Introduction to endocrinology

Dr. Shafali Singh

Coordinated functions

Nervous system Endocrinal system

Control system

Control systems vary in their speed & specificity– Specificity

• Nervous control very specific• Hormonal control more general

– Nature of signal• Nervous system uses both electrical and chemical• Endocrine system uses only chemicals

– Speed• Nervous system much faster, up to 120 m/sec• Endocrine system much slower

– Duration of action• Nervous system shorter, very brief• Endocrine system lasts longer

• Coordinating multiple activities of the cell, tissues and organs of the body.

• Maintain homeostasis• Regulating almost all body functions.

Chemical messenger system

Classic definition- hormones are secretory product of the ductless glands, which are released in catalytic amount into blood stream and transported to specific target cells(or organs),where they elicit physiological, morphological and biochemical responses.

hormone "to set in motion,"

What is the Endocrine System?

• Mutiple hormone system controls numerous body processes

•Numerous organs and tissues located throughout the body

• Works in conjunction with the nervous system

• growth and development• metabolism• reproduction• behavior• Water and electrolyte balance• homeostasisRegulating all most all body functions

• Proteins and polypeptides,

• Steroids

• Derivatives of the amino acid tyrosine,

Classification of hormones

1. Feed back control

2. Neural control

3. Chronotrophic control

Regulation of hormone secretion

1.Feedback control-

(i)Negative feedback control Gland

Hormone

Target

Product

• (ii) Positive feedback control

Gland

Hormone Target

Product

(-) (+)

Depending upon the product involved ,the feedback mechanism may be

Regulation of hormone secretion

Hormone -hormone feed back

control

Substrate hormone feed back control

Hormone Hormone feedback control

• A rise in blood glucose level promotes the secretion of insulin, while a fall in blood glucose promotes the secretion of glucagon

Substrate Hormone feedback control

• Act to evoke or suppress hormone secretion in response to both external and internal stimuli.

• External stimuli-visual, auditory, olfactory, gustatory and tactile

• Internal stimuli-pain, emotion, stress, fright, change in blood volume

2. Neural control

• Oscillating and pulsatile release of certain hormones

• Diurnal variation in hormonal levels• Menstural rhythm• Seasonal rhythm• Developmental rhythm

3. Chronotropic control

A circadian pacemaker directs numerous endocrine and body functions, each with its own daily profile. The nighttime rise in plasma melatonin may mediate certain other circadian patterns

Hypothalamus and Pituitary

Objectives

After the end of this lecture a student should be able to

• Name the hypothalamic hormones that regulate anterior pituitary function.

• Explain hypothalamo – hypophyseal portal system• Name the hormones that are released from anterior

pituitary• Name the hormones that are released from posterior

pituitary• Explain the functions of pituitary hormones

Hypothalamus and Pituitary

• The hypothalamus-pituitary unit is the most dominant portion of the entire endocrine system.

• The output of the hypothalamus-pituitary unit regulates the function of the thyroid, adrenal and reproductive glands and also controls somatic growth, lactation, milk secretion and water metabolism.

• Pituitary function depends on the hypothalamus and the anatomical organization of the hypothalamus-pituitary unit reflects this relationship.

Reituclar activating substance

Thalamus

neocortex

Limbic system

Optical system

Heat regulation (temperature)

Energy regulation (hunger,

BMI)

Autonomic regulation

(blood pressure etc)

Water balance (blood volume, intake--thirst, output—urine volume)

Metabolic rate, stress response, growth,

reproduction, lactation)

Sleep/wake

pain Emotion, fright, rage, smell vision

Anterior pituitary

hormonesposterior pituitary

hormones

Regulation of

Hypothalamus

Releasing Hormones

• secreted by hypothalamic neurons and transported to the anterior pituitary by the hypothalamic-hypophyseal portal system

• function as trophic hormones to either stimulate or inhibit release of anterior pituitary hormones

Hypothalamic releasing hormonesHypothalamic releasing hormone Effect on pituitary

Corticotropin releasing hormone (CRH) Stimulates ACTH secretion

Thyrotropin releasing hormone (TRH) Stimulates TSH and Prolactin secretion

Growth hormone releasing hormone (GHRH)

Stimulates GH secretion

Somatostatin Inhibits GH (and other hormone) secretion

Gonadotropin releasing hormone (GnRH) a.k.a LHRH

Stimulates LH and FSH secretion

Prolactin inhibiting hormone (dopamine)

Inhibits PRL secretion

endocrine axes have the following important features

1. The activity of a specific axis is normally maintained at a set point, which varies from individual to individual, usually within a normal range.

2. Hypothalamic hypophysiotropic neurons are often secreted in a pulsatile manner and are entrained to daily and seasonal rhythms through CNS input

3. Abnormally low or high levels of a peripheral hormone (e.g., thyroid hormone) may be due to a defect at the level of the peripheral endocrine gland (e.g., thyroid), the pituitary gland, or the hypothalamus. Such lesions are referred to as primary, secondary, and tertiary endocrine disorders, respectively.

Structure of the Pituitary Gland

Two distinct lobes:

• anterior pituitary (adenohypophysis; pars distalis)

• posterior pituitary (neurohypophysis; pars nervosa)

- true endocrine tissue- secretes classic hormones

- neural tissue- secretes neurohormones

Anterior pituitary cells and hormones

Cell type Pituitary population

Product Target

Corticotroph 15-20% ACTHb-lipotropin

Adrenal glandAdipocytesMelanocytes

Thyrotroph 3-5% TSH Thyroid gland

Gonadotroph 10-15% LH, FSH Gonads

Somatotroph 40-50% GH All tissues, liver

Lactotroph 10-15% PRL Breasts

Adrenocorticotropic hormone (ACTH; also called corticotropin)

The Lactotrope

• The lactotrope is not part of an endocrine axis. This means that PRL acts directly on nonendocrine cells (primarily of the breast) to induce physiological changes.

• Production and secretion of PRL are predominantly under inhibitory control by the hypothalamus.

• Thus, disruption of the pituitary stalk and the hypothalamohypophysial portal vessels (e.g., secondary to surgery or physical trauma) results in an increase in PRL levels but a decrease in ACTH, TSH, FSH, LH, and GH.

FSHandLH

GHRH CRHTRH PIHSS

AnteriorPituitary

GROWTH HORMONE

TSH PROLACTIN ACTH

Hypothalamus

GnRH

Hormones-estrogen, progesterone and testosterone

Other organs

IGF-1

Gonads Adrenal

cortex

Liver Thyroid Breasts

GrowthMetabolism

Germ cell development

DevelopmentMilk

CortisolThyroxineT3

Summary of Hypothalamic-anterior-pituitary system

Action of Growth Hormone

1. stimulates growth of bone, cartilage & connective tissue through somatomedins & other growth factors.

2. Growth Hormone Promotes Protein deposition in Tissues.

3. Growth Hormone Enhances Fat Utilization for Energy.

4. Growth Hormone Decreases Carbohydrate Utilization

Hypothalamic Control of Posterior Pituitary

• Hypothalamic neuron cell bodies produce:

ADH: supraoptic nuclei.Oxytocin: para ventricular nuclei.• Transported along

hypothalamo-hypophyseal tract.

• Stored in posterior pituitary.

• Release controlled by neuroendocrine reflexes.

ACTIONS OF ADH

1.ACTION ON KIDNEY Maintenance of ECF volume & Osmolarity Acts on DCT and CD of kidney Reabsorbs water Maintenance of volume more important that

maintenance of osmolarity2.VASOCONSTRICTOR EFFECT

3.ACTION ON ANTERIOR PITUITARY-cause increased ACTH secretion from the corticotroph

Actions of oxytocinIn females: Milk ejection Contraction of uterus during labor In females: during coitus causes uterine contraction and transport

of sperms

In Males: Increases at time of ejaculation May cause increase contraction of smooth muscle of vas deferens

- propelling sperm through urethra

IN THE CLINIC-- hypophysectomy (pituitary removal)

• Because posterior pituitary hormones are synthesized in the hypothalamus rather than the pituitary, hypophysectomy (pituitary removal) does not necessarily permanently disrupt synthesis and secretion of these hormones.

• Immediately after hypophysectomy, secretion of the hormones decreases.

• However, over a period of weeks, the severed proximal end of the tract will show histological modification and pituicytes will form around the neuron terminals. Secretory vacuoles are seen, and secretion of hormone resumes from this proximal end. Secretion of hormone can even potentially return to normal levels.

• In contrast, a lesion higher up on the pituitary stalk can lead to loss of neuronal cell bodies in the PVN and SON.