Nervous System
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The Cerebellar Cortex
The cerebellar cortex is highly organized into three distinct layers, containing five main types of neurons. Bipolar cells are NOT found here (they are special sensory neurons found primarily in the retina of the eye, the olfactory epithelium, and the inner ear).
The actual cells of the cerebellar cortex, sorted by layer (from outside to inside), are:
- Molecular Layer: Contains Stellate cells and Basket cells (both are inhibitory interneurons).
- Purkinje Cell Layer: Contains Purkinje cells (flask-shaped, highly branched cells that provide the only output from the cerebellar cortex; they are inhibitory).
- Granular Layer: Contains Granule cells (the only excitatory cells in the cerebellar cortex) and Golgi cells(inhibitory).
The Cerebral Cortex
The cerebral cortex has six layers and is dominated by two main cell types:
- Pyramidal Cells: The most prominent neurons in the cerebral cortex. They have a distinct pyramid-shaped cell body and a long apical dendrite. They act as the major output neurons of the cortex. A famous high-yield example is the Betz cell, a giant pyramidal cell found in the primary motor cortex.
- Non-Pyramidal Cells (Interneurons): These include Martinotti cells, Chandelier cells, and Basket cells. They generally keep the local circuitry in check (mostly inhibitory).
The Hippocampus
Crucial for memory formation, its histology is distinct:
- Pyramidal Cells: Found packed tightly in the Cornu Ammonis (CA1, CA2, CA3, CA4) regions.
- Granule Cells: Found densely packed in a distinct C-shaped structure called the Dentate Gyrus.
Neurons and Glia: CNS vs. PNS
While neurons share the same basic structure (soma, dendrites, axon) everywhere, their environment, supporting cells, and terminology change drastically depending on whether they are in the Central Nervous System (CNS) (brain and spinal cord) or the Peripheral Nervous System (PNS) (cranial and spinal nerves).
Here is a high-yield comparison table:
| Feature | Central Nervous System (CNS) | Peripheral Nervous System (PNS) |
| Location | Brain and Spinal Cord | Cranial nerves, spinal nerves, and ganglia |
| Cluster of Cell Bodies | Called a Nucleus (e.g., Basal ganglia is technically misnamed; should be basal nuclei) | Called a Ganglion (e.g., Dorsal root ganglion) |
| Bundle of Axons | Called a Tract (e.g., Spinothalamic tract) | Called a Nerve (e.g., Sciatic nerve) |
| Myelinating Cell | Oligodendrocytes | Schwann Cells |
| Myelination Ratio | One Oligodendrocyte can myelinate up to 50different axons. | One Schwann cell myelinates only onesingle segment of an axon. |
| Regeneration Capacity | Virtually none. Astrocytes form glial scars, and oligodendrocytes release growth-inhibiting proteins. | Good. Schwann cells form a regeneration tube to guide a severed axon back to its target. |
| Major Phagocytic Cell | Microglia (specialized macrophages of the brain) | Standard tissue Macrophages |
| Environment Regulators | Astrocytes (form the Blood-Brain Barrier, regulate ions, structural support) | Satellite Cells (surround and support cell bodies in ganglia) |
Glial Cell Classification
Macroglia (Ectodermal Origin)
Macroglia are larger supporting cells derived from the embryonic neuroectoderm (neural tube and neural crest). This category includes:
- Astrocytes: Star-shaped cells in the Central Nervous System (CNS) that maintain the blood-brain barrier and regulate the chemical environment.
- Oligodendrocytes: Cells responsible for forming the myelin sheath around axons in the CNS.
- Schwann Cells: Cells responsible for forming the myelin sheath around axons in the Peripheral Nervous System (PNS). (Note: Historically, macroglia referred strictly to CNS ectodermal glia, but modern neuroscience firmly groups Schwann cells and satellite cells under the peripheral macroglia umbrella).
- Ependymal Cells: Ciliated cells lining the ventricular system of the brain.
Microglia (Mesodermal Origin)
Microglia are entirely distinct from macroglia. They are small, specialized resident macrophages of the CNS derived from embryonic yolk sac mesoderm. They function as the primary immune defense mechanism of the central nervous system.