Endocrine - Hypothalamus and pituitary Flashcards
Define a hormone
Substance released by a cell, gland or organ in one part of the body that exerts its effects on tissues elsewhere in the body
How are hormones classified
PEPTIDE HORMONES
- Short chain
- -> TRH
- -> ADH
- -> ACTH
- -> Insulin - Longer chain
- -> GH
- -> PRL - Glycopeptides (peptide chain plus CHO attached)
- -> LH
- -> FSH
- -> TSH
LIPID and PHOSPHOLIPID DERIVED HORMONES
- Steroid hormones
- -> Cortisol
- -> Aldosterone
- -> Testosterone
- -> Oestrogen - Eicosanoids
- -> Prostaglandins
- -> Thromboxanes
- -> Leukotrienes
MONOAMINE DERIVATIVES
- Catecholamines
- Serotonin
- Thyroxine
How do peptide hormones bring about their effects at target tissues
Stored as granules
Released by exocytosis
Bind to CELL SURFACE RECEPTORS
From which substance are the steroid hormones derived
from which substance are eicosanoids derived
Steroids –> from cholesterol
Eicosanoids –> Phospholipid bilayer of cell membranes
Are the steroid hormones synthesized and stored?
No. They are synthesized as required and immediately released into circulation
What are the mechanisms of action of steroid hormones and eicosanoids
Steroids
–> High lipid soulbility –> cross cell membrane and bind to cytosolic receptors –> steroid receptor complex enters the cell nucleus to influence gene transcription.
Eicosanoids
–> complex mechanisms of action with a wide range of functions throughout the body
Describe the mode of action of the monoamine-derived hormones
Catecholamines and serotonin
- -> Stored as granules prior to release
- -> Cell membrane receptors
Thyroxin
- -> Incorporated into thyroglobulin
- -> Binds to receptors at the cell nucleus
Summarise the functions of the hypothalamus
- Autonomic nervous system (e.g. limbic system –> fear –> hypothalamus –> SNS response)
- Thermoregulation
- Hunger
- Water regulation
- Sleep-wake cycles (circadian rhythms)
- Control of Pituitary
- Behaviour (punishment & reward)
- Regulation sexual function
How is thermoregulation regulated by the hypothalamus
It integrates signals from the peripheral and central (hypothalamic) thermoreceptors. and controls the balance of activities between the two hypothalamic centers: heat loss and heat gain center
How does the hypothalamus regulate hunger
Controlled by the relative activities of the feeding and satiety centres in the hypothalamus. These centers are influenced by glucose, GIT hormones (CCK and glucagon) and adipose hormones (Leptin)
how does the hypothalamus regulate water
- Thirst center
- Osmoreceptors in the hypothalamus control renal water excretion in conjunction with ADH secretion by the pituitary gland
How does the hypothalamus control sleep wake cycles
Stimulate anterior hypothalamus –> sleep
Stimulate posterior hypothalamus –> wakefulness
Circadian rhythms originate here
How does the hypothalamus control pituitary function
Anterior lobe is controlled by secretion of hypothalamic hormones into the long portal vein
Posterior lobe is controlled by direct neural connections from the hypothalamus
Describe the anatomy of the pituitary/hypothalamus
Pituitary is a pea sized gland located in the sella turcica, a depression in the sphenoid gland at the base of the skull. It is situated directly below th hypothalamus and is connected to the hypothalamus by the pituitary stalk
The diaphragma sella is a fold of the dura mater which covers the pituitary superiorly. A gap allows the pituitary stalk to pass through
What important structures lie adjacent to the pituitary
Superior: pituitary stalk and hypothalamus
Laterally: Cavernous sinus
What important structures lie within the cavernous sinus
3, 4, 5(1), 5(2), 6 and the internal carotid artery
Describe the anatomy of the pituitary
Larger lobe = anterior
Smaller lobe = posterior
Separated by pars intermedia
Describe the blood supply to the pituitary gland
ANTERIOR LOBE
Superior hypophyseal artery (branch of internal carotid artery)
Long portal veins
–> connect hypothalamic capillary network to anterior pituitary capillary network
Short portal veins
–> connect posterior pituitary lobe capillary network to anterior lobe capillary network
POSTERIOR LOBE
Inferior hypophyseal artery (branch of internal carotid)
VENOUS BLOOD FROM BOTH DRAINS INTO THE CAVERNOUS SINUS
List the hormones secreted by the hypothalamus
- TRH (secretion of TSH)
- GnRh (secretion of FSH/LH)
- CRH (secretion ACTH)
- GHRH (Secretion GH)
- Somatostatin (Inhibits GH release. Slight inhibition TSH)
- Dopamine (Inhibits PRL release)
List the hormones secreted by the pituiatry
ANTERIOR LOBE Stimulating hormones 1. TSH 2. FSH 3. LH 4. ACTH
Directly acting hormones
- PRL
- GH
POSTERIOR LOBE
- ADH
- Oxytocin
What is meant by the term ‘hypothalmic-pituitary axis’
Two negative feedback loops
1. Short loops –> anterior pituitary lobe hormones inhibit hypothalamus
- Long loops –> peripheral endocrine glands secrete hormones in response to hormones secreted by anterior pituitary. These peripheral hormones inhibit the hypothalamus
What is the function of TSH
Stimulate synthesis and release of T3 (and T4) from the thyroid gland
What is the function of LH in males and females
FEMALES
Rapid increase LH –> ovulation in females
Following ovulation, LH promotes the development of the corpus luteum
MALES
LH stimulates the secretion of testosterone by Leydig cells
What is the function of FSH in males and females
FEMALES
- Promotes oestrogen synthesis
- Promotes development of ovarian follicles
MALEs
1. FSH aids sperm maturation
What is the function of ACTH
Cortisol release from the zona fasiculata
What causes skin hyperpigmentation in Addison’s disease
Excessive ACTH is degraded to produce alpha-MSH (alpha - melanocyte stimulating hormone)
What is the function of GH
DIRECT EFFECTS
–> Stimulates lipolysis through its action on adipose cell GH receptors –> increasing circulating ffas
INDIRECT EFFECTS
–> Stimulates liver secretion of IGF-1 (insulin-like growth factor 1) –> promotes cell growth and development.
Describe the release and inhibition of GH
Pulsatile release from anterior pituitary –> particularly at night.
GHRH –> stimulates GH secretion
Somatostatin –> inhibits GH secretion
IGF-1 –> inhibits GH release
Most of the time GH levels are low
Increased levels during physiological stress, hypoglycaemia and exercise
What is the function of prolactin
Promotes breast development during pregnancy
Promotes milk production following delivery
How does PRL secretion differ from other pituitary hormones
There is no releasing hormone for PRL
Hypothalamus controls PRL secretion through tonic dopamine release –> inhibits PRL release
Stimulation of PRL release
- Suckling
- Sex
- Stress response
- Epileptic seizure
What is the seventh hormone secreted by the pars intermedia of the hypothalamus and when does the secretion of this hormone become clinically apparent
alpha-MSH (melanocyte stimulating hormone)
This is responsible for the skin pigmentation during pregnancy
Name the hormones secreted by the posterior lobe of the pituitary and state where these hormones are synthesized. How are these hormones released into circulation
ADH and Oxytocin
These are synthesized in the hypothalamus, packaged into tiny vesicles and transported to the posterior lobe through nerve axons and then stored in granules within the nerve terminals. Action potential from the hypothalamus cause release of the granules into systemic circulation.
What are the physiological effects of ADH
- Insertion of aquaporins and urea transporters (UT-A1) into the cell membrane of collecting ducts –> increased water absorption.
- Vasoconstriction
- -> at normal concentrations –> minimal contribution of ADH to arterial tone
- -> At high concentrations –> pwerful vasoconstrictor
What stimulates and inhibits ADH secretion
Stimulates
- Increased plasma osmolarity
- Decreased blood pressure (lesser extent)
Inhibits
1. Alcohol (leading to diuresis)
What are the effects of oxytocin
- Contraction of uterine smooth muscle during labour
- Let-down reflex in lactation (milk into duct system)
- Psychological ‘cuddle hormone’
What stimulates oxytocin release
- Stretching of the cervix by the fetal head
2. Suckling –> myoepithelial cells contract –> allowing newly formed milk into the duct system
What are the four ways that a benign tumour of the anterior lobe of the pituitary can present
- Hormone hypersecretion (e.g. galactorrhoea with excessive PRL)
- Hormone hyposecretion (e.g. hypothyroidism / adrenocortical insufficiency / infertility)
- Mass effect
E.g. 1. optic chiasm –> bitemporal hemianopia)
E.g. 2. Cranial nerve palsy
E.g. 3. 3rd ventricle obstruction –> hydrocephalus - Incidental finding (CT/MRI for another reason)
Describe the anaesthetic concerns with regard to pituitary debulking surgery
Most commonly excised via the trans-sphenoidal approach
- Shared airway
- Minimal haemodynamic instability and a smooth extubation
