AHD
Oct/22/08
Amal Al-Hashmi
R4
Quiz
• List the different parts of a neuron?
• What are the functions of a neuron?
• List the different types of glia cells and
name one function for each?
Out line
1. Structure of neurons
2. Classifications of neurons
3. Axonal transport and its clinical applications
4. Electrical properties of neurons
5. Glial cell functions and clinical application
6. Conclusion/quiz
The structure of neurons
• Dendrites
• Cell body
• Axons and axons terminals
Dendrites
• *Usually brch extensively in the vicinity of the cell body
• *receive signals either from other neurons or from the
environment via specialized receptors
• *Info travels from distal to proximal along dendrites to
converge at the cell body
• *Small bud like extension (dendrite spines) of variety of
shapes is frequently seen on the more distal brch of
dendrite contact
Cell body
• Cell body of neuron is also called the soma or
the perikaryon
• Varies widely in shape depending on the neuron
type
• Active protein synthesis is indicated by the large
size of the nucleus and its content of diffuse
chromatin and at least one prominent nucleolus
(site of ribosomal RNA synthesis),
Cell body
The cell body of multipolar neurons vary widely in shape so their
profiles in tissue sections may appear fusiform , flask shaped ,
triangular , polygon or stellate
Variations of stellate polygon are most common
Bipolar have a round or oval perikaryon, with a single large process
emanating from each end of the cell body
The pseudo unipolar has a spherical cell body with centrally placed
nucleus
Axons and Axons Terminals
•
The axon arises from the cell body at a small elevation called axon hillock.
•
The proximal part of the axon, adjacent to the axon hillock, is the initial
segment
•
The cytoplasm of the axon (axoplasm) contains dense bundles of
microtubules and neurofilaments
•
Axons are typically devoid of ribosomes
•
Axon in CNS often end with fine terminals called terminal arobes which are
capped with small terminal boutons
•
The surface area of axon can be several 1000 times the surface area of the
parent cell body
Classification of neurons
• Multipolar 99%
• Bipolar
• Pseudounipolar/unipolar
Type of neuron
Unipolar
bipolar
Location of cell bodies
Post root or CN ganglion
Retina, olfactory epithelium,vestibular
ganglion , auditory ganglion
Multipolar
Stellate
Fusiform
Pyriform
Pyramidal
Purkinje
Mitral
Chandelier
Granule
amacrine
Many areas of CNS
Hippocampal
Cerebellar cortex
Olfactory bulb
Visual area of cerebral cortex
Cerebral and cerebellar cortex
retina
Axonal transport
Anterograde
Retrograde
Allows neurons to respond to molecules
Slow
1mm/day
Not clearly
understood
Continual recycling of components
of axon terminal. driven by the protien dynein
Fast 50-250mm/day
Fast
@ speed 400mm/day,
driven by the protien
Kinesin
Implication in of axonal transport in
pathogenesis of some human neurological
diseases e.g.
Rabies virus
Clostridium tetani ( Tetanus)
Axonal transport as a research tool
Injecting a fluorescent substance into region
containing axons terminals ( retrograde)
Injecting radioactively labeled a. a into cell
body ( anterograde)
Electrical properties of neuron
Neurons carry a negative electrical charge relative
to the extra cellular fluid bathing them
The plasma membrane is a semi permeable
because certain ions can cross at certain times
but there is not a free exchange
The opening and closing of specific ion channels
can be controlled by chemical signals including
neurotransmitter
Neurons as info receivers
•
Primary sensory neurons
photoreceptors ( cones. rods)
chemoreceptors (taste/smell, PH receptor
mechanoreceptors ( vestibular, auditory, somatosensory system)
thermoreceptors ( pain/temp)
Neurons as info transmitters
• Synapses
• Location at which a process of one neuron
communicates with a second neuron or a effector cell
• Chemical synapse consists of presynaptic elements ,
postsynaptic elements and synaptic cleft
• Unidirectional
• The strength of effect on postsynaptic membrane is
variable
Glial cells
• Major types in CNS are:
astrocytes
oligodendrocytes
microglia
• Major types in PNS:
satellite
schwann cells
MQ
Astrocytes
• Occur through out CNS
• Highly branched cells with processes that
contact most of of the surface of neuronal
dendrites and cell bodies as well as axonal
surface
• Role in structural support and response to injury
• Secrete growth factors, Cholesterol and
lipoprotein ,
• Contribute to the regulation of inflammatory
processes ( cytokines and IL-1, IL6)
Astrocytes
• Environmental modulation
• Metabolism
• Regional heterogeneity
• Role at the BBB
Oligodendrocytes
Arise during development from oligodendrocte
precursor cells
Occurs in both grey and white matter
Main function is myelination
One oligodendrocyte can myelinates 30 to 50 –
nearby axons
Oligodendrogial cell body located at some –
distance form myelin sheath, linked by thin
cytoplasmic bridge
oligodendrocytes
Microglia
Immune effector cells of CNS
Predominant cells involved in CNS
inflammation
Makes up to 1% of CNS cell population
They have phagocytic role
They secrete IL-IB, TNF, PG
microglia
Clinical implications
Excessive secretion of Cytokines IL-IB &TNF induces
endothelial cells to open the BBB allowing leukocytes
infiltration into the brain parenchyma
( bacterial meningitis)
some molecules secreted by activated microglial cells are
neurotoxin( including glutamate, oxygen radicals and
TNF ) and neurons may be directly damaged by
inflammatory rxn in the CNS
thus the microglial rxn maybe a 2nd cause of neuronal
damage following stroke or tumor
Clinical implications
• release of O2 radicals by microglia and
astrocytes can lead to oxidative damage
• in MS , demylination results from inflammatory
signaling that allows T lymphocytes to
extravasate and enter CNS tissue
• HIV virus leads to neuronal damage through
targeting microglia cells. ?!dementia
Glia-derived tumors
Astrocytomas
the most frequently encountered glial cell
tumor
Grade I
astrocytoma are slowly growing, arising from
white matter
Grade II
Slow, but more infiltrative
Glia-derived tumors
Grade III
Rapid growing with mitotic figures
grade IV
Also called glioblastoma multiform
Highly malignant , rapidly growing invasive
tumor
• Oligodendrogliomas
slow growing tumors
Summery
• Neurons are excitable cells that are
specialized for the perception of stimuli
and conduction of nerve impulses
• They vary in shape and size
• There are two ways of axonal
transportation
• Chemical synapse consists of presynaptic
elements , postsynaptic elements and
synaptic cleft
Summery
Glial cells provide structural support for the
nerve cells and control the environment
within the CNS
quiz
• Define the term synapse?
• List the different types of glia cells and
name one function for each.
• What is the role of microglial cells in CNS
truma or inflammation?
Thank you
questions