THE NERVOUS SYSTEM I
The study of nervous system histology could easily comprise the entire subject matter
of this course and a couple more. Thus, we will only be considering a broad overview
of the subject.
I. Two major components of nervous
system.
A. central nervous system (CNS) brain, spinal cord
B. peripheral nervous system (PNS)
1) axonal processes extending toward or
away from CNS (nerves)
2) ganglia (aggregations of nerve cell
bodies outside CNS).
http://www.student.loretto.org/anatomyphys/Coordination-%20nervous%20system.htm
III. If unstained brain or spinal cord are sectioned, we find that two major
areas of brain tissue may be defined on the basis of color.
A. gray matter - neuron perikarya (cell bodies), glial cells, axons, dendrites,
synapses
B. white matter - axons + myelin sheaths, oligodendroglia, other glial cells, no
neuron perikarya, no synapses.
II. Nervous tissue consists of two
major types of cells
1. neurons - responsible for conduction,
propagation, and reception of nervous
impulses. Processes called axons or
dendrites extend from these cells.
2. glial cells - (neuroglia) cells
associated with neurons. No axons or
dendrites. These cells are involved in
nutrition, support, insulation, protection
of neurons.
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
IV. BASIC NEURON STRUCTURE
IV. BASIC NEURON STRUCTURE
Nissl bodies
A. Perikaryon - nerve cell body, contains nucleus and
typical cell organelles
1. nucleus - large, central in most,
large amount of euchromatin (intense
synthetic activity), nucleolus, Barr body
in females (Dormant X chromosome of
females).
http://www.udel.edu/Biology/Wags/histopage/colorpage/cne/cn
emnns.GIF
2. rough endoplasmic reticulum (RER) - lots of
RER for synthesis of structural and transport
proteins, Nissl bodies/substance seen with
light microscope are condensations of this
RER and free ribosomes.
3. golgi apparatus - only found near
nucleus in perikaryon. Expected, since
intense synthetic activity of
neurotransmitters and/or neurohormones
that must be packaged in vesicles.
IV. BASIC NEURON STRUCTURE
A. Perikaryon
4. mitochondria - abundant for
high energy requirements
5. Neurofilaments - intermediate
filaments (10 nm)
6. Microtubules - important in
transport of materials (e.g.
neurotransmitters)
7. inclusions - pigment deposits function unknown. Lipofuscin
deposits - residual bodies from
autophagosome activity. Increase
with age.
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
IV. BASIC NEURON STRUCTURE
B. Dendrite - cell process
extending away from the
perikaryon
1. may be branched
2. forms receptive area for synaptic
contacts from other neurons
3. has tiny rough projectons or spines
called gemmules or dendritic spines that
are points of synaptic contact
4. dendrites from larger neurons may be
lightly myelinated by oligodendroglia
5. Cytoplasm in these processes similar to that
of perikaryon, but no golgi bodies or nucleus.
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
IV. BASIC NEURON STRUCTURE
C. Axon - one long thin cell process extending away
from perikaryon
1. may be branched
2. ends of branches form synapses with
other neurons or muscle cells
3. may be
unmylenated or may
be myelinated by
either oligodendroglia
in CNS or Schwann
cells in PNS
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
IV. BASIC NEURON STRUCTURE
4. Axon structure
a. axon hillock (pyramid shaped region associated with
perikaryon) - where axon begins - cytoplasm in this
region lacks ribosomes and organelles; however,
neurofilaments and microtubles are present.
b. initial segment (unmyelinated intitial portion of
axon) between axon hillock and beginning of
myelination.
c. remainder of axon (may be
myelinated, may be branched)
d. axons carry electrical impulses (action
potentials) to synapses at end of axon.
e. accept for the synapse, the axon cytoplasm
(axoplasm) has few organelles. Not much
synthetic activity in this part of neuron.
f. Microtubules and neurofilaments present.
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm
http://vv.carleton.ca/~neil/neural/neuron-a.html
IV. BASIC NEURON STRUCTURE
D. Synapse
1. specialized junctions with other
cells that are along the length or at
end of an axon.
2. act as transmission points for
electrical impulses or chemical (ionic)
changes.
3. synapse can transmit action
potential, or can polarize or
depolarize the postsynaptic cell.
4. synapses at the end of an axon or
axon branches that are swollen into a
club shape are called boutons
terminaux.
5. a synapse along length of an axon
results in varicosities (swellings) in
the axon that are called boutons en
passage.
http://www.nature.com/neuro/journal/v4/n11/full/nn744
.html
IV. BASIC NEURON STRUCTURE
6. General structure of synapse
a. synaptic vesicles - many small
vesicles in the bouton that contain
neurotransmitter (e.g. acetylcholine or
serotonin).
b. terminal or presynaptic membrane this is part of the neuron plasmalemma
c. synatic gap/cleft is present - this is a
space between the presynaptic
membrane of the axon and the
plasmalemma of the cell that receives
the synaptic input
d. postsynaptic membrane - part of
plasmalemma of cell that receives input
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
IV. BASIC NEURON STRUCTURE
6. General structure of synapse
e. when action potential reaches
synapse, the synaptic vesicles are
exocytosed at the presynaptic
membrane and their contents
(neurotransmitter) are released into the
synaptic gap.
f. neurotransmitter binds to receptors on
postsynaptic membrane and
propagates polarity change in postsynaptic cell.
Copyright Dennis Kunkel
http://www.pbrc.hawaii.edu/~kunkel/gallery
V. TYPES OF NEURONS
A. Types of neurons based on shape/morphology
1. multipolar - more than two processes arising from perikaryon, most of
neurons in brain and spinal cord are of this type.
http://www.lab.anhb.uwa.edu.au/mb140/
2. bipolar - two processes arising from perikaryon, may be branched at
ends, some sensory neurons in retina and cochlea, are of this type.
Bipolar
neurons in
vestibular
ganglion
http://www.csus.edu/org/nrg/carter/NeurosylActive/histology/neuron/bipolar.htm
3. Unipolar or pseudounipolar - one process arising from
perikaryon, really two processes that are fused along portions
closest to perikaryon - found in spinal ganglia and some crainial
ganglia. Developmentally, this type of neuron starts out as a bipolar
neuron.
4. Regardless of the type of neuron, the general structure dendrite(s)-perikaryon-axonsynapse is the same. Appearance may differ due number of processes and branching or
fusion of processes.
B. Types of neurons based on function
A. Motor neurons - efferent, axon extends out of CNS to an effector organ/tissue (e.g.
muscle) in peripheral regions
B. Sensory neurons - afferent, dendrite/axon extends from peripheral sensory structure
(e.g. pacinian corpuscles, touch, pressure) into CNS
C. Interneurons - form connections between neurons
VI. GLIAL CELLS
•More glial cells in the nervous system
than there are neurons.
•Cells are situated among the neurons and
are generally smaller.
•Special staining techniques needed to
differentiate from each other and from
surrounding neurons.
•With hematoxylin - eosin, only the glial
cell nuclei show up.
http://www.lab.anhb.uwa.edu.au/mb140/
VI. GLIAL CELLS - major types
A. Astrocytes
a. provide physical support for neurons
b. store glycogen
c. isolate synaptic areas from one and other
d. In the brain, processes abut against the basement
membrane of capillary endothelium (pedicles)
forming the blood-brain barrier
e. other processes are closely applied to
neurons (pedicles)
f. may form a conduit for nutrients from blood
vessels to neurons
http://members.tripod.com/blustein/Astrocytes/astrocytes.htm
Two types of astrocytes
1. Protoplasmic astrocytes
a. granular cytoplasm, many branches on short
processes
b. found mainly in gray matter
http://www.benbest.com/cryonics/protocol.html
2. Fibrous astrocytes
a. have longer slender processes
b. found mainly in white matter (but also occur
in gray matter).
VI. GLIAL CELLS - major types
B. Oligodendroglia (plural), also called oligodendria (plural) or oligodendrocytes (plural).
oligodendrocyte (singular)
1. smaller than astrocytes, fewer
processes
2. found in both grey and white matter of
CNS
3. particularly in white matter,
processes from these cells form the
myelin sheaths that are around many
axons
4. analogous to Schwann cells of
peripheral nervous system
5. these cells must be cultured with
neurons in order to get neurons to grow
in tissue culture. Suggests intimate
http://members.tripod.com/blustein/Oligodendrocytes/oligodendrocytes.htm
interactive association
C. Microglia
1. small cell body that is usually elongated and
stains densely
2. sometimes an elongate nucleus with mostly
heterochromatin (Other glia have spherical
nucleus)
3. many of what were thought to be microglia
under the light microscope, have turned out to
be oligodendroglia when cells were examined
with the electron microscope.
4. microglial cells are derived from
mesoderm.
5. microglial cells function in phagocytosis components of immune system, act as brain
macrophages.
http://members.tripod.com/blustein/Microglia/microglia.htm
6. known to migrate and accumulate at
the site of nerve damage within the
central nervous system.
Microglial cell engulfing nucleus of dead
cell in brain slice culture.
Microglial movement in brain slice culture
http://www.biology.uiowa.edu/daileylab/movies.html
VII. GLIAL CELLS - major types
D. Ependymal cells
1. ciliated cells forming single layer of simple cuboidal to low columnar
epithelium that lines the entire neurocoel
2. ciliary action acts to circulate cerebral spinal fluid.
http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Nervous/Nervous.htm
Peripheral Nervous System Glial Cells
E. Schwann cells - peripheral nervous system
Animation of myelination
http://www.siumed.edu/~dking2/ssb/neuron.htm
http://members.tripod.com/blustein/Oligodendrocytes/oligodendrocytes.htm
Schwann cells
1. Provide myelin sheaths for axons in the PNS
http://education.vetmed.vt.edu/Curriculum/VM8054/Labs
/Lab9/Lab9.htm
Peripheral Nervous System Glial Cells
F. satellite cells
1. Found surrounding neurons in ganglia.
VIII. STRUCTURE OF NERVES
A. Nerves are surrounded by a thick connective tissue sheath composed of
collagenous fibers and containing small blood vessels that is called the
epineurium - also extends into the nerve between the fascicles of axons.
B. Within the epineurium are bundles of nerve fibers called fascicles.
1. Nerve fibers - axons, each one of which is ensheathed by a single or
multiple layers of plasmalemma from a Schwann cell.
C. Each bundle (or fascicle) of nerve fibers is surrounded by a layer of connective
tissue called the perineurium
D. Each nerve fiber within each bundle is individually surrounded by a layer of
reticular connective tissue called the endoneurium
1. Small diameter axons usually ensheathed by a single layer of Schwann cell
plasmalemma and are not myelinated.
2. Larger axons are myelinated - ensheathed by multiple layers of specialized
Schwann cell plasma membrane called myelin.
Nerve Histology
http://www.meddean.luc.edu/lumen/medEd/Histo/frames/h_frame6.html
VIII. STRUCTURE OF NERVES
nerve fibers
epineurium
extending
between
fascicles
fascicle)