Spinal Cord
Chapter 12 - continued
The Spinal Cord


The spinal cord
extends from
the foramen
magnum to the
level of the 1st
or 2nd lumbar
vertebrae
It is enclosed
within the
vertebral
column
The Spinal Cord


The spinal cord
is a provides a
two way
conduction
pathway to and
from the brain
It is a major
reflex center
The Spinal Cord

The spinal cord is protected by bone, cerebrospinal fluid, and meninges
– Dura mater, arachnoid, pia mater
The Spinal Cord


Between the bony vertebrae and the dural sheath is
a large epidural space filled with a soft padding of
fat and a network of veins
Cerebrospinal fluid fills the subarachnoid space
The Spinal Cord


Inferiorly, the
dural and
subarachnoid
membranes
extend to the
level of S2 while
the spinal cord
ends at L1
Subarachnoid
space beyond
L1 is an ideal
site for a spinal
tap
The Spinal Cord

The spinal cord
terminates in a
tapering cone
shaped
structure called
the conus
medullaris
The Spinal Cord

A fiberous
extension of the
pia mater, the
filum terminale
extends
inferiorly from
the conus
medullaris to
attach to the
posterior
surface of the
coccyx
The Spinal Cord


There are 31 pairs of spinal nerves that
arise from the cord by paired roots and
exit from the vertebral column via the
intervertebral formina
Each segment of the spinal cord is
defined by a pair of spinal nerves that lie
just superior to their corresponding
vertebra
The Spinal Cord

The spinal cord
has obvious
enlargements
where the
nerves serving
the upper and
lower limb arise
– Cervical
enlargement
– Lumbar
enlargement
The Spinal Cord



Because the cord does not reach the end
of the vertebral column, the lumbar and
sacral spinal nerve roots angle sharply
downward and travel inferiorly before
reaching their intervertebral foramina
This collection of nerve roots at the
inferior end of the vertebral canal is
called the cauda equina
The arrangement reflects the fact that
during vertebral column grows more
rapidly than does the spinal cord
Embryonic Development


The spinal cord
develops from the
caudal portion of the
embryonic neural
tube
By the end of the 6th
week each side of the
developing cord has
two clusters of
neuroblasts that have
migrated outwarded
from the neural tube
Embryonic Development



The two clusters are
the dorsal alar plate
and a ventral basal
plate
Alar plate neurons
become interneurons
The basal plate
neurons become
motor neurons that
sprout axons that
grow out to the
effector organs
Embryonic Development


Axons that emerge
from alar plate cells
form the external
white matter of the
cord by growing
outward along the
length of the CNS
The alar plates
expand dorsally and
the basal plates
expand vertically to
become the H-shaped
mass of gray matter
Embryonic Development

Neural crest cells
that come to lie
alongside the cord
form the dorsal root
ganglia containing
sensory nerve cell
bodies, which send
their axons to the
dorsal aspect of the
brain
Neural crest cells
Cross Section of Spinal Cord

The spinal cord has two grooves that mark its
surface
– Anterior median fissure / Posterior medial sulcus
Gray Matter and Spinal Roots


These grooves run the length of the cord and
partially divide it into right and left halves
Gray matter inside, the white matter outside
Gray Matter and Spinal Roots

The gray matter consists of a mixture of neuron
cell bodies, their unmyelinated processes, and
neuroglia (support cells)
Gray Matter and Spinal Roots

The white matter is composed of myelinated and
unmyelinated nerve fibers that represent
ascending, descending and transverse pathways
Gray Matter and Spinal Roots

The gray matter consists of mirror-image lateral gray
masses connected by a cross-bar of gray matter called
the gray commissure that encloses the central canal
Gray Matter and Spinal Roots

The two posterior projections of gray matter are the
posterior (dorsal) horns; the anterior pair are the
anterior (ventral) horns with lateral horns in the lumbar
and thoracic portions of the cord
Gray Matter and Spinal Roots


The anterior horns house nerve cell bodies of the
somatic motor neurons
These send their axons out via ventral roots of the spinal
cord to the skeletal muscles
Gray Matter and Spinal Roots


The amount of ventral gray matter
present at a given level of the spinal cord
reflects the amount of skeletal muscle
innervated at that particular level
Thus, the anterior horns are the largest
in the areas where the innervation for
limbs is present
– Cervical enlargement / arms
– Lumbar enlargement / legs
Gray Matter and Spinal Roots


The lateral horn neurons are autonomic (sympathetic)
motor neurons that serve the visceral organs
Their axons also leave the cord via the ventral root
Gray Matter and Spinal Roots

Afferent fibers carrying impulses from peripheral
sensory receptors form the dorsal roots of the spinal
cord
Gray Matter and Spinal Roots

The cell bodies of the associated sensory neurons are
found in an enlarged region of the dorsal root called the
dorsal root ganglion or spinal ganglion
Gray Matter and Spinal Roots


After entering the cord, the axons take a number of
routes
Some enter the posterior white matter of the cord or
brain, others synapse with interneurons
Gray Matter and Spinal Roots

The dorsal and ventral roots are very short and fuse
laterally to form the spinal nerves which are then
considered part of the peripheral nervous system (PNS)
Gray Matter and Spinal Roots
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
The spinal gray matter can be divided further according
to its neurons relative involvement in the innervation of
the somatic and visceral regions of the body
The four zones are; somantic sensory (ss); visceral
sensory (vs); visceral motor (vm); somatic motor (sm)
White Matter

The white matter of the spinal cord is composed of
myelinated and unmyelinated nerve fibers that allow
communication between different parts of the spinal
cord and between the cord and the brain
White Matter

Nerve fibers run in three directions
– Ascending / up to higher centers (sensory inputs)
– Descending / down to the cord from the brain or from within
the cord to lower levels (motor outputs)
– Transversely / across from one side of the cord to the other
(commissural fibers)
White Matter

The ascending and descending tracts make up most of
the white matter of the spinal cord
– Ascending tracts are shown in blue and labeled at left
– Descending tracts are shown in red and labeled at right
White Matter


The white matter on each side of the column is divided
into three white columns or funiculi and labeled
according to their position (posterior, lateral, anterior)
Each funiculi contains several fiber tracts, and each
tract is made up of axons with similar destinations and
functions
White Matter


All major spinal tracts are actually part of multineuron pathways that connect the brain to the body
These ascending and descending pathways contain not
only spinal cord neurons but also parts of peripheral
neurons and neurons in the brain
White Matter

Generalizations about spinal pathways
– Most pathways cross over from one side of
the CNS to the other at some point
– Most consist of a chain of two or three
neurons that contribute to successive tracts
– Most exhibit somatotopy, a precise spatial
relationship among the tract fibers that
reflects the orderly mapping of the body
– All pathways and tracts are paired (right
and left) with a member of the pair on each
side of the spinal cord or brain
Ascending (Sensory) Tracts


The ascending pathways conduct sensory
impulses upward, typically through chains of
three successive neurons (first-, second, and
third-order neurons) to various regions of the
brain
Most of the incoming information results
from stimulation of
– General sensory receptors
• Touch / pressure / temperature / pain
– Stimulation of proprioceptors
• Muscle stretch / tendon / joint
Ascending (Sensory) Tracts

In general, sensory information is conveyed
along six main pathways on each side of the
spinal cord
– Four transmit impulses to the sensory cortex for
conscious interpretation
•
•
•
•
Fasciculi cuneatus
Fasciculi gracilis
Lateral spinothalamic tract
Anterior spinothalamic tract
– Two transmit impulses to the cerebellum to
coordinate muscle activity
• Anterior spinocerebellur tract
• Posterior spinocerebellur tract
Ascending (Sensory) Tracts

Posterior funiculi (dorsal white column)
– Fasciculi cuneatus
– Fasciculi gracilis

Transmit information from the fine touch and
pressure receptors and joint proprioceptors
– These tracts comprize what is referred to as discriminative
touch and conscious proprioception
Ascending (Sensory) Tracts

Lateral and anterior funiculi
– Lateral spinothalamic tract
– Anterior spinothalamic tract

Convey information on pain, temperature, deep
pressure and course touch (undiscriminated)
Ascending (Sensory) Tracts

Anterior and posterior funiculi
– Anterior spinocerebellar tract
– Posterior spinocerebellar tract

Convey information from proprioceptors (muscle and
tendon stretch) to the cerebellum which uses this
information to coordinate skeletal muscle activity
Ascending (Sensory) Tracts


Since the spinocerebellar tracts do not
terminate in the cortex, these pathways do
not contribute to conscious sensation
The spinocerebellar tracts do not decussate
and thus contribute to ipsilateral innervation
Descending (Motor) Tracts

The descending motor tracts that deliver impulses
from the brain to the spinal cord are divided into two
groups
– Pyramidal tracts
– All others
Descending (Motor) Tracts
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

Motor pathways involve two neurons,
referred to as upper and lower motor
neurons
The pyramidal cells of the motor cortex,
as well as the neurons in subcortical
motor nuclei that give rise to other
descending motor pathways, are called
upper motor neurons
The anterior horn motor neurons, which
actually innervate the skeletal muscles
are called lower motor neurons
Descending (Motor) Tracts

The lateral (pyramdial) and anterior corticospinal
tracts are the major motor pathways concerned with
voluntary movement, particularly precise or skilled
movement
Descending (Motor) Tracts

The pyramdial tracts are also called the direct
pathways because their axons descend without
synapsing from the pyramidal cells of the primary
motor cortex all the way to the spinal cord
Descending (Motor) Tracts


Pyramidal tracts synapse primarily with
interneurons, but also directly with
anterior horn motor neurons, principally
those controlling limb muscles
The anterior horn motor neurons activate
the skeletal muscles with which they are
associated
Descending (Motor) Tracts

The remaining descending tracts include:
–
–
–
–
–
Rubrospinal
Anterior reticulospinal
Lateral reticulospinal
Vestibulospinal
Tectospinal
Descending (Motor) Tracts



The remaining tracts originate in different subcortical
motor nuclei of the brain stem
These tracts were formerly lumped together as the
extrapyramidal tracts
The current term is to label them indirect pathways or
just the names of the individual pathway
Descending (Motor) Tracts


Although the cerebellum coordinates
voluntary muscle activity, no motor
efferents descend directly from the
cerebellum to the spinal cord
The cerebellum influences motor activity
by acting through relays on the motor
cortex
Spinal Cord Trauma

Damage to the spinal cord is associated
with some form of loss of function
–
–
–
–

Paralysis / loss of function
Paresthesis / sensory loss
Flaccid paralysis / motor loss
Spastic paralysis / upper motor neuron loss
Body regions below lesion
– Quadriplegia / spinal cord injury - 4 limbs
– Paraplegia / spinal cord injury - 2 limbs
– Hemiplegia / brain injury - one side of body
Developmental Aspects of CNS
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Fetal alcohol syndrome
Cerebral palsy
Anencephaly (without brain)
Spina bifida (forked spine)