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Chapter 12
Nervous Tissue
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The Nervous System Objectives
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Describe the two anatomical divisions of the
nervous system.
List the general functions of both divisions.
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Major Structures of the Nervous System
Copyright 2009 John
Wiley & Sons, Inc.
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The Nervous System
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The nervous system functions to:
 monitor the internal and external environment
 integrate sensory information
 coordinate voluntary and involuntary responses of
other organ systems
Functions are performed by neurons, the functional
unit of the nervous system.
Neuroglia support and protect the neurons.
Two major subdivisions of the nervous system are:
 central nervous system (CNS)
 peripheral nervous system (PNS)
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Central Nervous System
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consists of the brain and the spinal cord.
integrates and coordinates sensory data and
motor commands.
is the seat of higher functions (intelligence,
memory, and emotion)
Peripheral Nervous System
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provides communication between the CNS
and the rest of the body.
includes all the neural tissue outside the
CNS.
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Peripheral Nervous System
afferent division - brings sensory
information to CNS
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Ex. From touch receptor to brain
efferent division - brings motor
commands to muscles and glands
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Ex. From brain to triceps brachii
Subdivisions of the PNS
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Somatic (voluntary) nervous system (SNS)
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Autonomic (involuntary) nervous systems (ANS)
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motor neurons to skeletal muscle tissue – controls skeletal
muscle contractions
motor neurons to smooth & cardiac muscle and glands –
involuntary control
 sympathetic division (speeds up heart rate)
 parasympathetic division (slow down heart rate)
Enteric nervous system (ENS)
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involuntary sensory & motor neurons control GI tract
neurons function independently of ANS & CNS
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Organization of the Nervous System
Copyright 2009 John
Wiley & Sons, Inc.
9
Cellular Organization
Objective: Distinguish between neurons and
neuroglia on the basis of their structure and
function.
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Cellular Organization
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All neural tissue consists of two kinds of cells.
Neurons
 functional unit of the nervous system
 all neural functions involve the communication of
neurons with each other and with other cells
 Cannot divide
Neuroglia
 regulate the environment around the neurons
 provide a support framework for neural tissue
 act as phagocytes
 retain the ability to divide
 much smaller and much more numerous than
neurons
Neurons - Structure
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cell body
several branching
sensitive dendrites
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receive incoming signals
elongated axon
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Dendrites
carry outgoing signals
one or more synaptic
terminals
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Cell body
Nuclei of
neuroglia
communicates with other
cells
Axon
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Neurons - Structure
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Cell body
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large nucleus with large nucleolus
mitochondria
free and fixed ribosomes
nissl bodies - gray clusters of RER and free
ribosomes
usually no centrioles
Neurons
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Membrane of the cell body and dendrites are
sensitive to chemical, mechanical, and electrical
stimulation.
Stimulation results in an action potential from the
axon hillock.
Axon may branch into collaterals each with a
synaptic terminal.
Synapse is the site where communication occurs.
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Structure of a Multipolar Neuron
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Structural & Functional
Classification of Neurons
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Structural
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multipolar neuron
unipolar neuron
bipolar neuron
Functional
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sensory neurons
motor neurons
interneurons
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Multipolar Neuron
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Multiple processes
extending away
from the cell body.
Very common in
the CNS
Unipolar Neuron
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Dendrites and axon are
continuous and the cell
body lies to one side.
Action potential begins at
the base of the dendrites
and the rest of the process
is considered an axon.
Most sensory neurons of
the PNS are unipolar.
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Bipolar Neurons
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Cell body lies
between the one
dendrite and one
axon.
Rare, occur in
special sense organs
such as the eye and
ear.
Sensory Neurons (afferent)
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There are about 10 million neurons in the
afferent division.
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These are all sensory neurons
Receptors are categorized based on the
information that they carry
 somatic sensory receptors
 external receptors
 proprioceptors
 visceral receptors
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Motor Neurons (efferent)
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There are about .5 million motor neurons of the
efferent division.
There are two efferent divisions of the PNS:
 Somatic motor neurons of the somatic
nervous system innervate the skeletal
muscles.
 Visceral motor neurons of the autonomic
nervous system innervate smooth muscle,
cardiac muscle, and glands
Interneurons
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There are about 20 billion interneurons.
Located within the brain and spinal cord.
Interconnect other neurons.
Responsible for the distribution of sensory
information and the coordination of motor
activity.
A stimulus that requires a more complex
response involves a greater number of
interneurons.
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Neuroglia Objectives
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
Describe the locations of neuroglia.
Explain the functions of each type of
neuroglia.
Neuroglia
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found in the CNS and PNS.
The CNS has the greatest variety of glial cells.
There are four types of glial cells in the CNS.
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astrocytes
oligodendrocytes
microglia
ependymal
Schwann cells are the major glial cells in the
PNS.
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Neuroglia of the CNS
Copyright 2009 John
Wiley & Sons, Inc.
25
Ependymal Cells
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Epithelial cells that line the central canal of
the spinal cord and the ventricles of the
brain.
Produce cerebrospinal fluid (CSF) in
certain regions of the brain.
Cilia help to circulate the CSF in brain
ventricles.
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Astrocytes
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Largest and most numerous glial cells.
Secrete chemicals that maintain blood brain
barrier.
Create a structural framework for the CNS.
Perform repairs in damaged neural tissue.
Direct growth and interconnection for developing
neurons in the embryonic brain.
Adjust composition of ECF.
Oligodendrocytes
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Cytoplasmic extensions create a myelin sheath
around axons.
Myelin increases the speed of an action
potential.
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nodes of Ranvier (nodes): gaps between adjacent
oligodendrocytes
internodes: areas covered in myelin.
Lipid rich myelin causes axons to appear glossy
white.
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Microglia
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Smallest and rarest of the neuroglia in the
CNS.
Phagocytic cells.
Schwann Cells
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Most important glial cells in the PNS.
Cover every axon outside the CNS whether
myelinated or not.
The Schwann cell covering of the axon is
called the neurilemma.
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Neuroglia of the PNS
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Demyelination
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The progressive destruction of myelin sheaths.
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inflammation
axon damage
scarring of neural tissue
Results in gradual loss of sensation and motor
control.

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numbness / weakness
paralysis
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Anatomical Organization
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Neurons and their axons form bundles with distinct
anatomical boundaries.
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PNS
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neuron cell bodies are located in ganglia
axons are bundled in nerves
 spinal nerves connected to spinal cord
 cranial nerves connected to brain
Anatomical Organization
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CNS
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group of neuron cell bodies with a common function is a
center (or nucleus if it has a distinct boundary).
brain surface is covered by a thick layer of gray matter called
neural cortex
the most complex integration centers, nuclei, and cortical
areas in the brain are called higher centers.
white matter containing bundles of axons sharing common
origins, destinations, and functions are tracts
tracts in the spinal cord form larger groups called columns.
Pathways link the centers of the brain with the rest of the
body
 sensory (ascending) pathways take info to the brain
 motor (descending) pathways result in motor control at
the skeletal muscles
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?????
1.
2.
3.
What would damage to the afferent division of
the nervous system affect?
Examination of a tissue sample shows unipolar
neurons. Are these more likely to be sensory
neurons or motor neurons?
Which type of glial cell would you expect to be
present in large numbers in brain tissue from a
person suffering from an infection of the central
nervous system?
?????
1.
2.
3.
Afferent division of the nervous system is
composed of nerves that carry sensory
information to the brain and spinal cord.
Damage would interfere with a persons ability to
experience a variety of sensory stimuli.
Sensory neurons of the peripheral nervous
system are usually unipolar. This tissue is most
likely associated with a sensory organ.
Microglial cells are small phagocytic cells found
in increased number in damaged and diseased
areas of the CNS.
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