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 system The 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 peptides   Amines derived from amino acids Steroids

Hormones

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

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

located below the

hypothalamus

.

is  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)

thyroid gland to produce

thyroxine

– stimulates the 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 (Ca 2+ ) homeostasis in mammals.

Calcium Homeostasis



Calcitonin

, secreted by the thyroid gland, stimulates Ca secretion by the kidneys, thus lowering blood Ca 2+ levels.

2+ deposition in the bones and 

PTH

, secreted by the parathyroid glands, has the opposite effects on the bones and kidneys, and raises Ca 2+ levels.

 Also has an indirect effect, stimulating the kidneys to activate vitamin D, which promotes intestinal uptake of Ca 2+ 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.