D’YOUVILLE COLLEGE
BIOLOGY 108/508 - HUMAN ANATOMY & PHYSIOLOGY II
LECTURE # 7
ENDOCRINE SYSTEM I
1.
Introduction:
• exocrine glands secrete to an “external” surface via ducts
• endocrine glands are ductless and secrete directly into the bloodstream;
their products are called hormones
• endocrine system (fig. 16 – 1) is a regulating or coordinating system, like the
nervous system; it tends to be slow responding, but enduring in effect as opposed to
more rapid, shorter term nervous responses; the endocrine system, thus governs
long-term phenomena such as growth, reproduction, development, and metabolic
regulation; the most important endocrine glands include the pituitary gland
(hypophysis), thyroid gland, parathyroid glands, pancreatic islets, adrenal glands,
and the gonads.
2.
Hormones:
a. Chemical Nature:
• protein or glycoprotein, e.g. insulin, parathormone, thyrotropin
• peptide, e.g. oxytocin, antidiuretic hormone, releasing hormones
• amino acid derivative, e.g. thyroxine, epinephrine
• steroid, e.g. cortisol, testosterone, progesterone
• prostaglandins, leukotrienes: from arachidonic acid of cell membranes
(pseudohormones)
b. Mechanisms of Action:
• polar (nonsteroid) hormones find the cell membrane impenetrable and
thus bind to a cell surface receptor; formation of a hormone-receptor complex
activates a G protein that activates an enzyme, which produces an intracellular
second messenger, e.g. cyclic AMP; the second messenger leads to a cascade of
enzyme activations producing a tremendous amplification of the hormonal effect
(fig. 16 – 2)
• steroid hormones pass through the cell membrane with ease and bind to
a nuclear receptor; the hormone-receptor complex acts upon nuclear DNA (genome
modulation) (fig. 16 – 3)
• hormones affect only target cells, cells that express receptors for a
particular hormone; some hormones may affect a target cell’s population of receptors
for itself or for other hormones, resulting in modulation of hormonal sensitivity
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3.
Control of Secretion:
• blood level of various substances, nervous system (sympathetic,
hypothalamus) may signal secretion of certain hormones (fig. 16 – 4)
a. Hypothalamus: controls hypophysis (pituitary) via neurohormones
(releasing hormones – RH, or inhibiting hormones); neurosecretion (nerve cells
release hormone instead of neurotransmitter) into hypophyseal portal system (fig.
16 – 5b)
- hypothalamus controls hypophysis and it, in turn, controls target glands,
constituting an endocrine axis, e.g. hypothalamo-hypophyseo-adrenal cortical axis
or hypothalamo-hypophyseo-thyroid axis; control is mediated by release/inhibiting
hormones from specialized neurosecretory cells of the hypothalamus; the products
of these cells are neurohormones, released (like neurotransmitters) from synapses
b. Adenohypophysis (Anterior Pituitary): controls activity of several other
endocrines (target glands) via secretion of tropic hormones
c. Target Gland: elevated product level in bloodstream has inhibitory
influence on hypothalamic activity (long loop negative feedback) or hypophysial
activity (short loop negative feedback)
4.
Selected Glands and Their Products:
Hypophysis (fig. 16 – 5): consists of adenohypophysis (glandular in character
- sometimes known as the anterior lobe), and neurohypophysis (neural tissue
originating from embryonic hypothalamus - also known as posterior lobe)
• adenohypophysis (table 16 – 1)
• growth hormone (GH) promotes growth (protein buildup in muscle, &
cell proliferation in epiphysial discs of long bones), and stimulates metabolism:
- a) by increasing fat mobilization from adipose and
- b) by stimulating glucose release from liver, while impeding its
uptake by other tissues (anti-insulin effect) (fig. 16 – 6)
- GH release is stimulated by growth hormone RH and inhibited by
GHIH (somatostatin) from the hypothalamus
• prolactin stimulates milk production in mammary glands of the
female; its release is controlled by prolactin inhibiting hormone from the
hypothalamus
• ACTH (Adrenocorticotropic Hormone or Corticotropin) stimulates
glucocorticoid release from the adrenal cortex; it’s controlled by corticotropin RH
from the hypothalamus
• TSH (Thyroid Stimulating Hormone/thyrotropin) stimulates thyroid
hormone release; it’s controlled by thyrotropin RH from hypothalamus (fig. 16 – 7)
• GTH (Gonadotropic Hormones):
1) FSH (Follicle Stimulating Hormone) stimulates proliferation of
follicles in the ovary and of spermatogonia in the testis
2) LH (Luteinizing Hormone) promotes maturation of follicles &
estrogen release in the ovary; a high concentration (spike) causes ovulation; LH also
promotes formation and maintenance of the corpus luteum of the ovary (following
ovulation)
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• in males, LH promotes maturation of spermatozoa and release of
testosterone by interstitial cells of the testis
• gonadotropin release is controlled by a GnRH from the
hypothalamus