Anatomy, composition and
physiology of neuron, dendrite,
axon, and synapses
Glial cells/ Astrocyets, Oligodendrocyt , Schwan cells/
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Support neuronal cells
Produce myelin
Act as scavengers
Takes up released neurotransmitters
Guide migrating neurons and the out growth of axons
Form BBB
Release growth factor and help nourish nerve cells
Neurons type
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Unipolar cells: single process serving as receptor and releasing terminal
e.g. In autonomic nervous system
Biplar cells : two process dendrite and axon
e.g. in retina , olfactory nerves
Pseudo_unipolar cells:
e.g. dorsal root ganglia
Multipolar cells : most common type
e.g. spinal motor neurons ,pyramidal cells,pukrinje cells
NERONS COULD ALSO BE CLASSIFIED AS :
SENSORY
MOTOR
LOCAL INTERNEURON
PROJECTION INTERNEURON
NEUROENDOCRINE CELL
Structure of a neuron
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Cell body
Dendrites:
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apical and basal types,
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Axons: transmitting element,
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are input elements together with the cell body contains nucleus and gives rise to axon and dendrite
could be longer than 3m,
covered with myelin interrupted by node of Ranvier
is the out put element of neuron
single axon may form synapses with as many as 100000 neurons
Axon hillock: initial segment of neuron
Synapse:
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presynaptic terminal,
synaptic cleft ,
postsynaptic membrane
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dendrites
cell body
axon hillock
muscle
synaptic connections could be divergent or convergent
Functional organization of neurons
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Input component /receptor or synaptic potential/ :
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signal electrical
input signal is graded in amplitude and duration,
proportional to amplitude and duration of stimulus
Integrative component :
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signal electrical
action potential is generated only if input signal is greater than spike
threshold
stimulus intensity is represented by frequency of action potential
duration of stimulus is represented by number of action potentials
Functional organization of neurons
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Conductile component/action potential/ :
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Signal is electrical
action potentials are all or none
every action potentials have same amplitude and duration
information in the signal is represented by frequency and duration
Out put component :
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signal chemical transmitter
total number of action potential determine how much
neurotransmitter should be released
Comparison of local and propagated signals
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Input signals
amplitude
small
duration
brief
summation
graded
signal effect depolarizing
propagation
passive
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Action potential
amplitude
large
duration
brief
summation
all/none
signal effect
depolarize
propagation
active
Cytology of Neurons
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Nucleus
Nuclear envelope
Cytoplasm
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cytosol including cytoskeletal matrix
membranous organelle
Plasmalemma
myelin
Membranous organelles
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Mitochondria and peroxisomes
Rough endoplasmic reticulum/smooth endoplasmic reticulum
Golgi complex
Secretory vesicles, endosomes, lysosomes
Most of these structures are abundant in the cell body and dendrite
and there are no synthetic function differences between cell body and
dendrite.
The axon has few mitochondria and smooth endoplasmic reticulum
with abundant secratory vesicles
Synthesis and trafficking of neural proteins
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Most proteins are synthesized in the cell body
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The neuron express more the total genetic material than any other
organ
Neural cells are engaged in protein synthesis more often than other
cells and hence their chromosomes uncoiled
Ribosomal and m RNA are synthesized in the nucleus and exported
through nuclear pore
Some genetic information is also contained in the mitochondria
Protein synthesis occurs in cytosol where mRNA ribosome and tRNA
form complex
Secretory proteins and vacuolar apparatus and plasmalemma are
synthesized and modified in the endoplasmic reticulum
Secretory proteins are processed further in the golgi complex and then
transported
cytoskeleton
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Microtubules
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Neurofilaments
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Microfilaments
largest diameter fibers
helical cylinders
made of protofilaments
undergo cycles of polymerization and
depolymerization
monomers twist to form dimer
protofilament
protofibril
filament
polymerized actin monomers
smallest-diameter fibers
undergo cycles of polymerization and
depolymerization
Anterograde Axonal Transport
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Fast phase of axonal transport
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70-400 mm/d
20-70mm/d
4-20 mm/d
Convey mainly plasma membrane proteins such as
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acetyl cholinesterase
mitochondria
multivesicular bodies and secretoty vesicles
ATPases
The physiological properties of fast phase axonal transport has been
important for tracing connections in the brain
Slow phase of axonal transport
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1-4 mm/d
0.2-1.2mm/d
Conveys mainly
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Neurofilaments and Microtubulins
Actin and certain glycolytic enzymes
Retrograde Axonal Transport
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Toxins, drugs, heavy metals
Neurotropic viruses
Nerve Growth Factors and neurotrophines
Mitochondria, endosomes
Axonal vs dendrite transport
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Axonal
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Nerofilaments abundant
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Microtubules are of tau type
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Microtubules are uniformly
arranged
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Dendrite
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Microtubules abundant
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MAP-2 type
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Microtubules are bidirectionaly
arranged
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These difference in
arrangement explain the
polarization of organelles
myelin
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Insulate axons and facilitates speed of action potential transmission
Arranged in concentric bimolecular layers
Has a composition similar to plasma membranes
Schwan cells form myelin of peripheral nerves and Oligodendrocyts
that of central nerves
Schwan cells express their myelin gene in response to contact with
axon while Oligodendrocytes depend also on the presence of
Astrocytes
Myelin Proteins
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Myelin Basic Protein
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Myelin Associated glycoprotein
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Protiolipids
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Myelin protein zero
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Peripheral myelin protein 22
important for myelin compaction
strongly immunogenic
used to produce experimental allergic
encephalomyelitis
supper family of immunoglobulin
involved in cell to cell recognition
is an adhesion molecule that initiate
myelination
important for compaction of myelin
mutation in the gene causes
hypomyelination and degeneration
major protein in peripheral myelin
immunoglobulin family
important for myelin compaction
mice that lack the protein have poor motor
coordination
encoded by chromosome 17
DNA duplication results in CMT disease
Ion channels
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Conduct ions at fast rate
Selective for specific ions
Ion channels are proteins that span the cell mme
Flux of ions through the ion channel is passive
Opening and closing of ion channel involves conformational change
Open and close in response to specific stimulus
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Voltage –gated
Ligand –gated
Mechanically –gated
Gap-junction channels
The binding of exogenous ligands /toxins, poisons and drugs/can make channels
open or close
Ion channels are composed of several subunits
Channels are also important targets of diseases
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myasthenia gravis
hyperkalemic periodic paralysis
Synaptic transmission
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The average neuron makes 100000 connections
Two basic forms of transmission
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Chemical
Electrical
Electrical transmissions are
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Short lasting
Only excitatory
Do not induce long lasting postsynaptic changes
Gap junction channels
Bidirectional transmission
Chemical transmissions are
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Variable signaling :inhibitory or excitatory
Produce complex behavior
Longer lasting / delay in transmission
Amplify signals
Modify post synaptic receptors both functionally and anatomically
Ionotropic receptors :conformational change that opens the channels on binding transmitter
Metabotropic receptors: act by altering intracellular metabolic reaction
Chemical transmitters
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Classical
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Peptides
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Acetylcholine
Cathecolamines
Glutamates
GABA
Serotonine
histamine
Substance p
Enkephaline
Endorphine
Prolactin, oxytocine, vasopresin
Soluble gases
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NO
Cellular basis of connectionist approach
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Principle of dynamic polarization : electrical signals within a
nerve flow only in one direction
Principle of connectional specificity : nerve cells do not connect
indiscriminately with one another to from a network
Specificity and modifiability of neuronal connections
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Specific networks
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Parallel processing:
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Brain has at least two types of neuronal map/ motor and sensory maps/
which are interconnected with each other by interneuron. The neurons that
make up these map do not differ greatly in their electrical properties.
Rather, They have different function because of the connections they make.
deployment of several neuron groups or several pathways to convey similar
information
Plasticity :
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functional transformation in neurons as a result of appropriate stimulation
How dose nerve cells differ ?
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Lack of axon
Location of synaptic in puts on the cell
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Difference in
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cell body
dendrite
axon hillock
type of target cell
cell body size and shape,
distribution of axon and dendrite tree
Expressing different combination of ion channels
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providing them with different thresholds ,excitability and firing patterns.
Thus ,neurons with different ion channels encode the same class of
synaptic potential into different firing patterns and thereby convey different
signals
Cont.
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Chemical transmitter
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The type of neurotransmitter they use
The type of Receptors they have
Myelin content
Location in the nervous system –central/peripheral
These differences and others may account for different patterns of
disease