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what two body systems in control and communication overlap and integrate at many levels?

-The nervous system
-The endocrine system


what is a hormone?

a chemical mediator that is secreted by an endocrine tissue or gland into the blood, which transports it to its target cells


what is endocrinology?

The study of the homeostatic chemical adjustments and other activities that hormones accomplish


what are the 4 types of intercellular communication mediated via extracellular chemical messengers?

-paracrine secretion - local chemical messengers that only act on neighbouring cells

-neurotransmitter secretion - act over short distances

- hormonal secretion - released from endocrine cells in glands

-neurohormone secretion - hormones produced and secreted from nerve cells i.e ADH


what is the main difference between the nervous system and the endocrine system?

The nervous coordinates rapid and precise response whilst acting to the external environment but the endocrine system regulates activities that typically require duration rather than speed


what are the overall functions of the endocrine system?

-Regulating metabolism, water and electrolyte balance
-Regulation of nutrient supply
-Inducing adaptive changes to help the body cope with stressful situations
-Promoting smooth, sequential growth and development
-Controlling sexual differentiation and reproduction
-Regulating red blood cell production
-Controlling and integrating activities of both the ciruculatory and digestive systems


what are the complexities of endocrine function?

-A single endocrine gland may produce multiple hormones (E.g. anterior pituitary)
-A single hormone may be secreted by more than one endocrine gland (E.g. somatostatin)
-A single hormone has more than one type of target cell and therefore can induce more than one type of effect (E.g. vasopressin)
-Rate of secretion of some hormones can vary over time in a cyclic pattern (E.g. menstual cycle)
-A single target cell can be influenced by more than one type of hormone (E.g. liver cells)
-The same chemical messenger may be either a hormone or a neurotransmitter (E.g. Noradrenaline)
-Some organs are exclusively endocrine in function (e.g. anterior pituitary) whereas other organs of the endocrine system perform non-endocrine functions as well as secreting hormones (e.g. testes).


what is a tropic hormone classified as?

-a hormone that has as its primary function the regulation of hormone secretion by another endocrine gland (e.g. thyroid stimulating hormone (TSH)

-Non-tropic hormones do not act on endocrine cells but on target cells (e.g. insulin)


what is a trophic hormone classified as?

hormones stimulate and maintain their endocrine target tissues
E.g. thyroid stimulating hormone (TSH)


what happens to the thyroid gland in the absence of TSH?

the thyroid gland atrophies (shrinks) and produces very low levels of thyroid hormone


what are the two classes of hormones?



what are hydrophilic hormones?

-water-loving, low lipid solubility
-Peptide (e.g. insulin)
-Catecholamines (adrenaline)


what are lipophilic hormones?

-lipid-loving, poorly soluble in water
-Thyroid hormone (Amine)
-Steroid hormones (derived from cholesterol)


what does solubility properties of a hormone determine?

1) How the hormone is processed by the endocrine cell
2) How the hormone is transported in the blood
3)How the hormone exerts its effects at the target cell


what happens during processing of hydrophilic peptide hormones?

1) Large, inactive precursor proteins (preprohormones) are synthesised by ribosomes on rough ER.

2) They then migrate to the Golgi complex in transport vesicles that pinch off from the smooth ER.

3) Enzymes in the ER prune the preprohormones to active hormones.

4) The Golgi complex packages the finished hormones into secretory vesicles that are pinched off and stored in the cytoplasm until an appropriate signal triggers their secretion.

5) On stimulation, the secretory vesicles fuse with the PM and release their contents by exocytosis


where are peptide hormones stored?

stored in endocrine cell within secretary vesicles – ready made cells that can be released when the endocrine cell is stimulated


what happens during the processing of lipophilic steroid hormones?

-Cholesterol is the common precursor for all steroid hormones.
-Synthesis of steroid hormones from cholesterol requires a series of enzymatic reactions.
-Enzymes required are expressed and limited to certain organs/cells (e.g. adrenal cortex and cortisol)
-Steroid hormones are not stored. (Only the precursor cholesterol is stored.)
-Once formed, the lipid-soluble hormones immediately diffuse across the PM and into the blood


what does the rate of steroid hormone secretion depend on?

depends on rate of hormone synthesis


how are peptide hormones carried in the blood?

transported simply dissolved in the blood


how are lipophilic steroid hormones and thyroid hormones carries in the blood?

-Reversibly bind to plasma proteins
-Some plasma proteins carry one type of hormone, whereas others (e.g. albumin) carry any ‘hitchhiking’ hormone
-Only the freely dissolved (unbound) lipophilic hormone is biologically active
-The carrier-bound hormone is in dynamic equilibrium with the free hormone pool so can be used to replenish the active free pool


how are catecholamines carried in the blood?

Unusual; 50% circulate as free hormone, 50% loosely bound to albumin
Hydrophilic (water soluble)


where do hydrophilic peptides and catecholamines bind?

Bind to specific receptors on the outer Plasma membrane surface of the target cell


where do lipophilic steroids and thyroid hormone bind?

Bind with specific receptors located inside the target cell


how does hydrophilic hormones influence their target cells?

Activating 2nd messenger pathways within the target cell.
-This activation directly alters the activity of pre-existing intracellular proteins (enzymes)


how does lipophilic hormones influence their target cells?

activating specific genes in the target cell to cause formation of new intracellular proteins (enzymatic or structural)


what are the factors that effect the effective plasma concentration of a hormone?

-The hormone’s rate of secretion into the blood by the endocrine gland.
-For a few hormones, its rate of metabolic activation or conversion. (E.g. thyroxine (T4) to tri-iodothyronine (T3))
-For lipophilic hormones its extent of binding to plasma proteins. (E.g. pregnancy and thyroid hormone)
-Its rate of removal from the blood by metabolic inactivation and excretion in the urine


what general mechanisms control secretion of effective plasma concentration of a hormone?

-Negative feedback control
E.g. Control of free, circulating thyroid hormone
-Neuroendocrine reflexes
E.g. increased secretion of cortisol during a stress-response
-Diurnal (day-night) (aka circadian (around a day)) rhythms
E.g. cortisol secretion (rises during the night, falls throughout the day)


what causes endocrine disorders?

-Endocrine disorders most commonly result from abnormal plasma concentrations of a hormone caused by inappropriate rates of secretion;
-Too little hormone secreted (hyposecretion), or
-Too much hormone secreted (hypersecretion)
-target cell responsiveness to the hormone is abnormally low (even though plasma concentration of the hormone is normal)


what can cause primary hyposecretion?

-Abnormality within the gland
-Factors that cause primary hyposecretion can include:
Genetic, dietary, chemical or toxic, immunologic, disease, iatrogenic, idiopathic


what can cause secondary hyposecretion?

Endocrine gland is normal but is secreting too little hormone because of a deficiency of its tropic hormone