Download The Brain (Handout)

The Brain (Handout)
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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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Figure 03a E. coli's Response to Chemical
Gradient
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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.
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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.
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This in turn regulates the strength of the
signal sent by chemical messengers from
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 multicellar 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
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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.
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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.
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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.
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Figure 03b Maternal Care
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Young reptiles function as 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.
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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 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,
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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.
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The human brain can be divided into three
parts: the hindbrain, which has been
inherited from the reptiles; the limbic
system, which was first emerged in
mammals; and the forebrain, which has its
full development in human. Different
views of the human brain are shown in
Figure 03c and 04d. Tables 01 lists the
functions of the different parts of the
human brain. The brain is separated into
two hemispheres.
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Apart from a single little organ -- the
pineal gland in the centre base of the
brain -- every brain module is duplicated
in each hemisphere. The left brain is
calculating, communicative and capable of
conceiving and executing complicated
plans -- the reductionistic brain; while the
right one is considered as gentle,
emotional and more at one with the
natural world -- the holistic brain.
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The cerebral cortex is covered in a thin skin of
deeply wrinkled grey tissue called the grey
matter (densely packed neurons for information
processing). Each infold on the surface is known
as a sulcus, and each bulge is know as a gyrus.
While the white tissue inside are axons -tentacles which reach out to other cells (to relay
information).
 The cortex can be broken down into many
functional regions, each containing thousands of
cortical columns (oriented perpendicular to the
cortical surface).
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Columns are typically about half a
millimeter in diameter and contain about
one hundred thousand neurons. They are
the units of cognition (the mental process
of acquiring knowledge by the use of
reasoning, intuition or perception). Table
02 below lists the location and functions of
the major components in the human
brain.
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Figure 03e Brain Waves
The third, or parietal, eye is
a light-sensitive spot thought
to sense changing light
conditions. Opsin proteins
sensitive to blue and green
light has been identified in
the cell
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It is well known that the brain is an
electrochemical organ; a fully functioning brain
can generate as much as 20 watts of electrical
power. Even though this electrical power is very
limited, it does occur in very specific ways that
are characteristic of the human brain. Electrical
activity emanating from the brain can be
displayed in the form of brainwaves. There are
four categories of these brainwaves, ranging
from the most active to the least active. Figure
03e is produced by an EEG
(ElectroEncephaloGraph) chart
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Figure 03e is produced by an EEG
(ElectroEncephaloGraph) chart recorder to show
the different kind of brainwave according to the
different state of the brain. These are all
oscillating electrical voltages in the brain, but
they are very tiny voltages, just a few millionths
of a volt. Electrodes are placed on the outer
surface of the head to detect electrical changes
in the extracellular fluid of the brain in response
to changes in potential among large groups of
neurons. The resulting signals from the
electrodes are amplified and recorded.
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Autonomic Nervous System
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One division of the autonomic nervous system,
called the sympathetic nervous system,
dominates in times of stress. It controls the
"fight or flight" reaction, increasing blood
pressure, heart rate, breathing rate, and blood
flow to the muscles. Another division, called the
parasympathetic nervous system, has the
opposite effect. It conserves energy by slowing
the heartbeat and breathing rate, and by
promoting digestion and elimination (of waste).
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Most glands, smooth muscles, and cardiac
muscles constantly get inputs from both the
sympathetic and parasympathetic systems. The
CNS controls the activity by varying the ratio of
the signals. Depending on which motor neurons
are selected by the CNS, the net effect of the
arriving signals will either stimulate or inhibit the
organ. Figure 07 shows the various organs and
actions, which are related to the two different
divisions.
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Figure 07 ANS Side View
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Brain waves originate from the cerebral cortex,
but also reflect activities in other parts of the
brain that influence the cortex, such as the
reticular formation. Because the intensity of
electrical changes is directly related to the
degree of neuronal activity, brain waves vary
markedly in amplitude and frequency between
sleep and wakefulness. Beta wave rhythms
appear to be involved in higher mental activity,
including perception and consciousness. It
seems to be associated with consciousness, e.g.,
it disappears with general anesthesia.
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Other waves that can be detected are
Alpha, Theta, and Delta. When the
hemispheres or regions of the brain are
producing a wave synchronously, they are
said to be coherent. Alpha waves are
generated in the Thalamus (the brain
within the brain), while Theta waves occur
mainly in the parietal and temporal
regions of the cerebrum.
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The Alpha and Theta waves
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seem to be associated with creative, insightful thought.
When an artist or scientist has the "aha" experience,
there's a good chance he or she is in Alpha or Theta.
These two kinds of brain waves are also associated with
relaxation and, stronger immune systems. Therefore,
many people try to train themselves to enter such states
through various biofeedback7 techniques (with varying
degree of success).
Delta Waves occur during sleep. They originate from the
cerebral cortex when it is not being activated by the
reticular formation. In slow-wave sleep, the entire brain
oscillates in a gentle rhythm quite unlike the fragmented
oscillations of normal consciousness.
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Peripheral
Nervous System
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Figure 05 Cranial
Nerves
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Figure 05 Cranial Nerves
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Figure 06 Spinal
Nerves
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The peripheral nervous system
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The peripheral nervous system is outside the CNS. It
consists of the various nerves that connect particular
parts of the CNS with particular organs. Humans have 12
pairs of cranial nerves and 31 pairs of spinal nerves.
Cranial nerves (Figure 05) are either sensory nerves,
motor nerves, or mixed nerves. All of them, except the
vagus nerve, control the head, the face, the neck, and
the shoulders. The vagus nerve controls the internal
organs. Table 03 lists the functions of the various cranial
nerves. All spinal nerves (Figure 06) are mixed nerves
that take impulses to and from the spinal cord. Table 04
describes the symptom of spinal cord injury (SCI) with
the particular spinal nerve(s).
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Autonomic Nervous System

One division of the autonomic nervous system,
called the sympathetic nervous system,
dominates in times of stress. It controls the
"fight or flight" reaction, increasing blood
pressure, heart rate, breathing rate, and blood
flow to the muscles. Another division, called the
parasympathetic nervous system, has the
opposite effect. It conserves energy by slowing
the heartbeat and breathing rate, and by
promoting digestion and elimination (of waste).
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
Most glands, smooth muscles, and cardiac
muscles constantly get inputs from both the
sympathetic and parasympathetic systems. The
CNS controls the activity by varying the ratio of
the signals. Depending on which motor neurons
are selected by the CNS, the net effect of the
arriving signals will either stimulate or inhibit the
organ. Figure 07 shows the various organs and
actions, which are related to the two different
divisions.
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Figure 08 ANS Front View
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Figure 07 ANS Side View
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Motor fibers that govern involuntary responses, do not
lead directly to the organs they innervate. Instead, they
make their trips in two stages. The first set of fibers
leads from the CNS to ganglia (which are collections of
nerve cell bodies) that lie outside the CNS (the
preganglionic fibers). At the ganglia the fibers form
synaptic junctions with the dendrites of as many as
twenty different cell bodies. The axons of these cell
bodies form a second set of fibers, the postganglionic
fibers. It is these postganglionic fibers that lead to the
organs. The chief ganglia involved in the autonomic
nervous system form two lines running down either side
of the spinal column. They are outside the bony
vertebrae.
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These two lines of ganglia outside the column resemble
a pair of long beaded cords. At the lower end, the two
cords join and finish in a single central stretch. These
lines of ganglia are sometimes called the sympathetic
trunks (used by the sympathetic nervous system). Not
all ganglia are located in the sympathetic trunks. Some
are not; and it is possible for a preganglionic fiber to go
right through, making no synaptic junction there at all,
joining instead with ganglia located in front of the
vertebrae. For the parasympathetic nervous system,
some of the ganglia separating the preganglionic fibers
from the postganglionic fibers are actually located within
the organ the nerve is servicing.
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In that case, the preganglionic fiber runs almost
the full length of the total track, whereas the
postganglionic fiber is at most just a few
millimeters long.
The splanchnic nerves, which originate from
some of the thoracic nerves, have their
preganglionic fibers ending in a mass of ganglia
lying just behind the stomach. It represents the
largest mass of nerve cells that is not within the
CNS and is sometimes called the "abdominal
brain". It is a vital spot to be protected during
boxing.
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