Chemical Coordination Chapter 34 Hormones l A

Chemical Coordination Chapter 34

Hormones l. A hormone is a chemical signal that is secreted into the circulatory system and communicates regulatory messages within the body. l Hormones may reach all parts of the body, but only certain types of cells, target cells, are equipped to respond.

Systems of Internal Communication l Animals have two systems of internal communication and regulation: The nervous system l The endocrine system l

Systems of Internal Communication l The nervous system conveys highspeed electrical signals along specialized cells called neurons. l The endocrine system, made up of endocrine glands, secretes hormones that coordinate slower but longer-acting responses to stimuli.

Hormones l Advantages of using chemical messengers: Chemical molecules can spread to all tissues through the blood. l Chemical signals can persist longer than electrical ones. l Many different kinds of chemicals can act as hormones; different hormones can target different tissues. l

Glands l Many hormones are secreted by ductless endocrine glands. l Obtain raw materials from and secrete hormones directly into the bloodstream. l Exocrine glands have ducts for discharging secretions onto a free surface. l Sweat glands, salivary glands, enzymesecreting glands in the digestive tract.

Hormones l Hormones convey information via the bloodstream to target cells throughout the body. l Pheromones carry messages outside the body – to other individuals.

Hormones l Three major classes of molecules function as hormones in vertebrates: Proteins and peptides l Amines derived from amino acids l Steroids l

Hormones l Signaling by any of these molecules involves three key events: Reception l Signal transduction l Response l

Hormones l The hypothalamus regulates the neuroendocrine system, maintaining homeostasis in the body. l The hypothalamus can use motor nerves to send short-lived electrical messages or hormones to send chemical messages with a longer duration.

The Chain of Command The hypothalamus produces seven different “releasing” hormones that travel to the pituitary gland. l Each releasing hormone stimulates the pituitary to release a corresponding hormone which travels to an endocrine gland causes it to start producing a particular endocrine hormone. l

Membrane-Bound Receptors l Many hormones are too large, or too polar, to pass through plasma membranes. l Bind to transmembrane proteins that act as receptor sites on target cell membranes. l l Hormone is first messenger. Causes activation of a second messenger in the cytoplasm. l c. AMP

Nuclear Receptors l Steroid hormones are lipid soluble molecules that bind to hormone receptors in the cytoplasm of the target cell. l Site of activity is the nucleus. Steroids are manufactured from cholesterol. l Estrogen, progesterone, testosterone, cortisol. l

Nuclear Receptors l Thyroid hormones and insect-molting hormone (ecdysone) also act through nuclear receptors. l Binds to transmembrane protein that uses ATP to move it into the cell.

Control Pathways and Feedback Loops A common feature of control pathways is a feedback loop connecting the response to the initial stimulus. l Negative feedback regulates many hormonal pathways involved in homeostasis. l

Invertebrate Hormones Ecdysone regulates molting in insects. l Juvenile hormone favors the retention of juvenile characteristics. l

The Pituitary The pituitary gland is located below the hypothalamus. l Nine major hormones are produced here. l These hormones act primarily to influence other endocrine glands. l

The Pituitary l The posterior lobe of the pituitary regulates water conservation, milk letdown, and uterine contraction in women. l The anterior lobe regulates the other endocrine glands.

The Anterior Pituitary Thyroid stimulating hormone (TSH) – stimulates the thyroid gland to produce thyroxine which stimulates oxidative respiration. l Luteinizing hormone (LH) plays an important role in the menstrual cycle. It also stimulates the production of testosterone in males. l

The Anterior Pituitary Follicle-stimulating hormone (FSH) – plays an important role in the menstrual cycle. In males, it causes the testes to produce a hormone that regulates sperm production. l Adrenocorticotropic hormone (ACTH) – stimulates the adrenal gland to produce steroid hormones. Some regulate glucose production, others balance sodium & potassium in the blood. l

The Anterior Pituitary l Growth hormone (GH) – stimulates the growth of muscle and bone. l Prolactin – stimulates milk production. l Melanocyte-stimulating hormone (MSH) – in reptiles & amphibians, this hormone stimulates color change.

The Posterior Pituitary Antidiuretic hormone (ADH) regulates the kidney’s retention of water. l Oxytocin initiates uterine contraction during childbirth and milk release in mothers. l These hormones are actually synthesized in the hypothalamus and stored in the posterior pituitary. l

Biological Clocks l The pineal gland is located in the brain of most vertebrates. l l Evolved from a light sensitive “third eye”. Primitive fish & some reptiles still have a third eye.

Biological Clocks In other vertebrates it functions as an endocrine gland secreting melatonin. l Melatonin controls color change in amphibians & reptiles. l Release of melatonin is controlled by light/dark cycles. l The primary functions of melatonin appear to be related to biological rhythms associated with reproduction. l l Circadian rhythms – 24 hours long.

The Thyroid l The thyroid gland, located in the neck, produces: l l l Thyroxine – increases metabolic rate and promotes growth. Two iodine-containing hormones, triiodothyronine (T 3) and thyroxine (T 4). Calcitonin – stimulates calcium uptake by bones.

The Thyroid l The hypothalamus and anterior pituitary control the secretion of thyroid hormones through two negative feedback loops.

The Thyroid l The thyroid hormones play crucial roles in stimulating metabolism and influencing development and maturation.

The Parathyroids The parathyroid glands are four small glands attached to the thyroid. l The hormone they produce is parathyroid hormone (PTH) which regulates the level of calcium in the blood. l Essential that calcium is kept within narrow limits for muscle contraction, including the heart. l

Calcium Homeostasis l Two antagonistic hormones, parathyroid hormone (PTH) and calcitonin, play the major role in calcium (Ca 2+) homeostasis in mammals.

Calcium Homeostasis Calcitonin, secreted by the thyroid gland, stimulates Ca 2+ deposition in the bones and secretion by the kidneys, thus lowering blood Ca 2+ levels. l PTH, secreted by the parathyroid glands, has the opposite effects on the bones and kidneys, and raises Ca 2+ levels. l l Also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca 2+ from food.

The Adrenals l Mammals have an adrenal gland above each kidney. l l Adrenal medulla is the inner core which produces adrenaline (epinephrine) and norepinephrine. Adrenal cortex is the outer shell that produces the steroid hormones cortisol and aldosterone.

Adrenal Medulla l The adrenal medulla releases adrenalin (epinephrine) and norepinephrine in times of stress. l Identical to the effects of the sympathetic nervous system, but longer lasting. l Accelerated heartbeat, increased blood pressure, higher levels of blood sugar and increased blood flow to heart and lungs.

Adrenal Cortex l The adrenal cortex produces the steroid hormone cortisol (hydrocortisone). l Reduces inflammation. l l Synthetic derivatives such as prednisone are used as anti-inflammatory agents. Stimulates carbohydrate metabolism.

Adrenal Cortex l The adrenal cortex also produces aldosterone. l Aldosterone acts in the kidney to promote the uptake of sodium & other salts from the urine. l These salts are important in nerve conduction. l Aldosterone and PTH are the only two hormones essential for survival.

The Pancreas The pancreas is located behind the stomach and is connected to the small intestine by a small tube. l It secretes digestive enzymes into the digestive tract (exocrine function). l Endocrine function – production of insulin and glucagon. l

Glucose Homeostasis l The islets of Langerhans in the pancreas secrete insulin and glucagon. l l Insulin removes glucose from the blood. Glucagon returns glucose to the blood.

Diabetes l Diabetes mellitus, perhaps the bestknown endocrine disorder, is caused by a deficiency of insulin or a decreased response to insulin in target tissues. l Marked by elevated blood glucose levels.

Diabetes Type I diabetes mellitus (insulin-dependent diabetes) is an autoimmune disorder in which the immune system destroys the beta cells of the pancreas. l Type II diabetes mellitus (non-insulindependent diabetes) is characterized either by a deficiency of insulin or, more commonly, by reduced responsiveness of target cells due to some change in insulin receptors. l