Download The Spinal Cord

Figure 13-1 An Overview of Chapters 13 and 14
CHAPTER 14: The Brain
Sensory
receptors
Sensory input
over cranial nerves
Reflex
centers
in brain
Motor output over
cranial nerves
Effectors
Muscles
CHAPTER 13: The Spinal Cord
Glands
Sensory
receptors
Sensory input
over spinal nerves
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Reflex
centers
in spinal
cord
Motor output over
spinal nerves
Adipose tissue
An Introduction to the Spinal Cord, Spinal Nerves, and
Spinal Reflexes
• Spinal Reflexes
• Rapid, automatic nerve responses triggered by
specific stimuli
• Controlled by spinal cord alone, not the brain
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13-2 Spinal Cord
• Roots
•
Two branches of spinal nerves
1. Ventral root
•
Contains axons of motor neurons
2. Dorsal root
•
•
Contains axons of sensory neurons
Dorsal root ganglia
•
Contain cell bodies of sensory neurons
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13-2 Spinal Cord
• The Spinal Nerve
• Each side of spine
• Dorsal and ventral roots join to form a spinal nerve
• Mixed Nerves
• Carry both afferent (sensory) and efferent (motor) fibers
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Figure 13-3a The Spinal Cord and Spinal Meninges
White matter
Ventral root
Gray matter
Dorsal root
ganglion
Spinal nerve
Dorsal root
Meninges
Pia mater
Arachnoid mater
Dura mater
A posterior view of the
spinal cord, showing
the meningeal layers,
superficial landmarks,
and distribution of gray
matter and white matter
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Figure 13-3b The Spinal Cord and Spinal Meninges
Meninges
ANTERIOR
Subarachnoid
space
Dura mater
Arachnoid mater
Pia mater
Vertebral
body
Rami
communicantes
Spinal cord
Adipose tissue
in epidural space
A sectional view
through the spinal
cord and meninges,
showing the
peripheral
distribution of
spinal nerves
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Autonomic
(sympathetic)
ganglion
Ventral root
of spinal
nerve
Ventral
ramus
Dorsal
ramus
Denticulate Dorsal root
ligament
ganglion
POSTERIOR
13-2 Spinal Cord
• The Spinal Meninges
• Specialized membranes isolate spinal cord from
surroundings
• Functions of the spinal meninges include:
• Protecting spinal cord
• Carrying blood supply
• Continuous with cranial meninges
• Meningitis
• Viral or bacterial infection of meninges
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13-2 Spinal Cord
• The Interlayer Spaces of
Arachnoid Mater
• Cerebrospinal Fluid (CSF)
• Carries dissolved gases,
nutrients, and wastes
• Lumbar puncture or spinal
tap withdraws CSF
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13-3 Gray Matter and White Matter
• Sectional Anatomy of the Spinal
Cord
• White matter
• Is superficial
• Contains myelinated and unmyelinated
axons
• Gray matter
• Surrounds central canal of spinal cord
• Contains neuron cell bodies, neuroglia,
unmyelinated axons
• Has projections (gray horns)
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13-3 Gray Matter and White Matter
• Organization of Gray Matter
• The gray horns
• Posterior gray horns contain somatic and visceral
sensory nuclei
• Anterior gray horns contain somatic motor nuclei
• Lateral gray horns are in thoracic and lumbar
segments; contain visceral motor nuclei
• Gray commissures
• Axons that cross from one side of cord to the other
before reaching gray matter
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13-3 Gray Matter and White Matter
• Organization of Gray Matter
• The cell bodies of neurons form functional groups
called nuclei
• Sensory nuclei
• Dorsal (posterior)
• Connect to peripheral receptors
• Motor nuclei
• Ventral (anterior)
• Connect to peripheral effectors
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13-3 Gray Matter and White Matter
• Control and Location
• Sensory or motor nucleus location within the gray
matter determines which body part it controls
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13-3 Gray Matter and White Matter
• Organization of White Matter
• Tracts or fasciculi
• In white columns
• Bundles of axons
• Relay same information in same direction
• Ascending tracts
• Carry information to brain
• Descending tracts
• Carry motor commands to spinal cord
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Figure 13-5a The Sectional Organization of the Spinal Cord
Posterior white column
Dorsal root
ganglion
Lateral
white
column
Posterior
gray horn
Lateral
gray horn
Anterior
gray
horn
Anterior white column
The left half of this sectional view shows important
anatomical landmarks, including the three columns
of white matter. The right half indicates the
functional organization of the nuclei in the anterior,
lateral, and posterior gray horns.
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Figure 13-5a The Sectional Organization of the Spinal Cord
Posterior
median sulcus
Posterior gray
commissure
Functional Organization
of Gray Matter
The cell bodies of neurons in the
gray matter of the spinal cord are
organized into functional groups
called nuclei.
Somatic
Visceral
Visceral
Sensory nuclei
Motor nuclei
Somatic
Ventral root
Anterior gray commissure
Anterior white commissure
Anterior median fissure
The left half of this sectional view shows important anatomical
landmarks, including the three columns of white matter. The right
half indicates the functional organization of the nuclei in the
anterior, lateral, and posterior gray horns.
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13-4 Spinal Nerves and Plexuses
• Peripheral Distribution of Spinal Nerves
• Spinal nerves
• Form lateral to intervertebral foramen
• Where dorsal and ventral roots unite
• Then branch and form pathways to destination
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13-4 Spinal Nerves and Plexuses
• Peripheral Distribution of Spinal Nerves
• Motor nerves
• The first branch
• White ramus
• Carries visceral motor fibers to sympathetic
ganglion of autonomic nervous system
• Gray ramus
• Unmyelinated nerves
• Return from sympathetic ganglion to rejoin
spinal nerve
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13-4 Spinal Nerves and Plexuses
• Peripheral Distribution of Spinal Nerves
• Motor nerves
• Dorsal and ventral rami
• Dorsal ramus
• Contains somatic and visceral motor fibers
• Innervates the back
• Ventral ramus
• Larger branch
• Innervates ventrolateral structures and limbs
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Figure 13-7 Peripheral Distribution of Spinal Nerves
To skeletal
muscles of
back
The spinal nerve forms
just lateral to the
intervertebral foramen,
where the dorsal
and ventral roots unite.
Dorsal root
Dorsal root
ganglion
Postganglionic fibers
to smooth muscles,
glands, etc., of back
The dorsal ramus contains
somatic motor and visceral motor
fibers that innervate the skin and
skeletal muscles of the back.
The axons in the relatively
large ventral ramus supply
the ventrolateral body
surface, structures in the
body wall, and the limbs.
The ventral root of each
spinal nerve contains the
axons of somatic motor and
visceral motor neurons.
To skeletal
muscles of body
wall, limbs
Visceral motor nuclei
Somatic motor nuclei
Rami
communicantes
 Somatic motor
commands
 Visceral motor
commands
Postganglionic fibers
to smooth muscles,
glands, visceral organs
in thoracic cavity
Preganglionic fibers
to sympathetic
ganglia innervating
abdominopelvic
viscera
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Sympathetic
ganglion
Postganglionic fibers
to smooth muscles,
glands, etc., of body
wall, limbs
The white ramus is the first branch from the
spinal nerve and carries visceral motor fibers
to a nearby sympathetic ganglion. Because
these preganglionic axons are myelinated,
this branch has a light color and is therefore
known as the white ramus.
A sympathetic nerve
contains preganglionic
and postganglionic
fibers innervating
structures in the
thoracic cavity.
The gray ramus contains postganglionic
fibers that innervate glands and smooth
muscles in the body wall or limbs. These fibers
are unmyelinated and have a dark gray color.
13-4 Spinal Nerves and Plexuses
• Peripheral Distribution of Spinal Nerves
• Sensory nerves
• In addition to motor impulses
• Dorsal, ventral, and white rami also carry sensory
information
• Dermatomes
• Bilateral region of skin
• Monitored by specific pair of spinal nerves
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Figure 13-7 Peripheral Distribution of Spinal Nerves
From interoceptors
of back
From exteroceptors,
proprioceptors of back
The dorsal ramus carries
sensory information from
the skin and skeletal
muscles of the back.
Somatic
sensory nuclei
The ventral ramus carries
sensory information from the
ventrolateral body surface,
structures in the body wall,
and the limbs.
Dorsal
root
ganglion
From exteroceptors,
proprioceptors of
body wall, limbs
From interoceptors
of body wall, limbs
Rami
communicantes
Ventral
root
 Somatic
sensations
 Visceral
sensations
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The dorsal root of each
spinal nerve carries
sensory information to
the spinal cord.
The sympathetic nerve
carries sensory information
from the visceral organs.
Visceral
sensory nuclei
From interoceptors
of visceral organs
Figure 13-8 Dermatomes
C2C3
NV
C2C3
C2
C3
T2
C6
L1
L2
C8
C7
T1
L3
L4
L5
C3
C4
C5
T1
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
T2
T3
T4
T5
T6
T7
T8
T9
T10
T11
T12
L1
L2
L4 L3
L5
C4
C5
T2
C6
T1
C7
SS
S2
4 3
L1
S5
C8
S1 L5
L2 S2
L3
S1
L4
ANTERIOR
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POSTERIOR
13-4 Spinal Nerves and Plexuses
• Peripheral
Neuropathy
• Regional loss of
sensory or
motor function
• Due to trauma
or compression
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Figure 13-9 Shingles
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13-5 Neuronal Pools
• Functional Organization of Neurons
• Sensory neurons
• About 10 million
• Deliver information to CNS
• Motor neurons
• About 1/2 million
• Deliver commands to peripheral effectors
• Interneurons
• About 20 billion
• Interpret, plan, and coordinate signals in and out
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13-6 Reflexes
• Reflexes
• Automatic responses coordinated within spinal cord
• Through interconnected sensory neurons, motor
neurons, and interneurons
• Produce simple and complex reflexes
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13-6 Reflexes
• Neural Reflexes
• Rapid, automatic responses to specific stimuli
• Basic building blocks of neural function
• One neural reflex produces one motor response
• Reflex arc
• The wiring of a single reflex
• Beginning at receptor
• Ending at peripheral effector
• Generally opposes original stimulus (negative
feedback)
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13-6 Reflexes
• Five Steps in a Neural Reflex
• Step 1: Arrival of stimulus, activation of receptor
• Physical or chemical changes
• Step 2: Activation of sensory neuron
• Graded depolarization
• Step 3: Information processing by postsynaptic cell
• Triggered by neurotransmitters
• Step 4: Activation of motor neuron
• Action potential
• Step 5: Response of peripheral effector
• Triggered by neurotransmitters
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Figure 13-15 Events in a Neural Reflex
Activation of a
sensory neuron
Arrival of stimulus and
activation of receptor
Dorsal
root
Sensation
relayed to the
brain by axon
collaterals
Information processing
in the CNS
REFLEX
ARC
Receptor
Stimulus
Response by effector
Effector
Ventral
root
Activation of a
motor neuron
KEY
Sensory neuron
(stimulated)
Excitatory
interneuron
Motor neuron
(stimulated)
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13-6 Reflexes
• Four Classifications of Reflexes
1. By early development
2. By type of motor response
3. By complexity of neural circuit
4. By site of information processing
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13-6 Reflexes
• Development of Reflexes
• Innate reflexes
• Basic neural reflexes
• Formed before birth
• Acquired reflexes
• Rapid, automatic
• Learned motor patterns
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13-6 Reflexes
• Motor Response
• Nature of resulting motor response
• Somatic reflexes
• Involuntary control of nervous system
• Superficial reflexes of skin, mucous membranes
• Stretch or deep tendon reflexes (e.g., patellar,
or “knee-jerk,” reflex)
• Visceral reflexes (autonomic reflexes)
• Control systems other than muscular system
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13-6 Reflexes
• Complexity of Neural Circuit
• Monosynaptic reflex
• Sensory neuron synapses directly onto motor
neuron
• Polysynaptic reflex
• At least one interneuron between sensory neuron
and motor neuron
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13-6 Reflexes
• Site of Information Processing
• Spinal reflexes
• Occur in spinal cord
• Cranial reflexes
• Occur in brain
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Figure 13-16 The Classification of Reflexes
Reflexes
can be classified by
development
response
complexity of circuit
processing site
Innate Reflexes
Somatic Reflexes
Monosynaptic
Spinal Reflexes
• Genetically
determined
• Control skeletal muscle
contractions
• Include superficial and stretch
reflexes
Acquired Reflexes
Visceral (Autonomic) Reflexes
• Learned
• Control actions of smooth and
cardiac muscles, glands, and
adipose tissue
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• One synapse
Polysynaptic
• Multiple synapse
(two to several
hundred)
• Processing in
the spinal cord
Cranial Reflexes
• Processing in
the brain
13-7 Spinal Reflexes
• Spinal Reflexes
• Range in increasing order of complexity
• Monosynaptic reflexes
• Polysynaptic reflexes
• Intersegmental reflex arcs
• Many segments interact
• Produce highly variable motor response
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13-7 Spinal Reflexes
• Monosynaptic Reflexes
• A stretch reflex
• Have least delay between sensory input and motor
output
• For example, stretch reflex (such as patellar reflex)
• Completed in 20–40 msec
• Receptor is muscle spindle
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Figure 13-17 A Stretch Reflex
Receptor
(muscle
spindle)
Spinal cord
Stretch
REFLEX
ARC
Stimulus
Effector
Contraction
KEY
Sensory neuron
(stimulated)
Motor neuron
(stimulated)
Response
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13-7 Spinal Reflexes
• Muscle Spindles
• The receptors in stretch reflexes
• Bundles of small, specialized intrafusal muscle
fibers
• Innervated by sensory and motor neurons
• Surrounded by extrafusal muscle fibers
• Which maintain tone and contract muscle
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13-7 Spinal Reflexes
• The Sensory Region
• Central region of intrafusal fibers
• Wound with dendrites of sensory neurons
• Sensory neuron axon enters CNS in dorsal root
• Synapses onto motor neurons (gamma motor neurons)
• In anterior gray horn of spinal cord
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13-7 Spinal Reflexes
• Gamma Efferents
• Axons of the motor neurons
• Complete reflex arc
• Synapse back onto intrafusal fibers
• Important in voluntary muscle contractions
• Allow CNS to adjust sensitivity of muscle spindles
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Figure 13-18 A Muscle Spindle
Gamma
efferent
from CNS
Extrafusal
fiber
To CNS
Sensory
region
Intrafusal
fiber
Muscle
spindle
Gamma
efferent
from CNS
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13-7 Spinal Reflexes
• Postural reflexes
• Stretch reflexes
• Maintain normal upright posture
• Stretched muscle responds by contracting
• Automatically maintain balance
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13-7 Spinal Reflexes
• Polysynaptic Reflexes
• More complicated than monosynaptic reflexes
• Interneurons control more than one muscle group
• Produce either EPSPs or IPSPs
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13-7 Spinal Reflexes
• The Tendon Reflex
• Prevents skeletal muscles from:
• Developing too much tension
• Tearing or breaking tendons
• Sensory receptors unlike muscle spindles or
proprioceptors
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13-7 Spinal Reflexes
• Withdrawal Reflexes
• Move body part away from stimulus (pain or pressure)
• For example, flexor reflex
• Pulls hand away from hot stove
• Strength and extent of response
• Depend on intensity and location of stimulus
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Figure 13-19 A Flexor Reflex
Distribution within gray horns to other segments of the spinal cord
Painful
stimulus
Flexors
stimulated
Extensors
inhibited
KEY
Sensory neuron
(stimulated)
Motor neuron
(inhibited)
Excitatory
interneuron
Inhibitory
interneuron
Motor neuron
(stimulated)
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Figure 13-20 The Crossed Extensor Reflex
To motor neurons in other segments of the spinal cord
Extensors
inhibited
Flexors
stimulated
Extensors
stimulated
Flexors
inhibited
KEY
Painful
stimulus
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Sensory neuron
(stimulated)
Motor neuron
(inhibited)
Excitatory
interneuron
Inhibitory
interneuron
Motor neuron
(stimulated)
13-8 The Brain Can Alter Spinal Reflexes
• Integration and Control of Spinal Reflexes
• Reflex behaviors are automatic
• But processing centers in brain can facilitate or inhibit
reflex motor patterns based in spinal cord
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13-8 The Brain Can Alter Spinal Reflexes
• Voluntary Movements and Reflex Motor Patterns
• Higher centers of brain incorporate lower, reflexive
motor patterns
• Automatic reflexes
• Can be activated by brain as needed
• Use few nerve impulses to control complex motor
functions
• Walking, running, jumping
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13-8 The Brain Can Alter Spinal Reflexes
• The Babinski Reflexes
• Normal in infants
• May indicate CNS damage in adults
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Figure 13-21a The Babinski Reflexes
The plantar reflex
(negative Babinski
reflex), a curling of the
toes, is seen in healthy
adults.
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Figure 13-21b The Babinski Reflexes
The Babinski sign (positive
Babinski reflex) occurs in
the absence of descending
inhibition. It is normal in
infants, but pathological in
adults.
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