Lec 22: Endocrine System Flashcards
Difference between endocrine and exorcine galnds
Difference between endocrine system and autonomic nervous system
Hormones don’t have to use the bloodstream to reach their targets
Hormone
Chemical substance released into the
ECF that regulates the metabolic
function of other cells in the body
hormones must bind to specific
receptors
hormones are specific; level of target cell activation depends on:
(i) hormone concentration
(ii) target cell receptor content
(iii) affinity of hormone for receptor
Mechanism of Hormone Action:
hormones alter levels of cell activity:
» membrane permeability/potential (channels)
» synthesis of enzymes within cells
» enzyme activation/deactivation
» induction of secretory activity
» stimulation of mitosis
3 structural groups of hormones
(i) amino acids, peptides, proteins
(ii) steroid hormones (derivatives of cholesterol)
(iii) eicosanoids (derivatives of arachidonic acid) – e.g. prostaglandins
2 main mechanisms of action
Peptide/protein hormones:
water soluble, so cannot cross plasma membrane;
bind to a cell surface receptor » activation of membrane- bound G protein » production of 2nd messenger
Steroid hormones:
lipid soluble; they can enter the nucleus and activate gene transcription
hormones are potent
don’t need much to get a significant effect because the effects are amplified
Blood Level depends on:
- rate of synthesis
- rate of degradation/clearance from blood
Half-Life
persistence of a hormone in the blood; usually < 1 min to a week
time to onset
- if enzyme activation - rapid (minutes)
- if enzyme synthesis - hours to days
Control of Hormone Release
usually negative feedback (setpoint); sometimes positive feedback (goal)
3 types of stimuli:
humoral, neural & hormonal
Humoral stimuli
hormone secretion in direct response to change in blood level of a nutrient, ion [eg: parathyroid hormone (PTH) & blood
calcium; insulin & blood glucose]
Neural stimuli:
not as common, eg: sympathetic ns & epinephrine release by adrenal medulla, hypothalamic neurons & oxytocin release
Hormonal stimuli:
3-tiered system involving hypothalamus, pituitary & target endocrine gland - concept of hypothalamic-pituitary axis
Hormonal Stimulus (an example of the 3-tiered system)
Thyroid-releasing hormone (TRH)
(hypothalamus)
↓
Thyroid-stimulating hormone (TSH)
(anterior pituitary)
↓
Thyroid hormones (T3 & T4)
(thyroid gland)
Hypothalamus
is neural; produces a number of releasing factors (hormones) which travel to anterior pituitary via the hypophyseal portal system
Hypophyseal Portal System
a system of blood vessels in the microcirculation at the base of the brain, connecting the hypothalamus with the anterior pituitary
Hypophyseal Portal System (function)
is to quickly transport and exchange hormones between the hypothalamus arcuate nucleus and anterior pituitary gland
Pituitary Gland
- size & shape of a pea
- infundibulum connects pituitary to
hypothalamus
Posterior Lobe
- consists of axon terminals
- can think of it as a hormone storage area
- antidiuretic hormone (SON)
oxytocin (PVN)
