Fundamentals of the Nervous
System and Nervous Tissue
Chapter 11
Introduction
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The nervous system is the master
controlling and communicating system of
the body
It is responsible for all behavior
Along with the endocrine system it is
responsible for regulating and
maintaining body homeostasis
Cells of the nervous system communicate
by means of electrical signals
Nervous System Functions

The nervous system has three overlapping functions
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Gathering of sensory input
Integration or interpretation of sensory input
Causation of a response or motor output
Introduction
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Sensory input
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Integration
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The nervous system has millions of sensory
receptors to monitor both internal and
external change
It processes and interprets the sensory input
and makes decisions about what should be
done at each moment
Motor output

Causes a response by activating effector
organs (muscles and glands)
Organization of the Nervous System
Organization

There is only one nervous system;
however, for convenience the nervous
system is divided into two parts

The central nervous system
• Brain and spinal cord
• Integrative and control centers

The peripheral nervous system
• Spinal and cranial nerves
• Communication lines between the CNS and the
rest of the body
Organization

The peripheral nervous system has two
fundamental subdivisions

Sensory (afferent) division
• Somatic and visceral sensory nerve fibers
• Consists of nerve fibers carrying impulses to the
central nervous system

Motor (efferent) division
• Motor nerve fibers
• Conducts impulses from the CNS to effectors
– (glands and muscles)
Organization

The motor division of the peripheral
nervous system has two main subdivisions

The somatic nervous system
• Voluntary (somatic motor)
• Conducts impulses from the CNS to skeletal muscle

The autonomic nervous system (ANS)
• Involuntary
• Conducts impulses from the CNS to cardiac muscles,
smooth muscles, and glands
Organization

The autonomic nervous system has two
principle subdivisions

Sympathetic division
• Mobilizes body systems during emergency
situations

Parasympathetic division
• Conserves energy
• Promotes nonemergency functions
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The two subdivisions bring about opposite
effects on the same visceral organs
What one subdivision stimulates, the other
inhibits
Peripheral Nervous System
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Visceral organs are
served by motor fibers
of the autonomic
nervous system and by
visceral sensory fibers
The somata (limbs and
body wall) are served
by motor fibers of the
somatic nervous
system and by sensory
somatic sensory fibers
Arrows indicate the
direction of impulses
Histology of the Nervous Tissue
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Nervous tissue is highly cellular
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Less that 20% of the CNS is extracellular space
Cells are densely packed and tightly
intertwined
Nervous tissue is made up of two cell types
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Neurons
• Excitable cells that transmit electrical signals

Support cells
• Smaller cells that surround and wrap the delicate
neurons

These same cells are found within CNS and
PNS
Supporting Cells
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All neurons associate closely with
nonnervous support cells of which there
are 6 types
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Support cells of the CNS
•
•
•
•

Astrocytes
Microglial
Ependymal
Oligodendrocyte
Support cells of the PNS
• Schwann cells
• Satellite cells
Supporting Cells in the CNS
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The supporting cells of the CNS are
collectively called neuroglia or simply
glial cells
Like neurons, glial cells have branching
processes and a central cell body
Neuroglia can be distinguished by their
much smaller size and by their darker
staining nuclei
They outnumber neurons in the CNS by a
ration of 9 to 1
Make up half of the mass of the brain
Astrocytes
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Star shaped
Most abundant type
of glial cell
Radiating projections
cling to neurons and
capillaries, bracing
the neurons to their
blood supply
Astrocytes play a role
in exchanges between
capillaries and
neurons
Astrocytes
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Cells function as
antigen presenting
cells of the immune
response
Control chemical
environment around
neurons, recapturing
potassium ions and
released neurotransmitters
Astrocytes signal each
other via intracellular
calcium pulses
Microglial
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Small ovid cells with
relatively long
“thorny” processes
Their branches touch
nearby neurons to
monitor health of the
neuron
Microglial migrate
toward injured
neurons
Microglial
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Small ovid cells with
relatively long
“thorny” processes
Their branches touch
nearby neurons to
monitor health of the
neuron
Microglial migrate
toward injured
neurons
Microglial
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When invading microorganisms are present
or damaged neurons
have died, the microglial transforms into a
special type of macrophage that protects
the CNS by
phagocytizing the
microorganisms or
neuronal debris
Important because
cells of the immune
system can enter CNS
Ependymal
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Range in shape from
squamous to columnar
and many are cilated
Line the central cavities
of the brain and spinal
cord
Form a fairly
permeable barrier
between cerebrospinal
fluid of those cavities
and the cells of the CNS
Beating cilia circulates
cerebrospinal fluid
Oligodendrocytes
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Fewer branches than
astrocytes
Cells wrap their
cytoplasmic
extensions tightly
around the thicker
neurons in the CNS
Produce insulating
coverings called
myelin sheaths
Oligodendrocyte
Supporting Cells of the PNS
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There are two supporting cells in the PNS
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Satellite cells
Schwann cells
These cells are similar in type and differ
mainly in location
Satellite Cells
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Somewhat flattened
satellite cells
surround cell bodies
within ganglia
Thought to play
some role in
controlling the
chemical
environment of
neurons with which
they are associated
Satellite Cells
Schwann Cells
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Surround and form
myelin sheaths
around the larger
nerve fibers in PNS
Similar to the
oligodendrocytes of
CNS
Schwann cells are
vital to peripheral
nerve fiber
regeneration
Schwann
Cells
Neurons
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Neurons are the structural units of the
nervous system
Neurons are highly specialized cells that
conduct messages in the form of nerve
impulses from one part of the body to
another
Neuron Characteristics
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Extreme longevity
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Amitotic
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Live and function optimally for a lifetime
As neurons assume their role in the nervous
system they lose their ability to divide
Neurons cannot be replaced if destroyed
High metabolic rate
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Require continuous and abundant supplies
of oxygen and glucose
Homeostatic deviations often first appear in
nervous tissue which has specific needs
Neurons
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The plasma membrane of neurons is the
site of electrical signaling, and it plays a
crucial role in most cell to cell interaction
Most neurons have three functional
components in common
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A receptive component
A conducting component
A secretory or output component
Each component is associated with a
particular region of a neuron’s anatomy
Neuron structure
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Typically large, complex cells, they all
have the following structures

Cell body
• Nuclei
• Nissl bodies
• Axon hillock

Cell processes
• Dendrites
• Axon
• Myelin sheath or neurilemma
Neuron Cell Body
Neuron Cell Body
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The cell body consists
of a large, spherical
nucleus with a
prominent nucleolus
surrounded by
cytoplasm
The cell ranges from 5
to 140m in diameter
The cell body is the
biosynthetic center of
the neuron
Neuron Cell Body
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The cell body contains
the usual organelles
with the exception of
centrioles (not needed
in amitotic cells)
The rough endoplasmic
reticulum or Nissl
bodies is the protein
and membrane making
machinery of the cell
The cell body is the
focal point for neuron
growth in development
Neuron Cell Bodies
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Clusters of cell bodies in the CNS are
called nuclei
The relatively rare collection of cell
bodies in the PNS are called ganglia
Neuron
Processes
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Cytoplasmic
extension called
processes extend
from the cell body
of all neurons
The CNS contain
both neuron cell
bodies and their
processes
The PNS consists
chiefly of processes
Motor
neuron
Neuron
Processes
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Motor
neuron
Bundles of neuron
processes are called
tracts in the CNS
Bundles of neuron
processes in the PNS
are called nerves
Two types of neuron
processes
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Dendrites
Axons
Note: Convention of “typical” neuron
Dendrites
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Dendrites are short, tapering diffusely
branching extensions
Motor neurons have hundreds of
dendrites clustering close to the cell body
Dendrites are receptive to input and
provide an enormous surface area for the
reception of signals
In many areas of the brain the finer
dendrites are highly specialized for
information collection
Dendrites
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Dendritic spines
represent areas of
close contact with
other neurons
Dendrites convey
information toward
the cell body
These electrical
signals are not nerve
impulses but are
short distance
signals call graded
potentials
Axons
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Motor
neuron
Each neuron has a
single axon
The axon arises
Axon
from the cone
hillock
shaped axon hillock
It narrows to form
a slender process
that stays uniform
in diameter the rest
of its length
Length varies;
short, absent to 3
feet in length
Axons
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Each axon is called
a nerve fiber
Each neuron has
only one axon and
may possess a
collateral branch
It branches
profusely at its end
more than 10,000
telodendria
Motor
neuron
Axon
hillock
Myelinated
Axon
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Many nerve fibers,
particularly those
that are long or
large in diameter
are covered with a
whitish, fatty
segmented myelin
sheath
Myelin protects
and electrically
insulates fibers
from one another
Myelinated
Axon
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Myelin increase the
speed of
transmission of
nerve impulses
Myelinated axons
transmit nerve
impulse rapidly;
150 meters/second
Unmyelinated
axons transmit
quite slowly; 1
meter/second
Myelinated Processes
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Myelin sheaths are associated only with
axons and their collaterals as these are
impulse conducting fibers and need
insulation
Dendrites which carry only graded
potentials are always unmyelinated
Myelination of
an Axon
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Myelin sheaths in the
PNS are formed by
Schwann cells
The cells first become
indented to receive the
axon and then wrap
themselves around it in a
jelly roll fashion
Initially the wrappings
are loose but the
Schwann cytoplasm is
gradually squeezed out
Myelination of
an Axon
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When the wrapping
process is complete
many concentric layers
wrap the axon
Plasma membranes of
myelinating cells have
less protein which makes
them good electrical
insulators
Myelinated
Axons
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The nucleus and most
of the cytoplasm of
the Schwann cell is
located just beneath
the outer layer of the
plasma membrane
The outer layer is
called the sheath of
Schwann
Gaps, called Nodes of
Ranvier occur
between Schwann cell
Myelinated
Axons
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Nodes of Ranvier
occur at regular
intervals along the
axon
Since the axon is only
exposed at these
nodes nerve impulses
are forced to jump
from one node to the
next which greatly
increases the rate of
conduction
Myelinated
Axons
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Schwann cells that
surround but do not
coil around
peripheral fibers are
considered
unmyelinated
Each axon occupies a
separate tubular
recess
Fibers are typically
thin
CNS Axons
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Oligodendrocytes form
the CNS myelin sheaths
In contast to Schwann
cells, oligodendrocytes
can form the sheaths of
as many as 60 processes
at one time
Nodes at spaced more
widely than in PNS
Axons can be
myelinated or
unmyelinated
Oligodendrocyte
CNS Axons
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Regions of the brain containing dense
collections of myelinated fibers are
referred to as white matter and are
primarily fiber tracts
Gray matter contains mostly nerve cell
bodies and unmyelinated fibers
Classification of Neurons
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Neurons can be classified structurally or
functionally
Both classifications are described
Functional classification is usually used to
describe how the neurons work within us
Structural
Classification
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Multipolar - many
processes extend from cell
body, all dendrites except
one axon
Bipolar - Two processes
extend from cell, one a
fused dendrite, the other
an axon
Unipolar - One process
extends from the cell body
and forms the peripheral
and central process of the
axon
Multipolar Neurons
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Multipolar
neurons have
three or more
processes
Most common
type in humans
Major neuron of
the CNS
Most have many
dendrites and one
axon, some
neurons lack an
axon
Bipolar Neurons
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Bipolar neurons are
rare in the human body
Found only in special
sense organs where they
function as receptor
cells
Examples include those
found in the retina of
the eye and in the
olfactory mucosa
Unipolar Neuron
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Unipolar neurons have a single
process that emerges from the
cell body
The central process is more
proximal to the CNS and the
peripheral is closer to the PNS
Unipolar neurons are chiefly
found in the ganglia of the
peripheral nervous system
Function as sensory neurons
Functional Classification
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The functional classification scheme
groups neurons according to the direction
in which the nerve impulse travels
relative to the CNS
Based on this criterion there are three
neurons
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Sensory neurons
Motor neurons
Association neurons
Sensory
Neurons
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Neurons that transmit
impulses from sensory
receptors in the skin
or internal organs
toward or into the
CNS are called
sensory or affective
neurons
Virtually all primary
sensory neurons of the
body are unipolar
Sensory Neurons: Bipolar
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Bipolar sensory
neurons are
only found in
the special
sensory organs
of the eye or
olfactory
mucosa
Nuerons convey
sensory input to
higher CNS
levels (eye to
occipital lobe)
Sensory Neurons: Bipolar
Motor
Neurons
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Neurons that carry
impulses away from
the CNS to effector
organs (muscles and
glands) is called a
motor or efferent
neuron
Upper motor
neurons are in the
brain
Lower motor
neurons are in PNS
Association Neurons or Interneurons
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These neurons lie
between the motor
and sensory neurons
These neurons are
found in pathways
where integration
occurs
Confined to CNS
Make up 99% of the
neurons of the body
and are the principle
neuron of the CNS
Turn to Neural Integration
Page 395
Neural Integration
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The organization of the nervous system is
hierarchical
The parts of the system must be
integrated into a smoothly functioning
whole
Neuronal pools represent some of the
basic patterns of communication with
other parts of the nervous system
Neuronal Pools

Neuronal
pools are
functional
groups of
neurons that
process and
integrate
incoming
information
from other
sources and
transmit it
forward
One incoming presynaptic fiber synapses with
Several different neurons in the pool. When
Incoming fiber is excited it will excite some
Postsynaptic neurons and facilitate others.
Neuronal Pools
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Neurons most
likely to generate
impulses are those
most closely
associated with the
incoming fiber
because they
receive the bulk of
the synaptic
contacts
These neurons are
in the discharge
zone
Discharge Zone
Neuronal Pools
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Neurons farther
away from the
center are not
excited to threshold
by the incoming
fiber, but are
facilitated and can
easily brought to
threshold by stimuli
from another source
The periphery of the
pool is the
facilitated zone
Facilitated
zone
Neuronal Pools
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Note: The illustrations presented are a
gross oversimplification of an actual
neuron pool
Most neuron pools consist of thousands of
neurons and include inhibitory as well as
excitatory neurons
Types of Circuits
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Individual neurons in a neuron pool send
and receive information and synaptic
contacts may cause either excitation or
inhibition
The patterns of synaptic connections in
neuronal pools are called circuits and
they determine the functional capabilities
of each type of circuit
There are four basic types of circuits

Diverging, converging, reverberating, and
parallel discharge circuits
Diverging Circuits
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In diverging circuits
one incoming fiber
triggers responses in
ever-increasing
numbers of neurons
farther and farther
along in the circuit
Diverging circuits are
often called
amplifying circuits
because they amplify
the response
Diverging Circuits
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These circuits are
common in both sensory
and motor systems
Input from a single
receptor may be relayed
up the spinal cord to
several different brain
regions
Impulses from the brain
can activate a hundred
neurons and thousands
of muscle fibers
Converging Circuits
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The pattern of
converging circuits is
opposite to that of
diverging circuits
Common in both motor
and sensory pathways
In these circuits, the
pool receives inputs
from several
presynaptic neurons, Convergence:
and circuit as a whole multiple sources
has a funneling or
concentrating effect
Convergence:
single source
Converging Circuits

Incoming stimuli may
converge from many
different areas or from
the same source, which
results in strong
stimulation or inhibition
Reverberating (oscillating) Circuits
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In reverberating
circuits the incoming
signal travels through a
chain of neurons, each
Reverberating
of which makes
circuit
collateral synapses with
neurons in the previous
part of the pathway
As a result of this
positive feedback, the
impulses reverberate
through the circuit
again and again
Reverberating (oscillating) Circuits
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Reverberating circuits
give a continuous signal
until one neuron in the
circuit is inhibited and
Reverberating
fails to fire
circuit
These circuits are
involved in the control
of rhythmic activities
such as the sleep-wake
cycle and breathing
The circuits may
oscillate for seconds,
hours or years
Parallel After-Discharge Circuits
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
The incoming fiber
stimulates several
neurons arranged in
Input
parallel arrays that
eventually stimulate
a common output cell
Impulses reach the
output cell at
different times,
creating a burst of
impulses called an
after discharge that
may last 15 ms after
initial input ends
Output
Parallel After-Discharge Circuits
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This circuit has no
positive feedback and
once all the neurons Input
have fired, circuit
activity ends
These circuit may be
involved with
complex problem
solving activities
Output
Patterns of Neural Processing
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Processing of inputs in the various circuits
is both serial and parallel
In serial processing, the input travels along
a single pathway to a specific destination
In parallel processing, the input travels
along several different pathways to be
integrated in different CNS regions
Each patterns has its advantages
Brain derives its power from its ability to
process in parallel
Serial Processing
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In serial processing the whole system
works in a predictable all-or-nothing
manner
One neurons stimulates the next in
sequence, producing a specific, anticipated
response
Reflexes are examples of serial processing
but there are others
Reflexes
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
Reflexes are rapid, automatic responses
to stimuli, in which a particular stimulus
always causes the same motor response
Reflex activity is stereotyped and
dependable
Some your are born with and some you
acquire as a consequence of interacting
with your environment
Serial Processing: A Reflex Arc

Reflexes occurs over neural pathways called
reflex arcs that five essential components
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Receptor
Sensory neuron
CNS integration center
Motor neuron
Effector
Parallel Processing
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In parallel processing inputs are
segregated into many different pathways
Information delivered by each pathway is
dealt with simultaneously by different
parts of neural circuitry
During parallel processing several aspects
of the stimulus are processed


Barking dog
The same stimulus can hold common or
unique meaning to different individuals
Parallel Processing


Parallel processing is not repetitious
because the circuits do different things
with more information
Each parallel path is decoded in relation
to all the others to produce a total picture
of the stimulus
Parallel Processing

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
Even simple reflex arcs do not operate in
complete isolation
As an arc moves through an association
neuron this activates parallel processing
of the same input at higher brain levels
The reflex arc may cause you to pull away
from a negative stimulus while parallel
processing of the stimulus initiates
problem solving about what need to be
done
Parallel Processing

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Parallel processing is extremely for higher
level mental functioning
An integrated look at the whole problem
allows for faster processing
Parallel processing allows you to store a
large amount of information in a small
volume
This allows logic systems to work much
faster
Chapter 11: Fundamentals of
the Nervous System and
Nervous Tissue
End of Chapter