II.2 Biology
of the Brain-Mind Identity
A. What
is the brain? It is, among other things, an organ that functions as
the central control or operating system of the body. Networks of nerve
cells , some with fibers a few feet long, run throughout the body and
connect all body tissue with the brain. Also, it produces mind or higher
thought such as consciousness, self-awareness, thinking about thinking,
thinking how we think about thinking, memory, cognition, learning etc., and just like other organs produce
digestion, breathing, circulation of the blood, etc. The mind
emerged via the brain from the material world of which it is a part. For
details of the mind's workings see
"Mind and
Personality as
Mental Traits."
It controls all vital activities for survival including sleep, movement,
hunger, and thirst. It has many modules or nested structures with no clear
demarcation but a blurring of boundaries. The brain's parts seem to work as an
imperfectly integrated unit.
The human brain
has the same general structure as the brains of other mammals, but is over
five times as large as the "average brain" of a mammal with the same body
size.
From a philosophical point of view, it might be said that the most
important function of the brain is to serve as the physical structure
"underlying" the mind. However, from a biological point of view, the most
important function is to generate behaviors that promote the welfare of an
animal. Brains control behavior either by activating muscles, or by
causing secretion of chemicals such as hormones.
B. What is the brain-mind
identity?
Some say it is directly analogous, meaning similar in function but not in origin,
to the structure and output of the computer hardware-software link or connection. But in reality
there is no brain-mind connection because the two are identical. This so far
the most
plausible description is known as the brain-mind identity theory (also identity
theory or physicalism). It holds that mental states are identical to internal
states of the brain. In simple but elucidating terms:
A mental state M is nothing other than brain state
B. The mental state "desire for a cup of coffee" would thus be nothing more than
the "firing of certain neurons in certain brain regions.
One of the first to suggest this theory
was the psychologist E. G. Boring (1886-1968) in a book entitled
The Physical Dimensions of Consciousness
(1933).
He wrote that to him:
. . . a perfect
correlation is identity. Two events that always occur together at the same
time in the same place, without any temporal or spatial differentiation at
all, are not two events but the same event.
C. What is the brain's "hardware"?
It is
by far the most
complex structure known to
humankind and is still largely unexplored. This miniature universe developed naturally through
evolutionary processes that probably started with a few neurons (nerve cells) over 600
million years ago. See
"Timeline:
Evolution of the Human Brain."
Concerning the parts and workings of this "hardware":
-
It has roughly 50
to 100 billion (50
or 100 followed by 9 zeros)
neurons (nerve cells) that communicate with each other both
electrically and chemically (thru more than 50 different
neurotransmitters) at their "connections" which are gaps less than a millionth
of an inch wide and are named synapses.
-
It secretes
a variety of biochemical substances such as the neurotransmitters
acetylcholine or norepinephrine that transmit (or inhibit) nerve impulses
across a synapse to a postsynaptic element, as another nerve, muscle, or
gland. Also, it secretes endorphins
that relieve pain and produce feelings of well-being and euphoria. These
morphine-like natural opiates are part of the brain's reward system, e.g.,
for strenuous exercise, falling in love, etc.
-
It has about 100
billion supporting-tissue cells (neuroglia), and other tissues such as the
vascular (blood carrying ducts). In adults, the brain consumes about 20 percent
of the energy used by the body. This figure is about 60 percent for developing
infants.
-
There
are about 100 trillion (100 followed by 12 zeros) connections between
neurons in adults and young children having about twice as many. This high
number is possible because each neuron links to thousands of others. It is this
linking of neurons which forms complex circuits (functions, faculties, modules) that allow the various brain
parts to connect and function.
-
It has a 100 trillion
instructions per second processing capacity. This figure was extrapolated by a
researcher, Hans Moravec, from known capabilities of the retina to process image
inputs.
D. How can the human brain be understood?
It can be best perceived through its evolutionary development. And the best model is the triune, three brains united, brain theory advanced by Paul MacLean. Considerable evidence
from living fossils, animals that have not changed much over time, and the human
brain indicates that three distinct brains emerged successively. The newer
brains building on and connecting to the older ones. These three brains are
still identifiable in humans . All three now co-inhabit
the skull of humans and other animals. Although connected, the three brains,
the reptilian brain, the mammalian brain, and the primate brain
(neocortex) perform different functions:
1) The reptilian brain
(brainstem) evolved first and connects the spinal
cord with the rest of the brain. It allows instinctual and ritualistic behavior
such as preening and mating. Together with the spinal cord, it controls the
body's vital functions automatically, that is, reflexively without conscious
direction.
It controls functions such as the muscles of the heart,
blood pressure, concentration of salt in the blood, breathing, digestion, body
temperature, balance, certain glands, and homeostasis. Where homeostasis refers
to the regulation of these functions at a normal level as required for a healthy
organism or the internal environment of the body. Reactions such as blushing
indicate that regions higher in the brain that involve thinking are also
involved in reflexive responses.
2) The mammalian brain (limbic system) unfolded first in mammals. Its main parts are the hippocampus,
the amygdala, and the hypothalamus. It strongly influences our behavior thru
emotions
and instinctive discernments
that are triggered when it retrieves pleasant or unpleasant experiences from its
storage facility (memory). That is, emotions such as anger, disgust, fear, happiness,
hate, love, sadness, shame, and uncertainty. Instinctive discernments are often
unconsciously included in our decisions, actions, and value judgments.
3) The primate brain (neocortex)
emerged in nonhuman primates and reached its highest development in the human
brain with its two large cerebral hemispheres. They facilitate what makes us
distinctly human, that is, culture thru voluntary rather than instinctual
behavior, language, abstract thought, imagination, and consciousness.
E.
Where did neurons (nerve cells) come from? They developed naturally through
evolutionary processes that probably started with single-cell organisms where
the entire cell surface absorbed stimuli from the environment. In some of
these cells there are already discrete light-sensitive structures known
as "eyespots." However, the first complete neurons appeared about 600
million years ago in jelly fish.
(Photo
source: Wikimedia Commons)
F. How do neurons connect?
They do when
dendrites of one neuron receive signals from the axons of other neurons thru
chemicals known as neurotransmitters of which there are over 50 different
kinds. These neurotransmitters set off
electrical charges in the dendrites that move the signals electrochemically
to the cell body (soma). The cell body integrates the information and transmits it
electrochemically down the axon to its tip. As the diagram below shows,
these regions of axon to dendrite contact are known as synapses.
G. How do neurons communicate? Although
there is a very narrow gap named a synaptic cleft between, they are able to
communicate through what is called synaptic transmissions.
As shown in the
diagram below, at the tip of of an axon, there are small bubble like
structures called synaptic vesicles that release neurotransmitters that
carry the signal across the cleft between the axon tip of one neuron and the
neurotransmitter receptors at the tip of a dendritic spine of a second
neuron.
To reiterate, the illustration below depicts the major elements in chemical
synaptic transmission. An electrochemical wave called an action potential
travels along the axon of a neuron. When the wave reaches a synapse, it
provokes release of a puff of neurotransmitter molecules, which bind to
chemical receptor molecules located in the membrane of another neuron, on
the opposite side of the synapse.
(Picture
source: Wikimedia Commons)
H.
To how many other neurons is an individual one linked? Each neuron links to thousands of others. Hence, there are roughly
200 trillion (200 followed by 12 zeros) connections in adults and with
young children having about twice as many. It is this linking of neurons which forms complex
circuits that allow the various brain parts to connect and function. The
figure below, by Santiago Ramon y Cajal, illustrates the diversity of
neuronal structures in the auditory cortex.
(Image source: Wikimedia Commons)
I. How does the human brain compare to
others? The human brain is not the largest one as
the comparative pictures below indicate. What apparently counts, however, is
brain size relative to body size. Large animals need larger brains not for
higher functions such as thinking, but because they have more nerves
entering and leaving the brain to their muscles. Fish, amphibians, and
reptiles have similar brain sizes, but birds and mammals have much larger
brains relative to body size. Also, there are particular parts of the brain
that differ. For instance, in humans, the neocortex's two large hemispheres,
responsible for high-level brain functions, take up about 85 percent of the
brain's weight.
Picture source: University of Wisconsin and Michigan State Comparative
Mammalian Brain Collections and the National Museum of Health and Medicine.
In part funded by the National Science Foundation, as well as by the
National Institutes of Health. Website: http://en.citizendium.org/wiki/Image:ComparitiveBrainSize.jpg
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