Central and peripheral nervous system histology

Cells of the CNS & PNS - The Brain's Tiny Crew

• Central Nervous System (CNS)
  • Astrocytes: Provide physical support, repair, and form the blood-brain barrier (BBB).
  • Oligodendrocytes: Myelinate multiple axons (~50).
  • Microglia: Phagocytic scavengers; the macrophages of the CNS.
  • Ependymal Cells: Line ventricles and produce cerebrospinal fluid (CSF).
• Peripheral Nervous System (PNS)
  • Schwann Cells: Myelinate single axons.
  • Satellite Cells: Support neuron cell bodies in ganglia.

⭐ High-Yield: Microglia are mesodermal in origin, derived from circulating monocytes, unlike other CNS cells which are neuroectodermal.

CNS Architecture - Gray & White Layouts

• Gray Matter: Composed of neuronal cell bodies, dendrites, unmyelinated axons, and glial cells. It's the primary site of synaptic processing.
• White Matter: Consists mainly of myelinated axons, responsible for transmitting signals between different brain regions. The lipid-rich myelin gives it a white appearance.

CNS Layouts:

• Spinal Cord: Central, H-shaped gray matter surrounded by peripheral white matter.
  • 📌 Mnemonic: The arrangement is like a gray butterfly inside a white box.
• Brain: Predominantly peripheral gray matter (cortex) with deep, central white matter.
  • Exception: Deep gray matter nuclei (e.g., basal ganglia, thalamus) are embedded within the white matter.

⭐ The brainstem has a mixed arrangement, with gray matter nuclei (like cranial nerve nuclei) scattered within the white matter, acting as a transition between spinal and cerebral layouts.

PNS Architecture - Nerves, Ganglia, & Ends

• Nerve Connective Tissue Layers (superficial to deep):

  • Epineurium: Dense irregular CT; surrounds entire nerve.
  • Perineurium: Specialized cells around a fascicle; forms the blood-nerve barrier.
  • Endoneurium: Loose CT (reticular fibers) around a single axon & its Schwann cell.
  • 📌 Mnemonic (Outer to Inner): Eat Potato Everyday.

• Ganglia (PNS Neuron Body Clusters):

  • Sensory (Dorsal Root): Pseudounipolar neurons, central nuclei, many satellite cells. No synapses.
  • Autonomic: Multipolar neurons, eccentric nuclei, fewer satellite cells. Synapses present.

⭐ The perineurium's tight junctions create the blood-nerve barrier, protecting nerve fibers from toxins and maintaining endoneurial fluid homeostasis.

Myelination & Barriers - The Great Wall & Wrap

• Myelin Source:
  • CNS: Oligodendrocytes (one cell wraps multiple axons).
  • PNS: Schwann Cells (one cell wraps a single axon segment).
  • 📌 COPS: CNS = Oligodendrocytes, PNS = Schwann cells.
• Barriers:
  • Blood-Brain Barrier (BBB): Tight junctions of non-fenestrated capillary endothelium.
  • Blood-Nerve Barrier: Tight junctions between perineurial cells.

⭐ Demyelination in the CNS (e.g., Multiple Sclerosis) involves oligodendrocyte destruction.

High‑Yield Points - ⚡ Biggest Takeaways

• Nissl bodies (RER) are found in the soma and dendrites, but are absent from the axon hillock.
• Oligodendrocytes myelinate multiple axons in the CNS; they are the target in Multiple Sclerosis.
• Schwann cells myelinate a single axon in the PNS; they are damaged in Guillain-Barré syndrome.
• Astrocytes, marked by GFAP, form the blood-brain barrier and are responsible for glial scars.
• Microglia are the phagocytic immune cells of the CNS, derived from mesoderm.
• The perineurium surrounds a nerve fascicle, forming the protective blood-nerve barrier.