Nerve Cells and Glia - Quick Reference

Neurons and glia are the two main cell types in the nervous system.
Neurons receive information and transmit it to other cells; glia support many functions.
The adult human brain contains about $86 \times 10^9$ neurons.
Glia (from Greek for glue) have diverse functions beyond just binding neurons.
Major glia types: astrocytes, oligodendrocytes, Schwann cells, radial glia, microglia.
Tripartite synapse: astrocytes participate in and modulate synaptic signaling.

Neurons have a soma (cell body), dendrites, an axon, and presynaptic terminals.
Dendrites are branching fibers with synaptic receptors; dendritic spines increase surface area for synapses.
The soma contains the nucleus, ribosomes, and mitochondria; diameter ranges from $0.005 \text{ mm}$ to $0.1 \text{ mm}$ in mammals; up to $1 \text{ mm}$ in some invertebrates.
The axon is a thin fiber of constant diameter that conveys impulses to other neurons, organs, or muscles; axons can be longer than $1 \text{ m}$ .
Myelin sheath insulates many vertebrate axons; gaps are nodes of Ranvier.
Presynaptic terminals (end bulbs) release neurotransmitters across the synapse.
An afferent axon brings information into a structure; an efferent axon carries information away.
Interneurons have dendrites and axons entirely contained within a single structure.

Neurons vary in shape and size to suit their functions; multiple morphologies.
Examples: Purkinje cell, sensory neuron, pyramidal cell, bipolar cell, Kenyon cell.
Glia provide support and other functions; neurons’ shapes reflect their connections.
Oligodendrocytes form myelin segments for $30 \text{ to } 50$ axons each.

The blood–brain barrier (BBB) protects the CNS from pathogens and many chemicals via tight junctions.
Some viruses can cross; others are blocked (e.g., rabies example cited).
Active transport proteins pump essential chemicals into the brain: glucose, amino acids, purines, choline, some vitamins, iron.
Small uncharged molecules (e.g., water, oxygen, carbon dioxide) and many fats cross more freely.
Certain hormones (including insulin) can cross the BBB.
The BBB poses a challenge for treating brain cancers because most chemotherapeutic drugs do not cross well.

Vertebrate neurons rely almost entirely on glucose as a fuel.
The brain uses about $25 \%$ of the body's glucose and about $20 \%$ of the body's oxygen.
Thiamine (vitamin B1) is required to metabolize glucose.
Prolonged thiamine deficiency (e.g., Korsakoff’s syndrome) leads to neuron death and memory impairments.

Neurons receive information and convey it to other cells.
Santiago Ramón y Cajal established that the nervous system is composed of separate cells (neurons).
Neurons share internal structures with other animal cells.
Major parts: soma, dendrites, axon, presynaptic terminals; neuron shapes vary by function.
The BBB protects the CNS; many molecules cannot cross; some cross via active transport.
The BBB is an unbroken wall around brain vessels; water, oxygen, and CO2 cross freely; small uncharged molecules and fats cross; glucose, amino acids, and some hormones cross via transport.
Neurons rely on glucose; thiamine is needed to use it.