07b
Spinal cord and simple reflex arc
Chapter 13
Patterns in the NS as a whole are exemplified in
a basic/fundamental way at spinal cord level
• Anatomy:
– Aggregations of cell bodies and associated dendrites
= _________ matter (where synapses happen)
– Bundles of myelinated axons = _________ matter
(where action potentials are sent along axons)
• Physiology:
– Demonstrates synaptic relay of impulses along a
pathway, and potential patterns of breakdown
• Simple reflex arc at level of spinal cord:
– Sensory neuron  interneuron in spinal cord 
motor neuron
– Exemplifies nature of other reflexes (e.g. midbrain
reflexes), and their potential patterns of breakdown
Spinal Cord
(protected by meninges, and encased in
bones of spinal column)
Spinal Nerves
(emerge laterally from spinal cord and exit
the spinal column)
• Spinal cord covered by three layers of meninges
•
•
(same as brain!)
– pia mater (P)
– (subarachnoid space filled with CSF)
– arachnoid mater (A)
– dura mater (D)
CSF in
– Subarachnoid space
– Central canal, continuous caudally with 4th
ventricle (see Fig 3-9 in book)
Note on illustration:
– Spinal nerves emerge laterally from each
spinal cord segment, pass through
meninges, then exit spinal column between
vertebrae
– Many spinal nerves descend in subarachnoid
space before exit from spinal column, e.g.
cauda equina
Notice the paired lateral exit of the nerves through
the meninges?
Nerves are bundles of white matter (axons) in the
PNS that have a common function
Do you see how :
--the more rostral nerves exit
laterally without descending;
--the more caudal nerves
descend within the
subarachnoid space before
exiting through the arachnoid
mater and dura mater?
Spinal cord
• Location: Immediately
•
•
caudal to brain stem
Five regions (from rostral
to caudal); each region
divided into segments
– Cervical (8)
– Thoracic (12)
– Lumbar (5)
– Sacral (5)
– Coccygeal (1)
Each of the 31 segments
is associated with one
spinal nerve pair.
– How many spinal
nerve pairs are there?
– How many segments
are depicted here?
Spinal Cord (cont.)
Brain
• Transition between spinal cord
•
•
•
•
and medulla found at level of
foramen magnum
Spinal cord ends at coccygeal
segment, also called conus
medularis
Note: The cauda equina
(horse’s tail) is caudal to spinal
cord. It is composed of spinal
nerves, and is not part of
spinal cord.
Spinal cord diameter: 1 cm
Spinal cord encased in bones
called vertebrae (which make
up the spinal column)
Spinal cord
More details on the spinal cord
segments
• Each of the 31 segments is named for
an associated section of the spinal
column (composed of vertebrae) that
surround the spinal cord
–
–
–
–
–
8 cervical segment *
12 thoracic segments
5 lumbar segments
5 sacral segments
1 coccygeal segment
• Spinal cord does not extend complete
length of vertebral column
– At lower border of 1st lumbar vertebra in
adults
– At upper border of 3rd lumbar vertebra in
children
* There are 8 cervical nerves,
associated with the 7 cervical vertebrae.
More details on relationship
between spinal cord
segments, spinal nerves, and
vertebrae
Occipital bone is one of the bones that make up
the cranium (skull)
• Each spinal cord segment
•
•
associated with paired spinal
nerves of same name
– E.g. 1st thoracic cord
segment associated with
two, paired 1st thoracic
nerves
Spinal nerves exit the vertebral
column laterally, through
intervetebral foramina (holes
b/w vertebrae)
Exit point near (usually below)
the vertebra of the same
name.
– E.g. 1st thoracic nerves exit
below vertebra T1
Note: Two nerves (paired)
associated with each
segment. Only one
member of each pair is
depicted here. Nerve exits
laterally from spinal
column.
(anterior)
(left lateral view)
(posterior)
Summary of relationship between
spinal cord, spinal nerves, and
spinal column
• Each of the 31 spinal cord segments is
•
•
associated with a pair of spinal nerves
Most spinal nerves descend within the spinal
column (in the subarachnoid space) before
exiting the spinal column. This descent
becomes increasingly pronounced the more
caudal the cord segment.
Each spinal cord pair exits the spinal column
laterally, near the vertebra of the same name,
through intervertebral foramina.
Closer look at a
spinal cord segment
Horizontal cross-section of one spinal cord
segment: What’s in the middle?
• H-shaped gray matter in center of
each spinal segment
– Made up of neuron
• Bodies/somata
• Dendrites
• (unmyelinated portions of
axons)
– Synapses occur in the four horns
• dorsal/posterior horns mediate
sensory input (pre-synaptic sensory
neurons synapse here)
• ventral/anterior horns of gray
matter mediate motor output (postsynaptic motor neurons receive
their synaptic input here)
Antero-lateral view
Dorsal/posterior
Ventral/anterior
Transverse/
horizontal
section
Look at the lumbar
and cervical
enlargements of
spinal cord
• Reason for this
enlargement?
– Lots of gray matter
synapses for afferent
and efferent
pathways associated
with arms and legs
occur here
Horizontal cross-section of one spinal cord
segment: What surrounds the middle?
Antero-lateral view
• Peripheral portions of spinal cord are
white matter
– Bundles/fasciculi of myelinated
and unmyelinated axons
– Composed of columns of axons
ascending and descending along
the length of the neuraxis (to and
from brain)
• Dorsal or posterior column
• Ventral or anterior column
• Lateral columns
(continued….)
Picture of dorsal
and lateral
columns
Dorsal/posterior
Ventral/anterior
Transverse/
horizontal
section
Horizontal cross-section of one spinal cord
segment: What surrounds the middle? (cont.)
• Peripheral portions of spinal cord are made of white matter
– Organized in dorsal, ventral and lateral columns
– Bundles/fasciculi of myelinated axons make up the columns.
– Tracts are CNS fasciculi with common functions.
• Ascending/afferent to brain (bilateral, paired tracts!)
• Descending/efferent from brain (bilateral, paired tracts!)
– What tracts do you see?
• Name reflects origin +
•
•
destination (see boxes)
Tracts run the length of
the spinal cord
You see them here in
horizontal cross-section
Note: Tracts are paired,
bilaterally, but only one of
each pair is depicted here
• Example of white
matter pathway, as
it courses through
spinal cord
Horizontal cross-section of one spinal cord segment:
How does it connect with peripheral nervous system?
• Nerves are bundles of ______________
• Spinal nerves (bilateral, two for each
segment) are mixed (both sensory and
motor), formed by:
– Ventral root: efferent, motor
– Dorsal root: afferent, sensory
• Dorsal root ganglia (spinal ganglia, posterior
Sometimes called
axonal process (in this
case, found in dorsal
root)
Sometimes called
dendritic process (in
this case, runs from
periphery through
spinal nerve toward
dorsal root ganglion)
root ganglia) contain __________________
of sensory neurons (pseudo-unipolar, dorsal
root ganglion neurons).
Dorsal/posterior
Ventral/anterior
(Remember: Peripheral nerves emerging
from brain stem are called ______________)
1) Where is CNS? Where is PNS?
2) Are these pairs of dorsal roots or pairs of ventral roots? Are
they sensory or motor? How do you know? What other clues
would you look for?
3) Do you see the 4 white matter fasciculi (bundles of axons) on
the spinal cord? There are two _________ columns and two
______________ columns.
Use what you
have learned
to answer
these
questions
Summary of some terms you have learned to date,
related to gray-matter/white-matter distinction
CNS
Groups/ Aggregations
/Clusters of neuronal cell
bodies and their associated
dendrites (gray matter)
Bundles of (myelinated) axons
(white matter)
Cortex (e.g. cortex of cerebrum,
cortex of cerebellum)
Fasciculi : Bundles of white matter axons
(e.g. fasciculi that make up the ventral,
dorsal, and lateral columns of the spinal
cord)
The thalamus and basal ganglia
are aggregations of __________
Nuclei of thalamus and basal
ganglia
Nuclei in brainstem
PNS
Central gray matter of spinal cord
Tract: Bundles of white matter with a
common function in CNS (e.g. lateral
corticospinal tract; white matter tracts in
cerebral subcortical white matter
networks)
(Synapses in pathways are here.)
(Axonal portions of pathways are here.)
Ganglia (e.g. dorsal root ganglia
of the sensory, pseudounipolar
neurons)
Nerves: Bundles of white matter with a
common function in PNS (e.g., spinal
nerves, cranial nerves, and their branches)
Spinal reflexes: The reflex arc
Patterns in the NS as a whole are exemplified in
a basic/fundamental way at spinal cord level
• Anatomy:
– Aggregations of cell bodies and associated dendrites
= _________ matter (where synapses happen)
– Bundles of myelinated axons = _________ matter
(where action potentials are sent along axons)
• Physiology:
– Demonstrates synaptic relay of impulses along a
pathway, and potential patterns of breakdown
• Simple reflex arc at level of spinal cord:
– Sensory neuron  interneuron in spinal cord 
motor neuron
– Exemplifies nature of other reflexes (e.g. midbrain
reflexes), and their potential patterns of breakdown
Remember that spinal nerves are mixed
• Spinal nerves are mixed (both sensory and motor),
formed by convergence of:
– dorsal root: afferent, sensory
– ventral root: efferent, motor
Illustration of how
spinal nerves are
mixed. This pattern is
also seen at the level
of the brainstem,
where many of the
cranial nerves are also
mixed.
Spinal nerves and some cranial nerves carry
afferent and efferent fibers for two systems
• Somatic nervous system:
•
– afferent (sensory) from sense organs in the skin, joints,
and skeletal muscles
– efferent (motor) to the skeletal muscles
Autonomic nervous system:
– Afferent (sensory) from the cardiac muscles, smooth
muscles , and arteries (e.g. sensation in digestive tract;
levels of carbon dioxide oxygen and sugar in the blood)
– Efferent (motor) to the cardiac and smooth muscle, and
glands (e.g., muscles of heart, iris, intestines & arteries;
sweat & hormonal glands)
Both the somatic and autonomic
nervous system have reflexes
• A reflex is an involuntary, relatively
stereotyped motor (efferent) response to a
specific sensory (afferent) stimulus
– The location of the afferent stimulus determines
in a fixed way which particular muscles will
contract
• E.g. touch hot pot with finger  biceps will contract
• E.g. muscle of quadriceps senses a stretch (from a
tendon tap)  the quadiceps will contract
• E.g. shine light into eye  muscles of iris will contract
Spinal cord reflexes
“Simple reflex arc”
• Sensory neurons
•
•
•
Example of a simple
reflex arc: Touch hot
pot and pull arm away
(automatic)
– Sensory receptors excited by heat = input signal
– Dendritic projection (somatic afferent process) integrates
signal (Cell body in dorsal root ganglion / spinal ganglion)
– Axonal projection (somatic efferent process) carries action
potential = conductile signal
Sensory neurons excite post-synaptic interneurons with
excitatory neurotransmitter = output signal of sensory neuron
(Synapse in dorsal horn)
Interneurons excite post-synaptic motor neurons (Synapse in
ventral horn)
Motor neurons synapse with muscle  Effector (muscle)
moves
Extending your clinical
understanding of a reflex:
Questions for discussion
• If a reflex is absent, do you
know where in the reflex arc
the damage is?
• If the reflex is absent, and
you want to figure out if the
sensory component of the
reflex is intact, what would
you ask the client?
• Even though reflexes are
automatic, we can suppress
them. What part of the NS is
needed for this supression?
(Clue: The part that handles
conscious action.)
• Full reflex
complex is
combo of
activation
and
inhibition
• Still all
automatic!
Activation
Inhibition
Summary points
• Reflexes are involuntary, predictable motor
•
•
•
•
responses following a specific sensory stimulus
Reflexes occur at multiple levels (spinal cord, brain
stem, cerebellum). Our example was at the level of
the spinal cord.
Reflexes occur in both somatic and autonomic
nervous systems. Our example came from the
somatic system.
Reflexes can be consciously suppressed by the
cortex.
The reflex arc is a pathway, which can break down
at any point in the pathway.
Relationship of reflex arc with
dermatomes and myotomes
A somatic NS reflex (like the one we just
discussed) involve a dermatome and its associated
myotome
• The sensory skin area and
its corresponding spinal
nerve make up a
dermatome (blue afferent
pathway in lower
illustration, with its
associated skin area)
• The muscle and its
corresponding spinal nerve
make up a myotome
(orange efferent pathway
in the lower illustration,
with its associated muscle)
• The sensory skin area and its
corresponding spinal nerve
make up a dermatome. Note
that the sensory skin areas in
dermatomes of adjacent
spinal nerves overlap.
• The muscle and its
corresponding spinal nerve
make up a myotome. Note
that myotomes of adjacent
spinal nerves overlap.
Clinical applications of lecture 8b: Spinal cord, spinal nerves,
and NS reflexes
• Acetylcholine
–Major chemical messenger of the PNS
•Regulation of voluntary or reflexive
motor movement, & autonomic functions
•Reduced myoneural activity
–Myasthenia gravis
• Muscles for Respiration
–Patterned cycle of inhalation/exhalation
–Clinical Correlates:
•Lesion above C4: complete paralysis of
respirators
–Fatal if respiration is not immediately restored
•Lesion below C4
–Failure of forced exhalation
–Preserved inhalation (functioning of the
diaphragm)
• Syringomyelia
–Cyst or cavity within central spinal
region
•Bilateral loss of pain & temperature
sensations
•Bilateral paralysis in selected
muscles
–Lesion extension to motor nuclei
Clinical applications
• Damage in anywhere
in dermatome will
affect sensation
• Damage anywhere in
myotome will affect
movement
Clinical applications
• Damage of a spinal nerve will affect both the
sensory and motor functions associated with that
nerve Remember that spinal nerves are mixed.
Some examples important for communication:
– 3rd, 4th, and 5th cervical nerves (phrenic nerves)
innervate the breathing muscles, especially the
diaphragm, and (some) sensation from shoulders
and arm
– C8 involved with bending fingers, and also some
sensation in hand
Clinical applications
• Remember that white matter pathways run
•
up and down the spinal cord, some
sensory (ascending), some motor
(descending)
Damage to a spinal cord segment may produce partial or
complete loss of sensory or motor functions associated
with that segment and functions of all segments below the
damaged segment
• C3,4 and 5 supply the diaphragm
• C5 also supplies the shoulder muscles and the
•
•
•
•
muscle that we use to bend our elbow.
C6 is for bending the wrist back.
C7 is for straightening the elbow.
C8 bends the fingers.
T1 spreads the fingers.
Clinical application: Reflexes
(For the curious)
• Normal processes
– Some reflexes do not develop until after birth (e.g. segmental rolling
reflex appears by 6 months)
– Some reflex patterns are present at birth and then disappear (e.g.
rooting reflex appears at birth and disappears b/w 3 and 6 mos.)
– Some reflexes are present for a full lifetime (e.g. gag; swallow)
• Pathological processes
– Normal reflex patterns that do not appear on schedule (e.g. segmental
rolling reflex does not appear by 6 months)
– Reflex pattern that persists beyond the age at which it normally
disappears (e.g. rooting reflex continues past 6 months)
– Childhood reflexes that re-appear in adult following neurological
damage to cortex (e.g., rooting reflex re-appears in adult with TBI)
Indicates poor cortical inhibition of reflexes
– Absence of lifetime reflexes (e.g. gag reflex, swallowing reflex)
indicates damage somewhere in the reflex arc (afferent, central, or
efferent parts)