The Endocrine System
Chapter 15
Hormones
• Secreted by endocrine glands,
endocrine cells, and certain neurons
• Travel through the bloodstream to
nonadjacent target cells
Other Signaling Molecules
• Neurotransmitters
• Local signaling molecules
• Pheromones
Discovery of Hormones
• Bayliss and Starling severed nerves to dog’s
intestine; left blood vessels intact
• Pancreas still responded
• Extracts of glandular epithelium also
provoked pancreatic response
• Extracts contained secretin
Hormone Interactions
• Opposing interaction
• Synergistic interaction
• Permissive interaction
Endocrine System
Main Sources • Pituitary gland• Adrenal glands• Thyroid gland• Parathyroid
glands• Pineal gland• Thymus gland
Responses to Hormones Vary
• Different hormones activate different
responses in the same target cell
• Not all types of cells respond to a
particular hormone
Two Main Hormone Types
• Steroid hormones– Derived from cholesterol– Estrogens, progestins, androgens (such as
testosterone), cortisol, aldosterone
• Peptide hormones– Peptides, proteins, or glycoproteins– Glucagon, ADH, oxytocin, TRH, insulin,
somatotropin, prolactin, FSH, LH, TSH
Steroid Hormones
receptor
hormone-receptor complex
gene product
hormone
• Most diffuse across the plasma membrane and bind to a receptor
• Hormone-receptor complex acts in nucleus to inhibit or enhance transcription
Protein Hormone
• Hormone binds to a receptor at cell surface
• Binding triggers a change in activity of enzymes inside the cell
glucagon receptor
cyclic AMP + Pi
ATP
cAMP activatesprotein kinase A
glucagon
Protein kinase A converts phosphorylasekinase to active form and inhibits an enzyme required for glucagon synthesis.
The Hypothalamus
• Region in the
forebrain
• Contains hormone-
secreting cells
• Interacts with
pituitary
hypothalamus
pituitary gland
Pituitary Gland
• Pea-sized gland at base of hypothalamus
• Two lobes
– Posterior lobe stores and secretes hormones
that were synthesized in the hypothalamus
– Anterior lobe produces and secretes its own
hormones
Posterior Lobe
• Antidiuretic hormone (ADH)
• Oxytocin (OCT)
cell body in hypothalamus
axons
to the general circulation
Anterior Pituitary
• ACTH• TSH• FSH• LH• PRL• STH
Normal Hormone Production
• Generally, the body produces only very
small amounts of hormones
• To isolate 1 milligram of TRH,
researchers dissected 7 metric tons of
hypothalamic tissue
Abnormal Somatotropin Output
• Gigantism
• Pituitary dwarfism
• Acromegaly
Feedback Mechanisms
• Negative feedback
– An increase in concentration of a hormone
triggers activities that inhibit further secretion
• Positive feedback
– An increase in concentration of a hormone
triggers activities that stimulate further
secretion
Cortisol
• Cortisol secretion
– Inhibits blood glucose uptake by muscle and
other tissues
– Causes breakdown of proteins to amino
acids and conversion to glucose
– Causes degradation of adipose tissue to fatty
acids for use as energy source
Feedback Control of Cortisol Secretion
• Hypothalamus senses rise in glucose and
secretes less releasing hormone (CRH)
• Anterior pituitary responds by secreting
less ACTH
• Adrenal cortex slows its secretion of
cortisol
Localized Feedback in Adrenal Medulla
• Norepinephrine secreted by neurons in the medulla accumulates in the synaptic gap
• Some molecules bind to receptors on the axon endings that secreted them
• This prevents further secretion of norepinephrine by that axon
Thyroid Gland Disorders
• Goiter
• Hyperthyroidism
• Hypothyroidism
Calcium Regulation
• Parathyroid hormone (PTH) is the main regulator of calcium in the blood
• It is secreted when calcium levels drop
• PTH causes bone cells to digest bone tissue and release calcium
• PTH also stimulates calcium reabsorption by the kidneys and absorption by the gut
Control of Glucose Metabolism
insulin
Glucose rises
Glucose fallsGlucose is absorbed
Cells use glucose
glucagonGlycogen to glucose
Glucose uptake
Glucose to glycogen
Diabetes Mellitus
• Disease in which excess glucose accumulates in blood, then urine
• Effects include– Excessive urination– Constant thirst– Weight loss– Ketone formation and acid-base imbalances
Two Types of Diabetes
• Type 1
• Autoimmune disease
• Usually appears in
childhood
• Treated with insulin
injections
• Type 2
• Target cells don’t
respond
• Usually appears in
adults
• Treated with diet,
drugs
The Pineal Gland
• Photosensitive gland embedded in brain
• In the absence of light, secretes melatonin
• Affects the human biological clock
• May also play a role in human puberty and
in seasonal affective disorder
Local Signaling Molecules
• Prostaglandins
– Produced and secreted in response to local
changes
– Sixteen types with a variety of effects
• Growth factors
– Affect cell division rates in tissues