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.

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.

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.

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.

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).

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.

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.

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.

≈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 (