ANAPHY - CHAPTER 10: Endocrine System, Lecture notes of Anatomy

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CHAPTER 10: Module 8 Endocrine System

Chemical messengers - allow cells to communicate with each other

Gland - is an organ consisting of epithelial cells that specialize in secretion

Therefore, the study of the endocrine system includes several of the following categories:

1. Autocrine chemical messengers. stimulates the cell that originally secreted it, and sometimes nearby cells of the same type.

Secreted by cells in a local area ; influences the activity of the same cell or cell type from which it was secreted

EXAMPLE: Eicosanoids (prostaglandins, thromboxanes, prostacyclins, leukotrienes)

2. Paracrine chemical messengers. Paracrine chemical messengers are local messengers. Secreted by one cell type but affect neighboring cells of a different type.

EXAMPLE: Somatostatin, histamine, eicosanoids

3. Neurotransmitter. Produced by neurons ; secreted into a synaptic cleft by presynaptic nerve terminals ; travels short distances ; influences postsynaptic cells

EXAMPLE: Acetylcholine, epinephrine

4. Endocrine chemical messengers. Secreted into the blood by specialized cells; travels some distance to target tissues ; results in coordinated regulation of cell function

EXAMPLE: Thyroid hormones, growth hormone, insulin, epinephrine, estrogen, progesterone, testosterone, prostaglandins

10.2 FUNCTIONS OF THE ENDOCRINE SYSTEM

1. Metabolism
2. Control of food intake and digestion. regulates the level of satiety (fullness)
3. Tissue development
4. Ion regulation.
5. Water balance
6. Heart rate and blood pressure regulation.
7. Control of blood glucose and other nutrients.
8. Control of reproductive functions.
9. Uterine contractions and milk release.
10. Immune system regulation.

10.3 CHARACTERISTICS OF THE ENDOCRINE SYSTEM

Endocrine system

 is composed of endocrine glands and specialized endocrine cells.  endo , meaning within, and krino , to secrete  Endocrinology. study of the endocrine system

Exocrine glands

 have ducts that carry their secretions

Hormones

 Secreted by endocrine gland and cells in bloodstream rather than in duct.  Travel through the general blood circulation to target tissues or effectors.

10.4 HORMONES

 Hormone is derived from the Greek word hormon , which means to set into motion.  regulate almost every physiological process in our body

Lipid-Soluble Hormones

 Nonpolar , and include steroid hormones, thyroid hormones, and fatty acid derivative hormones , such as certain eicosanoids.  Are small molecules and are insoluble in water- based fluids , such as the plasma of blood.

 Lipid-soluble hormones travel in the bloodstream attached to binding proteins.  Binding proteins transport and protect hormones.  Without the binding proteins , the lipid-soluble hormones would quickly diffuse out of capillaries.

Water-Soluble Hormones

 Polar molecules ; they include protein hormones, peptide hormones , and most amino acid derivative hormones.  Quite large. Do not readily diffuse through the walls of all capillaries ; therefore, they tend to diffuse from the blood into tissue spaces more slowly.  Water-soluble hormones can dissolve in blood ; many circulate as free hormones , meaning that most of them dissolve directly into the blood and are delivered to their target tissue without attaching to a binding protein.  The organ regulated by water-soluble hormones are very porous, or fenestrated  short half-lives because they are rapidly degraded by enzymes, called proteases , 

Lysosomal enzymes – degrade the hormones once the hormones are inside the target cell.

Stimulation of Hormone Release

Humoral Stimuli

 Blood-borne chemicals can directly stimulate the release of some hormones.  Circulate in the blood.  Humoral refers to body fluids , including blood.  Sensitive to the blood levels of a particular substance, such as glucose, calcium, or sodium.

Neural Stimuli

 Releasing hormones, a term usually reserved for hormones from the hypothalamus.

Hormonal Stimuli

 The third type of regulation uses hormonal stimuli.  It occurs when a hormone is secreted that, in turn, stimulates the secretion of other hormones.  Hormones from the anterior pituitary gland, called tropic hormones. Stimulate the secretion of another hormone.  For example , hormones from the hypothalamus and anterior pituitary regulate the secretion of thyroid hormones from the thyroid gland.

Inhibition of Hormone Release

Inhibition of Hormone Release by Humoral Stimuli

 Companion hormone’s effects oppose those of the secreted hormone and counteract the secreted hormone’s action.

Inhibition of Hormone Release by Neural Stimuli

 Neurons inhibit targets just as often as they stimulate targets.  If the neurotransmitter is inhibitory , the target endocrine gland does not secrete its hormone.

Inhibition of Hormone Release by Hormonal Stimuli

 hormones prevent the secretion of other hormones

Regulation of Hormone Levels in the Blood

1. Negative feedback. A self-limiting system

 Most hormones are regulated by a negative- feedback mechanism.  hormone’s secretion is inhibited by the hormone itself

2. Positive feedback. A self-propagating system

 Some hormones, when stimulated by a tropic hormone  Stimulates further secretion of the original hormone.

and the G protein binds to it. GTP replaces GDP on the α subunit of the G protein.

3. The G protein separates from the receptor. The GTP-linked α subunit activates cellular responses, which vary among target cells.
4. When the hormone separates from the receptor, additional G proteins are no longer activated. Inactivation of the α subunit occurs when phosphate (Pi ) is removed from the GTP, leaving GDP bound to the α subunit.

G Proteins That Interact with Adenylate Cyclase

1. After a water-soluble hormone binds to its receptor, the G protein is activated.
2. The activated α subunit, with GTP bound to it, binds to and activates an adenylate cyclase enzyme so that it converts ATP to cAMP.
3. The cAMP can activate protein kinase enzymes, which phosphorylate specific enzymes activating them. The chemical reactions catalyzed by the activated enzymes produce the cell's response.
4. Phosphodiesterase enzymes inactivate cAMP by converting cAMP to AMP.

Signal Amplification

 Each receptor produces thousands of second messengers, leading to a cascade effect and ultimately amplification of the hormonal signal.

The combination of a hormone with a membrane- bound receptor activates several G proteins. The G proteins, in turn, activate many inactive adenylate cyclase enzymes, which cause the synthesis of a large number of cAMP molecules. The large number of cAMP molecules, in turn, activate many inactive protein kinase enzymes, which produce a rapid and amplified response.

10.7 ENDOCRINE GLANDS AND THEIR HORMONES

consists of ductless glands that secrete hormones into the interstitial fluid

Pituitary and Hypothalamus

The pituitary (pi-too′i-tār-rē; pituita, phlegm or thick mucous secretion) gland is also called the hypophysis (hī-pof′i-sis; hypo, under + physis, growth).

 is a small gland about the size of a pea  rests in a depression of the sphenoid bone inferior to the hypothalamus of the brain  Lies posterior to the optic chiasm and is connected to the hypothalamus by a stalk called the infundibulum (in-fŭn-dib′ŭ-lŭm; a funnel).  Anterior pituitary is made up of epithelial cells derived from the embryonic oral cavity; the posterior pituitary is an extension of the brain and is composed of nerve cells.

 control the functions of many other glands in the body, such as the ovaries, the testes, the thyroid gland, and the adrenal cortex  secretes hormones that influence growth, kidney function, birth, and milk production by the mammary glands.  Known as the body’s master gland because it controls the function of so many other glands.

The hypothalamus (hī′pō-thal′ă-mŭs; hypo, under + thalamos)

 Important autonomic nervous system and endocrine control center of the brain located inferior to the thalamus.  Controls the pituitary gland in two ways: hormonal control and direct innervation.

Pituitary Gland, Its Hormones, and Their Target Tissues

Hypothalamic-pituitary portal system. Capillary beds and veins that transport the releasing and inhibiting hormones

Hypothalamic neuropeptides function as either releasing hormones or inhibiting hormones

Direct Innervation of the Posterior Pituitary

 A storage location for two hormones synthesized by special neurons in the hypothalamus.  Stimulation of neurons within the hypothalamus controls the secretion of the posterior pituitary  cell bodies of these neurons are in the hypothalamus, and their axons extend through the infundibulum to the posterior pituitary

Hypothalamus and Anterior Pituitary

1. Stimuli within the nervous system regulate the secretion of releasing hormones (green circles) and inhibiting hormones (red circles) from neurons of the hypothalamus.
2. Releasing hormones and inhibiting hormones pass through the hypothalamohypophysial portal system to the anterior pituitary.
3. Releasing hormones and inhibiting hormones (green and red circles) leave capillaries and stimulate or inhibit the release of hormones (yellow squares) from anterior pituitary cells.
4. In response to releasing hormones, anterior pituitary hormones (yellow squares) travel in the blood to their target tissues (green arrow), which in some cases, are other endocrine glands

Hormones of the Anterior Pituitary

Growth hormone (GH)

 Stimulates the growth of bones, muscles, and other organs by increasing gene expression.  resists protein breakdown during periods of food deprivation and favors lipid breakdown  Too little growth hormone secretion can result from abnormal development of the pituitary gland.  pituitary dwarf - A young person suffering from a deficiency of growth hormone remains small, although normally proportioned  acromegaly – facial features abnormaly large