Endocrine System Dr. Michael P. Gillespie
Mediator Molecules in Nervous & Endocrine Systems • The nervous system utilizes neurotransmitters to control body functions. • The endocrine system utilizes hormones to control body functions.
Site Of Mediator Action In Nervous & Endocrine Systems • The neurotransmitters perform their action close to the site of release. • The hormones usually perform their action far from their site of release.
Types Of Target Cells In Nervous & Endocrine Systems • The nervous system acts upon muscle cells (smooth, cardiac, and skeletal), glands, and other neurons. • The endocrine system acts upon virtually all cells of the body.
Time To Onset Of Action In Nervous & Endocrine Systems • In the nervous system, action typically occurs within milliseconds of neurotransmitter release. • In the endocrine system, action can take seconds to days to occur after release of the hormone.
Duration Of Action In Nervous & Endocrine Systems • The actions tend to be briefer in duration in the nervous system and longer in duration in the endocrine system.
Hormones • A hormone is a mediator molecule that is released in one part of the body but regulates activity of cells in other parts of the body. • Most hormones enter the interstitial fluid and then the bloodstream. • Hormones travel through the bloodstream to cells throughout the body. • Several neurotransmitters are also hormones (i.e. norepinephrine).
Functions Of Hormones • Help regulate: • Chemical composition and volume of the internal environment (interstitial fluid). • Metabolism and energy balance. • Contraction of smooth and cardiac muscle fibers. • Glandular secretions. • Some immune system activities.
Functions Of Hormones • Control growth and development. • Regulate operation of reproductive systems. • Help establish circadian rhythms.
“Supersystem” • The nervous and endocrine systems function together. • Parts of the nervous system stimulate or inhibit the release of hormones. • Hormones can promote or inhibit the release of nerve impulses.
Exocrine Glands Versus Endocrine Glands • Exocrine glands (Exo = outside) – secrete their products into ducts that carry secretions into body cavities, into the lumen of an organ, or to the outer surface of the body. • Endocrine glands – secrete their hormones into the interstitial fluid surrounding the secretory cells.
Exocrine Glands Versus Endocrine Glands • Exocrine glands. • Sudoriferous (sweat). • Sebaceous (oil). • Mucous. • Digestive.
Exocrine Glands Versus Endocrine Glands • Endocrine glands. • Pituitary. • Thyroid. • Parathyroid. • Adrenal. • Pineal. • Other organs that secrete hormones: • Hypothalamus, thymus, pancreas, ovaries, testes, kidneys, stomach, liver, SI, skin, heart, adipose tissue, & placenta.
Endocrine System • All endocrine glands and hormone secreting cells comprise the endocrine system. • Endocrinology (-logy = study of) is the study of the science of endocrine glands, function of endocrine glands, diagnosis of endocrine disorders and treatment of endocrine disorders.
Hormone Receptors • Hormones bind with specific receptors. • Only target cells for a given hormone have specific receptors that bind and recognize that hormone. • A target cell can have anywhere between 2000 and 100,000 receptors for a particular hormone. • Receptors are constantly being synthesized and broken down to meet the needs of the body.
Down-regulation • If a hormone is present in excess, the number of target cell receptors may decrease. • Down-regulation decreases the responsiveness of the target cell to the hormone.
Up-regulation • When a hormone (or neurotransmitter) is deficient, the number of receptors may increase. • Up-regulation makes a target cell more receptive to a specific hormone.
Synthetic Hormones • Synthetic hormones in the form of drugs can block the receptors from naturally occurring hormones.
Circulating & Local Hormones • Circulating hormones – pass from secretory cells that make them into the interstitial fluid and then into the blood. • Most hormones are of this type. • Local hormones – act locally on neighboring cells or on the same cells that secreted them without first entering the bloodstream.
Local Hormones • Paracrines – (para = beside or near) act on neighboring cells. • Autocrines – (auto – self) act on the same cell that secreted them.
Example Of A Local Hormone • Interleukin 2 (IL-2) is an example of a local hormone. • It is released by helper T cells during immune responses. • It acts on nearby immune cells (paracrine) and on itself (autocrine). • This generates more helper T cells and boosts the immune response.
Duration of Local Versus Circulating Hormones • Local hormones usually are inactivated quickly. • Circulating hormones linger longer. • The liver eventually deactivates circulating hormones and the kidneys excrete them.
Chemical Classes Of Hormones • Lipid-soluble hormones. • Steroid hormones. • Thyroid hormones. • Nitric oxide (NO).
Chemical Classes Of Hormones • Water-soluble hormones. • Amine hormones. • Peptide hormones and protein hormones. • Eicosanoid hormones. • Prostaglandins. • Leukotrienes.
Hormone Transport In Blood • Most water-soluble hormones are transported in their “free” form (not attached to plasma proteins). • Most lipid-soluble hormones are bound to transport proteins.
Hormone Receptors • Lipid-soluble hormones – the receptors are located inside the target cells. • Water-soluble hormones – the receptors are located within the plasma membrane of the target cells.
Action Of Lipid-soluble Hormones • Lipid soluble hormones turn specific genes of the nuclear DNA on or off. • This directs the synthesis of a new protein (often an enzyme). • These new proteins alter the cells activity.
Action Of Water-soluble Hormones • Water soluble hormones are the first messenger. They activate the second messenger i.e. cyclic AMP (cAMP). • This initiates a cascade of events within the cell that produces millions of enzymes to catalyze reactions. • Phosphodiesterase inactivates cAMP.
Summary of Hormones By Class • Refer to Table 18.2
Responsiveness Of The Target Cell • The responsiveness of the target cell depends upon the following: • The hormone’s concentration. • The number of the hormone receptors on the target cell. • Influences exerted by other hormones.
Influences Of Other Hormones • Permissive effect – the action of a 2nd hormone is required for the 1st hormone to take effect. • Thyroid hormones (2nd) allow epinephrine to stimulate lipolysis.
Influences Of Other Hormones • Synergistic effect – the sum of the actions of the 2 hormones is greater than either hormone individually. • Estrogens and FSH promote development of oocytes. • Antagonistic effect – one hormone opposes the actions of another. • Insulin promotes synthesis of glycogen and glucagon stimulates breakdown of glycogen.
Control Of Hormonal Secretion • Hormone secretion is regulated by: • Signals from the nervous system. • Chemical changes in the blood. • Other hormones.
Hypothalamus • Serves as a major integrating link between the nervous system and the endocrine system. • Painful, stressful, and emotional experiences cause changes in hypothalamic activity. • Synthesizes at least 9 different hormones. • Regulates the pituitary gland.
Pituitary Gland (hypophysis) • Synthesizes at least 7 different hormones. • Release of anterior pituitary hormones is stimulated by releasing hormones and suppressed by inhibiting hormones from the hypothalamus. • Attaches to the hypothalamus by a stalk, the infundibulum (= a funnel).
Types Of Anterior Pituitary Cells & Their Hormones • Somatotrophs – secretes human growth hormone (hGH) or somatotropin, which stimulates tissues to secrete insulinlike growth factors (IGFs). • Thyrotrophs – secrete thyroid-stimulating hormone (TSH) or thyroptropin.
Types Of Anterior Pituitary Cells & Their Hormones • Gonadotrophs – secrete follicle-stimulating hormone (FSH) and luteinizing hormone (LH) which act on the gonads. • They stimulate the secretion of estrogen and progesterone and the maturation of oocytes in the ovaries. • They stimulate the secretion of testosterone and sperm production in the testes.
Types Of Anterior Pituitary Cells & Their Hormones • Lactotrophs – secrete prolactin (PRL), which initiates milk production. • Corticotrophs – secrete adrenocorticotropic hormone (ACTH) or corticotropin, which stimulates the adrenal cortex to secrete glucocorticoids.
Tropic Hormones (tropins) • Hormones that influence another gland are called tropic hormones or tropins.
Control Of Secretion By The Anterior Pituitary (adenohypophysis) • The hypothalamus secretes five releasing hormones and two inhibiting hormones. • Negative feedback loops from hormones released from target glands decrease the release from the anterior pituitary gland.
Human Growth Hormone & Insulinlike Growth Factors • The main function of hGH is to promote synthesis of IGFs. • IGFs cause cells to grow and multiply. • They help to maintain the mass of muscles and bones. • They promote healing of injuries and tissue repair. • They enhance lypolysis in adipose tissue.