The document provides an overview of the endocrine system, including: 1. The endocrine system uses chemical messengers called hormones to allow cells to communicate and regulate activities throughout the body. Hormones are secreted into the bloodstream and travel to target tissues. 2. The endocrine system consists of endocrine glands and specialized cells that secrete minute amounts of hormones. Hormones travel via the bloodstream to specific target sites where they elicit effects. 3. The endocrine and nervous systems both regulate homeostasis but differ in their modes of transport (hormones travel via bloodstream vs neurotransmitters acting locally) and speeds/durations of response.

1. Chapter 17: Functional Organization
of the Endocrine System

2. Principles of Chemical Communication
• Chemical Messengers
– Allow cells to communicate with each other to
regulate activities
– 4 classes of chemical messengers based on the
source of the chemical messenger and its mode of
transport in the body
• Autocrine chemical messengers
• Paracrine chemical messengers
• Neurotransmitters
• Endocrine chemical messengers

4. Characteristics of the Endocrine
System
• Composed of endocrine glands and
specialized endocrine cells located throughout
the body
• Secrete minute amounts of chemical
messengers (hormones) into the bloodstream
• Hormones travel a distance from their source
through the bloodstream to specific sites
(target tissues or effectors)

7. Comparison of the Nervous System
and Endocrine Systems
• Nervous system and the endocrine system
regulate and coordinate the activities of
essentially all body structures to achieve and
maintain homeostasis
– Nervous system functions as a communication
system
– Endocrine system sends information to the cells it
controls in the form of hormones, which are
carried by the bloodstream to all parts of the body

8. Similarities
• Structures associated with the brain
– E.g. hypothalamus
• Same molecule is a neurotransmitter for the
nervous system and a hormone for the
endocrine system
– E.g. epinephrine
• Work together to regulate critical body
processes
– E.g epinephrine

9. Similarities cont.
• Some neurons secrete hormones
– Neuropeptides (neurohormones)
– E.g. oxytocin

10. Differences
• Mode of transport
– Endocrine system secretes hormones which are
transported in the bloodstream
– Nervous system secretes neurotransmitters which
are released directly onto their target cells
• Speed of response
– Nervous system responds faster than the
endocrine

11. Differences cont.
• Duration of response
– Nervous system activates its targets quickly
– Endocrine system tends to have longer-lasting
effects

12. Hormones
• Characteristics
– Stability
• Life span of hormone varies with its chemical nature
• Expressed as half-life – amount of time it takes for 50% of
the circulating hormone to be removed from the circulation
and excreted
– Communication
• Able to regulate specific cellular pathways once they arrive
at their targets
– Distribution
• Transported by the blood to many locations

13. Chemical Nature of Hormones
• Lipid soluble
– Steroid hormones, thyroid hormones and fatty
acid derivative hormones
– Travel in bloodstream bound to binding proteins
• Water soluble
– Protein hormones, peptide hormones and most
amino acid derivative hormone
– Circulate as free hormones
– Relatively short half-lives

16. Patterns of Hormone Secretion
• 3 main patterns of hormone secretion
– Chronic hormone secretion
– Acute hormone secretion
– Episodic hormone secretion
• Steroid reproductive hormones

17. Control of Hormone Secretion
• 3 types of stimuli regulate hormone release
– Humoral
• Blood-borne molecules can directly stimulate the
release of some hormones
– Neural
• Involves neural stimuli of endocrine glands
– Hormonal
• Hormone is secreted that stimulates the secretion of
other hormones

18. Control by Humoral Stimuli

19. Control by Neural Stimuli

20. Control by Hormonal Stimuli

21. Regulation of Hormone Levels in the
Blood
• 2 major mechanisms
– Negative feedback
• Most hormones are regulated

22. • Positive feedback
– Example:

23. Hormone Receptors and Mechanisms
of Action
• Hormones exert their actions by binding to
proteins called receptors
– Only can stimulate cells that have the receptor for
that hormone
– Specific

24. • Down regulation
• Up regulation

26. Classes of Receptors
• Lipid-soluble hormones bind to nuclear
receptors
– Relatively small
– Able to diffuse through the plasma membrane and
bind to nuclear receptors

27. • Water-soluble hormones bind to membrane-
bound receptors
– Large molecules and cannot pass through the
plasma membrane
– Interact with membrane-bound receptors

28. Action of Nuclear Receptors

29. Membrane-Bound Receptors and
Signal Amplification
• Membrane-bound receptors activate
responses in 2-ways
– Receptors may alter the activity of G proteins at
the inner surface of the plasma membrane
– Receptors may directly alter the activity of
intracellular enzymes
– Second messenger system

30. Receptors that activate G Proteins
• G proteins
– 3 subunits
• Alpha, beta and gamma
• Guanine nucleotide bound to alpha subunit
– Inactive state – GDP bound
– Active state – GTP bound

32. G proteins that Interact with Adenylate
Cyclase

33. G Proteins that Activate other
Intracellular Mediators
• Alter the concentration of intracellular
mediators other than Ca2+ or cAMP

34. Receptors that Directly Activate
Intracellular Mediators

35. Receptors that Phosphorylate
Intracellular Proteins
• Hormones bind to
membrane-bound
receptors.
• Part of receptor protein
on inside of membrane
acts as an enzyme to
phosphorylate proteins
• E.g., insulin receptors
bound to insulin cause
phosphorylation of
proteins and cell
responds to presence of
insulin.

36. Signal Amplification