2019B Question 13
Briefly outline the roles of the hypothalamus.
Examiner Report
45.1% of candidates achieved a pass in this question.
The major domains assessed in this question were:
- Autonomic nervous system
- Thermoregulation
- Osmolarity
- Anterior pituitary
A pass could be attained with demonstration of understanding of at least 3 of the major domains, and credit was also given for other relevant correct material (feeding, emotion, oxytocin, circadian rhythm etc).
A common problem was focusing solely on one or two of the hypothalamic functions in detail, without “briefly outlining” all or most of the major roles. This topic is covered in physiology texts including those by Kam/Power, Guyton or Ganong.
Model Answer
Structure:
- Introduction
- Autonomic function
- Endocrine function
- Other specific homeostatic functions
Introduction
- Subcortical component of the limbic system
- Control centre for endocrine and involuntary neural functions
- Dense connections with higher and lower centres
Autonomic Function
| Region | Response |
|---|---|
| Anterior Stimulation | - Depressor area → PSNS response |
| Posterior Stimulation | - SNS response - Effect via hindbrain (e.g. RVLM) and spinal cord (intermediolateral horn) - Organ-specific projections: e.g. Skeletal muscle vs skin vs splanchnic - Stimulation by ARAS maintains wakefulness |
Endocrine Function
| Region | Function |
|---|---|
| Posterior Pituitary | - Releases into infundibulum from the SON and PVN: - ADH → Water retention, vasoconstriction - Oxytocin → Myometrial contraction in labour, behavioural effects |
| Anterior Pituitary | - Secretes into hypophyseal portal system - CRH → ↑ ACTH → ↑ Cortisol → Stress response - TRH → ↑ TSH → Thyroid hormones → Metabolic homeostasis - GHRH → ↑ GH → ↑ IGF → Tissue growth, metabolic effects - GnRH → ↑ FSH, LH → Reproductive function - ↓ Dopamine → ↑ PRL → Lactation |
Other Specific Homeostatic Functions
| Domain | Details |
|---|---|
| Temperature | - Senses temperature (anterior); controls set point (posterior); responds outside inter-threshold range - Anterior stimulation (pre-optic nuclei): Triggers autonomic and behavioural heat loss responses (e.g. Sweating, cutaneous vasodilatation) - Posterior stimulation (tuberal nuclei): Triggers autonomic and behavioral heat gain and conservation responses (e.g. Shivering, cutaneous vasoconstriction) |
| Water | - Paraventricular region: Contains a thirst centre activated by hypertonicity, hypovolaemia, dry mouth, other - ↑ ADH synthesis - ↑ Thirst → Water-seeking - Contains internal osmoreceptors: Supraoptic (and paraventricular) nuclei - Receives input from external osmoreceptors: SFO, OVLT |
| Food | - Ventromedial: Senses [glucose], induces satiety - Lateral: Induces hunger and food-seeking |
| Sleep-wake | - Suprachiasmatic nucleus: Generates circadian rhythm (input from photosensitive cells) |
| Emotion | - Well connected to other limbic structures (e.g. Limbic lobe, amygdala, hippocampus) - Mediates emotional expression via autonomic and somatic nervous systems - Defence area: Anterior perifornical region - Activated by the limbic system - Stimulation → ↑ HR, ↑ BP, vasodilatation of skeletal muscle arterioles, splanchnic and renal vasoconstriction - i.e. fight-or-flight response - Depressor area: Anterior region - Stimulation → ↓ HR, ↓ BP - ?Accompanies the ‘playing dead’ response in some young animals |
| Sexual Function | - By control of pituitary hormones: Onset of puberty; menstrual cycle - Medial region control sexual behaviour |