The Endocrine System

Intercellular chemical messengers

target cells

paracrines

histamine

neurotransmitters

acetylcholine

hormones

T₃ and T₄

neurohormones

GnRH

General principles

specificity of communication depends on specialization of target-cell receptors

complementarity of endocrine and nervous systems

type of chemical messenger

distance of action of chemical messenger

means of specificity

anatomic arrangement

“wired” vs. “wireless”

major functions

regulation of organic metabolism, water and electrolyte balance

inducing adaptive changes to stress

promoting smooth, sequential growth and development

controlling reproduction

regulating red blood cell production

along with ANS, controlling and integrating circulation, digestion

speed of response

fast vs. slow

duration of action

Hormones

Not a Complete Listing of Hormones
Organ		Hormone(s)
hypothalamus		releasing hormones: TRH, CRH, GnRH, GHRH, PRH, PIH
posterior pituitary		antidiuretic hormone (vasopressin)
posterior pituitary		oxytocin
anterior pituitary		thyroid-stimulating hormone (TSH)
		adrenocorticotropic hormone (ACTH)
		growth hormone (GH)
		follicle-stimulating hormone (FSH)
		luteinizing hormone (LH)
		prolactin
thyroid gland	follicular cells	tetraiodothyronine (T₄)
	follicular cells	triiodothyronine (T₃)
	C cells	calcitonin
adrenal cortex		aldosterone
		cortisol
		androgens (esp. DHEA)
adrenal medulla		epinephrine
adrenal medulla		norepinephrine
endocrine pancreas (islets of Langerhans)		insulin β cells
endocrine pancreas (islets of Langerhans)		glucagon α cells
parathyroid		parathormone
testis		testosterone
testis		inhibin
ovary		estrogen
		progesterone
		inhibin
pineal gland		melatonin
placenta		estrogen
		progesterone
		chorionic gonadotropin (hCG)
		chorionic somatomammotropin (hCS)
kidney		renin
kidney		erythropoietin
stomach		gastrin
duodenum		secretin
		cholecystokinin
		gastric inhibitory polypeptide (GIP)
liver		somatomedins
skin		vitamin D
thymus		thymosin
heart		atrial natriuretic hormone

Hormonal types

peptides

amines

derived from the amino acid tyrosine

catecholamines

thyroid hormones

steroids

behavior

hydrophilic vs. hydrophobic

lipophobic vs. lipophilic

mechanisms of synthesis

peptide hormones

steroid hormones

cholesterol

amines

transport

Modes of action

postreceptor events

hydrophilic hormones

intracellular second messenger

cAMP

adenylyl cyclase

lipophilic hormones

gene activation

hormone levels are regulated by changes in secretion rate

hormone-hormone negative feedback

output of a system opposes a change in input

substrate-hormone feedback

changes in substrate trigger alter secrection rates

neuroendocrine reflexes

diurnal or circadian rhythms

response of target cell can be regulated by altering number of receptors

normal

down regulation

permissiveness

one hormone enhances responsiveness to another

synergism

FSH + testosterone

antagonism

progesterone and estrogen during pregnancy

endocrine disorders can be caused by hormonal excess or deficiency, or to decreased target cell responsiveness

abnormal responsiveness

testicular feminization syndrome

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Questions for thought
1.		List the three groups of hormones on the basis of chemical structure, giving examples of each.
2.		Describe a design whereby a cell can respond to two different hormones at the same time.
3.		A peer of yours makes the claim that the hypothalamus is exclusively a part of the CNS. What argument(s) would you make to substantiate that the hypothalamus really is an interface between the nervous system and endocrine system, and further that it produces hormones of its own?
4.		Explain the mechanism used by the protein/peptide hormones to trigger an effect in their target cells.
5.		Explain how hormones may interact to produce coordinated physiological responses, giving examples.
6.		Explain why some people refer to the anterior pituitary as the “master gland of the body”.
Other questions to test your knowledge

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