This section is limited to a brief description of the gross structure of the brain,
its major divisions, its composition, the cranial nerves, and how it's protected. The
gross structure of brain consists of the forebrain,
midbrain, and hindbrain.
The brains functions are numerous and vary widely, and it's the main portion of
the central nervous system (CNS).
It helps make sense of our world and all that happens around us. Everything that
happens around us is brought in and received through the five senses of sight,
smell, hearing, touch and taste, often many at the same time. The brain controls
thoughts, memory and speech, arm and leg movements, and the function of many organs
within the body. It also determines how people respond to stressful situations by
regulating heart and breathing rates. The brain is an organized structure and is
divided into many components that serve specific and important functions.
The highest mental and behavioral activities characteristic of humans are mediated
by the cerebral hemispheres, in particular by the cerebral cortex. Important aspects
of these functions are learning and language. In addition, there are association
mechanisms for the integration of motor and sensory functions.
The brain is made up of two types of cells: neurons and glial cells. The neurons
are responsible for sending and receiving nerve impulses or signals. Glial cells
are non-neuronal cells that provide support and nutrition, maintain homeostasis,
form myelin, and facilitate signal transmission in the nervous system. In the
human brain, glial cells outnumber neurons by about 50 to 1.
The weight of the brain changes from birth through adulthood. At birth, the
average brain weighs about 1 pound, and grows to about 2 pounds during
childhood. The average weight of an adult brain is about 3 pounds. The brain has
billions of neurons that receive, analyze, and store information about internal
and external conditions. It's also the source of conscious and unconscious
thoughts, moods, and emotions.
The brain burns about 11 calories an hour, just going about its business of
keeping one alive and functioning in the world. Although it accounts for about
2% of body weight, it expends 20% of the body's energy.
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The brain functions through a complex set neural circuits. Communication between
neurons is both electrical and chemical and always travels from the dendrites of
a neuron, through its soma, and out its axon to the dendrites of another neuron.
Dendrites of one neuron receive signals from the axons of other neurons through
chemicals known as neurotransmitters. The neurotransmitters set off electrical
charges in the dendrites, which then carry the signals electrochemically to the
soma. The soma integrates the information, which is then transmitted
electrochemically down the axon to its tip.
At the tip of the axon, small, bubble like structures called vesicles release
neurotransmitters that carry the signal across the synapse, or gap, between two
neurons. There are many types of neurotransmitters, including norepinephrine,
dopamine, and serotonin. Neurotransmitters can be excitatory (they excite an
electrochemical response in the dendrite receptors) or inhibitory (they block
the response of the dendrite receptors).
Although the brain appears symmetrical, but how it functions is not. Each
hemisphere is specialized and dominates the other in certain functions. Research
has shown that hemispheric dominance is related to whether a person is
predominantly right-handed or left-handed. In most right-handed people, the left
hemisphere processes arithmetic, language, and speech. The right hemisphere
interprets music, complex imagery, and spatial relationships and recognizes and
expresses emotion. In left-handed people, the pattern of brain organization is
more variable.
Hemispheric specialization has traditionally been studied in people who have
sustained damage to the connections between the two hemispheres, as may occur
with stroke in which an interruption of blood flow to an area of the brain that
causes the death of nerve cells in that area. The division of functions between
the two hemispheres has also been studied in people who have had to have the
connection between the two hemispheres surgically cut in order to control severe
epilepsy, a neurological disease characterized by convulsions and loss of
consciousness.
Intelligence
Intelligence provides the means to utilize abstract ideas and implement
reasoning in arguments to justify the things we do. The degree of intelligence
in people is variable to a number of factors, such as genetics, environment,
illness, and even diet.
It's important to distinguish that:
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Knowledge is the accumulation and retention
of information acquired through experience or education; |
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Intelligence is the ability to analyze and
understand information to the persons advantages. |
An illness or even a disease such as MS doesn't tend to have a direct impact on
one's intelligence, but rather the side effects of the disease or medications
may hinder or slow down the process. It's important to know that this in no way
means that those with MS are any less intelligent or have less knowledge, they
may simply have to find different ways to attain and maintain it.
Vision
The visual system of humans is one of the most advanced sensory systems in the
body. More information is conveyed visually than by any other means. In addition
to the structures of the eye itself, several cortical regions (collectively
called primary visual and visual associative cortex) as well as the midbrain are
involved in the visual system. Conscious processing of visual input occurs in
the primary visual cortex, but reflexive responses (immediate and unconscious)
occur at the superior colliculus in the midbrain. Associative cortical regions
(specialized regions that can associate, or integrate, multiple inputs) in the
parietal and frontal lobes along with parts of the temporal lobe are also
involved in the processing of visual information and the establishment of visual
memories.
Language
Language involves specialized cortical regions in a complex interaction that
allows the brain to comprehend and communicate abstract ideas. The motor cortex
initiates impulses that travel through the brain stem to produce audible sounds.
Neighboring regions of motor cortex, called the supplemental motor cortex, are
involved in sequencing and coordinating sounds. Broca's area of the frontal lobe
is responsible for the sequencing of language elements for output. The
comprehension of language is dependent upon Wernicke's area of the temporal
lobe. Other cortical circuits connect these areas.
Memory
Memory is usually considered a diffusely stored associative process (puts
together information from many different sources.) Although research has failed
to identify specific sites in the brain as locations of individual memories,
certain brain areas are critical for memory to function. Immediate recall (the
ability to repeat short series of words or numbers immediately after hearing
them) is thought to be located in the auditory associative cortex. Short-term
memory (the ability to retain a limited amount of information for up to an hour)
is located in the deep temporal lobe. Long-term memory probably involves
exchanges between the medial temporal lobe, various cortical regions, and the
midbrain.
The Autonomic Nervous System
The autonomic nervous system (ANS) regulates the life support systems of the body
reflexively (without conscious direction.) It automatically controls the muscles
of the heart, digestive system, and lungs; certain glands; and homeostasis (the
equilibrium of the internal environment of the body.) The autonomic nervous
system itself is controlled by nerve centers in the spinal cord and brain stem
and is fine-tuned by regions higher in the brain, such as the midbrain and
cortex. Reactions such as blushing indicate that cognitive, or thinking, centers
of the brain are also involved in autonomic responses.
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The brain presents three main divisions: forebrain
(prosencephalon), midbrain (mesencephalon),
and hindbrain (rhombencephalon). The forebrain in turn
has two subdivisions, telencephalon (endbrain) and diencephalon (interbrain). The
hindbrain likewise has two subdivisions, the metencephalon (afterbrain) and the
myelencephalon (marrowbrain).
The forebrain is responsible for a variety of
functions including receiving and processing sensory information, thinking,
perceiving, producing and understanding language, and controlling motor
function. The two major divisions of forebrain are the diencephalon and the
telencephalon. The diencephalon contains structures such as the thalamus and
hypothalamus which are responsible for such functions as motor control, relaying
sensory information, and controlling autonomic functions. The telencephalon
contains the largest part of the brain, the cerebral cortex. Most of the actual
information processing in the brain takes place in the cerebral cortex.
The midbrain and the hindbrain together make
up the brainstem. The midbrain is the portion of the brainstem that connects the
hindbrain and the forebrain. This region of the brain is involved in auditory
and visual responses as well as motor function.
The hindbrain extends to the top of the
spinal cord and is composed of the metencephalon and myelencephalon. The
metencephalon contains structures such as the pons and cerebellum. These regions
assists in maintaining balance and equilibrium, movement coordination, and the
conduction of sensory information. The myelencephalon is composed of the medulla
oblongata which is responsible for controlling such autonomic functions as
breathing, heart rate, and digestion.
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