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Chapter 2

HMB200H1 Chapter Notes - Chapter 2: Cell Migration, Schwann Cell, Astrocyte


Department
Human Biology
Course Code
HMB200H1
Professor
John Yeomans
Chapter
2

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Chapter(2:(Neurons(and(Glia!
Introduction!
" neurons!sense!changes!in!the!environment,!communicate!these!changes!to!other!neurons,!and!command!the!
body’s!responses!to!these!sensations!
" glia!(glial!cells)!are!thought!to!contribute!to!brain!function!mainly!by!insulating,!supporting,!and!nourishing!
neighboring!neurons!
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The(Neuron(Doctrine!
" progress!in!cellular!neuroscience!was!not!possible!before!the!development!of!the!compound!microscope!because!
of!the!brain!cells’!small!size!
" but!the!brain!was!too!“jello”!!!in!order!to!study!the!anatomy!of!brain!cells,!had!to!wait!for!a!development!of!a!
method!to!harden!the!tissue!without!disturbing!its!structure!and!instrument!that!could!produce!very!thin!slices!
" in!early!1900s,!scientists!discovered!formaldehyde!can!be!used!to!harden!tissues!by!immersing!them!into!it!!!
developed!the!microtome!to!make!thin!slices!
" these!technical!advances!spawned!the!field!of!histology:!the!microscopic!study!of!the!structure!of!tissues!
" next!problem?!Freshly!prepared!brain!has!a!uniform,!cream"coloured!appearance!under!the!microscope!!!tissue!
had!no!differences!in!pigmentation!to!enable!histologists!to!resolve!individual!cells!
" next!breakthrough:!introduction!of!stains!that!could!selectively!colour!some!(not!all)!parts!of!the!cells!in!the!brain!
tissue!
" Franz(Nissl!(German!neurologists)!1900s:!
introduced!a!stain!used!today!still.!Showed!that!a!
class!of!basic!dyes!would!stain!the!nuclei!of!all!cells!
and!also!stain!clumps!of!material!surrounding!the!
nuclei!of!neurons!!!these!clumps!are!called!Nissl-
bodies!!!the!stain!was!called!Nissl*stain!
!!the!Nissl!stain!was!powerful!for!2!reasons:!!
1) distinguishes!neurons!and!glial!from!one!
another!
2) enables!histologists!to!study!the!arrangement,!or!cytoarchitecture,!of!neurons!in!different!parts!of!the!brain!
" the!study!of!cytoarchitecture!led!to!the!realization!that!the!brain!consists!of!many!specialized!regions!!!we!now!
know!each!region!performs!different!functions!
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The!Golgi!Stain!
" Problem!with!the!Nissl!stain?!he!Nissl!stain!does!not!tell!the!whole!story!!!Nissl"stained"neuron!looks!like!a!lump!
of!protoplasm!containing!a!nucleus!
" 1873(Camillo(Golgi((Italian!histologist):!discovered!that!
soaking!brain!tissue!in!a!silver!chromatic!solution!(Golgi(
stain),!a!small!percentage!of!neurons!became!darkly!
coloured!in!their!entirety!!
" revealed!that!in!the!neuronal!cell!body!(the!region!of!
the!neuron!around!the!nucleus!that!is!shown!with!
the!Nissl!stain)!is!actually!only!a!small!fraction!of!the!
total!structure!of!the!neuron!
" the!Golgi!stain!shows!that!neurons!have!at!least!2!
distinguishable!parts:!
1) central!region!that!contains!the!cell!nucleus!
2) numerous!tubes!that!radiate!away!from!the!
central!region!
" the!swollen!region!containing!the!cell!nucleus!has!several!
names:!cell!body,!some,!perikaryon!
" the!thin!tubes!that!radiate!away!from!the!soma!are!called!
neuritis!!!2!types!of!neuritis:!axons!and!dendrites!
" the!cell!body!gives!rise!to!a!SINGLE!axon!!
" axon!is!of!uniform!diameter!throughout!its!length,!and!if!it!
braches,!the!branches!generally!extend!at!right!angles!
" axons!can!travel!great!distances!in!the!body!=!recognized!by!histologists!that!axons!must!act!like!“wires”!that!carry!
output!of!the!neurons!
" dendrites!on!the!other!hand,!rarely!extend!more!than!2mm!!!many!extend!from!the!cell!body!!
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" since!dendrites!come!into!contact!with!many!axons,!they!must!act!as!the!antennae!of!the!neuron!to!receive!
incoming!signals!
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Cajal’s!Contribution!
" Santiago(Ramon(y(Cajal((histologist)!1888:!learned!about!
Golgi’s!method!
" In!a!series!of!publications!over!the!next!25!years!after!1888,!Cajal!
used!the!Golgi!stain!to!work!out!the!circuitry!of!main!regions!of!
the!brain!
" Golgi!and!Cajal!drew!completely!opposite!conclusions!about!
neurons!
" Golgi(was!correct!about!how!neuritis!of!different!cells!are!fused!
together!to!form!a!continuous!reticulum!or!network!(like!the!
veins!and!arteries!system)!
!!According!to!the!reticular!theory:!the!brain!is!an!exception!to!
the!cell!theory!(states!that!the!individual!cell!is!the!elementary!
functional!unit!of!all!animal!tissues)!
" Cajal(argued!that!the!neuritis!of!different!neurons!are!not!
continuous!with!one!another!and!must!communicate-by-contact,!
not!continuity-!
" The!idea!that!the!neuron!adhered!to!the!cell!theory!=!Neuron!
Doctrine!
" 1950s:!electron!microscope!!!with!increased!resolving!power!of!microscopes,!it!was!discovered!that!neuritis!of!
different!neurons!are!not!continuous!with!one!another!
" Cajal:!discovered!dendritic!spines!
" Willliam(Greenough:!discovered!that!spine!number!is!also!very!sensitive!to!the!quality!of!environment!
experienced!during!early!development!and!in!adulthood!
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(
(
The(Prototypical(Neuron!
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