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Neurons and Brain
Structure of Neurons
Neurons are brain cells. Usually, every neuron contains a
cell body, one axon, and many dendrites. The cell body is
a house for nucleus and other organelles. It stores genetic
information and produces proteins and other molecules to support
the cell's life.
An axon grows out of the cell body and it is an output channel
of a neuron. It transfers information to other neurons or
other cells of our body. Dendrites are the treelike branching
arms of a neuron. They act like antennae for neurons, receiving
information from other neurons or from sensory receptors.
Dendrites are input channels of a neuron..
Neurons look like white threads because axons of many nerve
cells are covered a white, fatty material called myelin. The
myelin sheath works like insulation on an electric wire. It
protects the axon and improves transmission of the signal.
The sheath is the expanded plasma membrane of an accessory
cells. Where the sheath meets another cell, the axon is unprotected
and these spots play an important part in the propagation
of the nerve impulse.
Dendrites have little knobs called spines which are possible
contact places with an axon of other neuron. Each axon branches
many times before it ends and a message from one neuron can
hit many other neurons.  A
single nerve may make up to 20,000 connections with other
cells. There is a space between each axon and dendrite called
a synaptic gap (synapse). That gap is about one-millionth
of an inch. Every human brain has about 100 billions of neurons
that make trillions of synaptic connections among one another.
Functional Classification
of Neurons
Sensory neurons that receive sensory signals from sensory
organs and send them via short axons to the central nervous
system
Motor neurons that conduct motor commands from the
cortex to the spinal cord or from the spinal cord to the muscles
Interneurons that interconnect various neurons within
the brain or the spinal cord
Glial Cells
The glial cells provide the neurons with nourishment, physical
support, and protection. Glial cells also dispose of the waste
materials generated when neurons die, and accelerate neural
conduction by acting as an insulating sheath around certain
axons.
Various types of glial cells, each in its own way, help
the neurons of the central nervous system to function properly.
 Star-shaped
astrocytes provide neurons with mechanical support, supply
them with nutrients, and maintain the equilibrium of the extracellular
environment. They also digest and eliminate all kinds of debris.
The microglial cells constitute the first line of defence
against foreign invaders. They are the macrophages of the
brain.
The oligodendrocytes form the myelin sheath that surrounds
the axons of many neurons. The very special way in which the
oligodendrocytes wrap around the axons accelerates neural
conduction.
The glial cells that perform these same functions in the peripheral
nervous system have different names. The glial cells that
provide mechanical support are called satellite cells, while
those that manufacture myelin are called Schwann cells.
Astrocytes may actually play a far more important role in
neural communication. For instance, astrocytes supply glucose
needed for nerve activity. Through the astrocytes’ end
feet, which are apposed to the walls of the capillaries in
the brain, glucose can enter the astrocytes, which partially
metabolize it, then send it on to the neurons. More intense
synaptic activity, it even seems, promotes a better supply
of glucose by activating this astrocytic metabolisis.
It is also known that astrocytes are connected with each
other via “gap junctions” through which they can
pass various metabolites. It is through these junctions that
astrocytes evacuate to the capillaries the excess extracellular
potassium generated by intense neuronal activity.
But what is being discovered more and more is that this network
of intercommunicating astrocytes forms a veritable syncytium–in
other words, it behaves like a single, unitary entity. For
example, through this network, the regulatory effects of waves
of calcium ions might be propagated to large numbers of synapses
simultaneously. The astrocytic extensions surrounding the
synapses might thus exert a broader control over the concentration
of ions and the volume of water in the synaptic gaps. The
network of astrocytes would thus constitute a non-synaptic
transmission system superimposed on the neuronal system to
play a major role in modulating neuronal activities.
Signal Transmission
Information is transmitted along the dendrites and axons by
electrical or nerve pulses. Synaptic gaps are obstacles for
these pulses. They cannot jump these gaps, like in a spark
plug. Some chemicals called neurotransmitters help electrical
pulses to jump the gaps. Hence, transferring information through
the brain neural network is a combined electrical and chemical
process.
 Propagation
of electrical impulses is based on the difference of concentrations
of charged particles (ions) in the outside and the inside
of a neuron. When a neuron is resting, the fluid inside the
cell membrane contains more negatively charged particles than
the fluid outside the cell membrane. Activation (stimulus)
of a neuron by other neurons initiates opening gate-like pores
in the cell membrane, allowing positively charged particles
to move into the cell and negatively charged particles to
flow out. For a short moment, the charges across the cell
membrane are reversed. The inside of the axon becomes positive
with respect to the outside and a wave of electrical change
propagates along the axon. These electrical impulses can fly
at speeds of 250 miles per hour.
A second stimulus applied to a neuron less than 1 millisecond
after the first will not trigger another impulse. This "dead"
interval is called the refractory period.
The amplitude of the impulse depends on a property of the
cell. Strong stimuli produce no stronger pulses than weak
ones.
As we said before the impulses cannot travel directly from
the neuron to the next cell because of the synaptic gaps.
They must be carried across this space by chemicals. When
an impulse arrives at a synapse, it stimulates vesicles (storage
of neurotransmitters in the axon) to open. Neurotransmitters
flow from the axon of the neuron across the synapse, where
they bind to special receptors in the dendrite membrane of
the next cell. This binding stimulates the postsynaptic cell,
causing cell membrane pores open and creating an electrical
pulse. This impulse is also called an action potential. A
neuron has thousands of connections (synapses) with other
neurons. Some of these release activating neurotransmitters,
others release inhibitory neurotransmitters. The receiving
cell is able to integrate these signals. So, the sum of many
weak signals from other neurons at the same time can produce
an action potential.
Synapse Chemistry
 The
terminal button of the presynaptic neuron’s axon contains
mitochondria as well as microtubules that transport the neurotransmitters
from the cell body (where they are produced) to the tip of
the axon. This terminal button also contains spherical vesicles
filled with neurotransmitters. These neurotransmitters are
secreted into the synaptic gap by a process called exocytosis,
in which the vesicles’ membranes fuse with that of the
presynaptic button.
Neurotrqansmitters help to transfer information between neurons
through synaptic gap. The main neurotransmitter is glutamate.
When glutamate is released from the vesicles it duffuses to
the glutamate receptors. These receptors are located in the
spines of the dendrites. When glutamate binds to the receptors,
pores in the postsynaptic cell open and positive charges move
inside the cell. If the number of these charges is large then
an action potential occurs and electrical signal moves along
the axon.
To reduce neuron activity another neurotransmitter, GABA
(gamma aminobutyric acid), can be also ejected and bind by
GABA receptors. Negative ions flow through these receptors
and the inside of the postsynaptic cell becomes more negative.
It prevents formation of an action potential. This neurotransmitter
is called ingibitor. GABA receptors are located on the cell
body or on part of dendrites close to the cell body and serve
as guards for the action potentials. Without GABA ingibition,
neurons would sent out action potentials continuously under
the influence of glutamate, and would eventually literally
fire themselves to death [1].
There is another important chemicals, which can be relased
from the end of the axon. They called modulators and they
are able to change efficiency of glutamate and GABA. Their
action lasts much longer then diffusion of glutamate and GABA
and they can act during many information transitions through
the synapses.
More about neurotransmitters
To be considered a neurotransmitter, a molecule must meet
several criteria.
1) It must be produced inside a neuron, found in the neuron’s
terminal button, and released into the synaptic gap upon the
arrival of an action potential.
2) It must produce an effect on the postsynaptic neuron.
3) After it has transmitted its signal to this neuron, it
must be deactivated rapidly.
4) It must have the same effect on the postsynaptic neuron
when applied experimentally as it does when secreted by a
presynaptic neuron.
Over 60 different molecules are currently known to meet these
criteria. The most important neurotramsmitters are shown in
the table.
| Acetylcholine is a very widely distributed
excitatory neurotransmitter that triggers muscle contraction
and stimulates the excretion of certain hormones. In the
central nervous system, it is involved in wakefulness,
attentiveness, anger, aggression, sexuality, and thirst,
among other things. |
Alzheimer’s
disease is associated with a lack of acetylcholine in
certain regions of the brain. |
| Dopamine is an inhibitory neurotransmitter
involved in controlling movement and posture. It also
modulates mood and plays a central role in positive reinforcement
and dependency. |
The
loss of dopamine in certain parts of the brain causes
the muscle rigidity typical of Parkinson’s disease. |
| GABA (gamma-aminobutyric acid) is
an inhibitory neurotransmitter that is very widely distributed
in the neurons of the cortex. GABA contributes to motor
control, vision, and many other cortical functions. It
also regulates anxiety. |
Some
drugs that increase the level of GABA in the brain are
used to treat epilepsy and to calm the trembling of people
suffering from Huntington’s disease. |
| Glutamate is a major excitatory neurotransmitter
that is associated with learning and memory. |
It
is also thought to be associated with Alzheimer’s
disease, whose first symptoms include memory malfunctions. |
| Norepinephrine is a neurotransmitter
that is important for attentiveness, emotions, sleeping,
dreaming, and learning. Norepinephrine is also released
as a hormone into the blood, where it causes blood vessels
to contract and heart rate to increase. |
Norepinephrine
plays a role in mood disorders such as manic depression.
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| Serotonin contributes to various
functions, such as regulating body temperature, sleep,
mood, appetite, and pain. |
Depression,
suicide, impulsive behaviour, and agressiveness all appear
to involve certain imbalances in serotonin |
More about Recepors
There are two kinds of receptors:
| Ionotropic receptors are ion channels to which
neurotransmitters bind directly in order to open them.
|
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Metabotropic receptors are separate from the
ion channels whose operation they regulate. They make
the linkage by means of a membrane protein from the G-protein
family.
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Long-Term Potentiation
The long-term potentiation (LTP) was experimentally discovered
in 1973 by Bliss & Toro. They used electrodes inserted
into the brain to study the electrical responce of the postsynaptic
neurons after applying an electrical pulse. They found that
after applying many rapidly repeated pulses the synaptic responce
becomes larger and this effect lasted for hours. It can be
roughly interpreted as modeling of the short-term memory.
There is a molecular mechanism, which can be related to these
experimental data. Glutamate, the main exitatory neurotransmitter,
after releasing from a presynaptic axon terminal can be catched
by glutamate receptors There are two main glutamate receptors:
AMPA and NMDA. Binding of glutamate to AMPA is the major way
to fire an action potential in the postsynaptic cell. NMDA
receptor is blocked at the beginning but after activation
of the postsynaptic cell (through AMPA) the block on the NMDA
is removed, and glutamate can open the receptor channel and
ions can move through the membrane of the postsynaptic cell.
In other words, a strong signal passed through a synapse
can leave a pass for ions open for some time and this synapse
becomes a better conductor for the information pathway. [1]
Mechanism of the long-term memory (days and years) is not
clear by now. Researchers believe that postsynaptic cells
start producing a special protein, which can make formation
of an action potential easier. Less number of glutamate molecules
is needed to fire an action potential. The long term memory
can also involve the formation of new synaptic connections.
Read more about the
memory.
Read more
1. Synaptic Self. How out brains become who we are. Joseph
LeDoux
2. The Brain and Nervous System. Pam Walker and Elaine Wood.
3. Kimball's
Biology Pages. John W. Kimball.
4. The
brain from to to bottom
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