Prakash Kr. Yadav

Lecturer

Department of Physiology

Chitwan Medical College, Bharatpur.

Introduction to Endocrine Physiology

Biological communicationCommunication among cells, organisms &

between organisms & their environment is absolutely central to life & reproduction.

II. Endocrine vs. Nervous SysteII. Endocrine vs. Nervous Syste

Major communication systems in the bodyIntegrate stimuli and responses to changes

in external and internal environmentBoth are crucial to coordinated functions of

highly differentiated cells, tissues and organs

Unlike the nervous system, the endocrine system is anatomically discontinuous.

Nervous systemNervous system

•The nervous system exerts point-to-point control through nerves, similar to sending messages by conventional telephone.

•Nervous control is electrical in nature and fast.

Hormones travel via the bloodstream to target cellsHormones travel via the bloodstream to target cells

•The endocrine system broadcasts its hormonal messages to essentially all cells by secretion into blood and extracellular fluid.

•Like a radio broadcast, it requires a receiver to get the message –

•in the case of endocrine messages, cells must bear a receptor for the hormone being broadcast in order to respond.

Together, the nervous and endocrine systems coordinate functions of all body systems.

NERVOUS SYSTEM VS. ENDOCRINE SYSTEM

NERVOUS

neurotransmitters hormones

ENDOCRINE

muscle contractions and glandular secretions

metabolic activities , growth & development, reproduction

acts in milliseconds acts in seconds to minutes to hours to days to months

brief effects long-lasting effects

Coordination of body functions by chemical messengers

The multiple activities of the cells, tissues ,and organs of the body are coordinated by the interplay of several types of chemical messenger systems:

Neurotransmitters are released by axon terminals of neurons into the synaptic junctions and act locally to control nerve cell functions . E.g. Acetylcholine, epinephrine etc

Endocrine hormones are released by glands or specialized cells into the circulating blood and influence the function of cells at another location. E.g. thyroid hormones, insulin etc

Neuroendocrine hormones are secreted by neurons into the circulating blood and influence the function of cells at another location in the body . Oxytocin, ADH, hypothalamic releasing hormones.

Coordination of body functions by chemical messengers

Paracrines are secreted by cells into the extracellular fluid and effect neighboring cells of different type . e. g. GI hormones like histamine, gastrin, somatostatin etc

Autocrines are secreted by cells into the extracellular fluid and affect the function of the same cells that produced them. E.g. Interferon

Cytokines are peptides secreted by cells into the extracellular fluid and can function as autocrines ,paracrine ,or endocrine hormones (interleukins,and lymphokines) .

5. Neuroendocrine

Interactions between the nervous and endocrine systems emphasize their coordinated communicative function.

Introduction to endocrine glandsIntroduction to endocrine glands

Endocrine gland:Endocrine gland: A gland that secretes a chemical A gland that secretes a chemical substance (a hormone) into the bloodstream. The substance (a hormone) into the bloodstream. The endocrine glands are "glands of internal secretion." endocrine glands are "glands of internal secretion."

Endocrine Glands

Hormones:Specific chemical product secreted by

endocrine glands transported by blood stream to reach target cells to elicit specific response.

Help regulate total body metabolism, growth, internal environment and reproduction.

it exerts its effect at very low concentrations.

For a chemical substance to be a classical endocrine hormone , it must fulfill following characteristics:

Must be synthesized by one or more endocrinal glands;Must be released directly into the blood (or into lymphatic channel), without being laid by duct;Must have a very low blood concentration;Needs a stimulus for its secretion, andActs on distant target cells, containing hormone specific receptors.Must have a feedback control system ( regulation)

Receptors:Specific molecular groups of protein present at cell membrane or inside cell which bind with hormone specifically with high affinity.

Hormones and their receptor relationship is expressed – in terms of Affinity (bind to receptors with high bond strength).Capacity (quantity of hormone binding)

Hormones binds to the receptor with high specificity and affinity !!

Membrane Receptor Signal Pathways

• Signal molecule (ligand)• Receptor• Intracellular signal• Target protein• Response

Maintenance of growth & development – Growth hormone, Thyroxine, insulin, Glucocorticoid, & Gonadal hormones

Maintenance of internal environment – ADH, Mineralocorticoids, PTH

Regulation of energy balance and metabolism –Insulin, glucagon , Leptin & Ghrelin

Reproduction & species propagation – Gonadal & Pituitary hormones

General Functions of the Endocrine system

V. Properties of the hormone effect

1. Specificity

The special feature of the the target cells is the presence of receptors which can “attract” and interact with the hormone.

The receptors may be present either on the plasma membrane, or in the cytoplasm, or in the nucleus.

These receptor molecules are protein in nature and may contain carbohydrate or phospholipid moieties.

2. Signal TransmissionThe role of the hormones is to transit the regulatory signals from the control (endocrine) system to the target cells (organs or glands).

It could enhance or inhibit some function of the target.

3. High Biological EfficiencyLow plasma concentration (nmol – pmol/L) great regulatory function

Signal amplification during the transmembrane and intracellular transmission

4. Interaction Between the Hormones(1) Synergistic effects. When two or more hormones work together to produce particular result their effects are said to be synergistic.

These effects may be additive or complementary.

Additive: Same effect of the hormones on one target organ, for example, epinephrine and norepinephrine on the heart rate

Complementary: Work on different stages of a physiological procedure, for example, FSH (initiation) and testosterone (maintenance) on spermatogenesis

(2) Permissive effect. A hormone is said to have a permissive effect on the action of a second hormone when it enhances the responsiveness of a target organ to the second hormone or when it increases the activity of the second hormone.

Estrogen – Expression of progesterone receptors on uterus – progesterone effect on the uterus.Glucocorticoids – effects of catecholamines on cardiovascular system Thyroid hormones- effects of catecholamines on cardiovascular system

(3) Antagonist Effects. In some situations the actions of one hormone antagonize the effects of another. Lactation during pregnancy is prevented because the high concentration of estrogen in the blood inhibits the milk secretion and action of prolactin.

EFFECTS OF HORMONE ON TISSUE RESPONSE

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Receptors are dynamic components

Down–regulation : decrease in the number of target-cell receptors for a given hormone in response to a chronic high concentration of that hormone. Cells may display hyporesponsiveness (Desensitization).

Up–regulation : increase in the number of target-cell receptors for a given hormone in response to chronic low concentration of that hormone. Cells may display hyper responsiveness (Priming effect)

Chemical Regulating Systems: Summary

Hormones: cell to cell communication moleculesMade in gland(s) or cellsTransported by bloodDistant or local target tissue receptorsActivates physiological responseHelp regulate total body metabolism, growth,

internal environment and reproduction.

Types of cell-to-cell signaling (communication system)

1. NeuralNeuron NT synaptic cleft NT(neuron) excitable

or secretory cell

2. EndocrineEndocrine cell hormone blood hormone target cell

3. ParacrineSecretory cell secretion IF secretion adjacent cell

4. Autocrine

Secretory cell secretion IF secretion