Endocrine System

Overview• Body system that consists of a group of glands

that produces regulatory chemicals called hormones, and along with the nervous system, controls and coordinates all other systems of the body

• Exocrine glands are not part of endocrine sys. –secrete prod.’s that are passed outside body (i.e. salivary, digestive, sweat,…)

• Endocrine-ductless, exocrine-ducts

Nervous system vs. Endocrine system• Nervous sys. coordinates

rapid activity &precise response

• by means of electrical stim. (action potentials) and chemical stim. (neurotransmitters)

• Neurotransmitters travel across a narrow space (synaptic cleft), binds to receptors on target cell and act locally to control nerve cell functions.

• Endocrine system effects occur slower & over longer period of time

• Involve chemical stimuli only (hormones)

• Hormones are chemical messenger released by glands or specialized cells into circulating blood and influence the function of cells at another location in the body. (target cells)

Target cells

• Hormones act by binding to receptor sites on target cell, producing a response by the target cell

Target Cell Response

• Attached to the cell, the hormone affects cell activities:1. Regulating the manufacture of proteins2. Changing permeability of the membrane3. Affecting metabolic reactions

Hormone Function

• Play key role in regulating almost all body functions, including:– Metabolism– Growth & development– Water and electrolytes– Reproduction – Behavior

Generalized

• Some hormones affect many diff types of cells of the body

• Examples:– GH– Thyroxin– Insulin

Specific

• Other hormones only affect specific target tissues

• Examples:– ACTH– Ovarian hormones

Antagonistic Pairs

• Some hormones come in antagonistic pairs that have opposite affects on target organs

• Ex. Insulin and glucagon

Classes of Hormones

• Hormones are grouped into three classes based on structure1. Steroids2. Peptides3. Amines

Steroids• Lipids• Chem. Structure sim. to

cholesterol • In most instances

synthesized from cholesterol itself

• Consist of 3 cyclohexyl rings and one cyclopentyl ring

• Very little hormone storage in steroid producing endocrine cells

• Large stores of cholesterol esters in cytoplasm vacuoles can be rapidly mobilized for steroid synthesis after a stimulus

• Secreted by the adrenal cortex:– Cortisol– Aldosterone

• Secreted by ovaries:– Estrogen– Progesterone

• Secreted by testes:– Testosterone

• Secreted from Placenta:– Estrogen– Progesterone

Peptides, Polypeptides, & Proteins• Short chains of Amino Acids• Most hormones are polypep.’s and proteins• Range and size from small peptides with 3 AA’s (thyrotropin

releasing hormone TRS) to proteins with almost 200 AA’s (GH and prolactin)

• Polpypepetides with 100 AA’s or more are called proteins• Fewer than 100 = peptides• They are water soluble• Secreted by pituitary, parathyroid, heart, hypothalamus

stomach, liver, kidney and pancreas• Precursor molecules processed by ER and golgi are stored in

secretory granules

Amines• Derived from amino acid

Tyrosine• Secreted from thyroid

and adrenal medulla• Water soluble• Stored as granules in

cytoplasm until needed• Ex. Epinephrine

(adrenaline)

Mechanism of Action• Endocrine sys. acts by releasing hormones that trigger actions

in specific target cells• Receptors selective to one type hormone• Receptors are located on target cell membrane whereas

others are in cytoplasm or nucleus• First step of hormone action is to bind to specific receptor on

target cell (first messenger)• First messenger triggers 2nd messengers to affect cell’s activity• Binding hormone changes shape of receptor causing the

response to the hormone• 2 mechanisms of hormone action on all target cells: non-

steroid hormones and steroid hormones

Non-Steroid Hormones• Water soluble• Don’t enter cell but bind to plasma membrane receptors

(first messenger)• Generates chem signal (second messenger) inside target cell• Second messenger activates other intracellular chemicals to

produce the target cell response• Resulting complex activates an enzyme that catalyzes the

synthesis of cyclic AMP (cAMP) from ATP • cAMP activates other enzymes to produce target cell

response

Steroid Hormones

• Pass through plasma membrane act in a 2 step process

• Steroid hormones bind to nuclear membrane receptors or receptors in cytoplasm producing activated hormone-receptor complex

• Complex enters nucleus binds with chromatin (DNA) and activates specific genes increasing production of proteins

• Act more slowly than peptide hormones

Onset of hormone secretion after stimulus and duration of action of different hormones

• Some hormones (i.e. norepinephrine and epi.) are secreted within seconds after glands stimulated and may develop full action within another few seconds or minutes

• While other hormones (i.e. thyroxine or GH) may require months for full effect

• Each hormone has own characteristic onset and duration of action – each tailored to perform its specific function

Hormone receptors - location

• The locations for the different types of hormone receptors are generally:

1. In or on the surface of the cell membrane (protein, peptide and catecholamine hormones)

2. In the cell cytoplasm (steroid hormones) 3. In the cell nucleus (thyroid hormones)• Hormonal receptors are large proteins• Each cell that is to be stimulated usually has 2,000

– 100,000 receptors

Transport of Hormones in the Blood

• Water soluble hormones are dissolved in the plasma & transported from their site of synthesis to target tissue

• They diffuse out of capillaries into the interstitial fluid & ultimately to target cells

• Steroid and thyroid hormones circ. In blood bound to plasma proteins (<10% free in soln)

• Protein bound hormones can’t easily diffuse across capillaries to get to target cells

• They are inactive until disassociate from plasma proteins

Feedback Control of Hormone Secretion

• Although hormone concen. Fluctuate in response to various stimuli, they are closely controlled

• In most instances negative feedback mechanisms insure proper level of hormone activity at the target tissue

• After stim causes release of hormone conditions or products resulting from hormone action suppress further release

• The control variable is not the secretory rate of the hormone, but the degree of activity of the target tissue

• Thus when target tissue activity rises to approp. Level than feedback signals to endocrine gland to slow further secretion of hormone

• Promotes balance and homeostasis

(this is the last slide )

• In a few instances positive feedback occurs when action of hormone causes additional secretion of the hormone

• i.e. LH luteinizing hormone • Also periodic variations in hormone release that are

influenced by seasonal changes, stages of development and aging, diurnal cycle, and sleep

• i.e. GH• Periodic variations due to changes in activity of neural

pathways involved in controlling hormone release