Cytology Nikon ©
Types of cell Neurons - nerve cells Glial cells 50:50 volume 100 billion Glial cells ≈ 10 X neurons 50:50 volume CNS vs PNS
Neuronal overview Basic function of a neuron Requirements: Transmit information from one place to another Requirements: Structural Functional
Neurons are polarized
Parts of a neuron Dendrites Soma Axon Axon terminal receive information Soma synthesize stuff electrical integration Axon information conduction Axon terminal transmit information
En masse Segregation: white gray
Same but different Multipolar (typical) Bipolar Pseudo-unipolar single axon multiple dendrites Bipolar Pseudo-unipolar
Pseudounipolar neurons…. ….are really bipolar
Classes of neurons Sensory Motor Interneurons Projection Week 6 - somatosensation Week 7 - motor
Synapses Dendritic shafts / spines inhibitory / excitatory (Week 4) MAP2 synaptotagmin
Axons are long ≈ 5ft motor neuron (Sciatic nerve) ≈99% cytoplasm How to accomplish fast signaling (week 4)? How to maintain structure? How to communicate between distant parts?
Axon growth cone Cytoskeleton (Ken Balazovich)
Cross section of dendrite Neurofilaments filamentous actin Microtubules Tubulin (10% brain protein) substrate for axonal transport MAPs
Active transport molecular motors Slow: Fast Retro Antero kinesin few mm / day Fast < 400 mm /day Retro Antero kinesin dynein
Ribosomes: Nissl substance In dendrites (not largely in axons) Local protein synthesis at the base of spines - plasticity (week 9)
High energy use 30-40 % total energy consumption at rest Mitochondria Maintain ionic gradients Protein synthesis Axonal transport Mitochondria Site of oxidative metabolism - ATP Brain exclusively dependent on glucose Found throughout the perikaryon, dendrites, axons and in synaptic terminals
Other organelles Similar to other cells Nucleus: Golgi: Only a few 1000 CNS specific genes - encode CNS proteins Extensive RNA splicing Golgi: post-translational modification
Relationship to other cells
Brain Glue
Glial cells Types Roles Phalloidin Tubulin DAPI
Special properties (Astrocytes) star-shaped & largely lack polarity? No synapses but cells communicate through gap-junctions Relatively low energy requirement; function well under anaerobic conditions Remove glutamate and other amino-acids from extracellular space - de-toxify the brain Form myelin to insulate axons Serve numerous homeostatic functions Can and do proliferate postnatally; tumors
Are astrocytes really star-shaped? (Bushong et al., 2002)
Macroglia <=> Microglia Classification Macroglia <=> Microglia Astrocyte protoplasmic astrocyte (Type 1) fibrous astrocyte (Type 2) Radial glia - development (week 4) Oligodendrocyte Schwann cell
Microglia engulfing a dying oligodendrocyte: phagocytotic cells in the nervous system blood derived cells comparable to macrophages remove debris from the brain following injury and constitute an important defense system against pathogens.
Radial glia Development neuronal guidance
Schwann cell Myelination in the PNS
Myelin sheet One-to-one
Gap junctions and disease Charcot-Marie-Tooth disease progressive loss of PNS axons - weakness, atrophy
Nodes of Ranvier
Saltatory conduction (week 4)
Oligodendrocytes
1:10 to 1:50
Unmyelinated CNS fibers
End feet…… contacting blood vessels
Induce the blood-brain-barrier Active transport Astrocytic endfoot
Nervous system regeneration The CNS does not regenerate while the PNS does This is NOT due to differences in central and peripheral neurons but due to differences in their glia CNS oligodendrocytes actively suppress regeneration PNS Schwann cells promote it
Injury to the CNS Reactive gliosis Degenerating Neuron Microglia
Buffering of extracellular ions Extracellular space is very narrow => small ionic fluxes cause large concentration changes
From here to there…..
Territorial coverage non-overlapping (Bushong et al., 2002)
Astrocytic Glutamate transport Around synapses
Transmitter “shuttle”
Glia versus neuron - difference? excitability (Bergles et al., 1997)