The Pituitary Flashcards
Describe the role and importance of the Hypothalamus and pituitary gland
The hypothalamo-pituitary axis - containing these - is the principal organiser of the endocrine system
Identify the parts of the Hypothalamo-pituitary axis
What is the pituitary gland?
A small, bean-shaped endocrine gland contained within the sphenoid bone.
It is directly below and connected both physically and neurally to the hypothalamus
It contains an anterior and posterior portion
Describe the differences between the anterior and posterior portions of the pituitary gland?
Anterior - “Adenohypophysis”:
- Truly endocrine portion
- Epithelial in origin
- Acted upon by the hypothalamus via a capillary portal system
Posterior - “Neurohypophysis”:
- Neuroendocrine portion - secretes hormones made in the hypothalamus
- Extension of the neural tissue from Infundibulum and hypothalamus
- Made up of axons & terminals of hypothalamic neurones
- Neural in origin
Describe the function of the hypothalamus with relation to the pituitary gland
Hypothalamus serves as the integration centre for endocrine stuff - basically connecting the nervous system to the endocrine system
It secretes neurohormones - these act on the pituitary gland in 2 ways:
- Neurohormones secreted in the hypothalamus can travel to the anterior pituitary (endocrine) portion via a capillary portal system
- Neurohormones can be released directly from the posterior portion - via the hypothalamic neurones in the infundibulum and posterior pituitary gland
What is a tropic hormone?
One that governs the release of another hormone
Hypothalamic hormones can either be tropic or non-tropic. Explain why this is
Non-tropic - neurohormones produced in the hypothalamus and travel to posterior pituitary (via axons of hypothalamic neurons) where they are released into blood.
Tropic – neurohormones secreted into capillaries travelling to anterior pituitary. Govern release of anterior pituitary hormones.
What hormones are released in the posterior pituitary
Posterior pituitary contains magnocellular neurones which have their cells bodies up in the hypothalamus
These neurones either store and release 2 neurohormones - Vasopressin & Oxytocin
Vasopressin (aka ADH) - antidiuretic hormone that maintains our water balance
Oxytocin - stimulates uterine contraction at parturition, and aids expression of milk in lactating breast
How do neurohormones enter the bloodstream in the posterior pituitary gland?
The terminals of the axons in the posterior pituitary act directly onto capillaries which drain into the systemic circulation
Identify the stages in the secretion of neurohormones in the posterior pituitary gland
What type of hormones are oxytocin and vasopressin?
oxytocin and vasopressin behave as typical peptide hormones
i.e. synthesis and storage in vesicles, cell surface receptors etc
What ‘releasing’ hormones does the hypothalamus release for the anterior pituitary?
What type of hormones are these?
- Thyrotropin Releasing Hormone (TRH)
- Corticotropin Releasing Hormone (CRH)
- Growth Hormone Releasing Hormone (GHRH)
- Gonadotropin Releasing Hormone (GnRH)
- Prolactin Releasing Hormone (PRH)
These are all tropic (releasing) neurohormones hormones as they govern the release of hormones from the anterior pituitary
What inhibiting hormones does the hypothalamus secrete which act on the anterior pituitary?
- Somatostatin - aka Growth Hormone Inhibiting Hormone (GHIH)
- Dopamine - aka Prolactin Inhibiting Hormone (PIH)
Although inhibitory - these are still tropic neurohormones
List the tropic hormones produced by the hypothalamus and state what structural type they are - amine, peptide etc
- Thyrotropin releasing hormone (TRH) - peptide
- Corticotropin releasing hormone (CRH) - peptide
- Growth hormone releasing hormone (GHRH) - peptide
- Gonadotropin releasing hormone (GnRH) - peptide
- Prolactin releasing hormone (PRH) - peptide
- Somatostatin (GHIH) - peptide
- Dopamine (PIH) - amine
What is the Hypothalamo-hypophyseal portal system?
Network of tiny vessels which transfer trophic hormones from hypothalamus to anterior pituitary
Formed of two capillary beds connected in series
How do tropic hormones from the hypothalamus enter the Hypothalamo-hypophyseal portal system?
Hormones released from neurosecretory neurons at the median eminence
What hormones does the anterior pituitary gland release?
Anterior pituitary release 6 peptide hormones, 5 of which are tropic
- Thyroid-stimulating hormone (TSH) aka Thyrotropin
- Adrenocorticotrophic Hormone (ACTH) aka Corticotropin
- Follicle-stimulating hormone (FSH)*
- Luteinising Hormone (LH)*
- Growth hormone (GH)
- Prolactin (not tropic)
Which of the hormones produced by the anterior pituitary gland are Gonadotropins?
Follicle-stimulating hormone (FSH)
Luteinising hormone (LH)
What does prolactin do?
Prolactin directly stimulates milk production from the breast during lactation
What organs do the AP hormones go to?
Broadly speaking, how does feedback control work for the anterior pituitary gland?
Control of hormone secretion is mediated by the Hypothalamus, Anterior pituitary and Endocrine gland specific to that hormone
Hormones (including Tropic hormones) work to inhibit their secretion from earlier in the reflex.
The feedback system can be long-loop or short-loop…
For the feedback control of the anterior pituitary gland - which mechanism is long-loop and which is short-loop?
Short-loop:
- Tropic Hormone 2 from AP inhibits hypothalamus
Long-loop:
- Hormone inhibits the target endocrine cell and upwards
Hypo or hypersecretion of hormones can result in disease. These can be separated into orders. What do these orders mean?
1° disorders - are those in which the defect is in the cells that secrete the hormone
2° disorders - are those in which there is too little or too much trophic hormone from pituitary
3° disorders - relate to hypothalamic defects
What is Hyporesponsiveness?
Hyporesponsiveness relates to alterations in receptor for hormone, disordered post-receptor events, failure of metabolic activation of hormone, (where this is required for function) or antagonistic effects.
Basically just generated too weak an effect in response to a hormone
What typically causes hypo/hyperresponsiveness to develop?
Prolonged exposure to low [hormone]plasma often leads to up-regulation of receptor number.
This increases tissue sensitivity/response to hormone
(And vice versa)
This process may also affect the receptors for other hormones
What is meant by ‘permissive’?
When the presence of one hormone enhances the effect of another
eg - Epinephrine causes only modest lipolysis in adipose tissue, but when thyroid hormones are also present, greatly increased lipolysis occurs.
The opposite of this is ‘antagonistic’ - when presence of 1 hormone reduces the effect of another
