The Endocrine System

General accepts of endocrine system  

• interacts with the nervous system to coordinate cellular metabolism. 

• influences the metabolic activities of cells via hormones (chemical messengers) 

• hormones have widespread and diverse effects 

• some functions

  • growth and development

  • maintenance of electrolyte balance

  • regulation of cellular metabolism

  • etc…

 

Endocrinology

study of the endocrine system and hormones 

Endocrine glands

• scattered throughput the body

• pituitary gland

• parathyroid gland

• thyroid gland

• pineal gland

• adrenal gland

• thymus, pancreas

• gonads

• Additionally, endocrine tissue and cells are also located throughout the body. 

• Some of these additional tissues and cells are located in the:

  • stomach

  • kidney

  • small intestine

  • heart.

Hypothalamus

is considered a neuroendocrine gland

Endocrine gland (ductless gland)

• secrete their produces into a body fluid (pituitary gland)

• primary build of transport for hormone is blood

Exocrine glands

secrete their products onto a internal or exteranl’s body surface (sweet glands & salivary glands)

Endocrine system

• composed of ductless gland that synthesize and secreat hormones

  • Hormone are released into the blood and transport thought out the body

• Target cells have the specific receptors for hormone

  • they bind hormone and respond

Amino acid-based hormones

• Amines, peptides, proteins

• use 2nd messenger systems and G proteins 

Steroid hormones

• Four, fused carbon rings

• remember that steroids are a type of lipid synthesized from cholesterol.

• Steroid hormones include those that are produced by the gonads as well as adrenocortical hormones – use direct gene activation. 

• if it ends in “one” = Steroid hormones

Eicosanoids

• potentially a 3rd class of hormones

• These “hormones” are lipids synthesized from arachidonic acid

• They act locally (paracrines) & are potent.   

leukotrienes

signaling chemicals that mediate inflammation and some allergic reactions

prostaglandins

have a variety of effects including increasing bp, uterine contractions and enhancing blood clotting and inflammation

target cells

• cells that a specific hormone influences.

• Target cells must have specific protein receptors on its plasma membrane or interior to which the hormone binds.

up-regulation

target cells form more receptors in response to rising blood levels of a specific hormone

down-regulation

loss of receptors in response to high concentrations of hormones over an extended period of time. (prevents overreacting by target cells)

Effects of hormonal stimulation on target cells

• overall effect: alter cell metabolism

• alteration of membrane permeability

• stimulation of protein synthesis or synthesis of certain regulatory molecules

• activation or deactivation of enzymes

• stimulates mitosis

• induces secretions

Direct gene activation

• used by steroid hormones and thyroid hormone.

• These molecules are lipid soluble and pass readily through the plasma membrane of cells. 

• Upon entering the cell they join with a protein.

• This complex passes through the nuclear envelope, and directly activates a gene by prompting the transcription of a gene by mRNA. 

Second messenger systems

• used by amino acid-based hormones 

1)Hormone (first messenger) binds to receptors on the plasm membrane

2) Once the hormone bind to the receptor, nearby G protein is activated

3) The activated G protein activates adenylate cyclase (effort enzyme)

4) Adenylate cyclase converts ATP to cAMP (second messenger)

5) cAMP production result in the desired cellular effects

**The G proteins is the single transducer**

Control of hormone release

negative feedback system

Humoral stimuli

• changing blood levels of certain ions and nutrients.

  • Example - The release of insulin due to an increase in the blood glucose level

Neural stimuli

• nerve fiber stimulation

  • Examples - The release of norepinephrine (NE) from the adrenal gland’s and medulla due to nervous stimulation

Hormonal stimuli

• in response to hormones released by other endocrine glands.

  • Examples - The release of thyroid hormone from the thyroid gland due to thyroid stimulating hormone (TSH) being sent to the thyroid gland from the anterior lobe of the pituitary gland