Correct Answer: A. Purkinje cells
Purkinje cells are the largest and most distinctive neurons in the cerebellar cortex, making them the most recognizable on histological sections. They are located in the Purkinje cell layer, a single row of cells sandwiched between the molecular layer (superficial) and the granular layer (deep). Their defining morphological features are: large soma (20–30 μm diameter), eccentric nucleus, abundant rough endoplasmic reticulum (Nissl substance), and an extensive dendritic tree that extends into the molecular layer in a single plane, resembling a "candelabrum" or branching tree. On H&E staining, they appear as large, pale-staining cells with prominent nucleoli. The Purkinje cell layer is a constant anatomical landmark in cerebellar histology, and these cells are the only output neurons of the cerebellar cortex—they receive input from parallel fibers (granule cell axons) and climbing fibers (from the inferior olivary nucleus) and project inhibitory GABAergic axons to the deep cerebellar nuclei. This unique position, size, and morphology make Purkinje cells unmistakable in standard histological preparations used in Indian medical curricula (Robbins, Bailey & Love).
Why the other options are wrong
B. Basket cells — Basket cells are interneurons in the molecular layer, much smaller than Purkinje cells, and are difficult to identify on routine H&E staining without special stains. They form inhibitory synapses on the axon initial segment of Purkinje cells. Their small size and scattered distribution in the molecular layer make them invisible as a distinct layer, unlike the prominent Purkinje cell layer. NBE may trap students who confuse cerebellar interneurons without recognizing the layer-specific anatomy. C. Granule cell — Granule cells are the smallest neurons in the CNS (4–5 μm soma), located in the granular layer deep to the Purkinje cell layer. They are so numerous and densely packed that they appear as a basophilic (dark-staining) layer on H&E, not as individual identifiable cells. Their tiny size and deep location are opposite to the large, prominent cells visible in the Purkinje layer. This is a common distractor for students who focus on cell count rather than morphology. D. Golgi cells — Golgi cells are inhibitory interneurons in the granular layer, similar in size to basket cells and equally difficult to identify on routine staining. They modulate granule cell input but do not form a distinct, recognizable layer. Their location in the granular layer and small soma make them invisible as a prominent histological feature. Students may confuse them with Purkinje cells if they do not recall the layer-specific distribution of cerebellar neurons.
High-Yield Facts
- Purkinje cell layer is a single row of large neurons between molecular and granular layers—the most distinctive landmark in cerebellar cortex histology.
- Purkinje cells are the only output neurons of cerebellar cortex; they are GABAergic and inhibit deep cerebellar nuclei.
- Soma diameter of Purkinje cells is 20–30 μm, making them among the largest neurons in the CNS and easily visible on H&E staining.
- Dendritic tree of Purkinje cells extends in a single plane into the molecular layer, receiving parallel fiber input (from granule cells) and climbing fiber input (from inferior olivary nucleus).
- Cerebellar cortex layers from superficial to deep: molecular layer (basket cells), Purkinje cell layer, granular layer (granule and Golgi cells).
Mnemonics
PCG (Purkinje-Cerebellar-Granule) Layer Order Purkinje (middle, large, prominent) → Cellular (granular layer, deep, dense) → Granule (smallest cells). Purkinje is the 'star' of cerebellar histology—largest, most visible, single row. GABA Output Rule Granule cells → parallel fibers → Purkinje cells → GABA to deep nuclei. Purkinje = only output; all others are interneurons or input.
NBE Trap
NBE may pair "cerebellar interneurons" (basket, Golgi) with "inhibitory" to lure students into selecting them without recognizing that Purkinje cells are the only identifiable layer on routine histology. The trap exploits confusion between functional role (all inhibitory) and morphological visibility (Purkinje cells alone form a distinct layer).
Clinical Pearl
In Indian clinical practice, cerebellar atrophy (seen in chronic alcoholism, spinocerebellar ataxias) preferentially affects Purkinje cells first, leading to loss of the Purkinje cell layer on imaging and histology—a hallmark finding in neurodegenerative disease workup. Recognizing Purkinje cell loss on biopsy is critical for diagnosing inherited cerebellar ataxias in Indian populations.
_Reference: Robbins & Cotran Pathologic Basis of Disease, Ch. 28 (CNS); Bailey & Love's Short Practice of Surgery, Ch. 63 (Neuroanatomy); OP Ghai Textbook of Anatomy, Ch. 8 (Cerebellum)_