Neurons, Glial Cells, and the Blood-Brain Barrier

Axon Terminals & Synaptic Outputs

• The axon behaves like an electrical wire that “frays” at its end, splitting into many terminals.
  • A single axon can branch to contact ~8 distinct post-synaptic cells in the classroom diagram.
• Branching tips are called axon terminals (a.k.a. synaptic boutons).
  • Serve as the output region of the neuron.
  • Most commonly form synapses onto the dendrites of another neuron, but later you will learn they can also target cell bodies, axons, muscles, glands, etc.

Neuron Morphologies (Structural Classes)

• Illustrated four canonical shapes:

  1. Pseudo-unipolar neuron
  • Cell body bulges off to the side of a single elongated process.
  • Appears to have two axonal branches and no true dendrites.
  1. Bipolar neuron
  • Cell body placed in the middle of two symmetrical processes.
  • One process functions as a dendrite, the other as an axon.
  1. Multipolar neuron (classic)
  • Cell body surrounded by many tree-like dendrites; single long axon exits.
  1. Multipolar neuron with extensive dendritic field (e.g., Purkinje-like)
  • Even greater dendritic arborization around the soma.

• Key identification rule:

  • A dendrite is whatever receives info; an axon is whatever sends it.
  • Branching pattern alone can be deceptive; functional directionality is the defining criterion.

Sensory Neuron Specializations

• Pseudo-unipolar neurons are exclusively sensory.
  • Major modality: touch (somatosensation).
  • Their “dendrite-looking” endings in the skin are actually receptor terminals that start the action potential de novo.
  • Because no previous neuron feeds them, the impulse-carrying process is classified functionally as an axon.
• Bipolar neurons appear in rare, specialized senses:
  • Olfaction (smell) in the olfactory epithelium.
  • Vision within the retina.

Touch Receptor Pathway (Applied Example)

• In skin, a pseudo-unipolar axon branches like a dendritic tree to cover a surface area.
• Mechanical pressure on any branch tip opens mechanosensitive channels → local depolarization → action potential propagates centrally toward the spinal cord/brain.

Glial Cell Types in the Central Nervous System (CNS)

• Astrocytes
  • Analogy: Person hugging a blood vessel while handing food to a neuron ("Uber Eats").
  • Structures called end-feet wrap around blood vessels and neurons.
  • Regulate what leaves blood → brain (part of the blood–brain barrier, BBB).
  • Shuttle nutrients (glucose, ions) directly to neurons.
• Oligodendrocytes
  • Produce myelin (electrical insulation) in the CNS.
  • One oligodendrocyte can myelinate multiple axon segments.
• Microglia
  • Brain-resident immune cells; first immunological defense inside BBB.
  • Analogy: Secret-service–style security restricted to “the White House” (brain).
  • Rationale: Regular blood-borne immune cells are barred; microglia handle surveillance and phagocytosis.
• Ependymal cells (briefly mentioned)
  • Line ventricles; secrete cerebrospinal fluid (CSF).

Blood–Brain Barrier (BBB)

• Structural basis: Tight junctions of capillary endothelial cells + astrocytic end-feet.
• Purpose: “Keep the brain VIP-only.”
• Never permitted across (regardless of circumstances):
  • White blood cells, antibodies → prevent auto-immunity.
  • Toxins & pathogenic microbes.
• Regulated transport (needs channels/carriers):
  • Water, major electrolytes (\text{Na}^+,\;\text{K}^+,\;\text{Cl}^-), glucose, amino acids, select hormones.
• Freely diffusing (small / lipid-soluble):
  • Gases: \text{O}2,\;\text{CO}2 (Fick diffusion law J=-D\frac{dC}{dx} applies).
  • Molecules such as caffeine and nicotine.
• Defensive layers hierarchy:
  1. Physical BBB.
  2. Immunologic BBB → microglia act as in-house immune system.

Peripheral Nervous System (PNS) Counterparts

• Cells perform analogous jobs but bear different names:
  • Satellite cells ≈ astrocytes ("astro = star; satellites are in space too – cute naming").
  • Schwann cells ≈ oligodendrocytes (named for Dr. Schwann).
  • Each Schwann cell myelinates one axon segment.
• No PNS equivalents for microglia or ependymal cells because:
  • Peripheral tissues already host standard immune surveillance.
  • No CSF production outside CNS.

Practical & Conceptual Connections

• Myelin’s insulating role explains why demyelinating diseases (e.g., multiple sclerosis) severely slow conduction.
• BBB selectivity underlies both therapeutic challenges (drug delivery) and safety (toxin exclusion).
• Sensory neuron architecture directly links receptor location (skin) with rapid, direct signal relay to CNS.
• Analogies (frayed wire, Uber Eats, Trojan Horse, Secret Service) aid memory by mapping cell functions to everyday images.

Quick Reference Numbers & Facts

• ≈8 axon terminal branches shown in lecture sketch.
• 1 action potential pathway per pseudo-unipolar neuron (single axon bifurcates).
• Immune-excluded substances are large and/or charged; allowed diffusers are small (