Classifying Neurons and Glia

Classifying Neurons

• Classification Based on Neuronal Structure

  • Number of Neurites

    • Total number of neurites = total number of axons and dendrites that extend from soma

    • Unipolar = neuron with single neurite

    • Bipolar = two neurites

    • Multipolar = three or more

      • Most neurons in brain are multipolar

  • Dendrites

    • Cerebral cortex

      • Stellate cells (star shaped)

      • Pyramidal cells (pyramid shaped)

  • Spine or no spine

    • Spiny = neurons with a spine

    • Aspinous = neurons without a spine

  • Dendritic classification schemes can overlap

    • All pyramid cells are spiny

    • Stellate cells can be spiny and aspinous

• Connections

  • Primary Sensory Neuron Connections

    • Info is delivered to NS by neurons w/neurites in sensory surfaces

    • Ex: Skin & Retina

  • Motor Neurons

    • Neurons that have axons that form synapses w/muscles & cause movement

  • Interneurons

    • Neurons that form connections only w/other neurons

• Axon Length

  • Golgi type I neurons

    • Long axons that extend from one part of brain to another

      • Ex: pyramidal cells

  • Golgi type II neurons (local circuit neurons)

    • Short axons that do not extend past cell body vicinity

      • Ex: stellate cells

• Gene Expression Classification

  • Differences in gene expression can cause pyramidal & stellate cells to develop diff shapes

  • Green fluorescent protein (GFP)

    • Encoded by a gene discovered in jellyfish

    • When illuminated fluoresces bright green allowing the neuron in which it is expressed to be seen

• Neurotransmitter Differences

  • Differences arise bc of differences in expression of proteins involved in transmitter:

    • Synthesis

    • Storage

    • Use

  • Ex: motor neurons that command voluntary movements release the neurotransmitter acetylcholine

    • Therefore classified as cholinergic

      • Cholinergic = expresses the genes that enable use of particular neurotransmitter

Glia

• Astrocytes

  • Most numerous glia

    • Fill most spaces between neurons

    • Space that remains between neurons and astrocytes in the brain is about 20 nm wide

  • Influence whether neurite can grow or retract

  • Regulates chemical content of extracellular space

    • Ex: Can envelop synaptic junctions in the brain > restricts spread of neurotransmitters released

    • Ex: Has proteins in membranes that remove neurotransmitters from synaptic cleft

  • Possess neurotransmitter receptors that trigger electrical and biochemical events inside glial cell

  • Control the extracellular concentration of substances that can interfere with neuro function

    • Ex: regulates concentration of potassium ions in extracellular fluid

  • Maintain blood-brain barrier

  • Repair and support

• Myelinating Glia

  • Oligodendroglia & Schwann cells provide layers of membrane that insulate axons

    • Wrapping called myelin spirals around axons in brain

    • Entire covering is called the myelin sheath

      • Node of Ranvier: short length where the axonal membrane is exposed, interrupts myelin sheath periodically

  • Oligodendroglia is found in CNS & contributes myelin to several axons

  • Schwann cells are found in peripheral nervous system (PNS) & myelinates single axon

• Non-Neuronal Cells

  • Ependymal cells

    • Line fluid filled ventricles in brain to produce CSF

    • Direct cell migration during development

  • Microglia

    • Phagocytes that remove debris left by dead or degenerating neurons and glia

    • Involved in remodeling synaptic connections by gobbling them up

  • Vasculature arteries, veins, & capillaries

    • Deliver nutrients and oxygen to neurons by blood

• Microglia v.s. Macroglia

  • Microglia

    • Primary immune defense of CNS

    • Travel through the brain and spinal cord to remove:

      • Damage neurons

      • Pathogens

      • Foreign substances

  • Macroglia

    • Astrocytes

    • Oligodendrocytes

    • Schwann cells

• Radial glia

  • Neurogenesis & neurodevelopment

    • Can develop new neurons

  • Movement of new neuros through the brain