Nervous Tissue

Neurons & Neuroglia - Brain's Building Blocks

• Neurons: Excitable cells; functional units.
  • Soma (perikaryon): Nissl bodies (RER), nucleus.
  • Dendrites: Receive signals.
  • Axon: Transmits signals; axon hillock. Nodes of Ranvier (saltatory conduction).
  • Myelin: ↑ conduction. Schwann (PNS, 1 axon), Oligodendrocytes (CNS, many axons).
• Neuroglia (Glial Cells): Support neurons. More numerous.
  • CNS Glia:
    • Astrocytes: Largest, star-shaped. Blood-Brain Barrier (BBB), K+ buffer, GFAP+.
    • Oligodendrocytes: Myelinate CNS axons (many). Damaged in MS.
    • Microglia: Phagocytic (macrophage-like). Monocyte-derived.
    • Ependymal cells: Line ventricles, central canal. CSF production (choroid plexus).
  • PNS Glia:
    • Schwann cells: Myelinate PNS axon (1). Aid regeneration. Damaged in GBS.
    • Satellite cells: Support ganglia neurons.

⭐ Microglia are of mesodermal origin; other glial cells are neuroectodermal.

Myelin & Fibers - Speedy Signal Conduits

• Myelin Sheath: Insulates axons; ↑ conduction velocity.
  • PNS: Schwann Cells (myelinate 1 axon segment; aid regeneration).
  • CNS: Oligodendrocytes (myelinate multiple axons; no significant regeneration).
  • Nodes of Ranvier: Myelin gaps; high Na+ channel density.
  • Saltatory Conduction: AP "jumps" node to node; ↑ speed, conserves energy.
• Nerve Fibers (Erlanger-Gasser Classification):
  • Type A: Large, myelinated (e.g., Aα: proprioception, motor). Fastest.
  • Type B: Smaller, myelinated (e.g., preganglionic autonomic).
  • Type C: Smallest, unmyelinated (e.g., pain, temperature, postganglionic autonomic). Slowest. 📌 Always Be Careful (A>B>C)

⭐ Oligodendrocytes in CNS myelinate multiple axons, while Schwann cells in PNS myelinate a single axon segment and aid in regeneration.

Synapses & Ganglia - Neural Network Hubs

• Synapse: Junction for neuronal communication.
  • Chemical: Neurotransmitters (e.g., ACh, GABA); unidirectional; synaptic delay. Key: presynaptic terminal, cleft, postsynaptic membrane.
  • Electrical: Gap junctions; rapid, bidirectional flow.
• Ganglia: Neuronal cell body clusters outside CNS.
  • Sensory (e.g., DRG): Pseudounipolar neurons. Satellite cells. No synapses.

    ⭐ Dorsal root ganglia (sensory) contain pseudounipolar neurons with centrally located nuclei and surrounding satellite cells, and NO synapses.

  • Autonomic (Symp/Parasymp): Multipolar neurons. Synapses present. Fewer satellite cells.

Nerve Coverings - Protective Nerve Wraps

• Peripheral nerves: wrapped in three connective tissue (CT) layers:
  • Epineurium: Outermost; dense irregular CT; surrounds entire nerve; contains vasa nervorum.
  • Perineurium: Middle; surrounds nerve fascicles; specialized CT.

    ⭐ The perineurium, composed of layers of flattened fibroblast-like cells with tight junctions, forms the blood-nerve barrier.

  • Endoneurium: Innermost; loose CT; surrounds individual nerve fibers & Schwann cells; contains capillaries.
• 📌 Mnemonic: Eat Pizza Everyday (Epineurium, Perineurium, Endoneurium - from out to in).

Nerve Injury & Repair - Damage & Fix Crew

• Seddon's Classification (📌 NAP for severity ↑):
  • Neuropraxia: Myelin damage, temporary conduction block. Full, rapid recovery.
  • Axonotmesis: Axon severed, endoneurium intact. Wallerian degeneration distally. Regeneration (1-4 mm/day) via Büngner bands.
  • Neurotmesis: Complete nerve transection (axon, sheaths). Surgery essential. Poor prognosis.
• Key Processes:
  • Wallerian Degeneration: Distal axon/myelin breakdown, macrophage cleanup.
  • Chromatolysis: Proximal cell body changes.
  • Regeneration: Axonal sprouting, Schwann cell guidance (Bands of Büngner).

⭐ Chromatolysis (central chromatolysis) involves swelling of the neuron cell body, eccentric displacement of the nucleus, and dispersal of Nissl substance, occurring after axonal injury.

High‑Yield Points - ⚡ Biggest Takeaways

• Neurons: functional units; Neuroglia: support (Astrocytes, Oligodendrocytes, Microglia, Ependymal cells).
• Nissl bodies (RER): in soma/dendrites; absent in axon hillock/axon.
• Myelin: Schwann cells (PNS), Oligodendrocytes (CNS); for saltatory conduction at Nodes of Ranvier.
• Astrocytes: largest glia, form blood-brain barrier, structural/metabolic support.
• Microglia: CNS macrophages (mesodermal origin), phagocytic.
• Ependymal cells: line ventricles/central canal, CSF production/circulation.