Chapter 45 – The Endocrine System Sara Aghaee Alia McKean Frank Chang Michael Kwett

Overview and Introduction Animals have two systems of internal communication and regulation 1) Nervous system; Conveys high-speed electrical signals along specialized cells called neurons 2) Endocrine system; All an animal’s hormone secreting cells constitute this system But.... WHAT IS A ???!!!! Hormone: a chemical signal that is secreted into the extracellular fluid, and communicates regulatory messages within the body. HORMONE

Overlapping of the Endocrine and Nervous Systems Several chemicals serve both as hormones in the endocrine system and as chemical signals in the nervous system. Examples Neurosecretory cells: specialized nerve cells that release hormones into the blood by extracellular fluid. Epinephrine: functions in the vertebrate body as “fight-or-flight” hormone AND as neurotransmitter, a local chemical signal that conveys messages in the nervous system. Lines between these two regulatory systems is blurred

Control Pathways and Feedback Loops Concepts to Remember!!! Receptor (or sensor) ~ detects stimulus Control center~ area where information is received After comparing information to a set point, control center sends out signal that directs an effector to respond In endocrine and neuroendocrine pathways this signal, efferent signal, is a hormone (or neurohormone), which acts on particular effector tissues and elicits specific physiological or developmental changes A common feature of control pathways is feedback loop. Negative feedback: effector response reduces in initial stimulus, and eventually response ceases Positive feedback: reinforces the stimulus and leads to an even greater response artist’s impression of a feedback loop 

Chemical Signaling  Hormones: long-distance chemical regulators; relay information to target cells  Amines (water-soluble)  Protein/peptides (water-soluble)  Steroids (lipid-soluble)  Pheromones: chemical signals carried between different organisms in a species  Local regulators: carry information between neighboring cells

Overview of Major Events in Hormone Signaling  Reception  A signal molecule binds to a specific receptor either on the cell surface or within the target cell  Signal Transduction  The events triggered by reception of the signal molecule  Response  A change in the target cell’s behavior in response to the signal transduction pathway; sometimes occurs in nucleus or cytoplasm

Types of Receptors and Examples  G-protein (Guanine-protein) linked receptors  G-protein activation (leads to binding of GTP molecule in the place of GDP — allows interaction of protein with target cell)  Gated Ion-channel receptors — channels that open and close in response to signals and specific molecules (ligands)  Neurotransmitters and muscle cells, action potentials  Tyrosine-kinase receptors  Catalyze transfer of phosphate groups from ATP to the amino acid tyrosine  activation, responses, multiple new proteins & pathways

Cell-surface Receptors for Water-soluble Molecules  Receptors are bound in plasma membrane, projecting outwards  When the signal molecule binds to a receptor, a signal transduction pathway is produced  This causes changes in cellular proteins converting chemicals into specific responses, activation of enzymes, secretion of molecules, rearrangement of the cytoskeleton, and regulation of transcription of genes in the nucleus of the target cell.  Hormones can produce different responses in contact with various cell-surface receptors.  Example: Multiple effects of epinephrine including decreased blood flow to digestive tract and increased glucose to skeletal muscles

Receptors for Lipid-soluble Molecules  Receptors are intracellular, either in the nucleus or in the cytoplasm, and they bind molecules traveling via the bloodstream.  They transduce signals inside the target cell.  The hormone-receptor complex binds to DNA, initiating gene transcription and expression  new proteins in cytoplasm  Again, the hormones produce different responses in contact with various target cell receptors.

Paracrine Signaling by Local Regulators  Local Regulator: send rapid messages between neighboring cells, are faster than hormones  Some have cell-surface receptors, others are intracellular  Types: neurotransmitters, cytokines (immune system), growth factors (cell proliferation, nitric oxide, and prostaglandins  Nitric Oxide  When oxygen levels in blood fall, blood vessels release NO  NO dilates the blood vessels, increasing blood flow  Oxygen levels increase again  Prostaglandins  Have a wide variety of functions in the body, including contraction of the reproductive tract, inducing labor, intensifying pain, and the accumulation of platelets during clotting

Relationship Between Hypothalamus and Pituitary Gland The hypothalamus receives information from nerves throughout the body and from other parts of the brain, initiating endocrine signals appropriate to environmental conditions. It contains different sets of neurosecretory cells, some producing direct-action hormones that are stored in and released from the posterior pituitary. Other hypothalamic cells produce tropic hormones that are transported by portal blood vessels to the anterior pituitary, an endocrine gland. These tropic hormones 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. Antidiuretic Hormone (ADH)

Anterior Pituitary Hormones Both tropic and nontropic hormones are produced by the anterior pituitary. The four strictly tropic hormones are thyroid- stimulating hormone (TSH), follicle-stimulating hormone (FSH), luteinizing hormone (LH), and adrenocorticotropic hormone (ACTH). Each acts on its target endocrine tissue to stimulate release of hormone(s) with direct metabolic or developmental effects. Prolactin, melanocyte- stimulating hormone (MSH), and β-endorphin are nontropic anterior pituitary hormones. Prolactin stimulates lactation in mammals but has diverse effects in different vertebrates. MSH influences skin pigmentation in some vertebrates and fat metabolism in mammals. Endorphins inhibit the perception of pain. Growth hormone (GH) promotes growth directly and has diverse metabolic effects; it also stimulates the production of growth factors by other tissues (a tropic effect).

Nonpituitary Hormones regulate: -metabolism -homeostasis -development -behavior

Thyroid Hormones -The thyroid gland consists of 2 lobes located on the ventral surface of the trachea. -This gland produces two similar hormones synthesized from the amino acid tyrosine: triiodothyronine (T 3 ) and tetraiodothyronine (T 4 ). -These hormones are responsible for vertebrate development and maturation such as skeletal growth, mental development, and the metamorphosis of a tadpole into a frog. -Goiter occurs when there is an iodine deficiency.

Parathyroid Hormone and Calcitonin: Control of blood calcium levels -The amount of calcium ions in the blood is essential in regulating skeletal muscles. The usual calcium level is maintained at 10mg/100mL. -When blood calcium level falls, the parathyroid glands, embedded in the surface of the thyroid, secrete parathyroid hormone (PTH) to increase release and absorption of calcium ions. -A rise in blood calcium level promotes the thyroid gland to release calcitonin, which reduces the release and uptake of calcium ions. -PTH stimulates osteoclasts in the bone to release calcium into blood. -PTH also directly stimulates uptake of calcium in kidneys. -PTH indirectly allows kidneys to convert vitamin D into its active hormonal form, which stimulate uptake of calcium in the intestines.

Insulin and Glucagon: Control of Blood Glucose Levels -The pancreas is considered both an endocrine and exocrine gland with important functions in endocrine and digestive systems. -Endocrine cells called islets of Langerhans are scattered throughout the tissues of the pancreas. -Each islet contains populations of either alpha or beta cells. -When blood glucose level rises, beta cells release the hormone insulin, which stimulates all body cells to take up glucose from blood and slows glycogen breakdown in the liver. -As blood glucose level drops, alpha cells release the hormone glucagon signaling liver cells to increase glycogen hydrolysis, convert amino acids and glycerol into glucose, and release glucose into the blood. -Glucose is a major fuel for cellular respiration and a source of carbon skeletons for the synthesis of organic compounds. The recommended blood glucose level is set at about 90mg/100mL.

Adrenal Hormones: Response to Stress -The adrenal glands are adjacent to the kidneys and each adrenal gland are made of the adrenal cortex, or outer portion, and adrenal medulla, or the central portion. -The adrenal medulla releases hormones called catecholamines synthesized from tyrosine. Among the catecholamines are epinephrine and nonepinephrine, which are secreted due to short-term stress response. -The adrenal cortex synthesizes and releases a family of steroids called corticosteroids. Glucocorticoids and mineralocorticoids are released as a result of long-term stress response. -The adrenal medulla is stimulated by nerve signals while the adrenal cortex is stimulated by ACTH from the blood vessels.

Gonadal Sex Hormones -The gonads produce and secrete three major categories of steroid hormones: androgens, estrogens, and progestins. -Estrogens, such as estradiol, are responsible for the development and maintenance of the female reproductive system. -Androgens, especially testosterone, are synthesized by the testes and they stimulate the development and maintenance of the male reproductive system. -Progestins are involved in preparing and maintaining the uterus, which supports growth and development of an embryo. -Both of the gonadotropic hormones (FSH and LH) in males and females stimulate activities of the gonads and control the synthesis of steroid hormones.

Melatonin and Biorhythms -The pineal gland is a small mass of tissue near the center of the brain. -This gland makes and secretes melatonin, a modified amino acid. -Melatonin’s primary functions are related to the biological rhythms associated with reproduction. Thymus Gland -The thymus gland is an organ that lies underneath the top of the breast bone.

The Three Invertebrate Hormones Three hormones play major role in molting and metamorphosis into adult shape and form Brain hormone: produced by neurosecretory cells in insect brain, stimulates the release of ecdysone from the prothoracic glands Ecdysone promotes molting and development of adult characteristics These two are balanced by the third hormone Juvenile hormone: promotes the retention of larval characteristics

Invertebrate Regulatory System Invertebrates produce a variety of hormones in typical hormone secreting endocrine cells and neurosecretory cells Most of these hormones function in reproduction and development. In Aplysia, for example, specialized nerve cells secrete a neurohormone that stimulates the laying of thousands of eggs and also inhibits feeding and locomotion

Arthropods All groups of arthropods possess extensive endocrine systems. Crustaceans have hormones that function in growth and reproduction, water balance, movement of pigments in the integument and eyes, and regulation of metabolism. Most insects get their adult characteristics in a single terminal molt and molting is usually triggered by a hormone

Hormones and Endocrine Glands in Insects Insects have: Prothoracic Glands--manufacture ecdysteroids: production of chitin and protein in epidermal cells. Results in flow of events that result primarily in molting. Prothoracic glands disappear after molting, when they are not needed. Release is stimulated by PTTH [prothoracicotropic hormone] from the corpora cardiaca (located behind brain). Corpora allata manufactures juvenile hormone. Neurosecretary cells regulate activity of corpora allata. Neurosecretary cells found in clusters, secrete brain hormone Ovaries and testes produce gonadal hormones that coordinate courtship and mating behaviors

Humans also have neurosecretory cells Secondary sex hormones in humans: androgens (testosterone and estrogen), similar to gonadal hormones in insects. Juvenile hormone promotes adult maturity in insects, as do many of the sex hormones in humans. Similarities in Humans and Insects Nervous and endocrine systems of insects, including the corpora cardiaca and the corpora allata