Hypothalamus-Pituitary axis

Question 1

What are the roles of the CNS in the regulation of endocrine system via the hypothalamus-pituitary axis?

Answer 1

• Hypothalamus controls autonomic function via brainstem autonomic centres and Controls endocrine function via the pituitary glands
  
  • Anterior pituitary hormones are regulated by secreted hypothalamic factors
  • Posterior pituitary hormone synthesised in the hypothalamus and transported via neuronal projections
    ○ The posterior pituitary gland is structurally continuous with the hypothalamus of the brain to which it remains attached to
    § During embryogenesis the posterior develops with the hypothalamus and anterior develops with at the pallet of the mouth then travel up to the posterior and attaches to it. This creates the 2 lobed shape
  • The hypothalamus sends this information/sends impulse to the pituitary gland, which then releases hormones that control other endocrine glands
  • Can also be caused by stress
  • There is a system of capillaries that carries hormone from the hypothallus to the anterior pituitary so the hormone will stay in this closed/portal system until relaced into the body
  • GW= growth hormone
  • MSH=darkens the skin (involved in changing skin colour in other Naomi also)
  • TSH=thyroid stimulating hormones
  • FSH and LH (gonadotropins) = for organises and testes
  • Prostaglandin=milk production
  • Oxytocin = milk production
  • ADH

Question 2

What are the hormones of the posterior pituitary gland and outline their regulation of secretion and their actions on target tissues?

Answer 2

• Vasopressin/ADH
  ○ Made in the hypothalamus as part of a large peptide in the ribosomes of neuronal cells bodies
  ○ Release is part of the neuroendocrine reflex
  ○ Has a half-life of 10 mins so is secreted when needed
  ○ Acts on the kidneys to increace water re-absorption by increasing the permeably of the collecting ducts to water to maintain the plasma osmolality close to 285mosmoles/KgH20
  ○ Change in the osmolality is detected by the osmoreceptors in the hypothalamus they detect the change and cause the relace of ADH
  ○ Baroreceptors in the heart (atrium) detest the change in blood volume and those in the arch of aorta and the carotid sinuses detects change in the pressure.
  ○ Causes Generalised vasoconstriction if there is a high concentration of ADH released
  ○ Released due to high plasma osmolality (can be due to low volume of water in the blood or a high concentration of osmolytes-so needs water to dilute the blood)
  ○ Increased extracellular osmolality will increace the relace if ADH as more water will move
  ○ Low blood volume will decrease the effective circulating fluid volume (lower ECFV and lower BP and BV) causes an increase in the release of ADH
  * Exercise and thinking about exercise can cause ADH relace
  ○ Decreased extracellular osmolality will cause water to move into the vessels. This will increase the blood volume and pressure and decrease the plasma osmolality. This will cause less ADH to be released as less water need to be reabsorbed
  § Can also be decreased
  * by alcohol which blocks the relace of ADH. Alcohol is a diuretic so more urine is produced as there CD is less permeable to water
  * Increased ECFV by the RAAS

    ○ Diabetes insipidus
        § Hypothalamus cannot make ADH/Pituitary gland cant secrete ADH/kidneys are unresponsive to ADH
        § Lack of ADH means kidneys can reabsorb water so lots of dilute urine is produced and can cause dehydration 
        § Both Diabetes insipidus and mellitus produce a lot of urine but Diabetes mellitus is due to the increased glucose levels due to lack of insulin (from pancreases)    * Oxytocin
    ○ Positive feedback cycle
    ○ Made in the hypothalamus as part of a large peptide in the ribosomes of neuronal cells bodies
    ○ Simulates contraction of smooth muscles in breast (for milk ejection) and uterus (birth). Can be used to induce labour.
    ○ Oxytocin causes contraction of the Nipple to release the milk
    ○ Tactile sense will send Action potential to the pituitary gland to release Oxytocin
    ○ Release is part of the neuroendocrine reflex
    1. Sensory Receptor receptors on the nipple are stimulated when the baby latches
    2. The receptors send sensory impulse to the spinal cord/CNS
    3. The spinal cord sends an impulse to the hypothalamic nuclei to stimulate the relace of oxytocin
    4. Pituitary gland will release oxytocin
    5. Oxytocin causes milk ejection

Question 3

What are the hormones of the anterior pituitary gland and outlie their regulation of secretion and their actions on target tissues?

Answer 3

• Produced by the secretory cells in the anterior pituitary
  ○ Thyrotropes
  ○ Gonadotrophs
  ○ Corticotropes
  ○ Somatotrophs
  ○ lactotrophs
  
  • Thyrotropin releasing hormone (TRH)
    ○ TSH=thyroid stimulating hormones
    ○ Glycoproteins
  • Follicle stimulating hormone (FSH)
    ○ Glycoproteins
    ○ Ovaries- stimulates the development of eggs and follicles
    ○ Testes- stimulates production of sperm
  • Luteinising hormone (LH)
    ○ Glycoproteins
    ○ Female- stimulates ovulation and corpus luteum to secrete progesterone
    ○ Males- stimulates interstitial cells of testes to secrete testeosteone
  • B subunit is the active hormone
  • The A sub is it is the same
  • Growth hormone (GH)
    ○ Proteins and polypeptide
    ○ Main effect is to stimulate liver to produce IGF-1
  • Adrenocorticotropic hormone (ACTH)
    ○ Proteins and polypeptide
    ○ Patients who over produce POMC there will be too much ACTH causing increased adrenal stimulation
    ○ Can also cause over production if y-MSH so can cause darkening of the skin
    * Will first cause changes of the gums
    ○ Responds to stress
    * Stimulates adrenal cortex to secrets corticosteroids
  • Prolactin
    ○ Proteins and polypeptide
    ○ Female- stimulates milk production after delivery
    ○ Male- lower sensitivity- cause testosterone secretion but this is unclear
  • THS- thyroid stimulating hormone
    ○ Stimulates growth of the gland and the secretion of thyroid hormone
  • Gonadotropin releasing hormone (GnRH)
  • Corticotropin releasing hormone (CRH)
  • Growth hormone releasing hormone (GRH / GHRH)
  • Somatostatin (Growth hormone inhibiting hormone)
  • Dopamine (Prolactin inhibiting hormone)
    ○ (catecholamine – neurotransmitter)
  • Prolactin releasing hormone (PRH) ??
  GH can have a small effect of glucose Absorbtion (same effect as insulin)
  • Evolution favours sexual reproduction and milk production
  • Compression of the optic nerve Due to growth/ tumour around the nerve can cause vision loss