Hypothalamus: Anatomy & Physiology

Anatomical Location & Gross Anatomy

Fig. 17.2 - Neuroanatomy through Clinical Cases, 3rd Ed.

• Forms the walls and floor of the inferior third ventricle
• Separated from the thalamus by the hypothalamic sulcus (a shallow groove on the third ventricle wall)
• On the ventral brain surface: visible posterior to the optic chiasm
• Tuber cinereum ("gray protuberance") - a bulge between the optic chiasm and mammillary bodies
• Mammillary bodies - paired structures forming the posterior portion
• Infundibulum - arises from tuber cinereum, continues as the pituitary stalk
• Median eminence - anterior infundibulum; where hypothalamic releasing factors enter the portal vessels

Hypothalamic Nuclei (Medial Area)

Fig. 17.3 - Neuroanatomy through Clinical Cases, 3rd Ed. Nuclei divided into four regions anterior to posterior.

Functional Map of Nuclei

Fig. 59.6 - Guyton & Hall Medical Physiology

Physiology: Major Control Functions

1. Neuroendocrine Control (Hypothalamic-Pituitary Axis)

• Anterior pituitary: Hypothalamic neurons release releasing and inhibiting hormones into the hypophyseal portal system at the median eminence. These include:
  • TRH (thyrotropin-releasing hormone)
  • CRH (corticotropin-releasing hormone)
  • GHRH / somatostatin (growth hormone)
  • GnRH (gonadotropin-releasing hormone)
  • Dopamine (inhibits prolactin)
• Posterior pituitary: Neurons in the supraoptic and paraventricular nuclei project axons directly into the posterior pituitary, releasing oxytocin and vasopressin (ADH) into systemic circulation.

2. Autonomic Nervous System

• Preganglionic parasympathetic nuclei (brainstem + sacral cord)
• Preganglionic sympathetic neurons in the intermediolateral cell column (T1-L2)
• Posterior/lateral hypothalamus stimulation → sympathetic (increased BP, HR, pupil dilation)
• Preoptic area stimulation → parasympathetic (decreased HR, BP)

3. Temperature Regulation

4. Hunger & Satiety

• Ventromedial nucleus - "satiety center": bilateral destruction causes hyperphagia and obesity
• Lateral hypothalamic area and perifornical nucleus - "feeding center": destruction causes anorexia
• Arcuate nucleus - responds to leptin, ghrelin; integrates appetite signals

5. Thirst & Water Balance

• Supraoptic nucleus secretes ADH in response to increased plasma osmolarity or decreased blood volume
• Osmoreceptors near the supraoptic nucleus monitor plasma osmolarity
• Lateral hypothalamic area drives thirst behavior

6. Circadian Rhythms

7. Emotional Behavior & Limbic Integration

• Hippocampus, amygdala, cingulate gyrus, septal nuclei
• Prefrontal cortex (orbitofrontal area)
• Brainstem reticular formation (via the medial forebrain bundle)

Summary Mnemonic: "TAN HATE"

• Neuroanatomy through Clinical Cases, 3rd Edition - pp. 822-824 (Blumenfeld)
• Guyton & Hall Textbook of Medical Physiology - pp. 733-734 (Chapters 59, 76)

Why Does Prolactin Increase? (Hyperprolactinemia)

The Core Mechanism First

Key principle: Anything that reduces dopamine action on lactotrophs OR directly stimulates prolactin secretion will raise prolactin levels.

Categories of Causes

1. Physiological (Normal) Causes

2. Pathological - Pituitary Tumor (Most Common Pathologic Cause)

• Prolactinoma - a benign lactotroph adenoma of the anterior pituitary. It is the most common hormone-secreting pituitary tumor (~50% of all pituitary adenomas). Prolactin secretion is proportional to tumor size - very high levels (>200 µg/L) strongly suggest prolactinoma.
• Other pituitary tumors (non-prolactinoma) - any suprasellar mass can compress the pituitary stalk, blocking dopamine delivery to lactotrophs, causing a moderate rise (up to ~2000 mU/L). This is called the "stalk effect."

3. Loss of Dopamine Inhibition ("Stalk Effect")

• Head trauma
• Craniopharyngioma or other suprasellar masses
• Infiltrative disease (sarcoidosis, lymphoma)
• Surgical disruption

4. Drugs (Very Common Cause - Must Always Be Excluded)

5. Systemic Diseases

6. Chest Wall Pathology

• Chest surgery or trauma
• Herpes zoster (shingles) affecting the chest
• Burns to chest wall

Clinical Consequences of High Prolactin

Quick Diagnostic Approach

Raised prolactin found
        |
Check medications first (most common reversible cause)
        |
If on no offending drugs → MRI pituitary
        |
Prolactin >200 µg/L → likely prolactinoma
Prolactin 20-200 µg/L → consider stalk compression, hypothyroidism, renal failure
        |
Check TFTs (thyroid function) and renal function

• Tintinalli's Emergency Medicine: A Comprehensive Study - Table 104-1
• Robbins, Cotran & Kumar Pathologic Basis of Disease - p. 737
• Scott-Brown's Otorhinolaryngology Head & Neck Surgery - p. 3735-3771

Somatomedins (IGF-1 & IGF-2): Role in the Fetus

What Are Somatomedins?

The Key Fetal Distinction: GH-Independent Growth

"Secretion of IGF-I is independent of growth hormone before birth but is stimulated by growth hormone after birth."

• Ganong's Review of Medical Physiology, 26th Ed.

• In the fetus, GH is largely unimportant for growth
• Fetal growth is driven primarily by IGF-2 (and to some extent IGF-1), both acting independent of GH
• After birth, GH becomes the dominant driver, signaling the liver to produce IGF-1

Specific Fetal Roles

1. IGF-2 - The Primary Fetal Growth Factor

• IGF-2 is the dominant somatomedin in fetal life
• Produced by diverse fetal tissues (liver, placenta, kidney, brain, muscle)
• Drives cell proliferation across essentially all fetal organs
• When IGF-2 is overexpressed in human fetuses, several organs grow out of proportion to the rest of the body - especially the tongue, muscles, kidneys, heart, and liver (this is the basis of Beckwith-Wiedemann syndrome)
• In adults, IGF-2 gene expression is confined to only the choroid plexus and meninges - it essentially "switches off" after fetal life
• IGF-2 levels are decreased in fetal growth restriction (FGR/IUGR)

2. IGF-1 - Supporting Fetal Growth via Placenta

• IGF-1 is an important modulator of fetal growth even before birth
• In pregnancy, IGF-1 levels may be regulated in part by placental GH (a variant of pituitary GH, not controlled by the pituitary-hypothalamic axis)
• Fetal cord plasma IGF-1 levels positively correlate with:
  • Birth weight
  • Fetal length
  • Gestational diabetes (elevated IGF-1)
• IGF-1 and IGF-2 are also implicated in the pathogenesis of pre-eclampsia

3. Role in Placenta

• IGF-II and IGF-I receptors in the placenta regulate cell proliferation
• The placenta produces IGFs to help divert maternal resources for fetal growth and cellular differentiation
• Key processes regulated: glucose and amino acid transport, DNA synthesis, cell replication, RNA and protein synthesis
• IGFs act in autocrine and paracrine modes within placental tissue

Receptor Mechanism in the Fetus

Postnatal Transition

• GH becomes essential - hypophysectomized animals show severe growth retardation
• IGF-1 rises throughout childhood, peaks at puberty (age 13-17), then declines in old age
• IGF-2 levels remain constant throughout postnatal life but are no longer the primary growth driver
• IGF-2 gene expression switches off in nearly all adult tissues

Clinical Relevance of Fetal IGF Disruption

Summary

FETUS:
   IGF-2 (dominant) → GH-independent → drives cell proliferation in all organs
   IGF-1 (supporting) → partly regulated by placental GH → correlates with birth weight
   Both act via placenta in autocrine/paracrine fashion

AFTER BIRTH:
   GH → liver → IGF-1 → bone/cartilage growth → peaks at puberty
   IGF-2 → constant levels, minor role

• Ganong's Review of Medical Physiology, 26th Ed. - pp. 333-334
• Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice - pp. 225-226 (Placenta and Growth Factors)