Comprehensive Notes – Uterine Physiology & Pregnancy Adaptations

Smooth Muscle Fundamentals

  • Composition & Architecture
    • Uterine body: three inter-digitating sheets
    • Outer layer → predominately longitudinal fibres
    • Middle layer → oblique/spiral fibres (power layer)
    • Inner layer → predominately circular fibres
    • Cervix: mainly circular fibres with very few longitudinal fibres
  • Neuro-physiology
    • No classical neuromuscular junctions; autonomic fibres end in bulbous varicosities
    • Neurotransmitter released onto many fibres → slow, synchronised sheet contraction
    • Action potentials spread cell-to-cell; some fibres act as pacemakers
  • Biochemistry of contraction
    • \text{Ca^{2+}} influx triggers actin–myosin interaction; ATP supplies energy
    • Contraction is slow, sustained, fatigue-resistant
    • Smooth muscle takes 30× longer to contract/relax than skeletal muscle
    • Same tension maintained for ( < 1\% ) of skeletal-muscle energy cost
  • Special properties
    • Less vigorous stretch reflex → hollow organs can distend without expulsive contractions
    • Greater length–tension adaptability than skeletal muscle
    • Ability to divide (hyperplasia) & to secrete collagen / elastin

General Growth & Size of the Uterus in Pregnancy

  • Predictable enlargement; fundal-height only reliable for dating up to 20 wks
  • Early enlargement by hyperplasia; second-half growth almost entirely hypertrophy of fibres
  • Muscle-fibre dimensions by term ↑ 3-fold in diameter & 10-fold in length
  • Key morphometric changes (non-pregnant → pregnant)
    • Shape: pear-shaped → globular (≤20 wks) → pear/cylindrical at term
    • Position: pelvic, anteverted/anteflexed → abdominal organ after 12 wks; right-obliquity common
    • Wall thickness: 10\,\text{mm} → 25\,\text{mm} (≈3–4 mths) → thins to 5–10\,\text{mm} at term
    • Lower-uterine wall: 7\,\text{mm} → softens/elongates to 25\,\text{mm} by ≈10 wks (origin of LUS)
    • Overall size: 7.5\times5\times2.5\,\text{cm} → 20\times25\times22.5\,\text{cm} at term
    • Weight: 50\,\text{g} → 80–1200\,\text{g} (≈20-fold)
  • Palpable fundal levels
    • 12 wks → just above symphysis pubis
    • 20 wks → at/just below umbilicus
    • 30 wks → midway umbilicus–xiphisternum
    • 36 wks → at xiphisternum (max)
    • >36 wks → height may fall as presenting part descends ("lightening")

Hormonal Regulation of Uterine Growth & Quiescence

  • Endocrine sources
    • Early: corpus luteum; Later: placenta
  • Oestrogen
    • Stimulates muscle-fibre growth & compliance
    • Acts via cytoplasmic receptor → nucleus → RNA ↑ → protein synthesis
  • Progesterone
    • Maintains myometrial quiescence (↑ resting membrane potential?)
    • Thickens cervical mucus → "operculum" barrier to ascending infection
  • Combined effect → profound but largely reversible uterine enlargement

Anatomical Layers of the Pregnant Uterus

  • Decidua (modified endometrium)
    • Thickens to 6–8\,\text{mm}; richly vascular/glycogen-loaded in fundus & body
    • Two functional strata produced by progesterone
    • Compact layer → enlarged, polygonal decidual cells
    • Spongy (cavernous) layer → dilated, secretion-filled glands
    • Basal layer unchanged; entire decidua shed post-partum (hence "deciduous")
  • Myometrium
    • Outer longitudinal layer → contraction & retraction
    • Middle spiral/oblique layer → expulsion & postpartum haemostasis ("living ligature" around vessels)
    • Inner circular layer → LUS distension & cervical dilatation
    • Muscle bundles embedded in thin collagen / elastin matrix → support & tension transmission
  • Perimetrium
    • Loose peritoneal coat; forms uterovesical & rectouterine (Douglas) pouches and broad ligaments → positional support yet allows unrestricted growth

Uterine Blood Flow & Vascular Control

  • Massive ↑ flow via ↑ vessel diameter & ↓ resistance
  • Prostacyclin \text{PGI}_2
    • Potent vasodilator, anti-platelet; sustains placental perfusion during labour
    • Minimal direct effect on contractility

Innervation of the Pregnant Uterus

  • Sparse overall; labour proceeds even after complete spinal transection → CNS not essential
  • Sympathetic (T10–T12)
    • Preganglionic fibres travel directly to uterus (unusual)
    • Neurotransmitters: acetylcholine (preganglionic) → noradrenaline (postganglionic)
    • Receptors: \alpha (excitatory); \beta1 (cardiac excitatory); \beta2 (uterine inhibitory)
    • \beta_2 agonists (salbutamol, ritodrine) → tocolysis but ↑ maternal heart rate
    • Non-selective \beta blockers (propranolol) → enhance uterine activity
  • Parasympathetic (S2–S4)
    • Synapse near target organs; neurotransmitter acetylcholine throughout
    • Plexi: bilateral paracervical (Lee–Frankenhäuser)

Cervical Transformation ("Ripening")

  • Gradual shift from rigid to highly elastic
    • Mass, water content & vascularity ↑ (appears purple – Chadwick/Jacquemenier sign)
    • Length remains ≈2.5\,\text{cm} until effacement late in gestation
  • Timeline of softening/dilatation
    • External os opens from ≈24 wks; internal os in \tfrac{1}{3} of primigravidae by 32 wks
    • Effacement integrates cervix into LUS; aided by longitudinal fundal fibres
  • Biochemical remodelling
    • Enzymatic collagen degradation (collagenase, elastase)
    • Proteoglycan matrix changes → water attraction
    • Smooth-muscle fibres become stretchable
  • Hormonal cues: rising oestrogen, relaxin, prostaglandins \text{PGE}2 & \text{PGF}{2\alpha}
    • Local \text{PGE}_2 used clinically for pre-induction ripening
  • Operculum: thick, viscous mucus plug under progesterone → infection barrier

Vaginal Adaptations

  • Oestrogen effects
    • Muscle-layer hypertrophy & connective-tissue remodelling → ↑ elasticity (second stage distension)
    • Epithelial desquamation → white discharge (leucorrhoea)
    • ↑ Glycogen → Döderlein flora → acidic pH (defence) but predisposes to Candida albicans (thrush)
  • Vascular signs
    • Reddish-purple colouring (Jacquemenier’s)
    • Enhanced lateral-fornix pulsation (Osiander’s)

Evolution of Uterine Activity

  • Electrical & mechanical maturation
    • From ≈7 wks: high-frequency (\approx2 contractions min^{-1}) low-amplitude (1–1.5\;\text{kPa}) twitches
    • Post 20 wks → progressive ↑ frequency & amplitude; marked acceleration last 6–8 wks
    • Development of gap junctions (low-resistance bridges) driven by oestrogen / progesterone / prostaglandins → whole-uterus AP spread in 2–3\,\text{s} at term
  • Pacemaker concept
    • No single cell identified; cornual regions likely initiation sites but any myocyte may act as pacemaker
  • Braxton Hicks contractions
    • Low frequency, high pressure, non-painful, non-dilating; maternal perception of uterine "tightening"

Pharmacologic Responsiveness

  • Oxytocin sensitivity
    • Early pregnancy (≤30 wks) → very low; infusion up to 128\,\text{mU·min}^{-1} needed for effect
    • >30 wks → sensitivity rises rapidly; 8\,\text{mU·min}^{-1} at ≈35 wks; 4\,\text{mU·min}^{-1} at term induce labour-like activity
  • Prostaglandins
    • \text{PGE}2 & \text{PGF}{2\alpha} stimulate contractions at any gestational age → likely final common pathway for labour

Consolidated Key Points for Revision

  • Three muscle types: skeletal, smooth, cardiac; uterus is smooth → involuntary, non-striated, slow-contracting
  • Four muscle functions: movement, posture, joint stability, heat generation
  • Four muscle properties: excitability, contractility, extensibility, elasticity
  • Pelvic-floor anatomy (diaphragm, levator ani, perineal body) underpins support & childbirth mechanics; damage predisposes to prolapse/incontinence
  • Myometrial layers specialise: outer for retraction, middle for expulsion/haemostasis, inner for LUS & cervical responses
  • Uterine enlargement: early hyperplasia, later hypertrophy; fundal levels provide clinical growth check
  • Hormone interplay (↑oestrogen vs ↑progesterone) governs growth vs quiescence; prostaglandins trigger activity
  • Innervation sparse; chemical & gap-junction changes, not nerves, drive labour; sympathetic \beta_2 agonists tocolyse, \beta blockers augment contractions
  • Cervical ripening is biochemical, not mechanical; prostaglandin-based interventions mimic natural process
  • Vaginal & decidual adaptations protect fetus and prepare birth canal—but entail altered infection risks
  • Prostaglandins are probable final mediators of uterine contractions; oxytocin sensitivity is gestation-dependent