Transcript Axons

PowerPoint® Lecture Slides
prepared by
Janice Meeking,
Mount Royal College
CHAPTER
11
Fundamentals
of the Nervous
System and
Nervous
Tissue: Part A
Copyright © 2010 Pearson Education, Inc.
*Functions of the Nervous System
1. Sensory input
•
Information gathered by sensory receptors
about internal and external changes
2. Integration
•
Interpretation of sensory input
3. Motor output
•
Activation of effector organs (muscles and
glands) produces a response
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Sensory input
Integration
Motor output
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Figure 11.1
*Divisions of the Nervous System
• Central nervous system (CNS)
• Brain and spinal cord
• Integration and command center
• Peripheral nervous system (PNS)
• Paired spinal and cranial nerves carry
messages to and from the CNS
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Peripheral Nervous System (PNS)
•
Two functional divisions
1. Sensory (afferent) division
•
Somatic afferent fibers—convey impulses
from skin, skeletal muscles, and joints
•
Visceral afferent fibers—convey impulses
from visceral organs
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*
2. Motor (efferent) division
•
Transmits impulses from the CNS to
effector organs
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*Motor Division of PNS
1. Somatic (voluntary) nervous system
•
Conscious control of skeletal muscles
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Peripheral nervous system (PNS)
Central nervous system (CNS)
Cranial nerves and spinal nerves
Communication lines between the
CNS and the rest of the body
Brain and spinal cord
Integrative and control centers
Sensory (afferent) division
Somatic and visceral sensory
nerve fibers
Conducts impulses from
receptors to the CNS
Somatic sensory
fiber
Motor (efferent) division
Motor nerve fibers
Conducts impulses from the CNS
to effectors (muscles and glands)
Somatic nervous
system
Somatic motor
(voluntary)
Conducts impulses
from the CNS to
skeletal muscles
Skin
Visceral sensory fiber
Stomach
Skeletal
muscle
Motor fiber of somatic nervous system
Sympathetic division
Mobilizes body
systems during activity
Sympathetic motor fiber of ANS
Structure
Function
Sensory (afferent)
division of PNS
Motor (efferent)
division of PNS
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Parasympathetic motor fiber of ANS
Autonomic nervous
system (ANS)
Visceral motor
(involuntary)
Conducts impulses
from the CNS to
cardiac muscles,
smooth muscles,
and glands
Parasympathetic
division
Conserves energy
Promotes housekeeping functions
during rest
Heart
Bladder
Figure 11.2
*Motor Division of PNS
2. Autonomic (involuntary) nervous system
(ANS)
•
Visceral motor nerve fibers
•
Regulates smooth muscle, cardiac muscle,
and glands
•
Two functional subdivisions
•
Sympathetic
•
Parasympathetic
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*Histology of Nervous Tissue
• Two principal cell types
1. Neurons—excitable cells that transmit
electrical signals
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*Histology of Nervous Tissue
2. Neuroglia (glial cells)—supporting cells:
•
Astrocytes (CNS) – most abundant, control
chemical environment (blood/brain barrier)
•
Microglia (CNS)
•
Ependymal cells (CNS)
•
Oligodendrocytes (CNS)
•
Satellite cells (PNS)
•
Schwann cells (PNS) just this one!
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*Satellite Cells and Schwann Cells
• Satellite cells
• Surround neuron cell bodies in the PNS
• Schwann cells (neurolemmocytes) just this
one!
• Surround peripheral nerve fibers and form
myelin sheaths
• Vital to regeneration of damaged peripheral
nerve fibers
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Satellite
cells
Cell body of neuron
Schwann cells
(forming myelin sheath)
Nerve fiber
(e) Satellite cells and Schwann cells (which
form myelin) surround neurons in the PNS.
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Figure 11.3e
*Neurons (Nerve Cells)
• Special characteristics:
• Long-lived ( 100 years or more)
• Amitotic—with few exceptions
• High metabolic rate—depends on continuous
supply of oxygen and glucose
• Plasma membrane functions in:
• Electrical signaling
• Cell-to-cell interactions during development
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Cell Body (Perikaryon or Soma)
• Biosynthetic center of a neuron
• Spherical nucleus with nucleolus
• Well-developed Golgi apparatus
• Rough ER called Nissl bodies (chromatophilic
substance)
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Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
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Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
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Processes
• Dendrites and axons
• Bundles of processes are called
• Tracts in the CNS
• Nerves in the PNS
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*Dendrites
• Short, tapering, and diffusely branched
• Receptive (input) region of a neuron
• Convey electrical signals toward the cell body
as graded potentials
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The Axon
• One axon per cell
• Long axons (nerve fibers)
• Occasional branches (axon collaterals)
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Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
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Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
The Axon
• Numerous terminal branches
• Knoblike axon terminals called synaptic knobs
or boutons
• Release neurotransmitters to excite or inhibit
other cells*
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*Axons: Function
• Conducting region of a neuron
• Generates and transmits nerve impulses
(action potentials) away from the cell body
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Dendrites
(receptive regions)
Cell body
(biosynthetic center
and receptive region)
Nucleolus
Axon
(impulse generating
and conducting region)
Nucleus
Nissl bodies
Axon hillock
(b)
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Impulse
direction
Node of Ranvier
Schwann cell
Neurilemma (one interTerminal
node)
branches
Axon
terminals
(secretory
region)
Figure 11.4b
*Myelin Sheath
• Segmented protein-lipoid sheath around most
long or large-diameter axons
• It functions to:
• Protect and electrically insulate the axon
• Increase speed of nerve impulse transmission
--conduction in myelinated axons is about
30 times faster
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Myelin Sheaths in the PNS
• Schwann cells wraps many times around the
axon
• Myelin sheath—concentric layers of Schwann
cell membrane
• Neurilemma—peripheral bulge of Schwann
cell cytoplasm
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Schwann cell
plasma membrane
Schwann cell
cytoplasm
Axon
1
A Schwann cell
envelopes an axon.
Schwann cell
nucleus
2
The Schwann cell then
rotates around the axon,
wrapping its plasma
membrane loosely around
it in successive layers.
Neurilemma
Myelin sheath
(a) Myelination of a nerve
fiber (axon)
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3
The Schwann cell
cytoplasm is forced from
between the membranes.
The tight membrane
wrappings surrounding
the axon form the myelin
sheath.
Figure 11.5a
*Unmyelinated Axons
• Thin nerve fibers are unmyelinated
• One Schwann cell may incompletely enclose
15 or more unmyelinated axons
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Myelin sheath
Process of
oligodendrocyte
Nerve
fibers
(d) Oligodendrocytes have processes that form
myelin sheaths around CNS nerve fibers.
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Figure 11.3d
*Multiple Sclerosis (MS)
• An autoimmune disease that mainly affects young
adults
• Symptoms: visual disturbances, weakness, loss of
muscular control, speech disturbances, and urinary
incontinence
• Myelin sheaths in the CNS become nonfunctional
scleroses
• Shunting and short-circuiting of nerve impulses
occurs
• Impulse conduction slows and eventually ceases
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*Multiple Sclerosis: Treatment
• Some immune system–modifying drugs,
including interferons and Copazone:
• Hold symptoms at bay
• Reduce complications
• Reduce disability
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*White Matter and Gray Matter
• White matter
• Dense collections of myelinated fibers
• Gray matter
• Mostly neuron cell bodies and unmyelinated
fibers
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*Functional Classification of Neurons
• Three types:
1. Sensory (afferent)
• Transmit impulses from sensory receptors
toward the CNS
2. Motor (efferent)
• Carry impulses from the CNS to effectors
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*Functional Classification of Neurons
3. Interneurons (association neurons)
•
Shuttle signals through CNS pathways;
most are entirely within the CNS
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Table 11.1 (3 of 3)
Reflex Arc*
•
Components of a reflex arc (neural path)
1. Receptor—site of stimulus action
2. Sensory neuron—transmits afferent impulses to the
CNS
3. Integration center—either monosynaptic or
polysynaptic region within the CNS
4. Motor neuron—conducts efferent impulses from the
integration center to an effector organ
5. Effector—muscle fiber or gland cell that responds to
the efferent impulses by contracting or secreting
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Stimulus
Skin
1 Receptor
Interneuron
2 Sensory neuron
3 Integration center
4 Motor neuron
5 Effector
Spinal cord
(in cross section)
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Figure 13.14
The patellar (knee-jerk) reflex—a specific example of a stretch reflex
2
Quadriceps
(extensors)
1
3a
3b
3b
Patella
Muscle
spindle
Spinal cord
(L2–L4)
Hamstrings
(flexors)
Patellar
ligament
1 Tapping the patellar ligament excites
muscle spindles in the quadriceps.
2 Afferent impulses (blue) travel to the
spinal cord, where synapses occur with
motor neurons and interneurons.
3a The motor neurons (red) send
+
–
Excitatory synapse
Inhibitory synapse
activating impulses to the quadriceps
causing it to contract, extending the
knee.
3b The interneurons (green) make
inhibitory synapses with ventral horn
neurons (purple) that prevent the
antagonist muscles (hamstrings) from
resisting the contraction of the
quadriceps.
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Figure 13.17 (2 of 2)
*Role of Membrane Ion Channels
•
Proteins serve as membrane ion channels
• Two main types of ion channels
1. Leakage (nongated) channels—always open
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*Role of Membrane Ion Channels
2. Gated channels (three types):
•
Chemically gated (ligand-gated)
channels—open with binding of a
specific neurotransmitter
•
Voltage-gated channels—open and
close in response to changes in
membrane potential
•
Mechanically gated channels—open and
close in response to physical
deformation of receptors
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Receptor
Neurotransmitter chemical
attached to receptor
Na+
Na+
Na+
Chemical
binds
K+
Closed
Membrane
voltage
changes
K+
Open
(a) Chemically (ligand) gated ion channels open when the
appropriate neurotransmitter binds to the receptor,
allowing (in this case) simultaneous movement of
Na+ and K+.
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Na+
Closed
Open
(b) Voltage-gated ion channels open and close in response
to changes in membrane voltage.
Figure 11.6
*The Synapse
• A junction that mediates information transfer
from one neuron:
• To another neuron, or
• To an effector cell
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*The Synapse
• Presynaptic neuron—conducts impulses
toward the synapse
• Postsynaptic neuron—transmits impulses
away from the synapse
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Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
(b)
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Cell body (soma) of
postsynaptic neuron
Figure 11.16
*Electrical Synapses
• Less common than chemical synapses
• Neurons are electrically coupled (joined by gap
junctions)
• Communication is very rapid, and may be
unidirectional or bidirectional
• Are important in:
• Embryonic nervous tissue
• Some brain regions
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*Chemical Synapses
• Specialized for the release and reception of
neurotransmitters
• Typically composed of two parts
• Axon terminal of the presynaptic neuron, which
contains synaptic vesicles
• Receptor region on the postsynaptic neuron
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*Termination of Neurotransmitter Effects
• Within a few milliseconds, the
neurotransmitter effect is terminated
• Degradation by enzymes
• Reuptake by astrocytes or axon terminal
• Diffusion away from the synaptic cleft
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*Neurotransmitters
• Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
• 50 or more neurotransmitters have been
identified
• Classified by chemical structure and by
function
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*Chemical Classes of Neurotransmitters
• Acetylcholine (Ach)*
• Released at neuromuscular junctions and
some ANS neurons
• Synthesized by enzyme choline
acetyltransferase
• Degraded by the enzyme acetylcholinesterase
(AChE)*
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Chemical Classes of Neurotransmitters
• Biogenic amines include:
• Catecholamines
• Dopamine, norepinephrine (NE), and
epinephrine*
• Indolamines
• Serotonin and histamine
• Broadly distributed in the brain
• Play roles in emotional behaviors and the
biological clock
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Chemical Classes of Neurotransmitters
• Amino acids include:
• GABA—Gamma ()-aminobutyric acid
• Glycine
• Aspartate
• Glutamate
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Chemical Classes of Neurotransmitters
• Peptides (neuropeptides) include:
• Substance P
• Mediator of pain signals
• Endorphins
• Act as natural opiates; reduce pain
perception
• Gut-brain peptides
• Somatostatin and cholecystokinin
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Chemical Classes of Neurotransmitters
• Purines such as ATP:
• Act in both the CNS and PNS
• Produce fast or slow responses
• Induce Ca2+ influx in astrocytes
• Provoke pain sensation
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Chemical Classes of Neurotransmitters
• Gases and lipids
• Nitric oxide (NO)
• Synthesized on demand
• Activates the intracellular receptor guanylyl
cyclase to cyclic GMP
• Involved in learning and memory
• Carbon monoxide (CO) is a regulator of cGMP
in the brain
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Chemical Classes of Neurotransmitters
• Gases and lipids
• Endocannabinoids
• Lipid soluble; synthesized on demand from
membrane lipids
• Bind with G protein–coupled receptors in the
brain
• Involved in learning and memory
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Functional Classification of
Neurotransmitters
• Neurotransmitter effects may be excitatory
(depolarizing) and/or inhibitory (hyperpolarizing)
• Determined by the receptor type of the postsynaptic
neuron
• GABA and glycine are usually inhibitory
• Glutamate is usually excitatory
• Acetylcholine
• Excitatory at neuromuscular junctions in skeletal
muscle
• Inhibitory in cardiac muscle
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Peripheral Nervous System (PNS)
• All neural structures outside the brain
• Sensory receptors
• Peripheral nerves and associated ganglia
• Motor endings
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Central nervous system (CNS)
Peripheral nervous system (PNS)
Sensory (afferent)
division
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Motor (efferent) division
Somatic nervous
system
Autonomic nervous
system (ANS)
Sympathetic
division
Parasympathetic
division
Figure 13.1
Sensory Receptors
• Specialized to respond to changes in their
environment (stimuli)
• Activation results in graded potentials that
trigger nerve impulses
• Sensation (awareness of stimulus) and
perception (interpretation of the meaning of
the stimulus) occur in the brain
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*Classification of Receptors
• Based on:
• Stimulus type
• Location
• Structural complexity
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*Classification of Nerves
• Most nerves are mixtures of afferent and efferent
fibers and somatic and autonomic (visceral) fibers
• Pure sensory (afferent) or motor (efferent) nerves are
rare
• Types of fibers in mixed nerves:
• Somatic afferent and somatic efferent
• Visceral afferent and visceral efferent
• Peripheral nerves classified as cranial or spinal
nerves
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Ganglia
• Contain neuron cell bodies associated with
nerves
• Dorsal root ganglia (sensory, somatic)
(Chapter 12)
• Autonomic ganglia (motor, visceral)
(Chapter 14)
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Cranial Nerves
• Twelve pairs of nerves associated with the
brain
• Most are mixed in function; two pairs are
purely sensory
• Each nerve is identified by a number
(I through XII) and a name
“On occasion, our trusty truck acts funny—very
good vehicle anyhow”
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• The cranial nerves are:
• I - Olfactory nerve
Old
• II - Optic nerve
Opie
• III - Occulomotor nerve
Occassionally
• IV - Trochlear nerve
Tries
• V - Trigeminal nerve/dentist nerve
Trigonometry
• VI - Abducens nerve
And
• VII - Facial nerve
Feels
• VIII - Vestibulocochlear nerve/Auditory nerve
Very
• IX - Glossopharyngeal nerve
Gloomy
• X - Vagus nerve
Vague
• XI - Accessory nerve/Spinal accessory nerve
And
• XII - Hypoglossal nerve
hypoactive
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• Odor Of Orangutan Terrified Tarzan After
Forty Voracious Gorillas Viciously Attacked
Him
• Old Opie Occasionally Tries Trigonometry
And Feels Very Gloomy, Vague And
Hypoactive
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Frontal lobe
Temporal lobe
Infundibulum
Facial
nerve (VII)
Vestibulocochlear
nerve (VIII)
Glossopharyngeal
nerve (IX)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Filaments of
olfactory
nerve (I)
Olfactory bulb
Olfactory tract
Optic nerve
(II)
Optic chiasma
Optic tract
Oculomotor
nerve (III)
Trochlear
nerve (IV)
Trigeminal
nerve (V)
Abducens
nerve (VI)
Cerebellum
Medulla
oblongata
(a)
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Figure 13.5 (a)
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A person attempting to
show his teeth and raise
his eyebrows with Bell's
palsy on his right side
Bell's palsy is the most
common acute
mononeuropathy
Cranial Nerve VII
Caused by a herpes
virus
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Cranial nerves
I – VI
I
II
III
IV
V
Olfactory
Optic
Oculomotor
Trochlear
Trigeminal
VI Abducens
Cranial nerves
VII – XII
VII Facial
VIII Vestibulocochlear
IX
X
XI
XII
(b)
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Glossopharyngeal
Vagus
Accessory
Hypoglossal
Sensory
function
Motor
function
PS*
fibers
Yes (smell)
Yes (vision)
No
No
Yes (general
sensation)
No
No
Yes
Yes
Yes
No
No
Yes
No
No
No
Yes
No
Sensory
function
Motor
function
PS*
fibers
Yes (taste)
Yes (hearing
and balance)
Yes
Some
Yes
No
Yes (taste)
Yes (taste)
No
No
Yes
Yes
Yes
Yes
Yes
Yes
No
No
*PS = parasympathetic
Figure 13.5 (b)
*Spinal Nerves
• 31 pairs of mixed nerves named according to
their point of issue from the spinal cord
• 8 cervical (C1–C8)
• 12 thoracic (T1–T12)
• 5 Lumbar (L1–L5)
• 5 Sacral (S1–S5)
• 1 Coccygeal (C0)
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Cervical plexus
Brachial plexus
Cervical
enlargement
Intercostal
nerves
Cervical
nerves
C1 – C8
Thoracic
nerves
T1 – T12
Lumbar
enlargement
Lumbar plexus
Sacral plexus
Cauda equina
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Lumbar
nerves
L1 – L5
Sacral nerves
S1 – S5
Coccygeal nerve Co1
Figure 13.6
*Spinal Nerves: Roots
• Each spinal nerve connects to the spinal cord
via two roots
• Ventral roots
• Contain motor (efferent) fibers from the ventral
horn motor neurons
• Fibers innervate skeletal muscles)
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*Spinal Nerves: Roots
• Dorsal roots
• Contain sensory (afferent) fibers from sensory
neurons in the dorsal root ganglia
• Conduct impulses from peripheral receptors
• Dorsal and ventral roots unite to form spinal
nerves, which then emerge from the vertebral
column via the intervertebral foramina
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Gray matter
White matter
Ventral root
Dorsal root
Dorsal root
ganglion
Dorsal ramus
of spinal nerve
Ventral ramus
of spinal nerve
Spinal nerve
Dorsal and
ventral rootlets
of spinal nerve
Rami communicantes
Sympathetic trunk
ganglion
Anterior view showing spinal cord, associated nerves, and vertebrae.
The dorsal and ventral roots arise medially as rootlets and join
laterally to form the spinal nerve.
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Figure 13.7 (a)
*
Sciatica -- A common condition arising from
compression of, or damage to, a nerve or
nerve root.
Restless legs syndrome (RLS) is a disorder of
the part of the nervous system that affects the
legs and causes an urge to move them.
Because it usually interferes with sleep, it also
is considered a sleep disorder
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*
• Guillain-Barré syndrome -- causes muscle
weakness, loss of reflexes, and numbness or
tingling in your arms, legs, face, and other
parts of your body. In GBS, the immune
system attacks the myelin sheath of certain
nerves. This causes nerve damage. May be
triggered by a viral or bacterial infection
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*Innervation of Skin
• Dermatome: the area of skin innervated by
the cutaneous branches of a single spinal
nerve
• All spinal nerves except C1 participate in
dermatomes
• Most dermatomes overlap, so destruction of a
single spinal nerve will not cause complete
numbness
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C2
C3
C4
C5
C6
C7
C8
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
C2
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T2
C5
C6
C6
C7
L1
C8
L2
T12
S2
S3
T2
C5
C6
L1
C8
L2
S1
L4
S2
S3
S4
S5
C6
C7
C6
C7
C8
C8
L2
S2
S1
L1
L3
L5
L4
T11
T12
L1
L3
L5
C7
C6
S1 S2
L3
C5
L2
L5
L4
L3
L5
L5
L4
S1
Anterior
view
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S1
(b) Posterior
view
L4
L5
L4
L5
S1
Figure 13.12