Introduction to the endocrine system & HPA Flashcards
How does the nervous system compare to the endocrine system?
NS- Allows very rapid, focused , precise communication
Endocrine- transmitters are released into carrier medium (blood) and rare that two organs are directly linked- usually CVS involvement (e.g. via blood pumping from heart)
-Sustained communication between group of cells
What is the endocrine system?
What are hormones?
A system of ductless glands which secrete chemical messengers into the blood
Chemical messengers carried in blood that are able to act on cells processing the appropriate hormone receptors (target tissues). Each hormone can act on several target tissues.
Identify 5 endocrine organs
Pineal gland Hypothalamus Pituitary gland Parathyroid gland Thyroid gland Thymus Adrenal gland ancreas Kidney Ovary Testes
Where is the pituitary gland found? Outline the anatomical structures found in and around it
Situated below the brain, encased in skull
Ant and post pituitary gland Infundibulum (connection to hypothalamus) Hypothalamus Neurosecretory cells Hypothalamico-hypophyseal tract Hypothalamico-hypophyseal portal vessels
Where are the thyroid and parathyroid gland found? Function?
In the neck
Important in control of metabolic rate and calcium homeostasis
Function of the pancreas
Secrete digestive enzymes directly into the GI tract (exocrine)
Also secretes insulin which regulated b/g and glucose utilisation
Shape and function of adrenal glands
Left- pyramidal
Right- semilunar
Medulla is concerned with stress response
Cortex with stress, sodium and glucose homeostasis
Function of the gonads (ovaries and testes)
Secrete female and male reproductive hormones respectively. Responsible for the development of secondary sexual characteristics and reproduction
Consider the mechanism of hormone action at steroid hormone receptors
Lipid soluble hormone crosses the cell membrane
Binds to receptor (cytoplasmic or nuclear)
Hormone-receptor complex binds to hormone-response element of DNA
Influences gene transcription
! Receptor may be constitutively active
Consider the mechanism of hormone action at G-Protein coupled hormone receptors
Receptor –> G-Protein–> Phospholipase C –>:
- PIP2
- DAG
- IP3
DAG and IP3 act as secondary messengers and phosphorylate intracellular, regulatory protein to influence cellular activity (e.g. smooth muscle contraction)
Consider the mechanism of hormone action at Tyrosine kinase hormone receptors
Membrane bound enzyme is a single strand of protein (often dimerised)
1 hormone molecule binds to each receptor
Receptor acts as an enzyme to phosphorylate ‘tyrosine’ components of intracellular regulatory proteins
Outline the characteristics of peptide hormones
- Chains of AA’s
- High water solubility (susceptible to protease attack therefore not orally active and short half-life)
- Cant cross cell membrane without carrier protein
- Act on membrane-bound receptors (RAPID onset)
- Short duration of action
Outline the characteristics of steroid hormones
- Poorly water soluble (transported in blood, bound to plasma protein)
- Orally active
- Crosses cell membrane to act on intracellular receptors (slow onset of action)
- Long plasma 1/2-life
- Long duration of action
Function of the anterior pituitary gland
Control secretion of a hormone from a gland
It in itself secretes stimulating (TROPHIC) hormones
In turn, the secretion of a hormone by the ant.Pituitary is controlled by the hypothalamus
Consider the control of the endocrine system
The anterior pituitary acts on a gland to stimulate secretion of a hormone.
Complete the name of the gland and hormone for each SH:
- Thyroid stimulating hormone (TSH)
- Adrenocorticotrophic hormone (ACTH)
- Follicle stimulating hormone (FSH)
- Luteinising hormone (LH)
- Thyroid gland, thyroxine (T4)
- Adrenal cortex, cortisol
- Ovary, oestrogens
- Ovary, Progesterone
The hypothalamus secretes “Hormone 1 “ to stimulate secretion of “Hormone 2 “ from the ant.Pituitary gland
Which “Hormone 1” leads to the release of :
- TSH
- ACTH
- FSH
- LH
- Thyrotrophin releasing hormone (TRH)
- Corticotrophin releasing hormone (CRH)
- Gonadotrophin releasing hormone (GnRH)
- Growth hormone releasing hormone (GHRH)
State the negative feedback function of the following hormones:
- Thyroid hormone
- Cortisol
- Oestrogens
- Acts to decrease secretion of TRH and TSH
- Acts to decrease secretion of CRH and ACTH
- Acts to decrease secretion of GnRH and FSH
What is the consequence of negative feedback of [hormone]
Cyclic variation
Varies at different times of the day (Always note at which time hormone conc. is being measured. Cortisol should be measured at 9am- shift workers excluded, measured first thing when waking up)
State 5 hormones secreted from the anterior pituitary gland
What is special about the destination of the R- hormones?
Growth hormone FSH LH TSH ACTH Prolactin
These travel from hypothalamus into hypothalamicoadenohyphophyseal portal vessel and do not enter systemic circulation
State 2 hormones secreted from the posterior pituitary gland
How do these two hormones compare?
Alternate name for posterior pituitary gland?
Function?
ADH
Oxytocin
Both peptide hormones, 9AA’s (7 identical–> overlap of activity)
Made by hypothalamic neurones (supraoptic nuclei and paraventricular nucleus)
Travel via hypothalamiconeurohypophyseal tract
Neurohypophysis
Post.PG converts these hormones into their active forms and stores.
Consider ADH
When is it secreted?
Mechanism of action?
Disorder?
Secretion is influenced by plasma osmolarity
Stimulation of V2 receptors causes translocation of aquaporins to the membrane of the kidney tubule –> water reabsorption
Stimulation of V1 receptors causes vasoconstriction (THIS DOES NOT OCCUR AT PHYSIOLOGICAL CONCS)
Disorders of ADH secretion –> disorders of fluid balance
Consider oxytocin
When is it secreted?
Mechanism of action?
How does synthesis differ at different times in a womans life?
Secretion stimulated by genital and nipple stimulation. Important in parturition (childbirth) as it causes contraction of SM of uterus aiding expulsion of child; and lactation (stimulated by sucking)
- Continues stimulation of the cervix causes continued secretion and SM contraction of the myoepithelial cells in . breast during breast feeding
- Not necessary for the initiation of a normal labour but oxytocin may be administered to induce labour
IP3 –> causes contraction of the smooth muscle of the genital tract (uterus) and breast
Synthesis increases during pregnancy, but there is a parallel increase in oxytokinase activity (to prevent premature contractions)
How has the body adapted to the delay in lactation?
There is a delay between suckling and milk ‘let down’
Pavlovian reflex- begin secretion in response to baby crying
- inappropriate secretion in response to other children crying
Function of prolactin?
Chemical structure?
Secretion?
Disorder?
Promote lactation and development of breast tissue
- Most important in women (post-puberty and during/after pregnancy) but also secreted in men
199 AA’s
- Synthesised in lactotrophe cels of anterior pituitary
- Secretion under hypothalamic control by means of Prolactin Release Inhibiting factor (DA); TRH stimulates prolactin synthesis
- Secretion stimulated by mild stress, nipple stimulation, coitus
Hyperprolactinaemia in breastfeeding mothers decreases fertility
Which cells synthesise TSH, ACTH and FSH?
- Thyrotrophe cells
- Corticotrophe cells
- Gonadotrophe cells
CLINICAL APPLICATION
Why is time of measurement of hormone relevant?
Diurnal rhythms (24 hour oscillations)
CLINICAL APPLICATION
In chronological order, outline how you would identify an endocrine disorder?
- Analyse signs and symptoms
- Biochemical tests: TSH & FT4, Cortisol, LH & FSH, Prl, Testosterone/’periods”
(compare with basal levels)
- Is it the gland that is the problem or hypothalamus? REMEMBERL: cant measure hypothalamus activity directly
e. g. Stimulate (ACTH) or suppress secretions (Dexmathsone suppression) - Imaging
- Visual field tests (because optic chiasm lies above pituitary gland)
- Bilateral hemianopia? (compression)
Consider growth hormone
Where is it synthesised?
Control of secretion?
Mechanism of action?
Function?
- Synthesised in somatotrophe cells of the anterior pituitary gland
- Secretion controlled by GHRH (most important) and GHRIH (somatostatin)
- Stimulated by low carbohydrates, fatty acids and AA’s
- In some tissues, acts via 2nd messengers produced in liver
e. g. Insulin-like GF-1, Insulin-like GF-2 - Promote linear growth of adolescence by increasing protein synthesis and collagen deposition (mediated by nucleus)
- FOETAL DEVELOPMENT IS RELATIVELY INDEPENDENT OF GH
