from bloodstream

      Bound to transport proteins

 

Mechanisms of hormone action

      Receptors for catecholamines, peptide hormones, eicosanoids are in the cell membranes of target cells

      Thyroid and steroid hormones cross the membrane and bind to receptors in the cytoplasm or nucleus

 

Control of endocrine activity

      Endocrine reflexes are the counterparts of neural reflexes

      Hypothalamus regulates the activity of the nervous and endocrine systems

    Secreting regulatory hormones that control the anterior pituitary gland

    Releasing hormones at the posterior pituitary gland

    Exerts direct neural control over the endocrine cells of the adrenal medullae

Endocrine System


The Pituitary Gland

Hypophysis

      Releases nine important peptide hormones

      All nine bind to membrane receptors and use cyclic AMP as a second messenger

 

Gland

The anterior lobe (adenohypophysis)

      Subdivided into the pars distalis, pars intermedia and pars tuberalis

      At the median eminence, neurons release regulatory factors through fenestrated capillaries

    Releasing hormones

    Inhibiting hormones

 

 

The Endocrine System

Hypophyseal portal system

      All blood entering the portal system will reach the intended target cells before returning to the general circulation

 

Hormones of the adenohypophysis

      Thyroid stimulating hormone (TSH)

    Triggers the release of thyroid hormones

    Thyrotropin releasing hormone promotes the release of TSH

      Adrenocorticotropic hormone (ACTH)

    Stimulates the release of glucocorticoids by the adrenal gland

    Corticotrophin releasing hormone causes the secretion of ACTH

 

Hormones of the adenohypophysis

      Follicle stimulating hormone (FSH)

    Stimulates follicle development and estrogen secretion in females and sperm production in males

      Leutinizing hormone (LH)

    Causes ovulation and progestin production in females and androgen production in males

      Gonadotropin releasing hormone (GNRH) promotes the secretion of FSH and LH

 

Hormones of the adenohypophysis

      Prolactin (PH)

    Stimulates the development of mammary glands and milk production

      Growth hormone (GH or somatotropin)

    Stimulates cell growth and replication through release of somatomedins or IGF

   Growth-hormone releasing hormone
(GH-RH)

   Growth-hormone inhibiting hormone
(GH-IH)

 

Melanocyte stimulating hormone (MSH)

      May be secreted by the pars intermedia during fetal development, early childhood, pregnancy or certain diseases

      Stimulates melanocytes to produce melanin

 

The posterior lobe of the pituitary gland (neurohypophysis)

      Contains axons of hypothalamic nerves

      neurons of the supraoptic nucleus manufacture antidiuretic hormone (ADH)

    Decreases the amount of water lost at the kidneys

    Elevates blood pressure

 

The posterior lobe of the pituitary gland (neurohypophysis)

      Neurons of the paraventricular nucleus manufacture oxytocin

    Stimulates contractile cells in mammary glands

    Stimulates smooth muscle cells in uterus


The Thyroid Gland

The thyroid

      Lies near the thyroid cartilage of the larynx

      Two lobes connected by an isthmus

 

Thyroid follicles and thyroid hormones

      Thyroid gland contains numerous follicles

    Release several hormones such as thyroxine (T4) and triiodothyronine (T3)

      Thyroid hormones end up attached to thyroid binding globulins (TBG)

    Some are attached to transthyretin or albumin

 

Thyroid hormones

      Held in storage

      Bound to mitochondria, thereby increasing ATP production

      Bound to receptors activating genes that control energy utilization

      Exert a calorigenic effect

 

Cells of the thyroid gland

      C cells produce calcitonin

    Helps regulate calcium concentration in body fluids


The Parathyroid Glands

Four parathyroid glands

      Embedded in the posterior surface of the thyroid gland

      Chief cells produce parathyroid hormone (PTH) in response to lower than normal calcium concentrations

      Parathyroid hormones plus calcitriol are primary regulators of calcium levels in healthy adults

 

 

 


The Adrenal Glands

Adrenal cortex

      Manufactures steroid hormones (corticosteroids)

      Cortex divided into three layers

    Zona glomerulosa (produces mineralocorticoids)

    Zona fasciculate (produces glucocorticoids)

    Zona reticularis (produces androgens)

 

Adrenal medulla

      Produces epinephrine (~75 - 80%)

      Produces norepinephrine (~25-30%)


The Pineal Gland

Pineal gland

      Contains pinealocytes

      Synthesize melatonin

      Suggested functions include inhibiting reproductive function, protecting against damage by free radicals, setting circadian rhythms


The Pancreas

The pancreatic islets

      Clusters of endocrine cells within the pancreas called Islets of Langerhans or pancreatic islets

    Alpha cells secrete glucagons

    Beta cells secrete insulin

    Delta cells secrete GH-IH

    F cells secrete pancreatic polypeptide

 

Insulin and glucagon

      Insulin lowers blood glucose by increasing the rate of glucose uptake and utilization

      Glucagon raises blood glucose by increasing the rates of glycogen breakdown and glucose manufacture by the liver


The Endocrine Tissues of Other Systems

The intestines

      Produce hormones important to the coordination of digestive activities

 

The kidneys

      Produce calcitriol and erythropoietin (EPO) and the enzyme rennin

    Calcitriol = stimulates calcium and phosphate ion absorption along the digestive tract

    EPO stimulates red blood cell production by bone marrow

    Renin converts angiotensinogen to angiotensin I

 

Angiotensin I converted to angiotensin II in the lungs

      Stimulates adrenal production of aldosterone

      Stimulates pituitary gland release of ADH

      Promotes thirst

      Elevates blood pressure

 

The heart

      Specialized muscle cells produce natriuretic peptides when blood pressure becomes excessive

    Generally oppose actions of angiotensin II

 

The thymus

      Produces thymosins

    Help develop and maintain normal immune defenses

 

The gonads

      Interstitial cells of the testes produce testosterone

    Most important sex hormone in males

      In females, oocytes develop in follicles

    Follicle cells produce estrogens

      After ovulation, the follicle cells form a corpus luteum that releases a mixture of estrogens and progesterone

 

Adipose tissues secrete

      Leptin, a feedback control for appetite

      Resistin, which reduces insulin sensitivity


Patterns of Hormonal Interaction

Hormones often interact, producing

      Antagonistic (opposing) effects

      Synergistic (additive) effects

      Permissive effects (one hormone is required for the other to produce its effect)

      Integrative effects (hormones produce different but complimentary results)

 

Hormones and growth

      Normal growth requires the interaction of several endocrine organs

      Six hormones are important

    GH

    Thyroid hormones

    Insulin

    PTH

    Calcitriol

    Reproductive hormones

 

Hormones and stress

      Stress = any condition that threatens homeostasis

      GAS (General Adaptation Syndrome) is our bodies response to stress-causing factors

      Three phases to GAS

    Alarm phase (immediate, fight or flight, directed by the sympathetic nervous system)

    Resistance phase (dominated by glucocorticoids)

    Exhaustion phase (breakdown of homeostatic regulation and failure of one or more organ systems)

 

Hormones and behavior

      Many hormones affect the CNS

      Changes in the normal mixture of hormones significantly alters intellectual capabilities, memory, learning and emotional states


Aging and Hormone Production

Endocrine system

      Few functional changes with age

    Chief change is a decline in concentration of reproductive hormones