Chemical Coordination

Chapter 34

Hormones

A 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.

Systems of Internal Communication

Animals have two systems of internal communication and regulation:The nervous systemThe endocrine system

Systems of Internal Communication

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 longer-acting responses to stimuli.

Hormones

Advantages of using chemical messengers:Chemical molecules can spread to all

tissues through the blood.Chemical signals can persist longer than

electrical ones.Many different kinds of chemicals can act as

hormones; different hormones can target different tissues.

Glands

Many hormones are secreted by ductless endocrine glands.Obtain raw materials from and secrete

hormones directly into the bloodstream.Exocrine glands have ducts for

discharging secretions onto a free surface.Sweat glands, salivary glands, enzyme-

secreting glands in the digestive tract.

Hormones

Hormones convey information via the bloodstream to target cells throughout the body.

Pheromones carry messages outside the body – to other individuals.

Hormones

Three major classes of molecules function as hormones in vertebrates:Proteins and peptidesAmines derived from amino acidsSteroids

Hormones

Signaling by any of these molecules involves three key events:ReceptionSignal transductionResponse

Hormones

The hypothalamus regulates the neuroendocrine system, maintaining homeostasis in the body.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.

Each releasing hormone stimulates the pituitary to release a corresponding hormone which travels to an endocrine gland and causes it to start producing a particular endocrine hormone.

Membrane-Bound Receptors

Many hormones are too large, or too polar, to pass through plasma membranes. Bind to transmembrane

proteins that act as receptor sites on target cell membranes.

Hormone is first messenger.

Causes activation of a second messenger in the cytoplasm.

cAMP

Nuclear Receptors

Steroid hormones are lipid soluble molecules that bind to hormone receptors in the cytoplasm of the target cell. Site of activity is the

nucleus. Steroids are

manufactured from cholesterol.

Estrogen, progesterone, testosterone, cortisol.

Nuclear Receptors

Thyroid hormones and insect-molting hormone (ecdysone) also act through nuclear receptors. 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.

Negative feedback regulates many hormonal pathways involved in homeostasis.

Invertebrate Hormones

Ecdysone regulates molting in insects.

Juvenile hormone favors the retention of juvenile characteristics.

The Pituitary

The pituitary gland is located below the hypothalamus.

Nine major hormones are produced here.

These hormones act primarily to influence other endocrine glands.

The Pituitary

The posterior lobe of the pituitary regulates water conservation, milk letdown, and uterine contraction in women.

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.

Luteinizing hormone (LH) plays an important role in the menstrual cycle. It also stimulates the production of testosterone in males.

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.

Adrenocorticotropic hormone (ACTH) – stimulates the adrenal gland to produce steroid hormones. Some regulate glucose production, others balance sodium & potassium in the blood.

The Anterior Pituitary

Growth hormone (GH) – stimulates the growth of muscle and bone.

Prolactin – stimulates milk production.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.

Oxytocin initiates uterine contraction during childbirth and milk release in mothers.

These hormones are actually synthesized in the hypothalamus and stored in the posterior pituitary.

Biological Clocks

The pineal gland is located in the brain of most vertebrates. 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.

Melatonin controls color change in amphibians & reptiles.

Release of melatonin is controlled by light/dark cycles.

The primary functions of melatonin appear to be related to biological rhythms associated with reproduction. Circadian rhythms – 24 hours long.

The Thyroid

The thyroid gland, located in the neck, produces: Thyroxine – increases

metabolic rate and promotes growth.

Two iodine-containing hormones, triiodothyronine (T3) and thyroxine (T4).

Calcitonin – stimulates calcium uptake by bones.

The Thyroid

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

The Thyroid

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.

The hormone they produce is parathyroid hormone (PTH) which regulates the level of calcium in the blood. Essential that calcium is

kept within narrow limits for muscle contraction, including the heart.

Calcium Homeostasis

Two antagonistic hormones, parathyroid hormone (PTH) and calcitonin, play the major role in calcium (Ca2+) homeostasis in mammals.

Calcium Homeostasis

Calcitonin, secreted by the thyroid gland, stimulates Ca2+ deposition in the bones and secretion by the kidneys, thus lowering blood Ca2+ levels.

PTH, secreted by the parathyroid glands, has the opposite effects on the bones and kidneys, and raises Ca2+ levels. Also has an indirect effect, stimulating the kidneys

to activate vitamin D, which promotes intestinal uptake of Ca2+ from food.

The Adrenals

Mammals have an adrenal gland above each kidney. 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

The adrenal medulla releases adrenalin (epinephrine) and norepinephrine in times of stress. Identical to the effects of the sympathetic

nervous system, but longer lasting.Accelerated heartbeat, increased blood

pressure, higher levels of blood sugar and increased blood flow to heart and lungs.

Adrenal Cortex

The adrenal cortex produces the steroid hormone cortisol (hydrocortisone).Reduces inflammation.

Synthetic derivatives such as prednisone are used as anti-inflammatory agents.

Stimulates carbohydrate metabolism.

Adrenal Cortex

The adrenal cortex also produces aldosterone.

Aldosterone acts in the kidney to promote the uptake of sodium & other salts from the urine.These salts are important in nerve

conduction.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.

It secretes digestive enzymes into the digestive tract (exocrine function).

Endocrine function – production of insulin and glucagon.

Glucose Homeostasis

The islets of Langerhans in the pancreas secrete insulin and glucagon. Insulin removes

glucose from the blood.

Glucagon returns glucose to the blood.

Diabetes

Diabetes mellitus, perhaps the best-known endocrine disorder, is caused by a deficiency of insulin or a decreased response to insulin in target tissues.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.

Type II diabetes mellitus (non-insulin-dependent 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.