The endocrine system is the second control system.It is integrated with and interactive with the nervous system.

Comparison of the nervous system with the endocrine system:

Nervous SystemEndocrine System

method of signalingelectrical impulsechemical agent ("messenger")transmissionrelease

delivery time framemillisecondsseconds to days

adjustment responsequickgradual

duration of signal eventbriefprolonged

system organizationcontinuousdiscontinuous

Other Characteristics of the Endocrine System:

* comprised of endocrine cells, tissues, and organs scattered throughout the body

* collectively a relatively tiny mass of tissue

* endocrine glands:

* ductless

* cells organized in cords and branching networks

* rich vascular supply maximize access to fluid compartments

* released agent/mediator is HORMONE

* include:

* pituitary

* thyroid

* parathyroid

* adrenal

* pineal

* thymus

* pancreas (exocrine + endocrine)

* gonads (gametogenesis + endocrine)

* hypothalamus (neuroendocrine)

* other scattered endocrine structures

* some tumors synthesize and release relatively large amounts of hormones

Contrast with Exocrine Glands (c.f., Chapter 4)

* all are ducted structures which release their secretory products through a tube onto a membrane 

surface or into a membrane-lined pouch

* include:

* sweat

* mucous

* oil

* salivary

* liver (bile)

* pancreas (digestive enzymes)

* released agent is NON-HORMONAL

Hormones:

* secreted by cells into extracellular fluids

* regulate metabolic function of other cells in body

* chemical substances

* most hormones are AMINO ACID -based

* simple amino acid derivatives and thyroxine

* peptides (short a.a. chains)

* proteins (long polypeptides)

* very few hormones are STEROIDS

* steroids are cholesterol derivatives/variants

* e.g., gonadal hormones and adrenocortical hormones

* EICOSANOID HORMONES are paracrine or "local" hormones

* biologically active lipids

* derivatives of ARACHIDONIC ACID

* LEUKOTRIENES are signaling chemicals that mediate inflammation and some allergic 

reactions

* PROSTAGLANDINS have multiple targets and effects; effects are typically highly localized, e.g.:

* raise blood pressure

* increase expulsive uterine contractions during birth

* enhance blood clotting and inflammation

Mechanisms of Hormone Action

* alter target cell activity (i.e., increase or decrease rates of normal processes)

* response is dictated by target cell type

* typical changes produced by hormones:

* change plasma membrane permeability and/or membrane potential by open/close ION 

CHANNELS

* synthesis of proteins (viz., enzymes = regulatory proteins)

* activate/deactivate enzymes

* induce secretory activity

* stimulate mitosis

 
 

PHYSIOLOGICAL SIGNALING:Three basic plans

I.At a distance, between individuals (e.g., pheromones)

II.At a distance, within an individual (e.g., traditional hormones)

III.Local messengers (e.g., neurotransmitters and prostaglandins)

Different communication mechanisms to signal for hormone production:

HUMORAL:e.g.The parathyroid glands are nested in the thyroid gland.Reduced Ca²⁺ levels in the capillary blood delivered to the parathyroids stimulates the release of PTH (parathyroid hormone) which stimulates release of Ca²⁺ from bone.

NEURAL:e.g.Sympathetic nerve fibers from the spinal cord directly stimulate adrenal medulla cells to quickly secrete CATECHOLAMINES (epinephrine and norepinephrine) into the blood, and those catecholamines have a rapid effect on elevating blood pressure, heart rate, and general level of agitation.

HORMONAL:e.g.TROPIC HORMONES (a.k.a., "releasing hormones") from the HYPOTHALAMUS stimulate the release of specific other hormones from the thyroid, adrenal cortex, and gonads.

Although the pituitary is commonly referred to as the "master gland", the HYPOTHALAMUS (in the brain stem of the central nervous system) can be thought of as the chief regulatory center for the endocrine system.It regulates:

·sleep

·arousal

·emotions

oanger

ofear

opleasure

ohunger

osexual response

opain

·body temperature

·blood pressure

·water balance

The PITUITARY (hypophysis)

·"master gland"

·controlled by the hypothalamus

·hormone-producing portion = anterior pituitary (adenohypophysis)

·neural portion = posterior pituitary is an extensions of the hypothalamus

oposterior pituitary + infundibulum = neurohypophysis

A PORTAL SYSTEM is an arrangement o blood vessels in which a capillary bed feeds into veins which, in turn, feed into another capillary bed.

e.g., The HYPOPHYSEAL PORTAL SYSTEM:capillaries from the superior hypophyseal artery deliver hormones produced and secreted by the hypothalamus into the hypophyseal portal vein which conducts the material directly to the adenohypophysis, where the hypothalamic hormones regulate the secretory activity of the adenohypophysis.

 
 

Amino Acid-based Hormones and Second-Messenger Systems

Cyclic Adenosine Monophosphate (cAMP)

Problem:Proteins and peptides cannot penetrate the plasma membranes of tissue cells

Question:So, how do amino acid-based hormones exert their signaling effect?

Answer:Through intracellular SECOND MESSENGERS activated by hormone binding to plasma 

membrane receptors.

Although their are several known second messengers, the cAMP mechanism is currently the best understood.

Characteristics of cAMP?

·intracellular substance that can be generated by event on surface of cell

·once generated, is free to diffuse throughout the cell

·can nonspecifically trigger (activate) one or more enzyme [systems], such as protein kinases

·kinases phosphorylate some proteins to activity, but inactivate others

·chain of events (referred to as [ENZYMATIC] CASCADE) amplifies the effect

·cAMP is rapidly degenerated by PHOSPHODIESTERASE which occurs through the cytoplasm

·enzymes are re-used until inactivated by separate control mechanism

Recall facts about ENZYMES (c.f. Chapter 2):

·enzymes are required for all biological reactions

·globular proteins

·act as biological CATALYSTS

·substances that regulate and accelerate rate of biochemical reactions

·not used up or changed in the reaction

·without specific enzymes, biological reactions proceed so slowly they are inconsequential

·enzymes increase reaction rates by factor of 10⁶

 

cAMP Mechanism:

3 plasma membrane components interact to determine intracellular levels of cAMP:

·hormone receptor (specific, amino acid-based)

·hormone binding to receptor activates the G protein signal transducer (which possesses GTP-ase)

·GTP - GDP reaction provides phosphate-bond energy to activate cAMP

Direct Gene Activation Systems:Steroid Hormones

STEROID HORMONES and THYROID HORMONE are LIPID SOLUBLE:

·easily diffuse into their target cells

·bind to and activate intracellular receptor

·hormone-receptor complex reaches nuclear chromatin

·"turns on" gene

Recall the Central Dogma of Biology regarding the relationship between gene (DNA) activation and protein synthesis:

Hormone-Target Cell Specificity:

·specific RECEPTORS on plasma membrane are required for hormones to bind

·specificity provides for selective responsiveness of tissues to hormones

Target Cell Response is related to:

·blood levels of hormone

·receptor density on target cells

·affinity (strength) of bond between hormone and receptor