AN64.1-3 | Histology & Embryology (Neuroanatomy) — Practice Quiz

Correct! Purkinje cells form a single row of cells BETWEEN the molecular layer (outer) and the granular layer (inner). Their large dendritic trees spread INTO the molecular layer; their axons project DOWNWARD into the white matter to the intracerebellar nuclei.

Cerebellar atrophy (alcoholic, paraneoplastic, SCA): loss of Purkinje cells is the histological hallmark. Medulloblastoma: arises from granule cell precursors in the external granular layer (a transient developmental layer in infants that normally involutes by 18 months).

Incorrect. Purkinje cell layer = middle layer of cerebellar cortex (between molecular = outer, granular = inner). Recognise: single row of large, distinctive, flask-shaped cells on H&E.

Correct! Betz cells (giant pyramidal cells) are in Layer V (internal pyramidal layer) of the PRIMARY MOTOR CORTEX (area 4). They give rise to the corticospinal tract. They are the largest neurons in the CNS (cell body diameter up to 120 μm).

Primary motor cortex histology is "agranular" (no prominent layer IV). Primary sensory cortex is "granular" (koniocortex, prominent layer IV). Line of Gennari in primary visual cortex (area 17) = thick band of myelinated fibres in layer IV visible macroscopically. These regional differences in cortical architecture are used to map functional areas.

Incorrect. Betz cells = Layer V of primary motor cortex. Layer IV (internal granular) is the THALAMIC INPUT layer — it is ABSENT in primary motor cortex (agranular cortex) and very prominent in sensory cortex (granular cortex).

Correct! Substantia gelatinosa = Rexed Lamina II. It occupies the TIP of the posterior horn and is composed of densely packed small neurons. It is the site of pain modulation (gate control theory), receives nociceptive C-fibre and A-delta input, and contains enkephalin + substance P.

Gate control theory of pain (Melzack and Wall): Substantia gelatinosa (Lamina II) neurons modulate pain transmission. Thick myelinated fibres (Aβ — touch/pressure) → inhibit gelatinosa → "close the gate" → less pain. Thin unmyelinated fibres (C — nociception) → activate gelatinosa → "open the gate" → more pain. Basis of transcutaneous electrical nerve stimulation (TENS) therapy.

Incorrect. Substantia gelatinosa = Lamina II (posterior horn tip). Lamina I = marginal zone (just above). Lamina VII = intermediate zone (Clarke's column, IML). Lamina IX = anterior horn motor neurons.

Correct! Dorsal root ganglion neurons are derived from NEURAL CREST cells. Neural crest cells detach from the dorsal neural tube and migrate extensively. Other derivatives: sympathetic and parasympathetic ganglia, adrenal medulla, Schwann cells, melanocytes, facial bones, meninges.

Neural crest = "the 4th germ layer." Phakomatoses (neurocutaneous syndromes) involve neural crest-derived tissues: NF1 (café-au-lait spots, neurofibromas), NF2 (acoustic neuromas), tuberous sclerosis, Sturge-Weber. NF1 = most common in India among phakomatoses; AD mutation in neurofibromin gene on chromosome 17.

Incorrect. DRG neurons = neural crest. Oligodendrocytes, astrocytes, and ependymal cells = neural TUBE derivatives (neuroectoderm of the tube). Microglia = mesodermal origin (monocytes/macrophage lineage).

Correct! Metencephalon = pons (ventral) + cerebellum (dorsal). Myelencephalon = medulla oblongata. Mesencephalon = midbrain. Diencephalon = thalamus + hypothalamus. Telencephalon = cerebral hemispheres + basal ganglia.

Developmental sequence: Week 4 = 3 primary vesicles. Week 5 = 5 secondary vesicles. Anomalies by timing: Week 3 failure = holoprosencephaly (prosencephalon fails to divide). Week 5 failure = hindbrain anomalies (Dandy-Walker). Week 8+ = cortical malformations (lissencephaly, pachygyria from failed neuronal migration).

Incorrect. Metencephalon = pons + cerebellum. Mnemonic: "Meta = Meet (Pons + Cerebellum); Myelo = My (Medulla); Meso = Middle (Midbrain); Dien = Dial (Thalamus + Hypothalamus); Telen = Television (Cerebral cortex + BG)"

Correct! Cortical neuronal migration is INSIDE-OUT. First-generated neurons (earliest) occupy the DEEPEST layers (layers V, VI). Later-generated neurons migrate PAST the earlier ones (on radial glial fibres) to form the more SUPERFICIAL layers (layers II, III). Layer I neurons (Cajal-Retzius cells) are born very early and remain at the surface.

Lissencephaly ("smooth brain"): failure of neuronal migration causes absence of normal gyration. Features: smooth brain surface, thick cortex (only 4 layers), severe intellectual disability, epilepsy. Causes: LIS1 gene mutation (chromosome 17p13.3), double cortex syndrome (doublecortin mutation, X-linked). MRI: smooth cortex, thick grey matter, figure-of-8 appearance.

Incorrect. Inside-out = early neurons deep (VI), late neurons superficial (II). Disruption of this migration = lissencephaly (smooth brain, agyria/pachygyria) from mutations in LIS1, DCX genes.

Correct! Open NTDs (anencephaly = most extreme open defect) allow AFP (synthesised by fetal yolk sac + liver) to leak through the unprotected defect into the amniotic fluid → crosses into maternal serum → elevated MSAFP. The more open the defect, the more AFP leaks (anencephaly > myelomeningocele > meningocele > closed defects).

MSAFP interpretation: (1) High MSAFP = open NTD (anencephaly, myelomeningocele), also ventral wall defects (gastroschisis, omphalocele), multiple pregnancy, incorrect dates. (2) Low MSAFP = Down syndrome (trisomy 21), trisomy 18. Triple test (India): MSAFP + hCG + unconjugated oestriol at 15–20 weeks for Down syndrome + NTD screening.

Incorrect. MSAFP elevation = AFP leakage through the open defect into amniotic fluid. Closed defects (meningocele with skin cover) do NOT significantly elevate MSAFP because the skin prevents leakage.

Correct! Neural tube closes in week 4 (days 22–28) — BEFORE most women know they are pregnant (most pregnancy tests are positive at 4–5 weeks, but the tube has ALREADY closed). Therefore, folic acid must be started AT LEAST 1 MONTH BEFORE CONCEPTION.

India government guidelines: 400 mcg/day folic acid for all women of reproductive age (preconception to 12 weeks). Reasons folic acid prevents NTDs: essential for DNA synthesis and cell division during rapid neural tube closure; deficiency impairs closure. Mandatory fortification of wheat flour and rice with folic acid in India (FSSAI 2024 directive) aims to prevent dietary deficiency even in women who do not take supplements.

Incorrect. By the time pregnancy is confirmed, the neural tube has already closed (week 4). Folic acid must be started PERICONCEPTIONALLY — at least 1 month BEFORE conception through 12 weeks. Starting at the first antenatal visit is TOO LATE for NTD prevention.

Correct! Chiari II (Arnold-Chiari): cerebellar tonsils + vermis + medulla herniate downward through the foramen magnum + medullary kinking + hydrocephalus (obstructed by hindbrain herniation). Associated with VIRTUALLY ALL cases of myelomeningocele. Must be addressed with VP shunt.

MMC management in India: (1) Immediate postnatal surgical closure of the defect (within 24–72 hours to prevent meningitis). (2) Concurrent VP shunt for hydrocephalus (present in >90%). (3) Long-term follow-up: urological (neurogenic bladder → CIC — clean intermittent catheterisation), orthopaedic (clubfoot, hip dislocation), neurological (tethered cord, syrinx). Preconceptional counselling + folic acid supplementation for the NEXT pregnancy.

Incorrect. Chiari II = hindbrain herniation through foramen magnum + hydrocephalus = found in nearly all MMC patients. Absent vermis + 4th ventricular cyst = Dandy-Walker. Occipital encephalocele = different cranial NTD.

Correct! Granule cells are the MOST NUMEROUS neurons in the entire CNS — an estimated 50–70 billion, more than all other brain neurons combined. They are EXCITATORY (glutamatergic). Their axons ascend into the molecular layer as PARALLEL fibres, synapsing on thousands of Purkinje cell dendrites.

Medulloblastoma is the most common malignant brain tumour in children (India: ~800 new cases/year). It arises from undifferentiated granule cell precursors of the external granular layer (transient during development). Histology: small round blue cells, Homer Wright rosettes. Molecular subtypes (WNT, SHH, Group 3, Group 4) determine prognosis. Treatment: surgery + radiotherapy + chemotherapy. 5-year survival 60–80% for standard risk.

Incorrect. Granule cells = MOST NUMEROUS (all of CNS), EXCITATORY (glutamate), SMALL cells in the granular layer. Purkinje cells = ONLY cortical output, INHIBITORY. Climbing fibres = from inferior olivary nucleus (NOT granule cells).