4 - Mammalian endocrine system I: The hypothalamus and pituitary gland Flashcards
What are the endocrine glands?
- Hypothalamus.
- Pineal gland
- Pituitary gland.
- Thyroid gland.
- Parathyroid gland.
- Thymus.
- Adrenal glands.
- Pancreas.
- Ovary (female).
- Testes (male).
What is the Endocrine System?
• One of the body’s primary communicating, regulating, and coordinating systems.
• It works in conjunction with other body systems, such as the nervous, hepatic, and renal system.
• It helps to regulate and maintain:
1) Appropriate responses to the external environment, stress, and injury.
2) Body energy.
3) Growth and development.
4) Internal homeostasis.
5) Reproduction.
What are pathologies of the endocrine system?
- In general, when there are pathologies of the endocrine system, there is either hyposecretion or hypersecretion of hormones.
- The underlying pathology may be of the endocrine gland itself (primary) or from a source outside the gland (secondary).
- Endocrine hormones affect every body system with a great range of diversity.
- Skillful assessment of the endocrine system is necessary to detect endocrine disorders
Factors that can result in endocrine dysfunction
1) Autoimmune destruction.
2) Benign and malignant tumors.
3) Congenital or genetic defects.
4) Infectious destruction.
5) Surgery.
6) Traumatic injuries.
How do endocrine glands function?
- They secrete their hormones into the interstitial fluid which surrounds these glands.
- The hormones then diffuse into capillaries and blood carries them to their target tissues.
Exocrine glands
• Exocrine glands secrete substances into ducts emptying outside (exo) of the body or into the lumen of an organ
What is the hypothalamus?
- The hypothalamus is a very small structure in the brain that lies above the pituitary gland.
- It consists of numerous, poorly defined nuclei.
- It receives input from almost every area of the brain and plays a major role in linking the nervous and endocrine systems.
Functions of the hypothalamus
1) Secretion of regulatory hormones to control the activity of the anterior pituitary.
2) Control of sympathetic output to adrenal medullae.
3) Production of ADH & Oxytocin.
How is the pituitary gland formed?
- Two protrusions meet
- Extension of roof of mouth breaks off and attaches to the extension of the hypothalamus.
- Bone develops around to protect (sella turcica).
What is the pituitary gland (hypophysis)?
- It’s located at the base of the skull in the sella turcica of the sphenoid bone.
- The sphenoid bone also separates the pituitary gland from the oral cavity.
- The hypophysis secretes seven different hormones and is divided into two lobes: posterior and anterior
Posterior pituitary (Neurohypophysis)
- An extension of the hypothalamus.
- Cell bodies of supraoptic and paraventricular neurosecretory cells release oxytocin & ADH.
- ADH is a peptide that promotes retention of water by the kidneys and is regulated by a salt/water balance.
- Oxytocin is a peptide that stimulates contraction of the uterus and mammary gland cells; it’s regulated by the Nervous system.
Anterior pituitary (Adenohypophysis)
- Production and release of the anterior pituitary hormones are regulated by the hypothalamus.
- Releasing and inhibiting factors are transmitted via the blood to the anterior pituitary: CRH, GnRH,GHRH, PRH, TRH, MSH, GHIH, PIH
Corticotropin-Releasing Hormone (CRH)
- stimulates the secretion of Adrenocorticotropic Hormone (ACTH), which is a peptide that stimulates the adrenal cortex to secrete glucocorticoids; regulated by hypothalamic hormones.
Gonadotropin-Releasing Hormone (GnRH)
stimulates the secretion of:
- Follicle-Stimulating Hormones (FSH) - a glycoprotein that stimulates ova/sperm production
- Luteinizing Hormone (LH) - a glycoprotein that stimulates ovaries/testes; regulated by hypothalamic hormones.
Growth Hormone-Releasing Hormone (GHRH)
stimulates secretion of Human Growth Hormone (GH), which is a protein that stimulates growth (especially bones) and metabolic functions; regulated by hypothalamic hormones
Prolactin-Releasing Hormone (PRH)
stimulates secretion of Prolactin (PRL), a peptide that stimulates milk production & secretion; regulated by hypothalamic hormones.
Thyrotropin-Releasing Hormone (TRH)
stimulates secretion of Thyroid-Stimulating Hormone (TSH), a glycoprotein that stimulates the thyroid; regulated by thyroxine in blood and hypothalamic hormones
Melanocyte stimulating hormone (MSH
regulates pigment containing cells, e.g. amphibians, fish, reptiles & some mammals; in mammals it acts on neurons to inhibits hunger.
Growth Hormone-Inhibiting Hormone (GHIH)
suppresses secretion of GH
Prolactin-Inhibiting Hormone (PIH
suppresses secretion of PRL.
How does regulation by the hypothalamus & pituitary generally occur?
- Releasing hormone (RH) causes Hormone 1 release from anterior pituitary.
- Hormone 1 causes Hormone 2 release from target endocrine organ.
- Hormone 2 inhibits release of RH and Hormone 1.
- Hormone 2 has effect upon target cells.
What is the circulatory system?
- An organ system that consists of the heart, the blood vessels and the blood.
- Its function is to transport materials around the body.
What is the hypophyseal portal system?
- System of blood vessels in the brain that connects the hypothalamus with the anterior pituitary.
- Enables communication from the hypothalamus to the anterior pituitary gland.
- Allows regulatory hormones to be transported from the hypothalamus to the adenohypophysis.
How MSH & Ghrelin regulate appetite?
- Corticotroph cells of the anterior pituitary secrete melanocyte-stimulating hormone (MSH).
- MSH produces melanin, the pigment that gives color to the skin, hair, substantia nigra of the brain, and the choroid of the eye.
- MSH receptors are present in the brain and influence brain activity.
- POMC is processed into MSH & released in pars-intermedia to stimulate hypothalamic neurons and reduce appetite.
- Ghrelin is released by the stomach to stimulate hypothalamic neurons in ARC to increase appetite.
What is a tropic hormone?
• A hormone that has an endocrine gland as its target and controls the release of other hormones.
What is TSH?
- TRH controls release of TSH (a Glycoprotein)
- TSH stimulates release of T3 and T4 from the thyroid
- Thyroid hormones T3 and T4, inhibit release of TRH and TSH(negative feedback)
Adrenocorticotropic hormone (ACTH)
- ACTH is a peptide
- CRH causes ACTH release from anterior pituitary
- ACTH stimulates adrenal cortex to release glucocorticoids
- Glucocorticoids have negative feedback effect on CRH and ACTH
Follicle Stimulating Hormone (FSH) and Luteinising Hormone (LH) (gonadotropins)
- FSH and LH are glycoproteins
- Gonadotropin Releasing Hormone (GnRH) controls FSH and LH production.
- Promote egg and sperm production and secretion of sex steroids
- Inhibin inhibits FSH production (both sexes)
- Inhibin may inhibit GnRH release
Prolactin (mammotropin)
- Prolactin is a peptide
- Prolactin release is stimulated by prolactin releasing factor (PRF) and inhibited by prolactin inhibiting hormone (PIH, dopamine)
- Stimulates milk production
Growth hormone (GH) (somatotropin)
- GH is a peptide
- GH release stimulated by growth hormone-releasing hormone (GHRH)
- GH release inhibited by growth hormone-inhibitory hormone (GHIH) (somatostatin)
- Stimulates somatomedin (Insulinlike Growth Factor - IGF) production
- Stimulates bone and cartilage growth; fat and glycogen breakdown, increasing blood glucose levels
Growth hormone abnormalities
- Pituitary Gigantism – excess GH before puberty
- Acromegaly – excess GH after puberty
- Pituitary growth failure – lack of GH
Acromegaly
excess GH after puberty; bones of hands, feet, cheeks and jaws thicken.
Pituitary growth failure
lack of GH
Nephron function
- ADH binds receptors in the DCT
- Increases expression of the Aquaporin-2 channel and its insertion in the membrane of DCT cells = Increase in permeability = water retention
How does ADH function?
- ADH binds receptors in the DCT
- This increases the expression of the Aquaporin-2 channel & its insertion in DCT cell membranes.
- This causes an increase in permeability, which leads to water retention.
How does Oxytocin function?
• It is a peptide that stimulates milk ejection by mammary glands and uterine contractions during childbirth via:
1) Contraction of the upper part of the uterus and relaxation of the lower part of the uterus.
2) Intermittent contractions. - It targets the brain, influencing behaviour e.g. pair bonding, maternal care and sexual activity.
