Myelin, Embryology & Introductory Brain Anatomy

Myelin & Schwann Cells (Peripheral Nervous System)

• Schwann cells (= neurolemocytes)

  • Every PNS axon is associated with at least one Schwann cell; only some axons actually become myelinated.

  • Myelin = spiral wrap of Schwann-cell plasma membrane (≈ $(80\%)$ lipid, up to $100$ compacted lamellae, virtually no cytoplasm between layers).

  • Each Schwann cell myelinates one internode → sheath is segmented; gaps = nodes of Ranvier.

  • Roles

  • Electrical insulation ➜ ↑ conduction velocity.

  • Mechanical protection (PNS is vulnerable to trauma).

  • Scaffold for axon regeneration after injury.

• Unmyelinated PNS fibres

  • Still enveloped by Schwann-cell cytoplasm (multiple small axons share one Schwann cell).

  • Conduction is slower but metabolically cheaper.

• Gross organisation of a peripheral nerve

  • Axons → fascicles → whole nerve, wrapped by epineurium.

  • A named nerve (e.g.00ulnar, femoral) contains $\gg10^3$ axons, both myelinated & unmyelinated.

Axon Size, Myelination & Conduction Velocity

• Determinants of speed

  • Surface area ($\propto$ diameter) → more charge accumulates.

  • Axoplasmic resistance ↓ with diameter (ions bump into fewer structures).

  • Myelin: main factor; reduces membrane leakiness ➜ saltatory conduction (node-to-node jumping).

• Functional hierarchy (metaphor: trains)

  • Large myelinated = express; conduct fastest (e.g.00proprioception, reflexes).

  • Small myelinated = regional trains (touch, pressure).

  • Unmyelinated = local train (temperature, pain, itch).

  • Evolutionary trade-offs: energy, space, prioritisation of critical signals.

Clinical Interlude – Tetanus Neurotoxin

• Clostridium tetani enters via wounds (e.g.00"rusty nail").

• Toxin binds presynaptic membrane at neuromuscular junction, is endocytosed & transported retrogradely to motor-neuron somata, then to interneurons.

• Blocks inhibitory neurotransmitter release ➜ alternating

  • Sustained contractions (spasms)

  • Flaccid periods when motor neurons exhaust.

• Incubation $3\text{ days} \rightarrow 3\text{ weeks}$.

• Mortality prevented only by immunisation (booster every $10$ years).

Central Nervous System Support Cells (Neuroglia)

• Oligodendrocytes – CNS analogue of Schwann cells; each sends multiple processes to myelinate many axons.

• Astrocytes

  • Structural scaffold; regulate extracellular milieu.

  • Form most of the blood–brain barrier (BBB) via perivascular end-feet.

  • Guide synaptogenesis & nourish neurons.

• Microglia – phagocytic CNS macrophages (immune surveillance, debris removal).

• Ependymal cells – ciliated epithelium lining ventricles & central canal; circulate CSF.

Embryological Basis of the Brain & Ventricles

• Timeline

  • $\approx2.5$ weeks: notochord induces surface ectoderm → neural plate (neuroectoderm + neural-crest margin).

  • Folding → neural groove → neural tube; zips up cranially & caudally, leaving two openings:

  • Cranial neuropore (closes ≈ $\text{day 25}$).

  • Caudal neuropore (closes ≈ $\text{day 27}$).

• Derivatives

  • Neural tube → brain & spinal cord.

  • Neural canal → ventricular system + central canal.

  • Neural crest → most PNS (sensory & autonomic ganglia), Schwann cells, etc.

• Closure failures

  • Cranial = anencephaly (face present, no brain) → lethal.

  • Caudal = spina bifida spectrum → motor/continence deficits; surgical repair often needed.

Macroscopic Anatomy of the Brain

Major subdivisions

• Cerebrum – two hemispheres (~ $85\%$ volume)

  • Lobes: frontal, parietal, temporal, occipital (+ hidden insula).

  • Surface: gyri (ridges) & sulci (shallow grooves); deep grooves = fissures.

  • Central sulcus separates pre- & post-central gyri (key for motor/sensory maps).

• Cerebellum – $\approx10\%$ volume yet $\approx50\%$ of neurons; coordination & balance.

• Brainstem (midbrain → pons → medulla oblongata) – autonomic centres, conduit for tracts, cranial-nerve nuclei.

• White-matter tracts

  • Projection (vertical) – cortex ⇄ spinal cord; decussate in medulla (explains contralateral deficits after stroke).

  • Commissural – e.g.00corpus callosum connects hemispheres.

  • Association – intra-hemispheric links.

Internal gray nuclei

• Basal nuclei (caudate, lentiform, etc.) – motor regulation.

• Thalamic & hypothalamic regions (not detailed today).

Skull & Cranial Fossae

• Anterior fossa ↔ frontal lobes.

• Middle fossa ↔ temporal lobes; contains sella turcica (pituitary sits in hypophyseal fossa of sphenoid).

• Posterior fossa ↔ cerebellum & brainstem; foramen magnum marks brain–spinal-cord junction.

• Cribriform plates (ethmoid) – olfactory-nerve filaments; potential infection portal.

• Optic canals transmit optic nerves; numerous other foramina for cranial nerves & vessels.

Meninges

Removal sequence in dissection: bone ➔ dura ➔ arachnoid (resembles cling-wrap) ➔ pia (inseparable from cortex).

Cerebrospinal Fluid (CSF) & Ventricular System

• Ventricles

  • $2$ Lateral ventricles ➔ interventricular foramina (of Monro) ➔ $3^{rd}$ ventricle ➔ cerebral aqueduct ➔ $4^{th}$ ventricle.

  • Exits $4^{th}$ via $1$ median & $2$ lateral apertures ➔ subarachnoid space ➔ circulates around brain & spinal cord ➔ absorbed via arachnoid villi into dural sinuses.

  • Central canal of spinal cord is distal continuation.

• Production & turnover

  • Choroid plexus = ependymal cells + capillaries in each ventricle.

  • Continuous secretion ≈ $500\,\text{mL day}^{-1}$; steady-state volume $100\text{–}160\,\text{mL}$.

  • Formed from plasma, returned to venous circulation – closed loop.

• Functions

  • Buoyancy: apparent brain mass reduced from $\approx1.5\,\text{kg}$ to $\approx50\,\text{g}$; prevents crushing of inferior neurons.

  • Protection: cushions against minor jolts (not severe TBI).

  • Chemical stability: regulates extracellular milieu, removes metabolites (glymphatic clearance during sleep, see later lecture).

• Clinical notes

  • Lumbar puncture (L3–L4) samples CSF for meningitis diagnosis, etc.

  • Hydrocephalus

  • Obstruction (aqueduct, apertures, villi) ➜ CSF accumulation, ↑ intracranial pressure, coma, death.

  • Infants: open sutures allow skull expansion (hydrocephaly); treated with shunt diverting CSF to venous system.

Famous-Brain Anecdote (Ethics & Preservation)

• In $1955$ Albert Einstein (aneurysm death) explicitly refused autopsy.

• Pathologist Thomas Harvey removed brain & eyes without consent, sectioned & stored pieces in jars/box for $23$ years.

• Subsequent study: brain unremarkable in size; intellectual prowess linked to microscopic connectivity patterns.

• Highlights historical ethical breaches & modern consent standards.

Brain: General Physiology & Metabolism

• Weight range $1.0 \text{–} 2.0\,\text{kg}$ (female < male primarily due to body size; functionally equivalent).

• Represents $\approx2\%$ body mass yet consumes $\approx20\%$ of resting energy – nearly constant (high basal metabolic rate).

• Primary fuel = glucose ➜ hypo/hyper-glycaemia impairs function (relevant in diabetes).

• Water content $\approx75\%$; remainder protein & lipids (hence soft "silken tofu / warm butter" consistency).

• Lacks nociceptors; headaches arise from vessels or meninges, not neural tissue.

Key Metaphors & Mnemonics Mentioned

• Local vs Express Train – unmyelinated vs myelinated conduction.

• Proprioception signals = "priority freight" (must reach CNS fastest to prevent falls → injury → potential death).

• "Cribriform" & "ethmoid" both mean sieve – remember olfactory fibres passing through like strands in a sieve.

Practical / Tutorial Pointers

• Histology: identify myelin as intensely staining concentric rings; look for vesicles & cleft to find synapses.

• When analysing micrographs, nodes of Ranvier appear as gaps in myelin.

• Coronal brain slices: find corpus callosum (thick white band) to orient; gray-matter "islands" inside white = cerebral nuclei.

(End of Friday lecture; continuation – basic functions, cranial nerves & spinal cord – scheduled for Monday.)