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Chapter 12 Part D
The Central
Nervous
System
© Annie Leibovitz/Contact Press Images
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PowerPoint® Lecture Slides
prepared by
Karen Dunbar Kareiva
Ivy Tech Community College
12.10 Spinal Cord
Gross Anatomy and Protection
• Spinal cord is enclosed in vertebral column
– Begins at the foramen magnum
– Ends at L1 or L2 vertebra
• Functions
– Provides two-way communication to and from
brain and body
– Major reflex center: reflexes are initiated and
completed at spinal cord
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Gross Anatomy and Protection (cont.)
• Protected by bone, meninges, and CSF
• Spinal dura mater is one layer thick
– Does not attach to vertebrae
• Epidural space
– Cushion of fat and network of veins in space
between vertebrae and spinal dura mater
• CSF fills subarachnoid space between
arachnoid and pia maters
• Dural and arachnoid membranes extend to
sacrum, beyond end of cord at L1 or L2
– Site of lumbar puncture or tap
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Figure 12.27a Gross structure of the spinal cord, dorsal view.
Cervical
enlargement
Dura and
arachnoid
mater
Cervical
spinal nerves
Thoracic
spinal nerves
Lumbar
enlargement
Conus
medullaris
Cauda
equina
Filum
terminale
Lumbar
spinal nerves
Sacral
spinal nerves
The spinal cord and its nerve roots, with the bony
vertebral arches removed. The dura mater and
arachnoid mater are cut open and reflected laterally.
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Figure 12.28 Diagram of a lumbar puncture.
T12
L5
Ligamentum
flavum
Lumbar puncture
needle entering
subarachnoid
space
L4
Supraspinous
ligament
L5
Filum
terminale
S1
Intervertebral
disc
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Arachnoid
mater
Dura
mater
Cauda equina
in subarachnoid
space
Gross Anatomy and Protection (cont.)
• Spinal cord terminates in cone-shaped structure
called conus medullaris
• Filum terminale extends to coccyx
– Fibrous extension of conus covered with pia
mater
– Anchors spinal cord
• Denticulate ligaments
– Extensions of pia mater that secure cord to dura
mater
– Cervical and lumbar enlargements: areas
where nerves servicing upper and lower limbs
arise from spinal cord
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Gross Anatomy and Protection (cont.)
• Spinal nerves
– Part of PNS
– Attach to spinal cord by 31 paired roots
• Cervical and lumbosacral enlargements
– Nerves serving upper and lower limbs emerge
here
• Cauda equina
– Collection of nerve roots at inferior end of
vertebral canal
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Figure 12.27a Gross structure of the spinal cord, dorsal view.
Cervical
enlargement
Dura and
arachnoid
mater
Cervical
spinal nerves
Thoracic
spinal nerves
Lumbar
enlargement
Conus
medullaris
Cauda
equina
Filum
terminale
Lumbar
spinal nerves
Sacral
spinal nerves
The spinal cord and its nerve roots, with the bony
vertebral arches removed. The dura mater and
arachnoid mater are cut open and reflected laterally.
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Figure 12.27b Gross structure of the spinal cord, dorsal view.
Foramen
magnum
Terminus
of medulla
oblongata
of brain
Sectioned
pedicles
of cervical
vertebrae
Spinal nerve
rootlets
Dorsal median
sulcus of
spinal cord
Cervical spinal cord.
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Figure 12.27c Gross structure of the spinal cord, dorsal view.
Vertebral arch
(cut)
Spinal cord
Cut edge of
dura mater
Dorsal root
ganglion
Dorsal median
sulcus
Dorsal root
Thoracic spinal cord.
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Figure 12.27d Gross structure of the spinal cord, dorsal view.
Spinal cord
First lumbar
vertebral arch
(cut across)
Conus
medullaris
Cauda equina
Filum terminale
Inferior end of spinal cord,
showing conus medullaris,
cauda equina, and filum
terminale.
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Spinal Cord Cross-sectional Anatomy
• Two lengthwise grooves that run length of cord
partially divide it into right and left halves
– Ventral (anterior) median fissure
– Dorsal (posterior) median sulcus
• Gray matter is located in core, white matter
outside
• Central canal runs length of cord
– Filled with CSF
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Figure 12.29a Anatomy of the spinal cord.
Epidural space
(contains fat)
Subdural space
Subarachnoid
space
(contains CSF)
Pia mater
Arachnoid mater
Dura mater
Spinal
meninges
Bone of
vertebra
Dorsal root
ganglion
Body
of vertebra
Cross section of spinal cord and vertebra
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Figure 12.29b Anatomy of the spinal cord.
Dorsal funiculus
White
Ventral funiculus
columns Lateral funiculus
Dorsal root
ganglion
Spinal nerve
Dorsal root
(fans out into
dorsal rootlets)
Ventral root
(derived from several
ventral rootlets)
Dorsal median sulcus
Gray commissure
Dorsal horn
Gray
Ventral horn
matter
Lateral horn
Central canal
Ventral median
fissure
Pia mater
Arachnoid mater
Spinal dura mater
The spinal cord and its meningeal coverings
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Spinal Cord Cross-sectional Anatomy (cont.)
• Gray matter and spinal roots
– Cross section of cord resembles butterfly or letter
“H”
– Three areas of gray matter are found on each
side of center and are mirror images:
• Dorsal horns: interneurons that receive somatic and
visceral sensory input
• Ventral horns: some interneurons; somatic motor
neurons
• Lateral horns (only in thoracic and superior lumbar
regions): sympathetic neurons
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Spinal Cord Cross-sectional Anatomy (cont.)
• Gray matter and spinal roots (cont.)
– Gray commissure: bridge of gray matter that
connects masses of gray matter on either side
• Encloses central canal
– Ventral roots: bundle of motor neuron axons
that exit the spinal cord
– Dorsal roots: sensory input to cord
– Dorsal root (spinal) ganglia: cell bodies of
sensory neurons
– Spinal nerves: formed by fusion of dorsal and
ventral roots
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Spinal Cord Cross-sectional Anatomy (cont.)
• Gray matter and spinal roots (cont.)
– Gray matter divided into four groups based on of
somatic or visceral innervation
• Somatic sensory (SS), visceral sensory (VS),
visceral (autonomic) motor (VM) and somatic
motor (SM)
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Figure 12.30 Organization of the gray matter of the spinal cord.
Dorsal root
(sensory)
Somatic sensory
neuron
Dorsal horn (interneurons)
Dorsal root
ganglion
SS
VS
VM
SM
Visceral sensory
neuron
Visceral motor
neuron
Somatic motor
neuron
Spinal nerve
Ventral root
(motor)
Ventral horn
(motor neurons)
SS Interneurons receiving input from somatic sensory neurons
VS Interneurons receiving input from visceral sensory neurons
VM Visceral motor (autonomic) neurons
SM Somatic motor neurons
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Spinal Cord Cross-sectional Anatomy (cont.)
• White matter
– Myelinated and nonmyelinated nerve fibers allow
communication between parts of spinal cord, and
spinal cord and brain
– Run in three directions
• Ascending: up to higher centers (sensory inputs)
• Descending: from brain to cord or lower cord levels
(motor outputs)
• Transverse: from one side to other (commissural
fibers)
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Spinal Cord Cross-sectional Anatomy (cont.)
• White matter (cont.)
• White matter is divided into three white
columns (funiculi) on each side
– Dorsal (posterior)
– Lateral
– Ventral (anterior)
• Each spinal tract is composed of axons with
similar destinations and functions
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Figure 12.29b Anatomy of the spinal cord.
Dorsal funiculus
White
Ventral funiculus
columns Lateral funiculus
Dorsal root
ganglion
Spinal nerve
Dorsal root
(fans out into
dorsal rootlets)
Ventral root
(derived from several
ventral rootlets)
Dorsal median sulcus
Gray commissure
Dorsal horn
Gray
Ventral horn
matter
Lateral horn
Central canal
Ventral median
fissure
Pia mater
Arachnoid mater
Spinal dura mater
The spinal cord and its meningeal coverings
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Figure 12.31 Major ascending (sensory) and descending (motor) tracts of the spinal cord, cross-sectional view.
Ascending tracts
Dorsal white column
• Fasciculus gracilis
• Fasciculus cuneatus
Spinocerebellar
tracts
• Dorsal
• Ventral
Spinothalamic
tracts
• Lateral
• Ventral
Descending tracts
Ventral white
commissure
Corticospinal tracts
(pyramidal tracts)
• Lateral
• Ventral
Rubrospinal tract
Reticulospinal
tracts
• Medial
• Lateral
Vestibulospinal
tract
Tectospinal tract
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Spinal Cord Trauma and Disorders
• Spinal cord trauma
– Localized injury to spinal cord or its roots leads
to functional losses
• Paresthesias: caused by damage to dorsal roots or
sensory tracts
– Leads to sensory function loss
• Paralysis: caused by damage to ventral roots or
ventral horn cells
– Leads to motor function loss
– Two types of paralysis: flaccid or spastic
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Spinal Cord Trauma and Disorders (cont.)
• Spinal cord trauma (cont.)
– Flaccid paralysis: severe damage to ventral
root or ventral horn cells
• Impulses do not reach muscles; there is no voluntary
or involuntary control of muscles
• Muscles atrophy
– Spastic paralysis: damage to upper motor
neurons of primary motor cortex
• Spinal neurons remain intact; muscles are stimulated
by reflex activity
• No voluntary control of muscles
• Muscles often shorten permanently
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Spinal Cord Trauma and Disorders (cont.)
• Spinal cord trauma (cont.)
– Transection (cross sectioning) of spinal cord at
any level results in total motor and sensory loss
in regions inferior to cut
• Paraplegia: transection between T1 and L1
• Quadriplegia: transection in cervical region
– Spinal shock: transient period of functional loss
caudal to lesion
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Spinal Cord Trauma and Disorders (cont.)
• Poliomyelitis
– Destruction of ventral horn motor neurons by
poliovirus
– Muscles atrophy
– Death may occur from paralysis of respiratory
muscles or cardiac arrest
– Survivors often develop postpolio syndrome
many years later from neuron loss
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Spinal Cord Trauma and Disorders (cont.)
• Amyotrophic lateral sclerosis (ALS)
– Also called Lou Gehrig’s disease
– Destruction of ventral horn motor neurons and
fibers of pyramidal tract
– Symptoms: loss of ability to speak, swallow, and
breathe
– Death typically occurs within 5 years
– Caused by environmental factors and genetic
mutations involving RNA processing
• Involves glutamate excitotoxicity
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Spinal Cord Trauma and Disorders (cont.)
• Amyotrophic lateral sclerosis (ALS) (cont.)
– Drug riluzole interferes with glutamate signaling:
only treatment
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12.11 Neuronal Pathways
• Major spinal tracts are part of multineuron
pathways
• Four key points about spinal tracts and
pathways:
– Decussation: Most pathways cross from one
side of CNS to other at some point
– Relay: Consist of chain of two or three neurons
– Somatotopy: precise spatial relationship in CNS
correspond to spatial relationship in body
– Symmetry: pathways are paired symmetrically
(right and left)
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Ascending Pathways
• Conduct sensory pathways upward through a
chain of three neurons:
– First-order neuron
• Conducts impulses from cutaneous receptors and
proprioceptors
• Branches diffusely as it enters spinal cord or medulla
• Synapses with second-order neuron
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Ascending Pathways (cont.)
– Second-order neuron
• Interneuron
• Cell body in dorsal horn of spinal cord or medullary
nuclei
• Axons extend to thalamus or cerebellum
– Third-order neuron
•
•
•
•
Also an interneuron
Cell bodies in thalamus
Axon extends to somatosensory cortex
No third-order neurons in cerebellum
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Ascending Pathways (cont.)
• Somatosensory signals travel along three main
pathways on each side of spinal cord:
– Two pathways transmit somatosensory
information to sensory cortex via thalamus
• Dorsal column–medial lemniscal pathways
• Spinothalamic pathways
• Provide for discriminatory touch and conscious
proprioception
– Third pathway, spinocerebellar tracts,
terminate in the cerebellum
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Ascending Pathways (cont.)
– Dorsal column–medial lemniscal pathways
• Transmit input to somatosensory cortex for
discriminative touch and vibrations
• Composed of paired fasciculus cuneatus and
fasciculus gracilis in spinal cord and medial lemniscus
in brain (medulla to thalamus)
– Spinothalamic pathways
• Lateral and ventral spinothalamic tracts
• Transmit pain, temperature, coarse touch, and
pressure impulses within lateral spinothalamic tract
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Ascending Pathways (cont.)
– Spinocerebellar tracts
• Ventral and dorsal tracts
• Convey information about muscle or tendon stretch to
cerebellum
– Used to coordinate muscle activity
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Figure 12.32a Pathways of selected ascending spinal cord tracts.
Primary somatosensory cortex
Axons of third-order neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
Dorsal
spinocerebellar
tract (axons of
second-order
neurons)
Medial lemniscus (tract)
(axons of second-order neurons)
Nucleus gracilis
Nucleus cuneatus
Medulla oblongata
Fasciculus cuneatus
(axon of first-order sensory neuron)
Axon of
first-order
neuron
Muscle spindle
(proprioceptor)
Joint stretch receptor
(proprioceptor)
Cervical spinal cord
Fasciculus gracilis
(axon of first-order sensory neuron)
Lumbar spinal cord
Touch receptor
Spinocerebellar pathway
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Dorsal column–medial lemniscal pathway
Figure 12.32b Pathways of selected ascending spinal cord tracts.
Primary somatosensory
cortex
Axons of third-order
neurons
Thalamus
Cerebrum
Midbrain
Cerebellum
Pons
Medulla oblongata
Lateral
spinothalamic
tract (axons of
second-order
neurons)
Pain receptors
Cervical spinal cord
Axons of first-order
neurons
Temperature
receptors
Lumbar spinal cord
Spinothalamic pathway
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Table 12.2-1 Major Ascending (Sensory) Pathways and Spinal Cord Tracts
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Table 12.2-2 Major Ascending (Sensory) Pathways and Spinal Cord Tracts (continued)
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Table 12.2-3 Major Ascending (Sensory) Pathways and Spinal Cord Tracts (continued)
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Descending Pathways and Tracts
• Deliver efferent impulses from brain to spinal
cord
• Two groups
– Direct pathways: pyramidal tracts
– Indirect pathways: all others
• Motor pathways involve two neurons:
– Upper motor neurons
• Pyramidal cells in primary motor cortex
– Lower motor neurons
• Ventral horn motor neurons
• Innervate skeletal muscles
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Descending Pathways and Tracts (cont.)
• Direct (pyramidal) pathways
– Impulses from pyramidal neurons in precentral
gyri pass through pyramidal (lateral and ventral
corticospinal) tracts
– Descend directly without synapsing until axon
reaches end of tract in spinal cord
– In spinal cord, axons synapse with interneurons
(lateral tract) or ventral horn motor neurons
(ventral tract)
– Direct pathway regulates fast and fine (skilled)
movements
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Figure 12.33a Three descending pathways by which the brain influences movement.
Pyramidal cells
(upper motor neurons)
Primary motor cortex
Internal capsule
Cerebrum
Midbrain
Cerebral
peduncle
Cerebellum
Pons
Ventral
corticospinal
tract
Pyramids
Decussation
of pyramids
Lateral
corticospinal
tract
Medulla oblongata
Cervical spinal cord
Skeletal
muscle
Lumbar spinal cord
Somatic motor neurons
(lower motor neurons)
Pyramidal (lateral and ventral corticospinal) pathways
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Descending Pathways and Tracts (cont.)
• Indirect pathways
– Also referred to as multineuronal pathways
– Complex and multisynaptic
– Includes brain stem motor nuclei and all motor
pathways except pyramidal pathways
– These pathways regulate:
• Axial muscles, maintaining balance and posture
• Muscles controlling coarse limb movements
• Head, neck, and eye movements that follow objects in
visual field
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Descending Pathways and Tracts (cont.)
• Indirect pathways (cont.)
– Consist of four major pathways:
• Reticulospinal and vestibulospinal tracts:
– maintain balance by varying tone of postural muscles
• Rubrospinal tracts: control flexor muscles
• Tectospinal tracts: originate from superior colliculi
and mediate head movements in response to visual
stimuli
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Figure 12.33b Three descending pathways by which the brain influences movement.
Cerebrum
Red nucleus
Midbrain
Cerebellum
Pons
Rubrospinal
tract
Medulla oblongata
Cervical spinal cord
Rubrospinal tract
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Table 12.3-1 Major Descending (Motor) Pathways and Spinal Cord Tracts
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Table 12.3-2 Major Descending (Motor) Pathways and Spinal Cord Tracts (continued)
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Developmental Aspects of Central Nervous
System
• Starting in a 3-week old embryo:
1. Ectoderm thickens, forming neural plate
• Invaginates, forming neural groove flanked by neural
folds
2. Neural crest forms from migrating neural fold
cells
3. Neural groove deepens, edges fuse forming
neural tube
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Figure 12.34-1 Development of the neural tube from embryonic ectoderm.
Head
Neural fold forming
Surface
ectoderm
Neural plate
Tail
1 The neural plate forms from surface ectoderm. It then
invaginates, forming the neural groove flanked by neural folds.
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Figure 12.34-2 Development of the neural tube from embryonic ectoderm.
Neural crest
Neural
groove
2 Neural fold cells migrate to form the neural crest, which will
form much of the PNS and many other structures.
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Figure 12.34-3 Development of the neural tube from embryonic ectoderm.
Head
Surface
ectoderm
Neural
tube
Tail
3 The neural groove becomes the neural tube, which will form
CNS structures.
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Developmental Aspects of Central Nervous
System
• Neural tube, formed by week 4, differentiates
into CNS
– Brain forms rostrally
– Spinal cord forms caudally
• By week 6, both sides of spinal cord bear a
dorsal alar plate and a ventral basal plate
– Alar plate becomes interneurons
– Basal plate becomes motor neurons
– Neural crest cells form dorsal root ganglia
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Figure 12.35 Structure of the embryonic spinal cord.
Dorsal root ganglion: sensory
neurons from neural crest
Alar plate:
interneurons
Axons
form
white
matter
Basal plate:
motor neurons
Neural tube
cells
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Central
cavity
Developmental Aspects of Central Nervous
System
• Gender-specific areas appear in both brain and
spinal cord
– Depends on presence or absence of fetal
testosterone
• Maternal exposure to radiation, drugs, or
infection can harm developing CNS
– Example: alcohol or opiates, various infections
such as rubella
• Smoking decreases oxygen in blood, which can
lead to neuron death and fetal brain damage
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Developmental Aspects of Central Nervous
System
• Hypothalamus is one of last areas of CNS to
develop
– Premature infants have poor body temperature
regulation
• Visual cortex develops slowly over first 11
weeks
• Neuromuscular coordination progresses in
superior-to-inferior and proximal-to-distal
directions along with myelination
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Developmental Aspects of Central Nervous
System
• Age brings some cognitive declines, but not
significant in healthy individuals until 80s
• Shrinkage of brain accelerates in old age
• Excessive alcohol use and boxing cause signs
of senility unrelated to aging process
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Clinical – Homeostatic Imbalance 12.2
• Cerebral palsy: neuromuscular disability
involving poorly controlled or paralyzed
voluntary muscles
– Due to brain damage, possibly from lack of
oxygen during birth
– Spasticity, speech difficulties, motor impairments
can be seen
– Some patients have seizures, are intellectually
impaired, and/or are deaf
– Visual impairment is common
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Clinical – Homeostatic Imbalance 12.2
• Anencephaly: cerebrum and parts of brain
stem never develop
– Neural fold fails to fuse
– Child is vegetative
– Death occurs soon after birth
• Spina bifida: incomplete formation of vertebral
arches
– Spina bifida occulata: least serious, involves only
one or few missing vertebrae
• Causes no neural problems
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Clinical – Homeostatic Imbalance 12.2
– Spina bifida cystica: more severe and most
common
• Saclike cyst protrudes dorsally from spine
• Cyst may contain CSF (meningocele) or portions of
spinal cord and nerve roots (myelomeningocele)
– Larger cysts cause neurological impairment
• Usually also see hydrocephalus
– Most cases caused by lack of B vitamin folic acid
• U.S. incidence has dropped with mandatory
supplementation
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