Hypothalamo pituitary axis Flashcards
Endocrine secretion
Hormones diffuse directly into capillaries to act on distant target organs
Paracrine secretion
Secreted and act more locally
Autocrine secretion
Act on themselves
What are the major endocrine glands?
- Pineal gland
- Hypothalamus
- Pituitary
- Thyroid
- Parathyroid
- Thymus
- Adrenal gland
- Pancreas
- Gonads - ovary and testes
What are the 3 categories of factors controlling hormone release from endocrine glands?
- Humoral
- Neural
- Hormonal
Describe the humoral factors affect hormone release
Response to changes in extracellular fluid e.g.
• Capillary blood has a low concentration of Ca2+ which stimulates the secretion of parathyroid hormone by parathyroid glands
• Release of insulin in response to blood glucose
Describe the neural factors affecting hormone release
Preganglionic sympathetic nervous system fibres stimulating the adrenal medulla cells to secrete catecholamines
What are the hormonal factors affecting hormone release?
- Hypothalamus secretes hormones/neurohormones that stimulate the anterior pituitary to secrete hormones that stimulate other glands to release hormones
- Amplifiable response
What is the pituitary gland enclosed by?
- Sella tunica of the sphenoid
* enclosed superiorly by diaphragma sellae
Where does the pituitary gland lie?
Immediately posterior to the optic chiasma and sphenoid sinus, on the floor of the midbrain
What are the parts of the anterior pituitary?
- Pars anterior (distalis)
- Pars tubercles (PT) surrounds the neural stalk
- Pars intermedia - exists between anterior and posterior pituitary
What is the anterior pituitary derived from?
• Rathke’s pouch
Describe the embryological development of the pituitary
- Anterior pituitary: upgrowth of epithelium from the oral cavity (Rathke’s pouch) - hypophysial diverticulum
- Posteiror pituitary: down growth from the brain (infundibulum) - neurohypophyseal
Craniopharyngiomas
Slow growing tumours that can develop along the track of Rathke’s pouch
Describe chromaphils
- Take up stain, there are two types
- Acidophils
- Basophils
What is the name given to cells that do not take up stian
Chromophobes
Describe the nuclei in the hypothalamus
• Large cell bodies: - paraventricular - supraoptic - infundibular • Small cell body nuclei
Describe the course of the axons of the large cell bodied nuclei
- Extend down through the median eminence
- Come together to form a nerve tract: hypothalmo-hypophyseal tract
- Fibres terminate in the posterior pituitary on a capillary network supplied by the inferior hypophyseal artery
- Herring bodies
Describe the course of the axons of the small cell bodied nuclei
- Axons terminate higher up on a capillary network in the median eminence or pituitary stalk
- Capillary network is supplied by the superior hypophyseal artery: called the primary capillary plexus
- Neurohormones released into the network drain into the hypothalmo-hypophyseal portal system which goes on to form a secondary capillary network within the anterior lobe of the pituitary
- The neurohormones diffuse out of the fenestrated capillaries and act on receptors (trophic cells) in the anterior pituitary to regulate secretions from adenohypophyseal cells
What ar the neurohormones released from the posterior pituitary?
- Vasopressin/ADH
* Oxytocin
What are the trophic hormones of the anterior pituitary
- Thyrotropin/ thyroid stimulating hormone
- Corticotropin/adrenocorticotropic hormone
- Gonadotropins: LH and FSH
What hormones released from he anterior pituitary act on peripheral targets?
- Somatotropin/ Growth hormone
- Prolactin
- Melanocyte stimulaitng hormone
Describe growth hormone release control
- Neurosecretory cells in the arcuate nucleus of the hypothalamus secrete growth hormone releasing hormone
- Reaches somatotrophs via the hypophyseal portal blood supply
- GHRH causes somatotrophs to synthesise and release growth hormone (in the anterior pituitary) by activating a stimulating g protein coupled receptor
Describe the inhibition of growth hormone release
- Somatotropin is released from the periventricular region
- Inhibits the release of GH by the somatotrophs
- Activates an inhibiting G protein coupled receptor
Describe feedback of growth hormone
• IGF-1 directly inhibits GH release by suppressing the somatotrophs
• IGF-1 indirectly inhibits GH release by:
- suppressing GHRH release
- increasing the secretion of somatostatin
• GH inhibits its own secretion via short loop feedback on somatotrophs
Describe the circadian rhythms for growth hormone secretion
- Pulsatile release of GH throughout the day
* Spikes during sleep
What are the physiological action fo growth hormone?
• Direct anti insulin effect:
- increased lipolysis in adipose tissue
- increased blood glucose
• Indirect actions (IGF-1 release from the liver)
- Increased cartilage formation and bone growth
- increased general protein synthesis and cell growth/division
What are the physiological consequences of growth hormone?
• Increased linear growth and lean body mass
• Vital importance for normal post natal development and rapid growth throughout puberty
• Maintenance of protein synthesis and tissue function in adults
- maintains protein turnover rate
GH deficiency
- Dwarfism in children
* Decreased availability of lipids and glucose for energy: treat with growth hormone
GH excess- before puberty
• Gigantism due to excess stimulation of the epiphyseal plates
GH excess after puberty
- Periosteal bone growth causing enlarged hands, jaw and foot size
- Soft tissue growth leading to enlargement of the tongue and coarsening of facial features
- Insulin resistance and gluten intolerance
- Treat with synthetic long acting somatostatins
Where is ADH synthesised?
In the neurosecretory cells in the supra-optic nucleus and paraventricular nucleus
What increases ADH release?
- Increased blood osmolarity detected by osmoreceptors in the brain
- Decreased blood volume detected by volume receptors (carotid, aorta, left atrium)
- RAAS - angiotensin II
What is the effect of ADH release?
Recruitment of aquaporin water channels -> water retention
ADH deficiency
- Diabetes insipidus
* Neural or nephrogenic
What are causes of cranial diabetes insipidus?
- Tumours - 30%
- Disease induced/trauma -> damage to the hypothalamus
- Familial disorders of the neurosecretory cells
What are the causes of nephrogenic diabetes insipidus?
- Sex linked genetic defect in the collecting tubule (receptor doesn’t recognise ADH)
- Non sex linked genetic defect in the aquaporin water channel
Describe the control of milk production
- Suckling stimulus travels through the spinal cord to the hypothalamus
- Neurones from the hypothalamus inhibit dopamine release from the arcuate nucleus leading to prolactin release -> milk production
- Neurones from the spinal cord stimulate the production and release of oxytocin from the paraventricular and supraoptic nuclei in the posterior pituitary -> systemic circulation -> breast and my-epithelial cells
- neurones in the spinal cord inhibit neurones in the arcuate nucleus and preoptic area of the hypothalamus causing a fall in GnRH, inhibiting the ovarian cycle
Where is the pineal gland?
• In the midline in the posterior part of the 3rd ventricle
What are the functions of the pineal gland?
- In darkness the pineal gland secretes melatonin
- Regulates circadian rhythm
- Regulates reproductive processes including the onset of puberty in humans
- Effects on ageing and regulation fo the immune system
- Accumulate calcium phosphate
