a&pi ichapter17

Chapter 17: Functional Organization of the Endocrine System

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

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)

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

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

Similarities cont.

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

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

Differences cont.

• Duration of response– Nervous system activates its targets quickly– Endocrine system tends to have longer-lasting

effects

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

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

Patterns of Hormone Secretion

• 3 main patterns of hormone secretion– Chronic hormone secretion– Acute hormone secretion– Episodic hormone secretion• Steroid reproductive hormones

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

Control by Humoral Stimuli

Control by Neural Stimuli

Control by Hormonal Stimuli

Regulation of Hormone Levels in the Blood

• 2 major mechanisms– Negative feedback• Most hormones are regulated

• Positive feedback– Example:

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

• Down regulation

• Up regulation

Classes of Receptors

• Lipid-soluble hormones bind to nuclear receptors– Relatively small– Able to diffuse through the plasma membrane and

bind to nuclear receptors

• Water-soluble hormones bind to membrane-bound receptors– Large molecules and cannot pass through the

plasma membrane– Interact with membrane-bound receptors

Action of Nuclear Receptors

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

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

G proteins that Interact with Adenylate Cyclase

G Proteins that Activate other Intracellular Mediators

• Alter the concentration of intracellular mediators other than Ca2+ or cAMP

Receptors that Directly Activate Intracellular Mediators

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.

Signal Amplification