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continued from the Brain Page of
Nervous System
Contents
Neurons and Nerves
neurotransmitter
The Brain & Spinal Cord
Cranial Nerves
Peripheral Nervous System
Autonomic Nervous System
Senses:
Eye diagrams,
Hearing,
Smell,
Taste, Taste
& Tongue Sensation,
Balance
Memory ,
Memory types, Creation of Memory,
Higher Functions
Altered States
[Top]
Neurotransmitters are chemicals that take a nerve signal across the
synaptic gap (Figure 02a) between a sending neuron, and a receiving
one. On the receiving neuron are receptors into which the
neurotransmitters fit like a key in a lock. Once a neuro-transmitter
is bound to its specific receptor, the likelihood of the receiving
cell "firing" to send its own message is affected. The excitatory
neurotransmitter-receptor systems make receiving cells more likely
to fire, whereas the inhibitory systems make the
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firing less likely (see
Figure 29a). It all depends on the type of
neurotransmitter. An individual nerve cell can possess both
kinds of synaptic connections (with a total of about 50000
synapses on the surface) to other nerve cells. Only if the
excitatory charges (positive charge) exceed a threshold does
the target neuron starting a nerve impulse of its own and is
known as transduction. Figure 02b shows the various
components in the synapse. The vesicle contains the neuro-transmitters
in the axon. The receptor is located on the surface of the
dendrite to pick up the neuro-transmitters. The
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transporter is for recycling un-used
neutrotransmitters back into the axon; while the glial cell
provides nutrition and support for the neurons. |
Figure 02c shows the process of signal transmission across the
synapse:
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- Release - As the action potential comes down the
axon, the calcium influx triggers an exocytosis of
vesicles that contain the neurotransmitters, which are
release into the synaptic cleft.
- Bind - The neurotransmitters then drifts across ,
binds to the postsynaptic receptors.
- Transduction - Depending on the integration of the
excitatory and inhibitory inputs, the receiving dendrite
may fire a signal for further transmission.
- Reuptake - The neurotransmitter transporters remove
the un-used neutrotransmitters in the synaptic gap back
to the axon for re-use. This step is to prevent
continuous stimulation of the postsynaptic neuron.
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There are other ways to turn the signal
off. One is simple diffusion into the extracellular
space. Another way is to break down the neuro-transmitters
with enzymes. Then there are the presynaptic
autoreceptors
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(not shown), which terminate the release once a
neutrotransmitter drifts back upstream and hits one of these
receptors.
Since the neurotransmitters are more accessible than the neuron
itself, it can be subjected to a lot of internal and external
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manipulations and abuses. Natural
neuromodulators can aid the release or inhibit the
reabsorption of neurotransmitters; still others delay the
breakdown after reabsorption, leaving them in the tip to be
reused by the next nerve impulse. Mood, pleasure, pain, and
other mental states are determined by particular groups of
neurons in the brain that use special sets of
neurotransmitters and neuromodulators. For example, mood is
strongly influenced by the neurotransmitter serotonin. It is
believed that depression results from a shortage of
serotonin. It is difficult to treat depression directly with
serotonin because the chemical has too many other side
effects. However, depression can be successfully treated
with drugs that act as serotornin neuromodulators (Figure
02d). Prozac, the |
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world's top-selling antidepressant, inhibits
the reabsorption of serotonin, increasing the amount in the
synapse by slowing down its removal.
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When a neuron cell is exposed to a
neurotransmitter for a prolonged period, it tends to lose
its ability to respond to the stimulus with its original
intensity. This is known as habituation, which is the result
of the cell producing fewer receptors for that particular
neurotransmitter. If someone takes a drug that acts as a
neuromodulator (such as cocaine), which causes abnormally
large amounts of neurotransmitter (dopamine in this case,
Figure 02e) to remain in the synapses for long periods of
time, it would generate more pleasure messages. Such action
reduces the number of receptors in the neuron. Next time a
higher dosage is required to maintain the pleasurable
sensation. The result is addiction. Cocaine is a stimulant
discovered in the mid-1800s. Many physicians at first
considered it a miracle drug, prescribing it for all sorts
of physical and mental ailments; it was even added to soft
drinks. Today United States law forbids the importation,
manufacture, and use of cocaine for nonmedical purposes, and
even the medical use is extremely limited. |
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Neurotransmitters can be broadly classified
into two groups - the "classical", small molecule
neurotransmitters and the relatively larger neuropeptide
neurotransmitters. The small molecule types are mainly
amino acids and amines (a nitrogen atom bonds to a
maximum of three hydrocarbon groups). The larger
neurotransmitters are combination of two or more amino acids
joined by peptide bonds. Some fifty different
neurotransmitters have been identified. The form of
receptors for the neurotransmitters varies depending on the
location in the body and produces different physiologic
symptom. Understanding the numerous neurotransmitters, their
receptors, locations and interactions with one another has
been central to the design of medicines for mental illness.
Figure 02f shows the effects of three major
neurotransmitters and the mental states induced by their
interactions.
Table 01 summarizes the properties of some important
neurotransmitters. |
Figure 02f Types of Neuro-transmitter
[view large image]
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| Name |
Type |
Postsynaptic Effect |
Location(s) |
Function(s) |
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Dopamine |
Amine |
Excitatory |
Brain, smooth muscle |
Control arousal levels |
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Serotonin |
Amine |
Excitatory |
Brain, smooth muscle |
Effects on mood, sleep, pain, appetite |
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Noradrenaline |
Amine |
Excitatory |
Brain, smooth muscle |
Induce arousal, heighten mood |
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Acetylcholine (ACh) |
Acetic acid |
Excitatory & Inhibitory |
Parasymathetic nervous system, brainstem |
Role in memory, vasodilation |
| GABA§ |
amino acid |
Inhibitory |
Brain |
Control anxiety level |
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Enkephalin (opiate) |
Neuropeptide |
Inhibitory |
Brain, spinal cord |
Reduce stress, promote calm, natural painkiller |
Table 01 Neurotransmitters
§GABA stands for gamma aminobutyric acid, which is
synthesized from
glutamate
by organisms.
[Top]
Brains exist because the distribution of resources necessary for
survival and the hazards that threaten survival vary in space and
time. There would be little need for a nervous system in an immobile
organism or an organism that lived in regular and predictable
environment. Brains are informed by the senses about the presence of
resources and hazards; they evaluate and store this input and
generate adaptive responses executed by the muscles.
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Some of the most basic features of brains
can be found in bacteria because even the simplest motile
organisms must solve the problem of locating resources and
avoiding toxins. They sense their environment through a
large number of receptors, which are protein molecules
embedded in the cell wall. The action taken in response to
the inputs usually depends on the gradient of the chemicals
(see Figure 03a). Thus memory is required to compare the
inputs from different locations. The strength of the signal
is modulated by immediate past experience. This in turn
regulates the strength of the signal sent by chemical
messengers from |
Figure 03a E. coli's Response to Chemical Gradient
[view large image]
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the receptor to the flagellar motors. Thus
even at the unicellular level, the bacteria have already
possessed the ability to integrate numerous analog inputs
and generate a binary (digital) output of stop or go.
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In multicellur organism, cells specialized for receptor function are
located on the surface. Other cells specialized for the transmission
and analysis of information are located in the protected interior
and are linked to effector cells, usually muscles, which produce
adaptive responses. As do unicellular organisms, neurons integrate
the diverse array of incoming information from the receptors, which
in neurons may result in the firing of an action potential (when the
summation is above a threshold level) rather than swimming toward a
nutrient source as in the unicellular organisms. Once the threshold
for generating an action potential is reached, the signal is always
the same, both in amplitude and shape (a nerve consists of many
neurons, it does not obey the all-or-none law).
Action potentials and voltage-gated sodium channels are present in
jellyfish, which are the simplest organisms to possess nervous
systems. The development of this basic neuronal mechanism set the
stage for the proliferation of animal life that occurred during the
Cambrian period. Among these Cambrian animals were the early
chordates, which possessed very simple brains. Some of these early
fish developed a unique way to insulate their axons by wrapping them
with a fatty material called myelin, which greatly facilitated
axonal transmission and evolution of larger brains. Some of their
descendants, which also were small predators, crawled up on the
muddy shores and eventually took up permanent residence on dry land.
Challenged by the severe temperature changes in the terrestrial
environment, some experimented with becoming warm-blooded, and the
most successful became the ancestors of birds and mammals. Changes
in the brain and parental care were a crucial part of the set of
mechanisms that enabled these animals to maintain a constant body
temperature.
Animals with large brains are rare -- there are tremendous costs
associated with large brains (the active human brain consumes about
20 watts). The brain must compete with other organs in the body for
the limited amount of energy available, which is a powerful
constraint on the evolution of large brains. Large brains also
require a long time to mature, which greatly reduces the rate at
which their possessors can reproduce. Because large-brained infants
are slow to develop and are dependent on their parents for such a
long time, the parents must invest a great deal of effort in raising
their infants. Young reptiles function as
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miniature versions of adults, but baby
mammals and birds are dependent because of their poor
capacity to thermo-regulate, the consequence of their need
to devote most their energy to growth. Most mammals solve
the problem with maternal care (Figure 03b), shelter,
warmth, and milk. In most birds, both parents cooperate to
provide food and shelter to their young. The expanded
forebrain and parental care provide mechanisms for the
extra-genetic transmission of information from one
generation to the next. This transmission results from the
close contact with parents during infancy, which provides
the young with opportunity to observe and learn from their
behavior; the expanded forebrain provides an enhanced
capacity to store these memories. The expanded forebrain and
the observation of parents |
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are probably necessary for the establishment
of successful care giving behavior itself, as the young
mature into adults that will in their turn have to serve
dependent young. During the period of infant dependency,
baby mammals and birds play, |
behavior that may be essential for the development of the forebrain.
The baby's playful interaction with its environment may serve to
provide the initial training of the forebrain networks that
ultimately will enable the animal to localize, identify, and capture
resources in its environment.Please go to the next page Human
Brain The Brain & Spinal Cord
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