Chapter 14
*Lecture Outline
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Chapter 14 Outline
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Organization of the Nervous System
Cytology of Nervous Tissue
Myelination of Axons
Axon Regeneration
Nerves
Synapses
Neural Integration and Neuronal Pools
Development of the Nervous System
Structural Organization
of the Nervous System
Structurally, the nervous system is divided
into two subdivisions:
1. Central nervous system (CNS)—
includes the brain and spinal cord
2. Peripheral nervous system (PNS)—
includes the cranial nerves, spinal
nerves, and ganglia
Structural
Organization
of the
Nervous
System
Figure 14.1
Functional Organization
of the Nervous System
The CNS and PNS perform three general
functions:
1. Collecting information—receptors are PNS structures
that detect changes in the internal and external
environment (sensory input) and pass the information on
to the CNS
2. Processing and evaluating information—CNS
determines what, if any, response is required
3. Responding to information—CNS initiates specific
nerve impulses, called motor output, to effectors
(muscles or glands) to react to changes in the body’s
environment
Functional Organization
of the Nervous System
There are two functional divisions of the
nervous system:
1. Sensory nervous system
2. Motor nervous system
Functional Organization
of the Nervous System
Sensory Nervous System
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Sensory nervous system (afferent)
receives sensory information from
receptors in the PNS and transmits it to
the CNS
Subdivided into two systems:
1. Somatic sensory (voluntary)—touch,
pain, pressure, vibration, and
proprioception
2. Visceral sensory (involuntary)—impulses
from viscera
Motor Nervous System
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Motor nervous system (efferent) sends
impulses from CNS to muscles and
glands
Subdivided into two systems:
1. Somatic motor (voluntary)—impulses
from the CNS that cause contraction of
skeletal muscles
2. Autonomic motor (involuntary)—
impulses from the CNS that regulate
smooth and cardiac muscle and glands
Cytology of Nervous Tissue
There are two distinct types of cells within
the nervous system:
1. Neurons (nerve cells)—electrically
excitable cells that initiate, transmit, and
receive nerve impulses
2. Glial cells—nonexcitable cells that
support and protect the neurons
Neurons
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The basic structural unit of the nervous
system
Conduct nerve impulses from one part of
the body to another part
They have the following features:
1. High metabolic rate
2. Extreme longevity
3. Nonmitotic
Neuron Structure
A neuron has three main structural regions:
1. Cell body
2. Dendrites
3. Axon
Neuron Structure
Figure 14.3
Cell Body
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Contains typical organelles such as:
─ Nucleus
─ Nucleolus
─ Mitochondria !!!!!!
─ Free ribosomes and rough
endoplasmic reticulum (Nissl
bodies)!!!!
Dendrites
• Short processes that branch from the cell
body
• Receive nerve impulses and carry them to
the cell body
Axon
• Neurons have either one axon or no axon
at all. Neurons without an axon are called
anaxonic.
• The region where the axon connects to the
cell body is the axon hillock.
• Axons transmit nerve impulses away from
the cell body and transmit information to
other cells.
Structures Associated
with Axons
1. Axon collaterals—side branches of the
main axon
2. Telodendria—fine terminal extensions at
the end of the axon and its collaterals
3. Synaptic knobs—expanded regions at
the tip of telodendria
Structural Classification of
Neurons
Classified according to the number of
processes emanating directly from the cell
body of the neuron:
1. Unipolar—single, short process that branches
like a T
2. Bipolar—two processes, one dendrite and one
axon
3. Multipolar—many dendrites and a single axon,
most common of all neurons
Structural Classification of Neurons
Figure 14.4
Functional Classification
of Neurons
Functionally, neurons are classified
according to the direction that the nerve
impulse is traveling relative to the CNS:
1. Sensory (afferent)—transmit impulses
from sensory receptors to the CNS
2. Motor (efferent)—transmit impulses from
CNS to muscles or glands
3. Interneurons—facilitate communication
between sensory and motor neurons
Functional Classification
of Neurons
Figure 14.5
Glial Cells=Neuroglias
• Sometimes referred to as neuroglia
• Found in both CNS and PNS
• Smaller than neurons and capable of
mitosis
• Physically protect and nourish neurons
• More numerous than neurons
• Brain tumors are more likely to be derived
from glial cells than neurons
Glial Cells of the CNS
Figure 14.6
Glial Cells of the CNS
There are four types of cells found in the
CNS:
1. Astrocytes
2. Ependymal cells
3. Microglial cells
4. Oligodendrocytes
See Table 14.4 for the function of each of
these glial cells.
Cellular Organization in Neural Tissue
Produce CSF
Regulate
Repair-Myelin
Myelin
BBB
Clean lady
Function of CNS Glial Cells
Astrocytes
Most abundant glial cells in the CNS, whose
functions include:
1. Helping to form the blood-brain barrier
(BBB)
2. Regulating tissue fluid composition
3. Forming a structural network
4. Replacing damaged neurons
5. Assisting neuronal development
Astrocytes
Figure 14.7
Ependymal Cells
• Ciliated cuboidal epithelial cells that line
the ventricles of the brain and the central
canal of the spinal cord
• In conjunction with other glial cells, the
ependymal cells produce cerebral spinal
fluid (CSF) and form the choroid plexus
Ependymal Cells
Figure14.7
Microglial Cells
• Small cells that are motile
• Wander through the CNS and exhibit
phagocytic activity, removing cellular
debris from dead or dying cells
Microglial Cells
Figure 14.7
Oligodendrocytes
• Associated with CNS axons only
• Wrap themselves around the axons like
electrical tape wrapped around a wire
• Produce myelin, which is an insulator of
electrical activity
Oligodendrocytes
Figure 14.7
Function of PNS Glial Cells
Glial Cells of the PNS
There are two types of cells found in the
PNS:
1. Satellite cells
2. Neurolemmocytes=Schwann Cells
See Table 14.4 for the function of each of
these glial cells.
Satellite Cells
• Flattened cells arranged around neuronal
cell bodies in ganglia
Figure 14.7
Neurolemmocytes
• Also called Schwann cells
• Associated with PNS axons only
• Wrap themselves around the axons like
electrical tape wrapped around a wire
• Produce myelin which is an insulator of
electrical activity
• Same structure and function as
oligodendrocytes
Neurolemmocyte Development
2 Neurolemmocyte
1 Neurolemmocyte
starts to wrap around
a portion of an axon.
Axon
Neurolemmocyte
Nucleus
Figure 14.8
Direction of
wrapping
cytoplasm and
plasma membrane
begin to form
consecutive layers
around axon.
Neurolemmocyte Development
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3 The overlapping inner
layers of the
neurolemmocyte
plasma membrane
form the myelin
sheath.
4 Eventually, the
Cytoplasm of the
neurolemmocyte
Myelin sheath
neurolemmocyte
cytoplasm and
nucleus are pushed
to the periphery of
the cell as the myelin
sheath is formed.
Myelin sheath
Neurolemmocyte
nucleus
Figure 14.8
Neurolemmocytes
Figure 14.7
Myelin
Sheaths
in the
CNS and
PNS
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Oligodendrocytes
Neurofibril
node
Axons
Myelin sheath
(a) CNS
Neurolemmocytes
(forming myelin sheath)
Neuron
cell body
Figure 14.9
Neurofibril
node
Axon
(b) PNS
Axon
Re
ge
ne
ra
tion
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Endoneurium
Neurolemmocytes
Trauma severs axon
1
Peripheral nerve injury
results in the severing
of axons (only one
axon is shown here).
Skeletal muscle fibers
Axon degenerates
in distal region
2
The proximal portion of each
severed axon seals off and
swells; the distal portion
degenerates.
Sealed, swollen end
Growth regeneration tube
formed by
neurolemmocytes
3
Neurolemmocytes form a
regeneration tube.
Remyelination accompanies
axon growth
4
Axon regenerates and
remyelination occurs.
Reconnection to effector
Figure 14.11
5 Reinnervation of the
effector (skeletal muscle
fibers) by the axon.
Nerves
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Cablelike bundle of parallel axons
Surrounded by three connective tissue
wrappings
– Endoneurium
– Perineurium
• Fascicles
– Epineurium
Nerve Structure
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Axon
Perineurium
Fascicle
Myelin sheath
Endoneurium
Fascicle
Perineurium
Endoneurium
Epineurium
Axon
Blood vessels
Myelin sheath
Blood vessels
SEM 450x
(b)
Myelin sheath
(a)
Axon
Neurolemmocyte
nucleus
Neurofibril node
LM 550x
(c)
Figure14.12
b: © Dr. Richard Kessel and Dr. Randy Kardon/ Tissues and Organs/ Visuals Unlimited; c: © Rick Ash
Synapses
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Specialized junctions between one axon
and another neuron, muscle cell, or
gland cell
– Presynaptic neuron
– Postsynaptic neuron
– Synaptic cleft
Electrical and Chemical Synapses
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Electrical
synapse
Smooth
muscle cells
Presynaptic
cell
Postsynaptic
cell
Nerve impulse
Axon of presynaptic neuron
Gap junction
Local current
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Positively
charged ions
Plasma membrane
Mitochondria
Calcium
(Ca2+;)ions
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+ +
Connexons
Microtubules
of cytoskeleton
Synaptic vesicles
containing
acetylcholine (ACh)
Voltage-regulated
calcium (Ca2+;)
channel
Synaptic
cleft
Inner surface
of plasma
membrane
(a) Electrical synapse
Acetylcholine
Acetylcholine binds
to receptor protein,
causing ion gates
to open
Sodium
(Na+) ions
Postsynaptic neuron
(b) Chemical synapse
Figure 14.14
Postsynaptic
membrane
Receptor protein
Nervous System Development
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Cut edge of amnion
Neural fold
Neural groove
Primitive node
Primitive streak
Neural groove
Neural crest
Neural folds
Notochord
1
Neural folds and neural groove form from the neural plate.
Neural groove
Neural folds
2
Neural folds elevate and approach one another.
Neural groove
Ectoderm
Neural crest
cells
3
Neural crest cells begin to "pinch off" from the neural folds
and form other structures.
Neural tube
Figure 14.16
Developing
posterior root
ganglion
4
Neural folds fuse to form the neural tube.
Developing
epidermis