NEUROHYPOPHYSEAL HORMONES - PowerPoint Presentation

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NEUROHYPOPHYSEAL HORMONES

ANTIDIURETIC HORMONE(ADH) OR VASOPRESSIN

OXYTOCIN

INTRODUCTION

These are hormones of posterior pituitary gland.

The posterior pituitary gland of mammals, including man stores two hormones which are:

Anti-diuretic hormone (vasopressin)

Oxytocin

PITUITARY GLAND

The pituitary gland is mainly composed of

pituicytes

.

The

pituicytes

are not known to secrete any hormones but support the several nerve fibres responsible for secreting anti-

diurectic

hormone and

oxytocin

.

The bodies of the cells that secrete the posterior pituitary hormones are not located in the pituitary gland itself but are large neurons, called

magnocellular

neurons,

located in the

supraoptic

and

paraventricular

nuclei

of the hypothalamus.

The

hormones are then transported

in combination with carrier protein called

neurophysin

in

the

axoplasm

of the neurons' nerve

fibers

passing from the hypothalamus to the posterior pituitary gland.

SYNTHESIS, STORAGE AND RELEASE OF NEUROHYPOPHYSEAL HORMONES

ADH

is

produced

primarily in the

supraoptic

nuclei,

while

oxytocin

is produced

primarily in the

paraventricular

nuclei.

Each

of these nuclei can

produce

about

1/6th

as much of the second hormone as of its primary hormone

.

The hormones are stored in the

secretory

granules found in the nerve endings.

When the stimuli for their release reach the nuclei, they are usually

released by mechanism

of

exocytosis

and

they are

absorbed into adjacent capillaries.

Chemical Structures of ADH and Oxytocin

ADH and

Oxytocin

are polypeptide hormones.

They contain nine amino acid residues.

Their amino acid sequences are the following:

Vasopressin

: Cys-Tyr-Phe-Gln-Asn-Cys-Pro-Arg-GlyNH

2

Oxytocin

: Cys-Tyr-Ile-Gln-Asn-Cys-Pro-Leu-GlyNH

2

The

two hormones are almost

similar

except that in vasopressin, phenylalanine and

arginine

replace

isoleucine

and

leucine

of the

oxytocin

molecule. The similarity of the molecules explains

why they have overlapping functions.

ACTIONS OF ANTI-DIURETIC HORMONE (ADH)

It causes retention of water by the kidney hence its name “anti-diuretic hormone.

It exerts a mild stimulatory influence upon smooth muscle and its action upon the walls of arterioles induces a rise in blood pressure. This

pressor

effect of the hormone resulted in the alternative name vasopressin. The hypertensive effect of the hormone occurs at pharmacological dose.

It decreases renal

medullary

blood flow.

It increases permeability of the collecting ducts to urea and possibly increases the rate at which Na ion is transported out of the ascending limb of

Henle’s

loop.

CONTROL OF ADH SECRETION

Many factors regulates ADH secretion. Among factors that increases its secretion are:

Increased plasma osmotic

presure

Decreased

extracelluar

fluid

volme

Severe exercise

Neurogenic

state such as pain, surgical stress and some emotions

Certain drugs

e.g

Nicotine, morphine and large doses of barbiturates

CONTROL OF ADH SECRETION CONT’D

Factors that decrease ADH secretion include:

Decreased plasma osmotic pressure

Increased

extracellular

fluid volume

Sympathetic effects

Alcohol

The

mechanism by which the

osmotic concentration of the

ECF

controls ADH secretion is not clear. Yet somewhere in or near the hypothalamus are modified neuron receptors called

osmoreceptors

.

When

the

ECF

becomes too concentrated, fluid is pulled by osmosis out of the

osmoreceptor

cell, decreasing its size and initiating appropriate nerve signals in the hypothalamus to cause additional ADH secretion.

Conversely

, when the

ECF becomes

too dilute, water moves by osmosis in the opposite direction, into the cell, and this decreases the signal for ADH secretion

.

ACTIONS OF OXYTOCIN

It assists in uterine contraction during parturition.

It may cause contractions of the non-pregnant uterus to facilitate sperm transport.

It aids

in

milk ejection

by the

breasts

.

It

also plays an

important role in

lactation.

In lactation,

oxytocin

causes

milk to be expressed from the alveoli into the ducts of the breast so that the baby can obtain it by suckling.

MECHANISM OF MILK LETDOWN

The suckling stimulus on the nipple of the breast causes signals to be

sent

through sensory nerves to the

oxytocin

neurons in the

paraventricular

and

supraoptic

nuclei in the hypothalamus, which causes release of

oxytocin

by the posterior pituitary gland.

The

oxytocin

is then

transported

by the blood to the breasts, where it causes contraction of

myoepithelial

cells

that lie outside of and form a latticework surrounding the alveoli of the mammary glands. In less than a minute after the beginning of suckling, milk begins to flow. This mechanism is called

milk letdown

or

milk ejection

.

REFERENCES

Guyton and Hall. Textbook of Medical Physiology. 9

th

Edition, W.B Saunders company, London,

TorontoPhiladelphia

.

Oyebola

D.O. (2002)Essential Physiology for students of

Medicin

e, dentistry, Pharmacy and Related disciplines. Volume 1.