Hormon

Endocrine System

Learning Objectives: - Differentiate between exocrine and endocrine glands. - Describe the structure of the thyroid gland. - Explain how thyroxine is produced, regulated and state its functions. - Differentiate the symptoms of hypothyroidism and hyperthyroidism.

• Overview: The Body’s Long-Distance Regulators • An animal hormone – Is a chemical signal that is secreted into the circulatory system and communicates regulatory messages within the body • Hormones may reach all parts of the body – But only certain types of cells, target cells, are equipped to respond • Exocrine glands release their secretion into ducts. • Endocrine glands release their secretion, hormones into the bloods

• Insect metamorphosis – Is regulated by hormones

• The endocrine system and the nervous system act individually and together in regulating an animal’s physiology • A gland which is both endocrine gland exocrine gland is pancreas. • Animals have two systems of internal communication and regulation – The nervous system and the endocrine system

The main endocrine glands in the human body PITUITARY GLAND

THYROID GLAND

PANCREAS

TESTES

OVARIES

• The nervous system – Conveys high-speed electrical signals along specialized cells called neurons • The endocrine system, made up of endocrine glands – Secretes hormones that coordinate slower but longeracting responses to stimuli via the bloodstream

• The endocrine and nervous systems – Often function together in maintaining homeostasis, development, and reproduction

• Specialized nerve cells known as neurosecretory cells – Release neurohormones into the blood • Both endocrine hormones and neurohormones – Function as long-distance regulators of many physiological processes

• A common feature of control pathways – Is a feedback loop connecting the response to the initial stimulus • Negative feedback – Regulates many hormonal pathways involved in homeostasis – High concentration of a hormone or some other substance inhibits further secretion.

• Four major classes of molecules function as hormones in vertebrates – – – –

Proteins and peptides (ADH and TSH) Amines (from tyrosine) derived from amino acids Steroids (from cholesterol) Prostaglandins (derivates from fatty acids)

• Signaling by any of these molecules involves three key events – Reception – Signal transduction – Response

Endocrine Disorders • Hyposecretion – abnormally reduced output of hormones • Hypersecretion – abnormally increased output of hormones

The thyroid gland • Located in the neck. Consists of two lobes located on the ventral surface of the trachea • Secretes thyroxine, which contains iodine. • The secretory cells form follicles, in which inactive precursor of the hormone, thyroglobulin, is stored before being converted into thyroxine and released into bloodstream. • Numerous capillaries lie between the follicles, their thin walls in close contact with the thyroid cells.

• The hypothalamus and anterior pituitary – Control the secretion of thyroid hormones through two negative feedback loops Hypothalamus

Anterior pituitary

TSH

Thyroid

T3 +

T4

How thyroxine is produced? - Thyroid cells takes up tyrosine and iodide ions from the blood by active transport. - Once inside the thyroid cells, iodide is oxidized into iodine. - Iodine is incorporated into thyroglobulin. - Thyroglobulin is transferred by exocytosis into the follicle and stored. - When the body needs thyroxine, thyroglobulin is taken up by endocytosis into the thyroid cells and is converted into thyroxine and secreted into the bloodstream when instructed by the TSH (Thyroid Stimulating Hormone). Thyroid cells contains a protease which removes the conjugated protein, thus allowing the free hormone to be released.

Functions of thyroxine - Thyroxine controls the basal metabolic rate (BMR), and is therefore important in growth. - If undersecretion – hypothyrodism. - Causes: damaged thyroid gland, iodine deficiency - If occurs during teenagers – patient is physically and mentally retarded – called cretinism - If occurs in adult – effect is not serious because the growth has completed – called myxoedema. Symptoms are decreased metabolic rate, increased subcutaneous fat, skin is coarse, physical and mental sluggishness.

- If oversecretion called hyperthyroidism. - Causes: thyroid tumour - Symptoms are exophthalmos (bulging of the eye anteriorly), goitre, increased metabolic rate, thin (loss of body fat), heart rate increased, physical and mental restlessness. • The thyroid gland also produces calcitonin –Which functions in calcium homeostasis

• Hyperthyroidism, excessive secretion of thyroid hormones – Can cause Graves’ disease in humans

Primary hypothyroidism

Cretinism

The control of thyroxine production • The secretion of thyroxine into the bloodstream is triggered by TSH. • TSH is produced by pituitary gland (anterior lobe). Production of TSH is regulated by throxine. • If thyroxine in blood decreased, it stimulates the pituitary to produce TSH ------ TSH in the blood increased ------ thyroid activity increases ----- more thyroxine produced. • If thyroxine increases in the blood ---- it inhibits the pituitary to produce TSH ---- TSH in the blood reduced ------ thyroid activity reduces ---- less thyroxine produced.

Learning Objectives: - Describe the structure and importance of the pituitary gland. - State the hormones produced by the organs in the human body and its functions. - Explain the model of hormonal action for steroid and peptide hormones.

Cell-Surface Receptors for Water-Soluble Hormones • The receptors for most water-soluble hormones – Are embedded in the plasma membrane, projecting outward from the cell surface

SECRETORY CELL

Hormone molecule VIA BLOOD Signal receptor

TARGET CELL Signal transduction pathway

Cytoplasmic response

(a) Receptor in plasma membrane

OR

DNA Nuclear response

NUCLEUS

• Binding of a hormone to its receptor – Initiates a signal transduction pathway leading to specific responses in the cytoplasm or a change in gene expression

Intracellular Receptors for Lipid-Soluble Hormones • Steroids, thyroid hormones, and the hormonal form of vitamin D – Enter target cells and bind to specific protein receptors in the cytoplasm or nucleus

• The protein-receptor complexes – Then act as transcription factors in the nucleus, regulating transcription of specific genes SECRETORY CELL

Hormone molecule VIA BLOOD

TARGET CELL

Signal receptor DNA

Signal transduction and response

mRNA NUCLEUS

(b) Receptor in cell nucleus

Synthesis of specific proteins

• The hypothalamus and the pituitary gland – Control much of the endocrine system

• The major human endocrine glands Hypothalamus Pineal gland Pituitary gland Thyroid gland Parathyroid glands

Adrenal glands Pancreas Ovary (female) Testis (male)

• Major human endocrine glands and some of their hormones

Relation Between the Hypothalamus and Pituitary Gland • The hypothalamus, a region of the lower brain – Contains different sets of neurosecretory cells

• The hypothalamus is connected to the anterior lobe via the portal blood vessels. • Portal blood vessel is one with capillaries at both ends. • When the hypothalamus is stimulated, neurosecretory cells secrete the ‘releasing hormones’ into the portal vessels. • The ‘releasing hormones’ are carried to the anterior lobe and regulate the secretion of various hormones. • An example of ‘releasing hormones’ is the thyroid releasing hormone (TRH) which regulates the TSH (Thyroid Stimulating Hormone).

• The hypothalamus is connected to the posterior lobe by neurones. • Neurons carry hormones from the hypothalamus to the posterior lobe and stored in the nerve terminals. • When appropriate the neurons transmit impulses to the posterior lobe causing the hormones to be released into the blood. • This is called the neurosecretion.

• Some of these cells produce direct-acting hormones – That are stored in and released from the posterior pituitary, or neurohypophysis

Hypothalamus

Neurosecretory cells of the hypothalamus

Axon

Posterior pituitary

HORMONE

TARGET

Anterior pituitary

ADH

Kidney tubules

Oxytocin Mammary glands, uterine muscles

• Other hypothalamic cells produce tropic hormones – That are secreted into the blood and transported to the anterior pituitary or adenohypophysis Tropic Effects Only FSH, follicle-stimulating hormone LH, luteinizing hormone TSH, thyroid-stimulating hormone ACTH, adrenocorticotropic hormone

Neurosecretory cells of the hypothalamus

Nontropic Effects Only Prolactin MSH, melanocyte-stimulating hormone Endorphin

Portal vessels

Nontropic and Tropic Effects Growth hormone Hypothalamic releasing hormones (red dots)

HORMONE

TARGET

FSH and LH

Testes or ovaries

TSH

Thyroid

Endocrine cells of the anterior pituitary Pituitary hormones (blue dots)

ACTH

Prolactin

MSH

Endorphin

Adrenal cortex

Mammary glands

Melanocytes

Pain receptors in the brain

Growth hormone

Liver

Bones

• The anterior pituitary – Is a true-endocrine gland • The tropic hormones of the hypothalamus – Control release of hormones from the anterior pituitary

Posterior Pituitary Hormones • The two hormones released from the posterior pituitary – Act directly on nonendocrine tissues

• Oxytocin – Induces uterine contractions and milk ejection • Antidiuretic hormone (ADH) – Enhances water reabsorption in the kidneys

Anterior Pituitary Hormones • The anterior pituitary – Produces both tropic and nontropic hormones

Tropic Hormones - are hormones that regulate the activity of various other endocrine glands. • The four strictly tropic hormones are – – – –

Follicle-stimulating hormone (FSH) Luteinizing hormone (LH) Thyroid-stimulating hormone (TSH) Adrenocorticotropic hormone (ACTH)

• Each tropic hormone acts on its target endocrine tissue – To stimulate release of hormone(s) with direct metabolic or developmental effects

Nontropic Hormones - are hormones that directly stimulate target cells to induce effects. • The nontropic hormones produced by the anterior pituitary include – Prolactin – Melanocyte-stimulating hormone (MSH)

• Prolactin stimulates lactation in mammals – But has diverse effects in different vertebrates • MSH influences skin pigmentation in some vertebrates – And fat metabolism in mammals

Growth Hormone • Growth hormone (GH) – Promotes growth directly and has diverse metabolic effects – Stimulates the production of growth factors by other tissues

• Nonpituitary hormones help regulate metabolism, homeostasis, development, and behavior • Many nonpituitary hormones – Regulate various functions in the body

Insulin and Glucagon: Control of Blood Glucose • Two types of cells in the pancreas – Secrete insulin and glucagon, antagonistic hormones that help maintain glucose homeostasis and are found in clusters in the islets of Langerhans

Adrenal Hormones: Response to Stress • The adrenal glands – Are adjacent to the kidneys – Are actually made up of two glands: the adrenal medulla and the adrenal cortex

Stress Hormones from the Adrenal Cortex • Hormones from the adrenal cortex – Also function in the body’s response to stress – Fall into three classes of steroid hormones

Gonadal Sex Hormones • The gonads—testes and ovaries – Produce most of the body’s sex hormones: androgens, estrogens, and progestins

• The testes primarily synthesize androgens, the main one being testosterone – Which stimulate the development and maintenance of the male reproductive system

• Testosterone causes an increase in muscle and bone mass – And is often taken as a supplement to cause muscle growth, which carries many health risks

• Estrogens – Are responsible for the maintenance of the female reproductive system and the development of female secondary sex characteristics • In mammals, progestins, which include progesterone – Are primarily involved in preparing and maintaining the uterus

How hormones control cells? • The model of hormonal action – 1st model • The model of hormonal action – 2nd model

The model of hormonal action – 1st model • What happens when a hormone reaches the target cell? • The hormone binds to a receptor on the cell membrane. • This activates the adenyl cyclase (AC) which catalyses the conversion of ATP to cAMP. • cAMP activates specific enzymes in the cytoplasm which bring about the appropriate response within the cell.

- cAMP acts as a second messenger. • The extent of the response depends on the [cAMP]. • [cAMP] depends on the balance between AC (stimulates synthesis) and phosphodiesterase (inhibit synthesis). • Example of hormones that use cAMP as a second messenger are ADH, adrenalin, glucagons.

The model of hormonal action – 2nd model • Thyroxine and steroid hormones are fat soluble so they can diffuse through the cell membrane. • In the cytoplasm, the hormone binds to a specific receptor, R which carries it into the nucleus. • In the nucleus, the hormone activates appropriate genes in the DNA. • The activated genes direct the mRNA transcription and then the synthesis of enzyme (protein synthesis) which brings about the appropriate response.

Puget Sound King Crab. Color changes are caused by changes in pigment cells, a process regulated by hormones.

The End….