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Anatomy and Physiology, Seventh Edition

Rod R. SeeleyIdaho State UniversityTrent D. StephensIdaho State UniversityPhilip TatePhoenix College

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

*See PowerPoint Image Slides for all figures and tables pre-inserted into PowerPoint without notes.

Chapter 17Chapter 17

Lecture OutlineLecture Outline**

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Chapter 17

Functional Organization of

Endocrine System

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General Characteristics• Glands that secrete chemical

signals (hormones) into circulatory system

• Hormone characteristics– Produced in small

quantities– Secreted into intercellular

space– Transported some distance

in circulatory system– Acts on target tissues

elsewhere in body• Regulate activities of body

structures• Ligands: more general term for

chemical signals

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Regulation of Activities:

Comparison of Endocrine and

Nervous Systems

• Endocrine: amplitude modulated signals. – Amount of hormone determines

strength of signal– Onset within minutes of secretion of

hormone• Nervous: frequency-modulated signals.

– Frequency of action potentials produced by neurons determines strength of signal.

– Onset within milliseconds• Two systems actually inseparable

– Nervous system secretes neurohormones into circulatory system

– Nervous system uses neurotransmitters and neuromodulators as ligands

– Some parts of endocrine system innervated directly by nervous system

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Intercellular Chemical Signals• Hormones: type of intercellular signal. Produced by cells of

endocrine glands, enter circulatory system, and affect distant cells; e.g., estrogen

• Autocrine: released by cells and have a local effect on same cell type from which chemical signals released; e.g., prostaglandin

• Paracrine: released by cells and affect other cell types locally without being transported in blood; e.g., somatostatin

• Pheromones: secreted into environment and modify behavior and physiology; e.g., sex pheromones

• Neurohormone: produced by neurons and function like hormones; e.g., oxytocin

• Neurotransmitter or neuromodulator: produced by neurons and secreted into extracellular spaces by presynaptic nerve terminals; travels short distances; influences postsynaptic cells; e.g., acetylcholine.

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Examples of Hormone Chemical Structure

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Control of Secretion Rate• Most hormones controlled by negative feedback

systems• Most hormones are not secreted at constant rate,

but their secretion is regulated by three different methods

1. The action of a substance other than a hormone on an endocrine gland.

2. Neural control of endocrine gland. 3. Control of secretory activity of one endocrine gland

by hormone or neurohormone secreted by another endocrine gland

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1. Action of Substance Other Than Hormone

• An increased blood glucose concentration stimulates increased insulin secretion from the pancreas

• Insulin increases glucose uptake by tissues, which decreases blood glucose levels.

• Autonomic nervous system also influences insulin secretion

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2. Nervous System Regulation

• Stimuli such as stress or exercise activate the sympathetic division of the autonomic nervous system

• Sympathetic neurons stimulate the release of epinephrine and smaller amounts of norepinephrine from the adrenal medulla. Epinephrine and norepinephrine prepare the body to respond to stressful conditions.

• Once the stressful stimuli are removed, less epinephrine is released as a result of decreased stimulation from the autonomic nervous system.

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3. Hormonal Regulation

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Positive and Negative Feedback

POSITIVE• During the menstrual cycle, before

ovulation, small amounts of estrogen are secreted from the ovary.

• Estrogen stimulates the release of gonadotropin-releasing hormone (GnRH) from the hypothalamus and luteinizing hormone (LH) from the anterior pituitary

• GnRH also stimulates the release of LH from the anterior pituitary

• LH causes the release of additional estrogen from the ovary. The GnRH and LH levels in the blood increase because of this positive-feedback effect.

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Positive and Negative FeedbackNEGATIVE

• During the menstrual cycle, after ovulation, the ovary begins to secrete progesterone in response to LH.

• Progesterone inhibits the release of GnRH from the hypothalamus and LH from the anterior pituitary.

• Decreased GnRH release from the hypothalamus reduces LH secretion from the anterior pituitary. GnRH and LH levels in the blood decrease because of this negative-feedback effect.

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Changes in Hormone Secretion Through Time

a) Chronic hormone regulation. Maintenance of relatively constant concentration of hormone. Thyroid hormone.

b) Acute hormone regulation. Epinephrine in response to stress.

c) Cyclic hormone regulation. Female reproductive hormones.

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Transport and Distribution • Hormones dissolve in blood plasma and

are transported in free form or are reversibly bound to plasma proteins

• Free form can diffuse from plasma into interstitial fluid and affect cells

• As concentration of free hormone molecules increase, more hormones molecules diffuse from capillaries into interstitial spaces to bind to target cells

• Lipid soluble hormones diffuse through capillary cells. Water soluble hormones diffuse through pores in capillaries called fenestrae.

• A large decrease in plasma protein concentration can result in loss of a hormone from the blood because free hormones are rapidly eliminated from circulation through kidney or liver

• Hormones are distributed quickly because they circulate in the blood

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Metabolism and Excretion

• Half-life: The length of time it takes for half a dose of substance to be eliminated from circulatory system

• Long half-life: regulate activities that remain at a constant rate through time. Usually lipid soluble and travel in plasma attached to proteins

• Short half-life: water-soluble hormones as proteins, epinephrine, norepinephrine. Have a rapid onset and short duration

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Hormone (Ligand) Interaction with Target Tissues

• Portion of molecule where ligand binds is called binding site.

• If the molecule is a receptor (like in a cell membrane) the binding site is called a receptor site

• Ligand/receptor site is specific; e.g., epinephrine cannot bind to the receptor site for insulin.

• The purpose of binding to target tissue is to elicit a response by the target cell.

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Down-Regulation

• Normally, receptor molecules are degraded and replaced on a regular basis.

• Down-regulation – Rate at which receptors are

synthesized decreases in some cells after the cells are exposed to a ligand.

– Combination of ligands and receptors can increase the rate at which receptor molecules are degraded. This combined form is taken into the cell by phagocytosis and then broken down.

• Tissues that exhibit down-regulation are adapted to short-term increases in hormone concentration.

• Tissues that respond to hormones maintained at constant levels normally do not exhibit down-regulation.

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Up-Regulation

• Some stimulus causes increase in synthesis of receptors for a hormone, thus increases sensitivity to that hormone

• For example, FSH stimulation of the ovary causes an increase of LH receptors. Ovarian cells are now more sensitive to LH, even if the concentration of LH does not change. This causes ovulation.

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Membrane-Bound Receptors• Receptor: integral proteins with

receptor site at extracellular surface. Interact with ligands that cannot pass through the plasma membrane.

• Ligands – Water-soluble or large-molecular-

weight hormones. Attachment of ligand causes intracellular reaction.

– Large proteins, glycoproteins, polypeptides; smaller molecules like epinephrine and norepinephrine

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Intracellular Receptors• Receptors: in the cytoplasm or

in the nucleus• Ligands

– Lipid soluble and relatively small molecules; pass through the plasma membrane.

– React either with enzymes in the cytoplasm or with DNA to cause transcription and translation

– Thyroid hormones, testosterone, estrogen, progesterone, aldosterone, and cortisol

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Membrane-Bound Receptors• Proteins or glycoproteins that have polypeptide chains

folded to cross cell membrane several times• Ligand binds reversibly to receptor site on receptor

protein• Three different results of ligand binding

1. Alteration of membrane permeability. Example: acetylcholine

2. Activation of G proteins associated with the membrane, causes production of intracellular mediator such as cyclicAMP, leads to activation of intracellular enzymes. Example: LH

3. Receptors linked to intracellular enzymes through intracellular mediators. Mediators alter activity of intracellular enzymes. Examples: nitric oxide, cyclicGMP, Ca ions.

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Alteration of Membrane Permeability

• Change in permeability of plasma membrane to ions.

• Ligand-gated ion channels.

• Causes a response in the cell.

• Example: neuromuscular junction

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Activation of G Proteins

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G Proteins and Intracellular Mediators• Intracellular mediators: ions or molecules that enter cell

or are produced in cell• Can be produced because of G protein activation• Regulate intracellular enzyme activities

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G Proteins and Ca2+ Channels

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G Proteins and Synthesis of cAMP• Membrane-bound receptors for

glucagon are associated with G proteins in liver cells

• When glucagon binds to glucagon receptors, the subunit of the G proteins dissociates from the other subunits and GTP binds to it

• The subunit binds to adenylate cyclase and activates it.

• Resulting increase in cAMP activates protein kinase enzymes, which phosphorylate other specific enzymes that break down glycogen and release glucose from the liver cells

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Activation of G proteins and increase in synthesis of inositol triphosphate

(IP3) and diacylglycerol (DAG)

• DAG causes production of prostaglandins, which increase smooth muscle contraction

• IP3 causes increase of Ca2+ in cytoplasm and increases contraction of smooth muscle

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Direct Activation of Intracellular Mediators

• Cyclic guanine monophosphate (cGMP) produced intracellularly in response to ligand attaching to receptor

• cGMP combine with and activate specific enzymes in cytoplasm.

• Cell responds• E.g., atrial natriuretic hormone

attaches to plasma membrane of kidney cells. cGMP activates enzymes that increase Na+ and water excretion by kidney

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Membrane-Bound Receptors That Phosphorylate Intracellular Proteins

• Ligands bind to membrane-bound receptors.

• Part of receptor protein on inside of membrane acts as an enzyme to phosphorylate proteins

• E.g., insulin receptors bound to insulin cause phosphorylation of proteins and cell responds to presence of insulin.

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Cascade Effect: Amplification

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Intracellular Receptors• Proteins in cytoplasm or nucleus• Hormones bind with intracellular receptor and

receptor-hormone complex activate certain genes, causes transcription of mRNA and translation. These proteins (enzymes) produce the response of the target cell to the hormone

• Latent period of several hours because time is required to produce mRNA and protein

• Processes limited by breakdown of receptor-hormone complex

• Estrogen and testosterone produce different proteins in cells that cause the differing secondary sexual characteristics of females and males.

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