Microscopic Anatomy of Nervous Tissues

Neurons - Brain's Building Blocks

• Function: Basic structural & functional units; excitability & signal conduction.
• Structure:
  • Soma (Perikaryon): Metabolic center; nucleus, Nissl bodies.
    • Nissl bodies: RER & ribosomes; protein synthesis (neurotransmitters).
  • Dendrites: Branched receptive processes; conduct graded potentials to soma.
  • Axon: Single projection from axon hillock (trigger zone); conducts action potentials away. Axoplasm, axolemma.
  • Axon Terminals: Branched endings; neurotransmitter release.
• Cytoskeleton: Maintains shape, aids transport.
  • Neurofilaments: Structural support.
  • Microtubules: Axonal transport (organelles, vesicles).
  • Microfilaments (Actin): Motility, growth cone.
• Axonal Transport: Bidirectional.
  • Anterograde: Soma → terminal (Kinesin).
  • Retrograde: Terminal → soma (Dynein).

⭐ Nissl bodies (RER) are found in the soma and dendrites, but are characteristically absent from the axon hillock and axon.

Neuroglia - Neuron's Nurturers

• Non-excitable cells; metabolic & structural support to neurons; outnumber neurons 5-10x.
• Types: CNS glia & PNS glia.

, oligodendrocytes (fried-egg), microglia (small, elongated), ependymal cells (cuboidal, ciliated))

⭐ Astrocytes, identified by GFAP (Glial Fibrillary Acidic Protein) intermediate filaments, are the most numerous glial cells in the CNS and form the blood-brain barrier.

Myelin & Nerve Fibers - Insulated Expressways

• Myelin Sheath:
  • Composition: Predominantly lipid (e.g., cerebrosides, cholesterol) & protein; acts as an electrical insulator.
  • Formation:
    • PNS: Schwann cells (one cell myelinates a single axon segment).
    • CNS: Oligodendrocytes (one cell myelinates segments of multiple axons).
  • Function: Facilitates saltatory conduction, significantly ↑ speed of nerve impulse transmission.
• Key Structures of Myelinated Fibers:
  • Nodes of Ranvier: Periodic gaps in the myelin sheath between adjacent Schwann cells or oligodendrocyte processes. High concentration of Na+ channels for action potential regeneration.
  • Internodal Segment: The myelinated portion of an axon between two Nodes of Ranvier.
  • Schmidt-Lanterman Clefts: Small pockets of Schwann cell cytoplasm within the myelin layers in PNS fibers; involved in myelin maintenance.

⭐ Oligodendrocytes myelinate multiple axons in the CNS, whereas Schwann cells myelinate only a single segment of one axon in the PNS and are crucial for regeneration.

Synapses & Ganglia - Critical Connections

• Synapse: Junction for signal transmission (neuron-neuron or neuron-effector).
  • Chemical: Unidirectional. Neurotransmitters (ACh, NE) from presynaptic vesicles to cleft. Synaptic delay.
  • Electrical: Bidirectional. Direct ion flow via gap junctions. Faster, minimal delay.
• Ganglia: PNS clusters of neuronal cell bodies (soma).
  • Supported by satellite glial cells.
  • Connective tissue capsule.

  ⭐ Sensory ganglia (e.g., Dorsal Root Ganglia) contain pseudounipolar neurons and do NOT have synapses, whereas autonomic ganglia contain multipolar neurons and do have synapses.

• Key Ganglia Differences:

  Feature	Sensory Ganglia (DRG)	Autonomic Ganglia (Sympathetic)
  Neuron Type	Pseudounipolar	Multipolar
  Synapses	No (cell bodies only)	Yes (pre- to post-ganglionic)
  Satellite Cells	Numerous, complete layer	Fewer, incomplete layer

High‑Yield Points - ⚡ Biggest Takeaways

• Nissl bodies (RER) are in neuron soma and dendrites, absent in axon hillock/axon.
• Oligodendrocytes (CNS) myelinate multiple axons; Schwann cells (PNS) myelinate single axon segments.
• Nodes of Ranvier are unmyelinated gaps crucial for saltatory conduction.
• Astrocytes (GFAP+) are the most numerous glia, forming the blood-brain barrier.
• Microglia are mesoderm-derived CNS macrophages with phagocytic function.
• Ependymal cells line ventricles and central canal, contributing to CSF production.
• Peripheral nerve layers: Endoneurium (single fiber), Perineurium (fascicle, blood-nerve barrier), Epineurium (entire nerve).