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General Overview of the Endocrine System

Questions to be thinking about to help organize yourlearning of the components of the Endocrine System:

Anatomy

1) Where are the different hormone producing cells located?

2) What hormone does a particular type of endocrine cell orneuron produce?

General Overview of the Endocrine SystemQuestions to be thinking about to help organize yourlearning of the components of the Endocrine System:

Hormones1) What is their general chemical structure?2) How does their structure affect their function?3) Where are they produced?4) What environmental or physiological stimuli regulatetheir production and secretion?5) Is their production and secretion under direct neural orhormonal control?6) What are their targets?7) What effects do they produce on their targets?8) How do they produce their effects on their targets (how isthe signal transduced? i.e. receptor mechanisms)

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Often endocrine cells are clumped together into a welldefined gland (e.g. pituitary, thyroid, adrenal, testes,ovaries), but not always (e.g. gut, liver, lung).

Often endocrine cells are clumped together into a welldefined gland (e.g. pituitary, thyroid, adrenal, testes,ovaries), but not always (e.g. gut, liver, lung).

Remember, it's cells that produce hormones, notglands.

Although many glands secrete more than one typeof hormone, most neurons or endocrine cells onlyproduce one type of hormone (there are a fewexceptions).

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Some Terminology

Neurohormone: a hormone that is produced by a neuron

All of the hormones we discuss can be classified asbelonging to one of 2 general functional categories:

1] Releasing hormone (or factors) — hormone that acts onendocrine cells to regulate the release of other hormones.(Note: the influence may be stimulatory or inhibitory)

2] Effector hormone —non-releasing factors; hormones thatproduce a regulatory effect on physiological/psychologicalfunction.

Some Terminology

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There's an especiallyimportant interactionbetween thehypothalamus, pituitaryand some of the otherendocrine glands.

There's an especiallyimportant interactionbetween thehypothalamus, pituitaryand some of the otherendocrine glands.

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There's an especially important interaction between thehypothalamus, pituitary and some of the other endocrineglands.

Neuroendocrine Arrangements3 "orders" of neuroendocrine arrangements: "starting

with a particular neurohormone, how many hormones must besecreted in order for an effector hormone to be secreted?"

first order — neurohormone itself is an effector hormone(released in posterior pituitary).

first order — neurohormoneitself is an effector hormone(released in posteriorpituitary).

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Example of a first orderarrangement:Oxytocin

The neurohormoneoxytocin is released in theposterior pituitary, where itenters the generalcirculation and travels todistant targets to producethe physiological effects ofmilk-ejection (milk let-down) in the mammarygland or contraction in theuterus during parturition(child-birth).

There's an especially important interaction between thehypothalamus, pituitary and some of the other endocrineglands.

Neuroendocrine Arrangements3 "orders" of neuroendocrine arrangements: "starting

with a particular neurohormone, how many hormones must besecreted in order for an effector hormone to be secreted?"

second order — neurohormone is a releasing factor for ananterior pituitary hormone that is an effector hormone.

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second order —neurohormone is areleasing factor for ananterior pituitary hormonethat is an effectorhormone.

Example of a secondorder arrangement:GHRH growth hormone

The neurohormone growthhormone releasinghormone (GHRH)stimulates cells in theanterior pituitary to secretegrowth hormone, whichenters the generalcirculation and travelsthroughout the body tostimulate growth ofvarious target tissues.

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There's an especially important interaction between thehypothalamus, pituitary and some of the other endocrineglands.

Neuroendocrine Arrangements3 "orders" of neuroendocrine arrangements: "starting

with a particular neurohormone, how many hormones must besecreted in order for an effector hormone to be secreted?"

third order — neurohormone is a releasing factor for ananterior pituitary hormone, which is a releasing factor foran effector hormone.

third order — neurohormone is areleasing factor for an anteriorpituitary hormone, which is areleasing factor for an effectorhormone.

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Example of a third order arrangement:GnRH luteinizing hormone testosterone

The neurohormone gonadotropin releasinghormone (GnRH) stimulates cells in theanterior pituitary to secrete the tropichormone, luteinizing hormone (LH), whichenters the general circulation and travels tothe testes where it stimulates production andsecretion of testosterone into the generalcirculation, which travels throughout the bodyto affect muscle growth, secondary sexcharacteristics, sexual behavior, etc.

Basic Anatomy of Hypothalamus:

neurohormones are produced by neurons located in the

hypothalamus

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Basic Anatomy of Hypothalamus:

Location of hypothalamus— below thalamus, baseof forebrain, surrounds third ventricle

A number of hypothalamic subnuclei contain the cellbodies of neurons that produce neurohormones.

• Some of these neurohormones are releasingfactors

• Some of these neurohormones are effectorhormones

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Subdivisions ofhypothalamus—Anterior portion: anteriorhypothalamic nucleus (n.),preoptic area,suprachiasmatic n.

Middle portion:paraventricular n.,periventricular n., supraopticn., arcuate n., dorsomedial n.,ventromedial n.

Posterior portion:mammillary bodies, posteriorhypothalamic area

Subdivisions ofhypothalamus—

Median eminence: part ofhypothalamus immediatelyabove pituitary stalk

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Features of Neurosecretory Neurons of the Hypothalamus

Median eminenceVarious

hypothalamicnuclei

Small“parvocellular

neuron”

Releasinghormone

Posteriorpituitary

Lateralparaventricularn. or supraoptic

n.

Large“magnocellular

neuron”

Effectorhormone

Location of axonterminal

Location of cellbody

Size of cell bodyGeneral class ofhormoneproduced

Why the difference in neuronal cell body size?

Hint #1: it has to do with the general class ofhormone produced and the quantity ofhormone required to do its task

Why the difference in neuronal cell body size?

Hint #2: it has to do with whether thehormone is a systemic hormone

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Basic Anatomy of Pituitary

pituitary—pitui, Latin = “snot gland”

AKA hypophysis—Greek = “undergrowth”

The pituitary is situated in the sella tursica (Latin = TurkishSaddle), indentation in sphenoid bone.

The pituitary is situated in the sella tursica (Latin = TurkishSaddle), indentation in sphenoid bone.

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Subdivisions (lobes) of pituitary:1] anterior pituitary (AKA adenohypophysisor pars anterior) developmentally originatesfrom roof of mouth

2] posterior pituitary (AKA neurohypophysisor pars nervosa) developmentally originatesfrom brain

3] intermediate pituitary, distinct junctionbetween anterior and posterior pituitary innon-human mammals

The pituitary is attached to the hypothalamusby the infundibulum (L. funnel) or pituitarystalk.

Portal Blood SystemSpecialized blood vessel system that serves to carryneurohormones:

from median eminence

to anterior pituitary

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Portal Blood System

Components:

Primay plexus—located in medianeminence

Secondary plexus—located inanterior pituitary

Portal vein—connects primary andsecondary plexus; in pituitary stalk;one-way blood flow fromhypothalamus