ASSIGNMENT
“PHYSIOLOGY”
NEURONS AND DIFFERENT TYPES,
CONDUCTIVITY OF IMPULSE
SUBMITTED TO:
MISS KALSOOM (LECTURER)
THE UNIVERSITY OF LAHORE
SUBMITTED BY:
IRFAN AHMAD
BS. BIOCHEMISTRY 1ST SMESTER
BB-307-033
fani3264@gmail.com
www.imbb07.wordpress.com
1
Neurons
Neurons
Neuron is unit of Nervous system.
Neurons are typically composed of a soma, or cell body, a dendritic tree and an axon. The
majority of vertebrate neurons receives input on the cell body and dendritic tree, and
transmits output via the axon. However, there is great heterogeneity throughout the
nervous system and the animal kingdom, in the size, shape, and function of neurons.
Neurons are usually considered amitotic; however, recent research shows that they do
indeed undergo adult neurogenesis.
Neurons communicate via chemical and electrical synapses, in a process known as
synaptic transmission. The fundamental process that triggers synaptic transmission is the
action potential, a propagating electrical signal that is generated by exploiting the
electrically excitable membrane of the neuron. This is also known as a wave of
depolarization.
History
The neuron's place as the primary functional unit of the nervous system was first
recognized in the early 20th century through the work of the Spanish anatomist Santiago
Ramón y Cajal. Cajal proposed that neurons were discrete cells that communicated with
each other via specialized junctions, or spaces, between cells. This became known as the
neuron doctrine, one of the central tenets of modern neuroscience. To observe the
structure of individual neurons, Cajal used a silver staining method developed by his
rival, Camillo Golgi. The Golgi stain is an extremely useful method for neuroanatomical
investigations because, for reasons unknown, it stains a very small percentage of cells in
a tissue, so one is able to see the complete microstructure of individual neurons without
much overlap from other cells in the densely packed brain.
Anatomy and histology
Neurons are highly specialized for the processing and transmission of cellular signals.
Given the diversity of functions performed by neurons in different parts of the nervous
system, there is, as expected, a wide variety in the shape, size, and electrochemical
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Neurons
properties of neurons. For instance, the soma of a neuron can vary from 4 to 100
micrometers in diameter.

The soma is the central part of the neuron. It contains the nucleus of the cell, and
therefore is where most protein synthesis occurs. The nucleus ranges from 3 to 18
micrometers in diameter.

The dendrites of a neuron are cellular extensions with many branches, and
metaphorically this overall shape and structure is referred to as a dendritic tree. This
is where the majority of input to the neuron occurs. Information outflow (i.e. from
dendrites to other neurons) can also occur, but not across chemical synapses; there,
the backflow of a nerve impulse is inhibited by the fact that an axon does not possess
chemoreceptors and dendrites cannot secrete neurotransmitter chemicals. This
unidirectionality of a chemical synapse explains why nerve impulses are conducted
only in one direction.
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
Neurons
The axon is a finer, cable-like projection which can extend tens, hundreds, or even
tens of thousands of times the diameter of the soma in length. The axon carries
nerve signals away from the soma (and also carries some types of information
back to it). Many neurons have only one axon, but this axon may - and usually
will - undergo extensive branching, enabling communication with many target
cells. The part of the axon where it emerges from the soma is called the 'axon
hillock'. Besides being an anatomical structure, the axon hillock is also the part of
the neuron that has the greatest density of voltage-dependent sodium channels.
This makes it the most easily-excited part of the neuron and the spike initiation
zone for the axon: in neurological terms it has the most negative hyperpolarized
action potential threshold. While the axon and axon hillock are generally involved
in information outflow, this region can also receive input from other neurons.

The
axon
terminal
contains
synapses,
specialized
structures
where
neurotransmitter chemicals are released in order to communicate with target
neurons.
Although the canonical view of the neuron attributes dedicated functions to its various
anatomical components, dendrites and axons often act in ways contrary to their so-called
main function.
Axons and dendrites in the central nervous system are typically only about one
micrometer thick, while some in the peripheral nervous system are much thicker. The
soma is usually about 10–25 micrometers in diameter and often is not much larger than
the cell nucleus it contains. The longest axon of a human motoneuron can be over a meter
long, reaching from the base of the spine to the toes. Sensory neurons have axons that run
from the toes to the dorsal columns, over 1.5 meters in adults. Giraffes have single axons
several meters in length running along the entire length of their necks. Much of what is
known about axonal function comes from studying the squid giant axon, an ideal
experimental preparation because of its relatively immense size (0.5–1 millimeters thick,
several centimeters long).
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Neurons
Types of Neurons
1) Sensory Neurons
2) Accessory Neurons
3) Motor Neurons
Sensory neurons
are nerve cells within the nervous system responsible for converting external
stimuli from the organism's environment into internal electrical motor reflex loops and
several forms of involuntary behavior, including pain avoidance. In humans, such reflex
circuits are commonly located in the spinal cord.
In complex organisms, sensory neurons relay their information to the central nervous
system or in less complex organisms, such as the hydra, directly to motor neurons and
sensory neurons also transmit information to the brain, where it can be further processed
and acted upon. For example, olfactory sensory neurons make synapses with neurons of
the olfactory bulb, where the sense of olfaction (smell) is processed.
At the molecular level, sensory receptors located on the cell membrane of sensory
neurons are responsible for the conversion of stimuli into electrical impulses. The type of
receptor employed by a given sensory neuron determines the type of stimuli it will be
sensitive to. For example, neurons containing mechanoreceptors are sensitive to tactile
stimuli, while olfactory receptors make a cell sensitive to odors.
Associate neurons
An interneuron (also called relay neuron, association neuron or bipolar neuron) is a term
used to describe a neuron which has two different common meanings.
PNS
In the peripheral nervous system, an interneuron is a neuron that communicates only to
other neurons. Interneurons are the neurons that provide connections between sensory
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Neurons
and motor neurons, as well as between themselves. Contrast to sensory neurons or motor
neurons, which respectively provide input from and output to the rest of the body.
CNS
According to the PNS definition, the neurons of the central nervous system, including the
brain, are all interneurons. However, in the CNS, the term interneurons is also used for
the general group of small, locally projecting neurons of the central nervous system.
These neurons are typically inhibitory, and use the neurotransmitter GABA or glycine.
However, excitatory interneurons using glutamate also exist as do interneurons releasing
neuromodulators like acetylcholine. A human brain contains about 100 billion
interneurons.
An example of interneurons are inhibitory interneurons in the neocortex which
selectively inhibit sections of the thalamus based on synaptic input both from other parts
of the neocortex and from the thalamus itself. This is theorized to help focus higher
attention on relevant sensory input and help block out behavioraly irrelevant or
unchanging input, such as the sensation of the backs of your thighs on a chair. The
neurophysiological measure short-latency intracortical inhibition (SICI) is believed to be
mediated by these inhibitory interneurons.
Spinal interneurons

1a Inhibitory Neuron: Found in Lamina VII. Responsible for inhibiting antagonist
motor neuron. 1a spindle afferents activate 1a inhibitory neuron.

1b Inhibitory Neuron: Found in Lamina V, VI, VII. 1b afferent or golgi tendon
organ activates it.
Cortical interneurons

Parvalbumin-containing interneurons

CCK-containing interneurons

VIP-containing interneurons
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Neurons
Cerebellar interneurons

Molecular layer interneurons (basket cells, stellate cells)

Golgi cells

Granule cells
Motor neuron
Motor neuron
Section through the spinal cord. Motor
neuron projection through ventral root is
shown in red.
In vertebrates, the term motor neuron (or motoneuron) classically applies to neurons
located in the central nervous system (CNS) that project their axons outside the CNS and
directly or indirectly control muscles. Motor neuron is often synonymous with efferent
neuron.
Upper motor neuron
Upper motor neurons are motor neurons that originate in motor region of the cerebral
cortex or the brain stem and carry motor information down to the final common pathway,
that is, any motor neurons that are not directly responsible for stimulating the target
muscle. The main effector neurons for voluntary movement lie within layer V of the
primary motor cortex and are called Betz cells. The cell bodies of these neurons are some
of the largest in the brain, approaching nearly 100μm in diameter.
These neurons connect the brain to the appropriate level in the spinal cord, from which
point nerve signals continue to the muscles by means of the lower motor neurons. The
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Neurons
neurotransmitter glutamate transmits the nerve impulses from upper to lower motor
neurons where it is detected by glutamatergic receptors.
Lower motor neuron
Lower motor neurons (LMNs) are the motor neurons connecting the brainstem and
spinal cord to muscle fibers, bringing the nerve impulses from the upper motor
neurons out to the muscles. The lower motor neuron's axon goes through a foramen
and terminates on an effector (muscle).
Classification
The axons of lower motor neurons are a type of motor fibers. Lower motor neurons are
classified based on the type of muscle fiber they innervate:

Alpha motor neurons (α-MNs) innervate extrafusal muscle fibers, the most
numerous type of muscle fiber and the one most involved in muscle contraction.

Gamma motor neurons (γ-MNs) innervate intrafusal muscle fibers, which are
involved with muscle spindles and the sense of body position.
Structural classification
Polarity
Most neurons can be anatomically characterized
as:

Unipolar or pseudounipolar: dendrite
and axon emerging from same process.
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
Bipolar: axon and single dendrite on opposite ends of the soma.

Multipolar: more than two dendrites:
o
Neurons
Golgi I: neurons with long-projecting axonal processes; examples are
pyramidal cells, Purkinje cells, and anterior horn cells.
o
Golgi II: neurons whose axonal process projects locally; the best example
are the granule cells.
Other
Furthermore, some unique neuronal types can
be identified according to their location in the
nervous system and distinct shape. Some
examples are:

Basket cells, neurons with dilated and
knotty dendrites in the cerebellum.

Betz cells, large motor neurons.

Medium spiny neurons, most neurons in
the corpus striatum.

Purkinje cells, huge neurons in the cerebellum, a type of Golgi I multipolar
neuron.

pyramidal cells, neurons with triangular soma, a type of Golgi I.

Renshaw cells, neurons with both ends linked to alpha motor neurons.

Granule cells, a type of as Golgi II neuron.

anterior horn cells, motoneurons located in the spinal cord.
Functional classification
Direction

Afferent neurons convey information from tissues and organs into the central
nervous system and are sometimes also called sensory neurons.

Efferent neurons transmit signals from the central nervous system to the effector
cells and are sometimes called motor neurons.
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
Neurons
Interneurons connect neurons within specific regions of the central nervous
system.
Afferent and efferent can also refer generally to neurons which, respectively, bring
information to or send information from the brain region.
Afferent nerve
In the nervous system, afferent neurons--otherwise known as sensory or receptor
neurons--carry nerve impulses from receptors or sense organs toward the central nervous
system. This is the case vice versa as well. This term can also be used to describe relative
connections between structures. Afferent neurons communicate with specialized
interneurons. (The opposite activity of direction or flow is efferent.)
In the nervous system there is a "closed loop" system of sensation, decision, and
reactions. This process is carried out through the activity of afferent neurons,
interneurons, and efferent neurons.
A touch or painful stimulus, for example, creates a sensation in the brain only after
information about the stimulus travels there via afferent nerve pathways. Afferent
neurons are pseudounipolar neurons, that have a single long dendrite and a short axon,
and a smooth and rounded cell body. The dendrite is structurally and functionally similar
to an axon, and is myelinated; it is these axon-like dendrites that make up the afferent
nerves. Just outside the spinal cord, thousands of afferent neuronal cell bodies are
aggregated in a swelling in the dorsal root known as the dorsal root ganglion.
Efferent nerve
In the nervous system, efferent nerves – otherwise known as motor or effector neurons –
carry nerve impulses away from the central nervous system to effectors such as muscles
or glands (and also the ciliated cells of the inner ear). The term can also be used to
describe relative connections between nervous structures. The opposite activity of
direction or flow is afferent.
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Neurons
The motor nerves are efferent nerves involved in muscular control. The cell body of the
efferent neuron is found in the central nervous system where it is connected to a single,
long axon and several short dendrites projecting out of the cell body itself. This axon then
forms a neuromuscular junction with the effectors. The cell body of the motor neuron is
satellite-shaped. The motor neuron is present in the grey matter of the spinal cord and
medulla oblongata, and forms an electrochemical pathway to the effector organ or
muscle.
Action on other neurons

Excitatory neurons excite their target neurons. Excitatory neurons in the central
nervous system, including the brain, are often glutamatergic. Neurons of the
peripheral nervous system, such as spinal motoneurons that synapse onto muscle
cells, often use acetylcholine as their excitatory neurotransmitter. However, this is
just a general tendency that is not always true. It is not the neurotransmitter that
decides excitatory or inhibitory action, but rather it is the postsynaptic receptor
that is responsible for the action of the neurotransmitter.

Inhibitory neurons inhibit their target neurons. Inhibitory neurons are often
interneurons. The output of some brain structures (neostriatum, globus pallidus,
cerebellum) are inhibitory. The primary inhibitory neurotransmitters are GABA
and glycine.

Modulatory neurons evoke more complex effects termed neuromodulation.
These neurons use such neurotransmitters as dopamine, acetylcholine, serotonin
and others.
Discharge patterns
Neurons can be classified according to their electrophysiological characteristics:

Tonic or regular spiking. Some neurons are typically constantly (or tonically)
active. Example: interneurons in neurostriatum.

Phasic or bursting. Neurons that fire in bursts are called phasic.
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
Neurons
Fast spiking. Some neurons are notable for their fast firing rates, for example
some types of cortical inhibitory interneurons, cells in globus pallidus.

Thin-spike. Action potentials of some neurons are more narrow compared to the
others. For example, interneurons in prefrontal cortex are thin-spike neurons.
Neurotransmitter released
Some examples are

cholinergic neurons

GABAergic neurons

glutamatergic neurons

dopaminergic neurons

5-hydroxytryptamine neurons (5-HT; serotonin)
Connectivity
Synapse
Neurons communicate with one another via synapses, where the axon terminal of one cell
impinges upon a dendrite or soma of another (or less commonly to an axon). Neurons
such as Purkinje cells in the cerebellum can have over 1000 dendritic branches, making
connections with tens of thousands of other cells; other neurons, such as the
magnocellular neurons of the supraoptic nucleus, have only one or two dendrites, each of
which receives thousands of synapses. Synapses can be excitatory or inhibitory and will
either increase or decrease activity in the target neuron. Some neurons also communicate
via electrical synapses, which are direct, electrically-conductive junctions between cells.
In a chemical synapse, the process of synaptic transmission is as follows: when an action
potential reaches the axon terminal, it opens voltage-gated calcium channels, allowing
calcium ions to enter the terminal. Calcium causes synaptic vesicles filled with
neurotransmitter molecules to fuse with the membrane, releasing their contents into the
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Neurons
synaptic cleft. The neurotransmitters diffuse across the synaptic cleft and activate
receptors on the postsynaptic neuron.
The human brain has a huge number of synapses. Each of the 1011 (one hundred billion)
neurons has on average 7,000 synaptic connections to other neurons. It has been
estimated that the brain of a three-year-old child has about 1016 synapses (10 quadrillion).
This number declines with age, stabilizing by adulthood. Estimates vary for an adult,
ranging from 1015 to 5 x 1015 synapses (1 to 5 quadrillion).
Mechanisms for propagating action potentials
The cell membrane in the axon and soma contain voltage-gated ion channels which allow
the neuron to generate and propagate an electrical impulse (an action potential).
Substantial early knowledge of neuron electrical activity came from experiments with
squid giant axons. In 1937, John Zachary Young suggested that the giant squid axon can
be used to study neuronal electrical properties.
As they are much larger than human neurons, but similar in nature, it was easier to study
them with the technology of that time. By inserting electrodes into the giant squid axons,
accurate measurements could be made of the membrane potential.
Electrical activity can be produced in neurons by a number of stimuli. Pressure, stretch,
chemical transmitters, and electrical current passing across the nerve membrane as a
result of a difference in voltage can all initiate nerve activity.
The narrow cross-section of axons lessens the metabolic expense of carrying action
potentials, but thicker axons convey impulses more rapidly. To minimize metabolic
expense while maintaining rapid conduction, many neurons have insulating sheaths of
myelin around their axons. The sheaths are formed by glial cells: oligodendrocytes in the
central nervous system and Schwann cells in the peripheral nervous system. The sheath
enables action potentials to travel faster than in unmyelinated axons of the same diameter,
whilst using less energy. The myelin sheath in peripheral nerves normally runs along the
axon in sections about 1 mm long, punctuated by unsheathed nodes of Ranvier which
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Neurons
contain a high density of voltage-gated ion channels. Multiple sclerosis is a neurological
disorder that results from demyelination of axons in the central nervous system.
Some neurons do not generate action potentials, but instead generate a graded electrical
signal, which in turn causes graded neurotransmitter release. Such nonspiking neurons
tend to be sensory neurons or interneurons, because they cannot carry signals long
distances.
Histology and internal structure
Nerve cell bodies stained with basophilic dyes show numerous microscopic clumps of
Nissl substance (named after German psychiatrist and neuropathologist Franz Nissl,
1860–1919), which consists of rough endoplasmic reticulum and associated ribosomes.
The prominence of the Nissl substance can be explained by the fact that nerve cells are
metabolically very active, and hence are involved in large amounts of protein synthesis.
The cell body of a neuron is supported by a complex meshwork of structural proteins
called neurofilaments, which are assembled into larger neurofibrils. Some neurons also
contain pigment granules, such as neuromelanin (a brownish-black pigment, byproduct
of synthesis of catecholamines) and lipofuscin (yellowish-brown pigment that
accumulates with age).
There are different internal structural characteristics between axons and dendrites. Axons
typically almost never contain ribosomes, except some in the initial segment. Dendrites
contain granular endoplasmic reticulum or ribosomes, with diminishing amounts with
distance from the cell body.
The neuron doctrine
The neuron doctrine is the now fundamental idea that neurons are the basic structural
and functional units of the nervous system. The theory was put forward by Santiago
Ramón y Cajal in the late 19th century. It held that neurons are discrete cells (not
connected in a meshwork), acting as metabolically distinct units. Cajal further extended
this to the Law of Dynamic Polarization, which states that neural transmission goes
only in one direction, from dendrites toward axons. As with all doctrines, there are some
exceptions. For example glial cells may also play a role in information processing. Also,
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Neurons
electrical synapses are more common than previously thought, meaning that there are
direct-cytoplasmic connections between neurons. In fact, there are examples of neurons
forming even tighter coupling; the squid giant axon arises from the fusion of multiple
neurons that retain individual cell bodies and the crayfish giant axon consists of a series
of neurons with high conductance septate junctions. The Law of Dynamic Polarization
also has important exceptions; dendrites can serve as synaptic output sites of neurons.
And axons can receive synaptic inputs.
Neurons in the brain
The number of neurons in the brain varies dramatically from species to species. One
estimate puts the human brain at about 100 billion (1011) neurons and 100 trillion (1014)
synapses. By contrast, the nematode worm Caenorhabditis elegans has just 302 neurons
making it an ideal experimental subject as scientists have been able to map all of the
organism's neurons. By contrast, the fruit fly Drosophila melanogaster has around
300,000 neurons (which do spike) and exhibits many complex behaviors. Many
properties of neurons, from the type of neurotransmitters used to ion channel
composition, are maintained across species, allowing scientists to study processes
occurring in more complex organisms in much simpler experimental systems.
How do neurons communicate with each other?
Neurons communicate at structures called synapses in a process called synaptic
transmission. The synapse consists of the two neurons, one of which is sending
information to the other. The sending neuron is known as the pre-synaptic neuron (i.e.
before the synapse) while the receiving neuron is known as the post-synaptic neuron (i.e.
after the synapse). Now, although the flow of information around the brain is achieved by
electrical activity, communication between neurons is a chemical process. When an
action potential reaches a synapse, pores in the cell membrane are opened allowing an
influx of calcium ions (positively charged calcium atoms) into the pre-synaptic terminal.
This causes a small 'packet' of a chemical neurotransmitter to be released into a small gap
between the two cells, known as the synaptic cleft. The neurotransmitter diffuses across
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Neurons
the synaptic cleft and interacts with specialized proteins called receptors that are
embedded in the post-synaptic membrane. These receptors are ion channels that allow
certain types of ions (charged atoms) to pass through a pore within their structure. The
pore is opened following interaction with the neurotransmitter allowing an influx of ions
into the post-synaptic terminal, which is propagated along the dendrite towards the soma.
Synaptic transmission can be excitatory or inhibitory
Neurotransmission
excitatory,
i.e.
can
it
be
increases
either
the
possibility of the post-synaptic neuron
firing an action potential, or inhibitory.
In this case, the inhibitory signal
reduces the likelihood of an action
potential being generated following
excitation. So how does inhibition work?
Well, this is where things get a little more complicated! We have seen that the action
potential is propagated by the leading edge of a depolarization wave activating sodium
channels further down the axon. We have also seen that the activation of these sodium
channels
is
achieved by a small
depolarization
the
of
neuronal
membrane.
But
what
happen
would
if
the
membrane potential
was stabilised? The depolarisation inside the neuronal axon would dissapate and the
action potential would not be able to propagate any further - i.e. it would be inhibited.
How, what, where, I hear you ask? The stabilisation of the membrane potential is
achieved by an influx of negatively charged chloride ions that is unaffected by the
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Neurons
depolarisation wave coming down the axon. Formerly, this is equivalent to an efflux of
positively charged sodium ions. Thus it is like punching a hole in a hose so that water
will leak out through the puncture and not get to the sprinkler!
Chemical
synapses
Are
specialized junctions through whom the
cells of the nervous system signal to
each other and to non-neuronal cells
such as those in muscles or glands.
Chemical synapses allow the neurons of
the central nervous system to form
interconnected neural circuits. They are
thus
crucial
to
the
biological
computations that underlie perception
and thought. They provide the means through which the nervous system connects to and
controls the other systems of the body. A chemical synapse between a motor neuron and
a muscle cell is called a neuromuscular junction; this type of synapse is well-understood.
The human brain contains a huge number of chemical synapses; young children have
about 1016 synapses (10 quadrillion). This number declines with age, stabilizing by
adulthood. Estimates for adults vary from 1015 to 5 × 1015 (1-5 quadrillion) synapses.
The word "synapse" comes from "synaptein", which Sir Charles Scott Sherrington and
his colleagues coined from the Greek "syn-" ("together") and "haptein" ("to clasp").
Chemical synapses are not the only type of biological synapse: electrical and
immunological synapses exist as well. Without a qualifier, however, "synapse"
commonly refers to a chemical synapse.
The signal across a synapse may be regarded as neurocrine, analogous to the types of
signaling of the endocrine system (endocrine, paracrine and autocrine).
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Neurons
References
1):- López-Muñoz, F.; Boya, J., Alamo, C. (16 October 2006). "Neuron theory, the
cornerstone of neuroscience, on the centenary of the Nobel Prize award to Santiago
Ramón
y
Cajal".
Brain
Research
Bulletin
70:
391–405.
doi:doi:10.1016/j.brainresbull.2006.07.010. PMID 17027775. Retrieved on 2007-04-02.
2):- Grant, Gunnar (9 January 2007 (online)). "How the 1906 Nobel Prize in Physiology
or Medicine was shared between Golgi and Cajal". Brain Research Reviews.
doi:doi:10.1016/j.brainresrev.2006.11.004. PMID 17027775. Retrieved on 2007-04-02.
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