The endocrine system
is the second control system.It
is integrated with and interactive with the nervous system.
Comparison of the
nervous system with the endocrine system:
Nervous
SystemEndocrine System
method of signalingelectrical
impulsechemical agent ("messenger")transmissionrelease
delivery time framemillisecondsseconds
to days
adjustment responsequickgradual
duration of signal
eventbriefprolonged
system organizationcontinuousdiscontinuous
Other Characteristics
of the Endocrine System:
*
comprised of endocrine cells, tissues, and organs scattered throughout
the body
*
collectively a relatively tiny mass of tissue
*
endocrine glands:
*
ductless
*
cells organized in cords and branching networks
*
rich vascular supply maximize access to fluid compartments
*
released agent/mediator is HORMONE
*
include:
*
pituitary
*
thyroid
*
parathyroid
*
adrenal
*
pineal
*
thymus
*
pancreas (exocrine + endocrine)
*
gonads (gametogenesis + endocrine)
*
hypothalamus (neuroendocrine)
*
other scattered endocrine structures
*
some tumors synthesize and release relatively large amounts of hormones
Contrast with Exocrine
Glands (c.f., Chapter 4)
*
all are ducted structures which release their secretory products through
a tube onto a membrane
surface
or into a membrane-lined pouch
*
include:
*
sweat
*
mucous
*
oil
*
salivary
*
liver (bile)
*
pancreas (digestive enzymes)
*
released agent is NON-HORMONAL
Hormones:
*
secreted by cells into extracellular fluids
*
regulate metabolic function of other cells in body
*
chemical substances
*
most hormones are AMINO ACID -based
*
simple amino acid derivatives and thyroxine
*
peptides (short a.a. chains)
*
proteins (long polypeptides)
*
very few hormones are STEROIDS
*
steroids are cholesterol derivatives/variants
*
e.g., gonadal hormones and adrenocortical hormones
*
EICOSANOID HORMONES are paracrine or "local" hormones
*
biologically active lipids
*
derivatives of ARACHIDONIC ACID
*
LEUKOTRIENES are signaling chemicals that mediate inflammation and some
allergic
reactions
*
PROSTAGLANDINS have multiple targets and effects; effects are typically
highly localized, e.g.:
*
raise blood pressure
*
increase expulsive uterine contractions during birth
*
enhance blood clotting and inflammation
Mechanisms of Hormone
Action
*
alter target cell activity (i.e., increase or decrease rates of normal
processes)
*
response is dictated by target cell type
*
typical changes produced by hormones:
*
change plasma membrane permeability and/or membrane potential by open/close
ION
CHANNELS
*
synthesis of proteins (viz., enzymes = regulatory proteins)
*
activate/deactivate enzymes
*
induce secretory activity
*
stimulate mitosis
PHYSIOLOGICAL
SIGNALING:Three basic plans
I.At
a distance, between individuals (e.g., pheromones)
II.At
a distance, within an individual (e.g., traditional hormones)
III.Local
messengers (e.g., neurotransmitters and prostaglandins)
Different
communication mechanisms to signal for hormone production:
HUMORAL:e.g.The
parathyroid glands are nested in the thyroid gland.Reduced
Ca2+ levels in the capillary blood delivered to the parathyroids
stimulates the release of PTH (parathyroid hormone) which stimulates release
of Ca2+ from bone.
NEURAL:e.g.Sympathetic
nerve fibers from the spinal cord directly stimulate adrenal medulla cells
to quickly secrete CATECHOLAMINES (epinephrine and norepinephrine) into
the blood, and those catecholamines have a rapid effect on elevating blood
pressure, heart rate, and general level of agitation.
HORMONAL:e.g.TROPIC
HORMONES (a.k.a., "releasing hormones") from the HYPOTHALAMUS stimulate
the release of specific other hormones from the thyroid, adrenal cortex,
and gonads.
Although
the pituitary is commonly referred to as the "master gland", the HYPOTHALAMUS
(in the brain stem of the central nervous system) can be thought of as
the chief regulatory center for the endocrine system.It
regulates:
·sleep
·arousal
·emotions
oanger
ofear
opleasure
ohunger
osexual
response
opain
·body
temperature
·blood
pressure
·water
balance
The
PITUITARY (hypophysis)
·"master
gland"
·controlled
by the hypothalamus
·hormone-producing
portion = anterior pituitary (adenohypophysis)
·neural
portion = posterior pituitary is an extensions of the hypothalamus
oposterior
pituitary + infundibulum = neurohypophysis
A
PORTAL SYSTEM is an arrangement o blood vessels in which a capillary bed
feeds into veins which, in turn, feed into another capillary bed.
e.g.,
The HYPOPHYSEAL PORTAL SYSTEM:capillaries
from the superior hypophyseal artery deliver hormones produced and
secreted by the hypothalamus into the hypophyseal portal vein which
conducts the material directly to the adenohypophysis, where the hypothalamic
hormones regulate the secretory activity of the adenohypophysis.
Amino Acid-based
Hormones and Second-Messenger Systems
Cyclic Adenosine
Monophosphate (cAMP)
Problem:Proteins
and peptides cannot penetrate the plasma membranes of tissue cells
Question:So,
how do amino acid-based hormones exert their signaling effect?
Answer:Through
intracellular SECOND MESSENGERS activated by hormone binding to plasma
membrane
receptors.
Although their
are several known second messengers, the cAMP mechanism is currently the
best understood.
Characteristics
of cAMP?
·intracellular
substance that can be generated by event on surface of cell
·once
generated, is free to diffuse throughout the cell
·can
nonspecifically trigger (activate) one or more enzyme [systems], such as
protein kinases
·kinases
phosphorylate some proteins to activity, but inactivate others
·chain
of events (referred to as [ENZYMATIC] CASCADE) amplifies the effect
·cAMP
is rapidly degenerated by PHOSPHODIESTERASE which occurs through the cytoplasm
·enzymes
are re-used until inactivated by separate control mechanism
Recall facts about
ENZYMES (c.f. Chapter 2):
·enzymes
are required for all biological reactions
·globular
proteins
·act
as biological CATALYSTS
·substances
that regulate and accelerate rate of biochemical reactions
·not
used up or changed in the reaction
·without
specific enzymes, biological reactions proceed so slowly they are inconsequential
·enzymes
increase reaction rates by factor of 106
cAMP Mechanism:
3 plasma membrane
components interact to determine intracellular levels of cAMP:
·hormone
receptor (specific, amino acid-based)
·hormone
binding to receptor activates the G protein signal transducer (which possesses
GTP-ase)
·GTP
- GDP reaction provides phosphate-bond energy to activate cAMP
Direct Gene
Activation Systems:Steroid Hormones
STEROID HORMONES
and THYROID HORMONE are LIPID SOLUBLE:
·easily
diffuse into their target cells
·bind
to and activate intracellular receptor
·hormone-receptor
complex reaches nuclear chromatin
·"turns
on" gene
Recall the Central
Dogma of Biology regarding the relationship between gene (DNA) activation
and protein synthesis:
Hormone-Target
Cell Specificity:
·specific
RECEPTORS on plasma membrane are required for hormones to bind
·specificity
provides for selective responsiveness of tissues to hormones
Target Cell Response
is related to:
·blood
levels of hormone
·receptor
density on target cells
·affinity
(strength) of bond between hormone and receptor