55.1 Functions of the Hypothalamus Flashcards
What are examples of the functions of the hypothalamus?
- Endocrine
- Autonomic
- Survival functions (eating, sleep, sexual behaviour, aggression)
What are the different hypothalamic nuclei?
What are the roles of the hypothalamic nuclei?
Homeostasis, rhythms, development (e.g. puberty), metabolism, control of ANS, and endocrine system control
How is the circuitry of the hypothalamus different to other brain areas?
- There are both neural and extensive non-neural interconnections
- Interconnections are bidirectional (besides a few exceptions like retinal input)
Which systems is the hypothalamus interconnected with?
- Limbic circuits
- Sensory and autonomic circuits
- Neurohumoral systems: Pituitary gland, circumventricular organs
What do the connections to the pituitary gland and circumventricular organs allow?
Monitoring of plasma levels of hormones, metabolites, and plasma osmolarity
How do hypothalamic neurons connect to the anterior posterior pituitary gland?
Secrete releasing factors into primary capillary plexus in the median eminence
Travel via hypothalamic-hypophyseal portal vessels to anterior pit
How do hypothalamic neurons connect to the posterior pituitary gland?
Directly travel down into posterior pit
Secrete vasopressin/oxy into blood supply
What are the circumventricular organs and their role?
Structures in the brain with extensive and highly permeable capillaries (no BBB)
Either sensory or secretory
In contact with CSF and blood so permit communication between CNS and peripheral blood
What are examples of sensory circumventricular organs?
Area postrema
Subfornical organ (SFO)
Organum vasculosum of Lamina terminalis (OVLT)
What are examples of secretory circumventricular organs?
Pituitary gland
Median eminence
Pineal gland
How does the hypothalamus control metabolism?
- Appetite control - leptin, insulin etc signalling
- Thyroid (T3) action
- Glucose sensing
- Amino acid sensing
What is the role of the hypothalamus in growth?
Releases GHRH to stimulate GH release
Releases somatostatin to inhibit GH release
(GH stimulates IGF-1 release from liver)
What is the role of the hypothalamus in triggering puberty?
Kisspeptin activates GnRH neurons to start releasing GnRH in 90 min pulses
Causes FSH and LH release from anterior pit
Which hypothalamic nuclei is involved in blood pressure and volume control?
Paraventricular
Supraoptic
Which circumventricular organ senses changes to blood osmolarity?
OVLT
What exactly does the OVLT sense?
Extracellular NaCl and angiotensin II concentrations
Describe the circuitry response to high dietary salt intake
Increases plasma sodium and osmolality - activates OVLT (and other sodium sensors)
Excites PVN and SON in hypothalamus
Triggers vasopressin release from posterior pit
Causes vasoconstriction and water retention in kidney
Activates arterial baroreceptors which inhibit firing - decreases BP
Which are the two main nuclei controlling the anterior pituitary gland?
Arcuate nucleus and paraventricular nucleus
Which hormones are under control by the arcuate nucleus?
GnRH
GHRH
Prolactin
Which hormones are under control by the paraventricular nucleus?
TRH
CRH
Oxytocin
Vasopressin (AVP)
What are the two release-inhibiting factors released by the hypothalamus?
Dopamine
Somatostatin
What does dopamine inhibit?
Prolactin secretion
What does somatostatin inhibit?
Growth hormone secretion
What does TRH stimulate?
TSH secretion from anterior pit
What does TSH stimulate?
T3 and T4 release from thyroid gland
Iodine uptake
Which molecules provide negative feedback of TRH/TSH release?
T3 and T4
What does CRH stimulate?
ACTH release from anterior pit
What does ACTH stimulate?
Glucocorticoid secretion from adrenal gland
How is stimulation of ACTH controlled by systemic circulation?
Inhibited by glucocorticoid –> negative feedback
What stimulates CRH release from the hypothalamus?
Stress, hypoglycaemia
What are the two types of feedback signals?
- Peripheral hormones
- Metabolic signals (e.g. leptin)
What is a common local effect of pituitary tumours?
Bitemporal hemianopia (and other visual disturbances)
Compression on optic chiasm
What is the most common pituitary tumour?
Prolactinoma - excessive growth of lactotrophs
What are the symptoms of prolactinoma? (women/men/both)
Women: low oestrogen - irregular periods, acne, excessive body and facial hair
Men: low testosterone - erectile dysfunction, decreased body and facial hair, enlarged breasts
Both: low bone density, headaches, visual disturbances, infertility
What does somatotroph adenoma cause?
Acromegaly (a rare condition where the body produces too much growth hormone, causing body tissues and bones to grow more quickly)
What does corticotroph adenoma cause?
Cushing’s syndrome
How does a tumour of some pituitary cells affect the other cells?
Impairs their survival
So less secretion of hormones from other cells
What are the main releasing and inhibitory hormones produced by the hypothalamus that act on the anterior pituitary?
[IMPORTANT]
Releasing:
- GnRH
- GHRH
- TRH
- CRH
Inhibitory:
- Somatostatin
- Dopamine
What are the roles of the hypothalamus in thermoregulation?
- Stimulating thermogenesis
- Shivering, piloerection, skin vasoconstriction
- Behaviours that increase body temperature
- Stimulating heat loss
- Sweating, skin vasodilation
- Behaviours that promote body temperature cooling
- Inducing fever
Which part of the hypothalamus determines the body temperature set point? How can this set point be changed?
- Anterior hypothalamus
- Set point can be elevated by pyrogens -> PGE2 acting on EP3 receptors
How can the supraoptic and paraventricular nuclei in the hypothalamus detect changes in blood osmolarity?
- They receive input from intrinsically osmosensitive neurons in the circumventricular organs (OVLT and SFO) and the nucleus tractus solitarius (NTS)
- The supraoptic and paraventricular nuclei are also osmosensitive but are less well understood
How can a cell be intrinsically osmosensitive?
- A change in osmolarity will cause cell swelling or shrinking, resulting in increased or decreased stretch of plasma membrane
- This causes opening/closing of membrane ion channels via:
- Direct stretch-activation of the ion channels
- The ion channel being tethered to the ECM or the cytoskeleton, which causes it to open as the cell stretches
- Indirect activation of the ion channels via a mechano-sensitive
What does the posterior pituitary secrete?
Oxytocin and ADH
