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Central Nervous System
Part II
Chapter 13
Protection to the Brain
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Nervous tissue is soft and delicate, and
the irreplaceable neurons can be injured
or destroyed by even slight pressure
The brain is protected from injury by…
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The skull
Surrounding membranes called meninges
A watery cushion of cerebrospinal fluid
The blood-brain barrier
Protection to the Brain
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The skull is a self-bracing arrangement
of bones that encapsulates the brain
It was presented in Chapter 7 and thus
will receive only passing reference in this
section
Meninges
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The meninges are three connective tissue
membranes that lie just external to the
brain and spinal cord
Meninges
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The meningeal membranes
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Cover and protect the CNS structures
Protect blood vessels and enclose venous sinuses
Contain cerebrospinal fluid
Form partitions within the skull
Meninges
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From external to internal, the meningeal layers
are
– Dura mater
– Arachnoid
– Pia mater
The Dura Mater
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The leathery dura mater is by far the strongest
of the meninges
Where it surrounds the brain it is a double
layer membrane
The Dura Mater
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The periosteal layer is the superficial and lines
the inner surface (periostium) of the skull
The deeper meningeal layer forms the true
external covering of the brain
The Dura Mater
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The brain’s dural layers are fused together except
in certain areas where they enclose the blood
filled dural sinuses
The dural sinuses collect venous blood and direct
it into the internal jugular veins of the neck
The Dura Mater
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In several places the meningeal dura mater
extends inward to form flat septa (partitions) that
limit movement of the brain within the skull
The Dura Mater
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The falx cerebri dips into the longitudinal fissure
It attaches to the crista galli of the ethmoid bone
The Dura Mater
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The falx cerebelli forms a midline partition that
runs along the vermis of the cerebellum
The Dura Mater
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The tentorium cerebelli extends into the transverse
fissure between the cerebral hemispheres and the
cerebellum
The Arachnoid Mater
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The middle membrane forms a loose brain
covering over the surface of the cerebrum
It is separated from the dura mater by a
narrow serous cavity, the subdural space
Beneath the arachnoid membrane is the wide
subarachnoid space
The Arachnoid Mater
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The subarachnoid space is filled with
cerebrospinal fluid and contains the largest
blood vessels serving the brain
Since the arachnoid is fine and elastic, these
blood vessels are rather poorly protected
The Arachnoid Mater
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Arachnoid villi protrude through the overlying
dura mater and into the dural sinuses overlying
the superior aspect of the brain
Cerebrospinal fluid is absorbed into the venous
blood sinuses through these valvelike villi
The Pia Mater
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The pia mater is a delicate connective tissue
that is richly invested with tiny blood vessels
It is the only membrane that clings tightly to
the brain, following its every convolution
The Pia Mater
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Meningitis is an inflammation of the meningeal
layers that is caused by either a bacterial or
viral infection that can spread to the
underlying nerve tissue
Brain inflammation is called encephalitis
Cerebrospinal Fluid (CSF)
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CSF is a watery “ broth”found in and
around the brain and spinal cord
It forms a liquid cushion that gives
buoyancy to the CNS organs
With the brain floating, CSF reduces brain
weight by 97% and thus prevents the brain
from crushing under its own weight
CSF also protects the brain and spinal cord
from trauma
Cerebrospinal Fluid CSF
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CSF also helps to nourish the brain
It also helps to remove wastes produced
by neurons
Finally, it carries chemical signals
between different parts of the CNS
Although it performs many functions
there is 100-160 ml of fluid (about a half
cup) present in the body at any one time
Cerebrospinal Fluid (CSF)
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CSF is a similar in composition to blood
plasma, from which it arises
It contains less protein and more sodium
and chloride ions
Cerebrospinal Fluid (CSF)
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The figure at the
right depicts the
sites of CSF
production and its
circulation
Most CSF is made
in the choroid
plexuses which are
membranes on the
roofs of the four
brain ventricles
Choroid Plexus
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Choroid plexus
hang from the
roof of each
ventricle
The plexuses are
clusters of thin
walled capillaries
enclosed by a
layer of
ependymal cells
Choroid Plexus
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The capillaries of
the choroid
plexus are fairly
permeable and
fluid filters
continuously
from the
bloodstream into
the ventricles
Choroid Plexus
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The choroid
plexus cells are
joined by tight
junctions and
have ion pumps
that allow them to
modify this
filtrate by actively
transporting only
certain ions across
their membranes
into the CSF pool
Choroid Plexus
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After entering the ventricles, the CSF
moves freely through these chambers
Some CSF enters the central canal of the
spinal cord, but most enters the
subarachnoid space through the lateral
and median apertures in the walls of the
fourth ventricle
In the subarachnoid space, the CSF
bathes the outer surface of the brain and
cord
The Choroid Plexus
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Cerebrospinal fluid arises from the blood
and returns to it at a rate of about 500 ml
a day
The choroid plexus also helps to cleanse
the CSF by removing waste products and
other unnecessary solutes
CSF Circulation
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The motion of the CSF is aided by the long microvilli
of the ependymal cells lining the ventricles
Blood-Brain Barrier
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The brain has a rich supply of capillaries
that provide its nervous tissues with
nutrients, oxygen, and all other vital
molecules
However, some blood-borne molecules
that can cross other capillaries of the
body cannot cross the brain capillaries
Blood-Brain Barrier
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Blood-borne toxins, such are urea, mild
toxins from food, bacterial toxins, are
prevented from entering brain tissue by
the blood-brain barrier
The barrier is a protective mechanism
that helps maintain a stable internal
environment for the brain
Blood-Brain Barrier
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The brain is very dependent on a constant
internal environment
Fluctuations in the concentration of ions,
hormones, or amino acids, would alter the
brain’s function
– Hormones and amino acids can influence
neurotransmitters
– Ions (K+) can affect neuron thresholds
Blood-Brain Barrier
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Blood-borne substances within the brain’s
capillaries are separated from the extracellular space and neurons by
– Continuous endothelium of the capillary walls
– Relatively thick basal lamina surrounding the
external face of the capillary
– To a limited extend the “feet” of the astrocytes
that cling to the capillaries
Blood-Brain Barrier
Basal lamina (cut)
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The capillary endothelial cells are joined almost
seamlessly by tight junctions
They are the least permeable capillaries in the body
The relative impermeability of brain capillaries
accounts for most of the blood brain barrier
Blood-Brain Barrier
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The blood-brain barrier is a selective,
rather than absolute barrier
Nutrients, such as glucose, essential
amino acids, and some electrolytes, move
passively by facilitated diffusion through
the endothelial cell membranes
Blood-Brain Barrier
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The barrier is ineffective against fats,
fatty acids, oxygen, and carbon dioxide,
and other fat-soluble molecules that
diffuse easily through all plasma
membranes
This explains why blood-borne alcohol,
nicotine, and anesthetics can affect the
brain
The barrier is not completely uniform
and not completely developed in infants
The Spinal Cord
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The spinal cord runs
through the vertebral canal
of the vertebral column
from the foramen magnum
superiorly to the level of
vertebra L1 or L2 inferiorly
The Spinal Cord
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The functions of the spinal cord include:
– Through the nerves that attach to it, the
spinal cord is involved in the sensory and
motor innervation of the entire body inferior
to the head
– It provides a two-way conduction pathway
for signals between the body and the brain
– It is a major center for reflexes
The Spinal Cord
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The spinal cord is protected by bone, cerebrospinal fluid, and meninges
– Dura mater, arachnoid, pia mater
The Spinal Cord
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Between the bony vertebrae and the spinal 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
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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
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The spinal cord does not extend the full
length of the vertebral column, ending in
the superior lumbar region
The spinal cord does not run all the way
to the coccyx because it grows slower
caudally than the spinal column
The Spinal Cord
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At 3 months after conception it extends to
the coccyx
At the time of birth it ends at L3
During childhood it attains the adult
position, terminating at the level of the
intervertebral disc between L1 and L2
But it does vary among people, ranging
from T12 to the superior margin of L3
The Spinal Cord
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The spinal cord
terminates in a tapering
cone shaped structure
called the conus
medullaris
The cone tapers into a
long filament of
connective tissue, the
filum terminale, which is
covered with pia mater
and attaches to the coccyx
inferiorly
The Spinal Cord
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There are 31 pairs of spinal nerves (PNS) that
arise from the spinal cord by paired roots and
exit from the vertebral column via the
intervertebral formina
The Spinal Cord
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Each segment of the spinal cord is
defined by a pair of spinal nerves that lie
just superior to their corresponding
vertebra
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8 cervical
12 thoracic
5 lumbar
5 sacral
1 coccygeal
The Spinal Cord
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The segments of the spinal cord all lie
superior to their corresponding vertebrae
because of the rostral shift of the spinal
cord during development
The Spinal Cord
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The spinal cord has
obvious enlargements
where the nerves serving
the upper and lower limb
arise
– Cervical enlargement
– Lumbar enlargement
The Spinal Cord
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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 SpinalCord
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The arrangement of the
cauda equina reflects the
fact that vertebral
column growth proceeds
more rapidly than does
the growth of the spinal
cord
The Spinal Cord
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The spinal cord is wider laterally than from an
anterior/posterior perspective
Two deep grooves, the posterior median sulcus
and the anterior median fissure run the length of
the cord and divide it into right and left halves
Gray Matter of the Spinal Cord
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The spinal cord consists of an outer
region of white matter and an inner
region of gray matter
As in other parts of the CNS, the gray
matter of the spinal cord consists of a
mixture of neuron cell bodies, short
unmyelinated axons and dendrites and
neuroglia
Gray Matter and Spinal Roots
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The gray matter consists of a mixture of neuron
cell bodies, their unmyelinated processes, and
neuroglia (support cells)
Gray Matter and Spinal Roots
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The white matter is composed of myelinated and
unmyelinated nerve fibers that represent
ascending, descending and transverse pathways
Gray Matter and Spinal Roots
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In cross section the gray matter is shaped like an H
The gray commissure contains the narrow central
cavity of the spinal cord, the central canal
Gray Matter and Spinal Roots
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The two posterior arms of the H are the posterior
horns, whereas the two anterior arms are the
anterior horns
Gray Matter and Spinal Roots
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In three dimensions these arms run the entire
length of the spinal cord and are called the
dorsal and ventral columns
Gray Matter and Spinal Roots
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Additionally, small lateral columns called lateral
horns are present in the thoracic and superior
lumbar segments of the spinal cord
Gray Matter and Spinal Roots
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This illustration depicts the basic organization
of the spinal gray matter
The posterior horns are almost entirely
comprised of interneurons
Gray Matter and Spinal Roots
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These interneurons receive information from
sensory neurons whose cell bodies lie outside
the spinal cord in dorsal root ganglia, and
whose axons reach the cord in dorsal roots
Gray Matter and Spinal Roots
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The anterior (and lateral horns) contain cell
bodies of motor neurons that send their axons
out of the cord in ventral roots to supply
muscles and glands
Gray Matter and Spinal Roots
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Interneurons also occur in the anterior horns,
but they are not emphasized in this picture
The size of the anterior motor horns varies
along the length of the spinal cord reflecting the
amount of skeletal musculature innervated
Gray Matter and Spinal Roots
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The anterior horns are the largest in the
cervical and lumbar regions of the cord, which
innervate the upper and lower limbs
respectively
Gray Matter and Spinal Roots
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The gray matter can be further divided
according to the innervation of the somatic and
visceral regions of the body
Gray Matter and Spinal Roots
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This scheme recognizes four zones of spinal
cord gray matter; somatic sensory (ss), visceral
sensory (vs); Visceral motor (vm), and somatic
motor (sm)
White Matter and Spinal Cord
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The white matter of the spinal cord is composed
of mylinated and unmylinated axons
White Matter and Spinal Cord
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Communication within the white matter
of the spinal cord occurs between
different parts of the spinal cord and
between the spinal cord and the bran
These fibers are of three types
– Ascending
– Descending
– Commissural
White Matter and Spinal Cord
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Ascending fibers in the spinal cord carry
sensory information from the sensory
neurons of the body to the brain
Descending fibers carry motor
instructions from the brain to the spinal
cord, to stimulate contraction of the
body’s muscles and secretion of its glands
Commissural fibers cross from one side
of the cord to the other
White Matter and Spinal Cord
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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 and Spinal Cord
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The white matter on each side of the spinal cord is
divided into three white columns, or funiculi, named
according to their positions in the cord, posterior,
anterior, and lateral funiculi
White Matter and Spinal Cord
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Posterior funiculi - also called dorsal white column
Anterior funiculi -adjacent the anterior median fissure
Lateral funiculi - adjacent the lateral horn
White Matter and Spinal Cord
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The three funiculi contain many fiber
tracts, each of which consists of axons
with similar destinations and functions
For the most part these spinal tracts are
named according to their origin and
destination
White Matter and Spinal Cord
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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
Sensory and Motor Pathways
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All major spinal
tracts are segments
of multi-neuron
pathways that
connect the brain to
the body
– Sensory information
to the brain
– Instructions to
effectors to the body
Sensory and Motor Pathways
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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) Pathways
Foot
Neuron Pathways
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Axons of sensory (1st
order) neurons enter
dorsal root of spinal
cord
Synapse with 2nd
order neurons in
medial lemniscal tract
and ascend to
Thalamus
Synapse with 3rd
order neurons which
transmit to somatosensory cortex
Ascending (Sensory) Tracts
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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) Pathways
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There are four main ascending pathways
– The dorsal column (fasciculus gracilis and
fasciculus cuneatus) and spinothalamic
(lateral and anterior) pathways transmit
sensory impulses to the primary
somatosensory cortex for interpretation
– The posterior and anterior spinocerebellar
pathways convey information on
proprioception to the cerebellum which uses
this information to coordinate body
movements
Ascending (Sensory) Tracts
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In general, sensory information is conveyed
along these main pathways on each side of
the spinal cord
– Four transmit impulses to the sensory cortex for
conscious interpretation
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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
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Posterior funiculi (dorsal white column)
– Fasciculi cuneatus
– Fasciculi gracilis
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Transmit information from the fine touch and
pressure receptors and joint proprioceptors
– These tracts comprise what is referred to as discriminative
touch and conscious proprioception
Ascending (Sensory) Tracts
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Lateral and anterior funiculi
– Lateral spinothalamic tract
– Anterior spinothalamic tract
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Convey information on pain, temperature, deep
pressure and course touch (undiscriminated)
Ascending (Sensory) Tracts
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Anterior and posterior funiculi
– Anterior spinocerebellar tract
– Posterior spinocerebellar tract
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Convey information from proprioceptors (muscle and
tendon stretch) to the cerebellum which uses this
information to coordinate skeletal muscle activity
Ascending (Sensory) Tracts
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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
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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
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The lateral (pyramdial) and anterior corticospinal
tracts are the major motor pathways concerned with
voluntary movement, particularly precise or skilled
movement
Descending (Motor) Tracts
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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
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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
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The remaining descending tracts include:
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Rubrospinal
Anterior reticulospinal
Lateral reticulospinal
Vestibulospinal
Tectospinal
Descending (Motor) Tracts
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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
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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
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Damage to the spinal cord is associated
with some form of loss of function
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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)
Embryonic Development
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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
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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
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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
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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
Gray Matter and Spinal Roots
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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
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Afferent fibers carrying impulses from peripheral
sensory receptors form the dorsal roots of the spinal
cord
Gray Matter and Spinal Roots
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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
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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
Ascending (Sensory) Pathways