Spinal Cord: Meninges
•
The spinal meninges (dura
mater, arachnoid mater, and
pia mater) are layers of
connective tissue that
protect the spinal cord and
supply it with nutrients
•
They are continuous with
the cranial meninges, which
perform the same functions
for the brain
•
Between the arachnoid
mater and the pia mater is
the subarachnoid space,
which contains cerebrospinal
fluid (CSF)
Spinal Cord: Spinal Tap
•
A spinal tap (lumbar puncture) is a
procedure in which a needle is inserted
into the arachnoid space in the lumbar
region to withdraw CSF or administer
medication
Spinal Cord: External Anatomy
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There are 31 pairs of spinal nerves (8 cervical,
12 thoracic, 5 lumbar, 5 sacral, and 1
coccygeal)
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Bundles of axons called roots connect each
nerve to the spinal cord
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The dorsal root is sensory, the ventral root is
motor
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Each dorsal root has a “swelling” composed of
cell bodies of sensory neurons, called the
dorsal root ganglion
•
Nerves to and from the upper limbs form the
cervical enlargement, nerves to and from the
lower limbs form the lumbar enlargement
•
The spinal cord tapers at its end to form the
conus medullaris
•
Lumbar, sacral, and coccygeal nerves hang
below the conus medullaris to form the cauda
equina (“horse’s tail”)
Spinal Cord: Internal Anatomy
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The white matter is
permeated by the
anterior median
fissure and the
posterior median
sulcus
•
The gray matter
forms an “H” or
butterfly, with the
central canal in the
middle of the gray
commissure
• The ventral horns contain somatic motor nuclei, whereas the dorsal horns contain somatic
and autonomic sensory nuclei
• The anterior white commissure connects the white matter on right and left sides
• The ventral and dorsal gray horns divide the white matter into the ventral white columns,
dorsal white columns, and lateral white columns
Spinal Nerve Anatomy
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Nerves are similar in
organization to muscles
•
Groups of axons are
bundled in fascicles
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The entire nerve is wrapped
in the epineurium
•
The perineurium surrounds
each fascicle
•
The endoneurium surrounds
each axon
Spinal Nerve Anatomy
Spinal Cord Tracts
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Spinal tracts are the “highways” for information traveling between the brain and the
body
•
Sensory tracts take information from sensory organs to the brain
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Motor tracts carry impulses from the brain to muscles and target organs
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Spinal cord gray matter is a site where excitatory and inhibitory impulses are summed
Dermatomes
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Dermatomes provide sensory input to the CNS
via the dorsal roots of spinal nerves, or via the
trigeminal nerve (cranial nerve V)
•
Can be used diagnostically to assess spinal
nerve damage
•
Can explain referred pain
•
Can be used to determine how to administer
anesthesia to portions of the body (e.g.
phantom pain)
•
May be a remnant of our earlier segmentation
Reflex Arcs
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The pathway followed by
a nerve impulse that
produces a reflex is a
reflex arc
•
A sensory receptor
responds to a stimulus
•
A sensory neuron conveys
the impulse to the spinal
cord
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Integration takes place in the spinal cord, conveying the impulse to a motor neuron
through a monosynaptic or polysynaptic reflex arc
•
The motor neuron conveys the impulse to the part of the body that will respond
•
The effector organ (an organ, muscle, or gland) responds to the motor impulse
•
Somatic reflexes involve skeletal muscles, autonomic reflexes involves smooth muscle
and glands
Reflex Arcs
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Reflex Arcs provide an illustration of homeostasis
Stretch Reflex
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The stretch reflex occurs in
response to stretching of muscle
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The stretch reflex is
monosynaptic and ipsilateral
•
Stretching simulates the muscle
spindle organ
•
Impulse is sent along a somatic
sensory neuron to the dorsal
horn
•
In the spinal cord the sensory
neuron synapses with a motor
neuron in the ventral horn
•
An excitatory motor impulse is conveyed along the motor neuron to the muscle, which
contracts
•
The sensory neuron also synapses with another motor neuron
•
An inhibitory motor impulse is sent to the antagonistic muscle
Stretch Reflex
•
The muscle spindle organ
“notifies” the spinal cord if
it is being stretched
Crossed Extensor Reflex
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The crossed extensor reflex is
polysynaptic and contralateral
•
The crossed extensor reflex is an
intersegmental reflex
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Painful stimulus to right foot sends
impulse along sensory neuron
•
In the spinal cord the sensory
neuron synapses with several
neurons on the same side and on the
opposite side
•
Motor neurons on the same side
cause flexion of the limb
•
Motor neurons on the opposite side cause extension of the opposite limb
•
Inhibitory motor neurons cause inhibition of necessary antagonistic muscles on both
sides (not shown)
Movements of the Lower Leg
Involuntary movement
Voluntary movement