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How Is the Nervous System Organized?
Central Nervous System
Peripheral Nervous System and Endocrine System
Note about the image on this slide: Perceived social isolation or connectedness can
produce changes in the cells of our immune system.
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•  The nervous system is organized into several major branches, each of which
serves a somewhat different function. This chart shows the overall structure of
the nervous system.
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•  At the highest level of organization, the nervous system is divided into the central
nervous system and the peripheral nervous system. The function of the central
nervous system is to process incoming information, and send commands to the
rest of the body. Unlike the peripheral nervous system, the central nervous
system is encased in bone – the skull and vertebrae.
•  The peripheral nervous system consists of nerves, or fibers made of multiple
neurons, that extend from the central nervous system to the rest of the body.
•  The peripheral nervous system can be divided into two smaller branches, which
sometimes run along the same nerve fibers. The somatic nervous system brings
in information from the senses about the state of the body and the environment.
•  The somatic nervous system also sends commands to voluntary muscles, like
the skeletal muscles of the head, neck, arms, and legs.
•  The other branch, the autonomic nervous system, sends commands to visceral
organs such as the heart, lungs, and stomach.
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•  The central nervous system consists of many structures, each of which serves a
somewhat different function.
•  Structures that are more posterior, or toward the “bottom” of the central nervous
system, tend to support simple, reflexive behaviors and the basic functions of life.
•  Structures that are more anterior, or toward the “front” of the central nervous
system, tend to support more complex cognitive and sensory processes.
•  However, all of these structures communicate extensively with each other in
normal brain function.
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•  The medulla is needed to control basic functions such as heartbeat, breathing,
and blood pressure. Damage to the medulla usually results in a quick death.
•  The pons is involved in managing energy and alertness. The pons regulates
sleep-wake cycles, as well as managing some aspects of attention.
•  The midbrain includes regions that control sensory reflexes and the experience
and management of pain. For example, the periacqueductal gray area of the
midbrain is rich in receptors for endorphins, which reduce the intensity of pain
messages traveling from the body to the brain.
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•  The cerebellum connects to the rest of the brain through fibers from the pons.
•  The cerebellum plays important roles in precise motor movement, including
balance and coordination.
•  Researchers only recently realized how complex the cerebellum is, with more
nerve cells than the rest of the brain combined, and receiving input from about 40
million cells throughout the brain. Recent studies suggest that the cerebellum
may be important for all kind of automatic behavior, including perception and
language as well as physical movement.
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•  Several important structures of the brain are above the brainstem and midbrain,
but buried beneath the wrinkly cortex on the surface of the brain.
•  The thalamus collects input from the eyes, ears, taste buds, and other sensory
organs. It integrates this information, and also sends some of it along to higher
cortical structures that will process the information in more complex ways.
•  The basal ganglia include a series of structures that wrap around the thalamus.
The basal ganglia play an important role in voluntary movement, helping people
to choose among multiple movement options rather than moving impulsively or
compulsively (as in ADHD and obsessive-compulsive disorder).
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•  Several structures form a network that links emotion and memory.
•  The hypothalamus is often called the body’s “thermostat.” It maintains
homeostasis by constantly assessing temperature, blood pressure, levels of
oxygen and glucose in the bloodstream, and other factors. It then directs the
autonomic nervous system to make changes needed to keep these factors in the
narrow range that supports human life. The hypothalamus also responds strongly
to stressful or emotional situations.
•  The amygdalae are structures that respond strongly to emotional situations,
especially threats such as spiders, snakes, and angry or intimidating people.
•  The hippocampi, which are connected to the amygdalae, play important roles in
explicit, long-term or “episodic” memory. Studies suggest that hippocampal
memory formation is strengthened in emotional situations, and amygdala
activation may help “tag” new emotional memories as especially important.
•  The cingulate cortex, which wraps around these structures, is also important for
many aspects of emotion and memory.
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•  The cerebral cortex is responsible for complex sensory processing, planning
motor activity, and for the complex, associative cognitive processes that take
place between sensation and action.
•  The frontal lobes are involved in planning, logical reasoning, problem solving,
and judgment. Some areas of the frontal lobe appear to integrate emotional and
rational factors in making good decisions. The primary motor cortex at the back
of the frontal lobes acts as a “map” of the skeletal muscles, directing voluntary
movement.
•  The parietal lobe is important for processing sensations of touch, including body
position, pressure, skin temperature, and pain. The primary somatosensory
cortex at the front of the parietal lobes acts as a sensory map, just as the motor
cortex acts as a motor map.
•  The occipital lobes process minute details of visual information, such as object
borders, shading, color, and movement.
•  The temporal lobes are responsible for complex processing of sound. The
temporal lobes also include regions that are important for understanding and
producing language, as well as recognizing and labeling visual objects.
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•  The two halves of the cortex are structurally separate, but connected by a bridge
called the corpus callosum. The right and left cortex can only communicate with
each other by this pathway.
•  Motor planning and sensation on the right half of the body are processed mainly
by the left hemisphere, whereas these functions on the left half of the body are
processed mainly by the right hemisphere.
•  Many myths about “right brain” versus “left brain” abilities and personalities have
not stood up to the tests of science. However, in most people language is
processed primarily by the left hemisphere, and positive emotions may be
lateralized to the left as well.
•  Evidence suggests that spatial orientation and most negative emotions may be
lateralized to the right hemisphere of the cortex.
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•  One branch of the peripheral nervous system, the somatic nervous system,
serves two functions. First, it carries sensory information from the sense organ
and body to the central nervous system.
•  Second, it carries commands for voluntary movement to the muscles.
•  Sensory and motor neurons fibers within this system often travel along the same
nerve fibers. This image shows the twelve pairs of nerves emerging directly from
the brain (one to each side of the body). As an example, the fifth cranial nerve
communicates sensations of touch and pain from the face, but also sends motor
commands to the jaw muscles. In addition to these twelve cranial nerves, 31
pairs of nerves emerge from the spinal cord.
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•  The other branch, the autonomic nervous system, carries commands from the
brain (especially the hypothalamus) to visceral organs such as the heart, lungs,
blood vessels, digestive system, and genitalia.
•  The ANS is divided into two further branches, which have largely opposing
effects although they are independent systems. The sympathetic division is often
called the “fight-flight” system, as it supports the body in physical exertion. When
sympathetic activation is dominant the body is focused on delivering energy to
the skeletal muscles.
•  The parasympathetic division is sometimes called the “rest and repair” system.
When parasympathetic activation is dominant the body is focused on digesting
food, storing energy for the future, and directing energy toward growth and
reproduction.
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•  The endocrine system also carries instructions from the brain to the body, but by
way of chemicals released into the bloodstream rather than nervous system
pathways. These chemicals are called “hormones.”
•  In some cases, the same chemical can act as a neurotransmitter or a hormone.
•  Particularly involved in sleep-wake cycles, metabolism, growth, and sex.
Triune Brain as an Organizing Concept:
Reptilian brain
(brain stem) oldest
Paleomammalian
brain (limbic
system) later
Neomammalian
brain (neocortex)
newest
•  The triune brain model has become a widely-used way of thinking about the
overall functional organization of the brain.
•  .Developed by Paul Maclean (1990) it is a powerful organizing concept for
understanding the evolution of the human brain and vertebrate brains in general.