The Spinal Cord

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The Spinal Cord

• Basic Neuroscience
• James H. Baños, Ph.D.

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Grey and White Matter
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Grey and White Matter
Grey Matter Cell Body
White Matter Myelinated axon
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Grey and White Matter

• Grey matter
• Cortex
• Nucleus (CNS)
• Ganglion (PNS) Exception Basal Ganglia

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Grey and White Matter

• White Matter
• Nerve (PNS)
• Tract (CNS)
• Fasciculus/Funiculus -- Group of fibers with
  common origin and destination
• Lemniscus -- Ribbon-like fiber tract
• Peduncle -- Massive group of fibers -- usually
  several tracts

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Grey and White Matter

• Tracts are named with origin first, then
  destination
• Corticospinal tract -- cortex to spinal cord
• Mammilothalamic tract -- Mammilary bodies to
  thalamus
• Spinocerebellar tract -- Spinal cord to
  cerebellum
• Corticobulbar tract -- Cortex to brain stem

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The Spinal Cord
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General Organization

• Spinal cord is SMALL!
• 42-45 cm long
• 1 CM wide at widest point
• Does not extend all the way to the bottom of the
  spinal column
• Pattern of grey/white matter is reversed in the
  cord
• White matter tracts on outside
• Grey matter on the inside
• Staining reverses this!!!

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General Organization
White matter (tracts of axons)
Grey Matter (cell bodies)
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General Organization

• Spinal cord is segmented anatomically
• Input and output occurs in groups of rootlets
  arranged in a series longitudinally along the
  cord
• Dorsal rootlets -- Input -- carry sensory
  information
• Ventral rootlets -- Output -- motor neurons

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General Organization

• Each set of rootlets forms a spinal nerve that
  innervates a corresponding segment of the body,
  called a dermatome

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General Organization
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General Organization

• There are 31 segments in the spinal cord
• 8 cervical (C1 - C8)
• 12 Thoracic (T1 - T12)
• 5 Lumbar (L1 - L5)
• 5 Sacral (S1 - S5)
• 1 Coccygeal

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General Organization

• The spinal cord is housed within the vertebral
  column

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General Organization

• Each cord segment has a corresponding vertebra of
  the same name (e.g., C3)
• Spinal nerves enter/exit underneath their
  corresponding vertebral segment

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General Organization

• But wait! Something doesnt add up!
• How can spinal nerves exit below their
  corresponding vertebral segment if the cord is
  only 42cm-45cm long?
• Answer Spinal nerves extend down to the
  appropriate vertebral segment forming the cauda
  equina
• This means cord segments and vertebral segments
  dont line up

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General Organization
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General Organization

• Cord is not of uniform thickness throughout its
  length. Why not?

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General Organization

• Cord is not of uniform thickness throughout its
  length. Why not?
• Answer
• Segments of the cord innervate parts of the body
  that differ in complexity
• There are fewer white matter tracts lower in the
  cord.

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General Organization
Cervical enlargement C5 - T1
Lumbar enlargement L2 - S3
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The Spinal Cord in Cross Section
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Cord Sections

• Segments of the spinal cord have a similar
  organization, but vary in appearance.
• Always know where you are in the cord (i.e.,
  cervical, thoracic, lumbar, sacral)

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Cord Sections -- Cervical

• Cervical cord is wide, flat, almost oval in
  appearance. Why?

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Cord Sections -- Cervical Enlargement

• Whats different about the cervical enlargement .
  Why?

Cervical
Cervical Enlargement
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Cord Section -- Thoracic

• Less White matter than cervical
• Rounder appearance
• Less prominent ventral horns than cervical
  enlargement

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Cord Section -- Lumbar

• Less White matter than thoracic
• Rounder appearance
• Larger ventral horns, especially in lumbar
  enlargement

Lumbar
Lumbar Enlargement
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Cord Section -- Sacral

• Not much white matter
• Mostly grey, although not much of that either

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Cross Sectional Organization
Posterior intermediate sulcus
Posterior median sulcus
Tract of Lissauer
Anterior white commissure
Anterior median fissure
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Grey Matter

• Laminar
• Laminae of Rexed

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Grey Matter

• Posterior (dorsal) Horn
• Intermediate Grey
• Anterior (ventral) Horn

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Grey Matter Posterior Horn

• Mostly Interneurons
• Substantia gelatinosa
• Pain/temp proc
• Body of the posterior horn
• Sensory proc

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Grey Matter Intermediate Grey

• Clarkes Column
• T1-L3
• Balance/proprio.
• Intermediolateral Column
• T1-L3
• Sympathetic neurons

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Grey Matter Anterior Horn

• Lower Motor Neurons

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White Matter The Big Four Pathways
Corticospinal tract
Dorsal Columns
Spinothalamic tract
Spinocerebellar tracts
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The Big Four

• Corticospinal tract
• Voluntary motor
• Dorsal columns/ medial lemniscus
• Discriminative touch
• Conscious proprioception
• Spinocerebellar tract (dorsal and ventral)
• Unconscious proprioception
• Spinothalamic tract
• Pain/temperature

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Corticospinal Tract

• Voluntary Motor

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Corticospinal Tract

• First order neuron (upper motor neuron)
  originates in precentral gyrus
• Passes through internal capsule
• 90 decussates in caudal medulla
• Lateral corticospinal tract
• 10 undecussated
• Anterior corticospinal tract
• Synapses on second order neuron (lower motor
  neuron) in ventral gray of the cord
• Second order neuron innervates muscle

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Motor Homunculus
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Motor Homonculus
HAL
Arms
Legs
Head
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Corticospinal Tract
Spinal Cord
Medulla
Pons
Midbrain
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Corticospinal Tract
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Upper Lower Motor Neurons

• Upper Motor Neuron
• Motor Cortex to Ventral Grey Horn
• Modulatory influence on stretch reflex arc
• Lower Motor Neuron
• Ventral Grey Horn to Neuromuscular Junction
• Efferent of stretch reflex arc
• Helps maintain tone
• Sensory Neuron
• Stretch receptors in muscle and tendons
• Helps maintain tone
• Afferent of basic stretch reflex arc

Motor Ctx
UMN
Ventral Grey Horn
LMN
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Upper Lower Motor Neurons

• Maintenance of Tone
• Input from stretch receptors causes lower motor
  neuron to supply tonic stimulation to the muscle
• The upper motor neuron modulates this -- will
  tend to override the tonic signal from the
  sensory neuron

UMN
LMN
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Upper Lower Motor Neurons

• Reflex Arc
• Afferent is sensory neuron detecting a sudden
  stretch
• Signal is strong and results in a strong response
  by the lower motor neuron
• Strong signal usually overcomes mild cortical
  input from the UMN

UMN
LMN
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Upper Lower Motor Neurons

• Upper Motor Neuron Signs
• Spastic paresis
• Hypertonia
• Hyperreflexia
• No muscle atrophy (until perhaps late in the
  course)
• Positive Babinski
• Why?
• Loss of voluntary UMN signal
• Loss of modulation of tone and reflexes by UMN --
  the circuit runs unchecked

Motor Ctx
UMN
Ventral Grey Horn
LMN
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Upper Lower Motor Neurons

• Lower Motor Neuron Signs
• Flaccid paresis/paralysis
• Muscle fasciculations
• Hypotonia
• Hyporeflexia
• Muscle atrophy
• Negative Babinski
• Why?
• Loss of LMN for voluntary movement
• Loss of efferent component of reflex arc and tone
  pathway

Motor Ctx
UMN
Ventral Grey Horn
LMN
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Babinskis Sign

• In response to stimulation of the sole of the
  foot, the toes will usually curl downward.
• When UMN inhibition is removed, the toes will
  curl upward (Dorsiflexion). This is referred to
  as a positive Babinski or presence of Babinskis
  sign.

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Related Terms

• Spasticity -- Increased muscle tone and increased
  reflex contraction (UMN)
• Clonus -- Rythmic contractions and relaxations
  seen when a spastic muscle is stretched (UMN)

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Basics of Localization

• If all limbs are checked for upper and lower
  motor neuron signs, you can begin to localize
  lesions
• Left-right differences are also very important

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Dorsal Column/ Medial Lemniscus

• Discriminative Touch
• Conscious Proprioception

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Dorsal Columns/Medial Lemniscus

• First order neuron begins in receptor
• Enters cord at tract of Lissauer
• Legs run in fasciculus gracilis (medial dorsal)
• Arms run in fasciculus cuneatus (lateral dorsal)
• Synapse on nucleus gracilis and nucleus cuneatus
  (caudal medulla)
• 2nd order neuron decussates and runs from NG NC
  to thalamus (as medial lemniscus)
• 3rd order neuron runs from thalamus to
  postcentral gyrus

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Dorsal Columns/Medial Lemniscus
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Spinocerebellar Tracts

• Unconscious Proprioception

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Dorsal (Posterior) Spinocerebellar Tract

• Involves Clarks Column, a longitudinal gray
  matter body from about T1 to L3
• Below Clarks Column
• Runs with f. gracilis, synapses in Clarks
  Column, joins dorsal spinocerebellar tract
• Level of Clarks Column
• Synapses in Clarks Column, joins dorsal
  spinocerebellar tract
• Above Clarks Column
• Runs with f. cuneatus, synapses in lateral
  cuneate nucleus (caudal medulla), projects to
  ipsilateral cerebellum

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Dorsal (Posterior) Spinocerebellar Tract
L3
T1
Spinal Cord
Medulla
Pons
Midbrain
To Cerebellum
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Ventral (Anterior) Spinocerebellar Tract

• Supplements Dorsal Spinocerebellar Tract
• Information from more diverse array or receptors
• Originates from scattered cells in the
  intermediate grey caudal to L1 (which in turn
  have input from proprioceptive axons or their
  collaterals
• Crosses twice, to end up in ipsilateral
  cerebellum

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Ventral (Anterior) Spinocerebellar Tract
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Spinothalamic Tract

• Pain and Temperature

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Spinothalamic Tract

• First order neurons originate in pain receptors,
  enter cord at tract of Lissauer, and synapse in
  substantia gelatinosa or nucleus proprius
• Second order neurons cross at the anterior white
  commissure, rising 1 or 2 cord levels in the
  process, and form contralateral spinothalamic
  tract
• A third order neuron (not technically
  spinothalamic tract) projects to the cortex

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Spinothalamic Tract
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Spinothalamic Tract
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L1
L1
L2
L2
L3
L3
L4
L4
L5
L5
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