a&pi ichapter17
TRANSCRIPT
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
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
