types of nerve fiber and myelination

📌 Neuron Classification (Based on Axon Length)

• Golgi Type 1: Long axons (e.g., median nerve)
• • Mnemonic: "Long" has 'G' sound like "Golgi"
• Golgi Type 2: Short axons (e.g., spinal cord neurons)

🖇 Nerve Fiber Coverings

• Naked nerve fibers: Uncovered
• • Found at axon beginning & end
• Myelinated with neurilemma: Myelin sheath + neurilemma
• • Most peripheral nerves (femoral, ulnar, median, sciatic)
• Myelinated without neurilemma: Myelin only
• • Found in CNS (no Schwann cells)
• Unmyelinated with neurilemma: Neurilemma only
• • Autonomic ganglia

🚨 Myelin Sheath Structure & Function

• Composition: Lipoproteins + glycolipids (fats + proteins + carbohydrates)
• EM appearance: Concentric layers
• • Dark lines = cell membrane
  • Pale lines = myelin itself
• Function: Insulation for nerve impulses
• • Absence → demyelination diseases → short circuits → sensory disturbances/paralysis
• Saltatory conduction: Nerve impulse jumps between Nodes of Ranvier
• • Node of Ranvier = site lacking sheath between Schwann cells

🟠 Myelinating Cells: PNS vs CNS

Peripheral Nervous System (PNS):

• Cell type: Schwann cells
• Mechanism: Wraps around axon multiple times
• Ratio: 1 Schwann cell → 1 axon (but 1 axon needs multiple Schwann cells)
• Innermost layer: Mesaxon (first wrap touching axon)
• Outermost layer: Neurilemma (contains nucleus)
• Nodes of Ranvier: Present

Central Nervous System (CNS):

• Cell type: Oligodendrocytes
• Mechanism: Extends multiple processes (arms)
• Ratio: 1 Oligodendrocyte → multiple axons
• Neurilemma: Absent
• Nodes of Ranvier: Present

🟣 Nerve Trunk Connective Tissue

• Fascicle: Bundle of axons
• Endoneurium: Light connective tissue around single axon (innermost)
• Perineurium: Connective tissue around fascicle
• Epineurium: Dense connective tissue around entire nerve trunk (outermost)
• • Mnemonic: E-P-I-N (Epi → Peri → Endo)

🖇 Staining Techniques

H&E Stain:

• Cannot visualize myelin sheath
• Clearing agents (Xylene/Zairod) dissolve lipids
• Can stain: axon + Schwann cell membrane (neurilemma)

Lipid Stains:

• Osmic acid: Stains myelin in black circles
• Sudan Black, Sudan III
• Required to visualize myelin sheath

🔥 Important Exam Questions

📝 Short Essay Questions:

Q1: Describe the cells that form the myelin sheath

• PNS: Schwann cells wrap around axon multiple times
• CNS: Oligodendrocytes extend multiple processes
• Key differences: ratio, neurilemma presence, location

Q2: Compare myelination by oligodendrocytes and Schwann cells

• Location: CNS vs PNS
• Ratio: 1 oligo → multiple axons vs 1 Schwann → 1 axon
• Neurilemma: absent in CNS vs present in PNS
• Mechanism: processes vs wrapping

Q3: Describe the EM image of myelin sheath

• Concentric layers appearance
• Alternating dark lines (cell membrane) and pale lines (myelin)
• Lipoprotein and glycolipid composition

Q4: Explain why myelin sheath cannot be stained by H&E

• Myelin is lipid-based
• Clearing agents (Xylene/Zairod) dissolve lipids during preparation
• Need special lipid stains (Osmic acid, Sudan Black/III)

🔘 MCQ Topics:

Q1: Which nerve fiber type is unmyelinated with neurilemma?

• Answer: Autonomic ganglia

Q2: What covering surrounds the fascicle?

• Answer: Perineurium

Q3: Where is the nucleus of Schwann cell located?

• Answer: Neurilemma (not mesaxon)

✏️ Fill-in-the-Blank:

Q1: Myelination in PNS occurs by _______, whereas in CNS it occurs by _______

• Answer: Schwann cells / Oligodendrocytes

🎯 Definition Questions:

Q1: Define Neurilemma

• Cell membrane of Schwann cell
• Outermost layer containing nucleus
• Present in PNS only

Q2: Name the long axon type and short axon type

• Long axon: Golgi Type 1
• Short axon: Golgi Type 2

Q3: Does one Schwann cell myelinate more than one axon?

• Answer: No, only one axon per Schwann cell