Chapter 14
The Spinal Cord and Spinal Nerves
Spinal Cord
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Begins at foramen magnum extends down to L1/L2.
Contiguous with the medulla of the brain stem.
Passes through the vertebral foramen of all vertebrae.
Consists of cervical, thoracic and lumbar divisions
same as vertebral column.
PNS - 31 paired spinal nerves one at each level C1-C8,
T1-T12 and L1-L5, S1-S5 and Co-1.
Contains gray and white matter, however gray matter
is central and white matter surrounds gray matter.
At L1/L2 level cona medullaris tapers into cauda
equina
Spinal cord levels:
C1-C8
T1-T12
L1-L5
Cauda eqiuna
S1-S5
The Meninges
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2.
3.
Consists of 3 layers around the brain and spinal cord
Dura mater- tough outer fibrous connective tissue
layer; two layers periosteal (periosteum) and
meningeal.
Arachnoid mater- inner layer attached to dura mater
- subarachnoid space between arachnoid and pia
mater filled with CSF.
Pia mater- thin connective tissue layer closely
adhered to brain surface and spinal cord. Pia mater
goes deep into the sulci (grooves) of the brain.
Vertebral column (x-section)
Spinal cord (x-section)
Gray matter of spinal cord
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Shaped like a “butterfly” with central canal.
Wings contain anterior, and posterior horns.
Thoracic and Lumbar levels also have lateral horns.
Contains neuron cell bodies, unmyelinated axons,
dendrites and neuroglia.
Gray commissure has fibers that communicate
between right and left halves of spinal cord.
Gray matter of spinal cord
White matter of spinal cord
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Contains myelinated ascending and descending spinal
tracts.
Divided into columns or funiculi; each column
consists of tracts or fasciculi.
Also has white commissural tracts that cross thru cord
Ascending tracts carry sensory information from body
to brain
Descending tracts carry motor information from brain
to motor nerves
Tracts named according to where they originate and
terminate e.g. Lateral spinothalamic tract
Tracts in white matter of spinal cord
Ascending spinal tracts
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Transmit sensory information from peripheral body.
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Receptors relay information to spinal cord via sensory
nerve in dorsal root and may synapse in dorsal horn
of gray matter, decussate or ascend to a higher level.
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Tracts are named according to their origin and
destination, e.g. Lateral spinothalamic tracts; anterior
spinothalamic; posterior spinalcerebellar tract, etc.
Ascending tracts
Three neuron chain:
First-order:
Receptor to afferent
nerve into spinal cord.
Second order:
Interneuron to relay
nucleus in thalamus.
Third order:
Thalamus projects
information to cortex.
Descending spinal tracts
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Take information from brain and descend in gray
columns to appropriate level to a lower motor neuron.
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Motor innervation is contralateral i.e. the left side of
the brain controls the right side of the body and vice
versa. Cross over at pyramidal decussation in medulla.
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Tracts are named according to their origin and
destination, e.g. Lateral corticospinal tract; anterior
corticospinal tract; reticulospinal tract, etc.
Descending tracts
Two neuron chain:
Upper motor neuron:
From cortex or brain
stem to LMN
Lower motor neuron:
From spinal cord to
muscle or target organ
Ascending tracts
Three neuron chain:
First-order:
Receptor to afferent
nerve into spinal cord.
Second order:
Interneuron to relay
nucleus in thalamus.
Third order:
Thalamus projects
information to cortex.
Nerve structure
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Nerves are only in the periphery
Cable-like organs in PNS = cranial and spinal nerves
Consists of 100’s to 100,000’s of myelinated and
unmyelinated axons (nerve fibers).
Endoneurium surrounds each axon (nerve fiber).
Axons are grouped into bundles of fascicles
Perineurium surrounds each fascicle
Epineurium surrounds each nerve bundle
Conduction is saltatory (i.e. jumps node to node) in
myelinated nerves and continuous in nonmyelinated.
Nerve anatomy
Spinal Nerves
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There are 31 pairs of spinal nerves. Spinal nerves are
all mixed nerves (i.e. sensory and motor). Each forms by
combining the dorsal and ventral roots at a segment
of the spinal cord.
As spinal nerves exit intervertebral foramen they
branch into a meningeal nerve and a dorsal and
ventral ramus.
The meningeal branch re-enters the spinal column to
supply the meninges, spinal ligaments and vertebrae.
After leaving the spinal cord, the ventral ramus of
select nerves combine to form nerve plexuses that
innervate the appendages and other parts of the body.
Dorsal and ventral ramus nerves
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Dorsal ramus nerves innervate the ligaments, muscles
and skin of the back.
Ventral ramus nerves innervates the anterior and
lateral muscles, skin and ligaments of the trunk. They
also give rise to the nerve plexuses that innervate the
appendages.
Thoracic level
-Dorsal ramus
-Ventral ramus
-Dorsal root
ganglion
-Sympathetic
chain ganglion
-white and gray
rami communicantes
Spinal nerve plexuses from ventral ramus
- major nerves off of them.
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Plexus= network of nerves
Cervical plexus – forms from C1 – C4
-Phrenic nerve is off of cervical plexus C3-C5
Brachial plexus – forms from C5 – C8, and T1
-Axillary, radial, median, ulnar nerves
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Lumbar plexus – forms from L1 - L4
-Femoral and obturator nerves
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Sacral plexus – forms from L4 – S4
-Sciatic nerve
Spinal nerve
plexuses
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Cervical plexus
-C1- C5
Phrenic nerve
-C3- C5
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Brachial plexus
-C5-C8 & T1
Axillary nerve
- C5 & C6
Median nerve
- C5-C8 & T1
Radial nerve
- C5-C8 & T1
Ulnar nerve
- C8 & T1
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Lumbar plexus
L1- L4
Femoral nerve
-L2-L4
Obturator nerve
-L2-L4
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Sacral plexus
-L4- S4
Sciatic nerve
-Tibial n. L4-S3
-Comn fibular
-L4-S2
DERMATOMES
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The area of skin that carries sensory information to
the CNS from all of the spinal nerves (except C1) and
cranial nerve V (facial).
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Entire body surface is supplied by a peripheral nerve.
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Knowing the distribution of these nerves is very
helpful diagnostically in determining whether a nerve
is affected at the central or peripheral level and at
what level the injury may be.
Reflex Arc
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Consists of 1. sensory receptor, 2. afferent nerve, 3.
interneuron, 4. efferent nerve and 5. an effector organ
(muscle or gland).
Occurs at the spinal level
Mono and Polysynaptic Reflexes
Receptors: Classification by location
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2.
3.
Exteroceptors- receptors sensitive to changes
outside of body. Are found on or near the body
surface: pain, pressure, touch, temperature and
special sense organs (eye, ear, nose, mouth).
Interoceptors- visceroceptors receive stimuli from
viscera; stretch, temperature, chemical, taste.
Proprioceptors- located in skeletal muscles and
joints, tendons, ligaments; perceive stretch in these
organs
Proprioceptors
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Encapsulated
receptors that monitor
stretch in muscles and
tendons.
Ex. Muscle spindles
Extrafusal fibers
Intrafusal
Classification by Stimulus
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Mechanoreceptors- respond to mechanical forces:
touch, pressure, stretch, vibration, and itch.
Thermoreceptors- respond to temperature changes
Chemoreceptors- respond to chemicals in solution
and blood chemistry.
Photoreceptors- respond to changes in light-eye
Nociceptors- respond to pain and harmful stimuli
leading to pain.
Classification by Structure
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Free nerve endings- in all tissues of body; abundant
in epithelial and connective tissue.
- respond to pain, temperature, itch and light pressure
Ex.- Merckels discs, hair follicle receptors
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Encapsulated nerve endings- enclosed in a capsule
of connective tissue; vary in shape and distribution.
Ex.- Meissner’s, Pacinian and Ruffini’s corpuscles
Reflex Arcs
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May be unilateral or contralateral
Unilateral knee jerk: Flexors contract extensor relax
Reflex Arcs
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Unilateral: Excess tension on tendon causes inhibition
of flexor and stimulation of extensor
Crossed-Extensor reflex arc
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Pain to one foot causes retraction of injured foot and
extension of opposite foot
PNS to CNS