SPINAL CORD AND SOMATOSENSORY SYSTEMS

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SPINAL CORD AND SOMATOSENSORY SYSTEMS
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Case Study 1

• A 62 year-old, right-handed man was brought to
  the hospital by his wife. She had noted weakness
  of his leg and hand he had dropped things held
  in his left hand and had fallen in the bathroom.
  The man denied that he had any problems. Sensory
  examination disclosed relatively normal responses
  to pin prick and light tactile stimulation of the
  left hand.

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Case Study 1 (contd)

• Prioprioception for the left hand was absent for
  small movements of the fingers. The patient had
  agraphesthesia on the left hand, face, or leg. He
  had left hand astereognosis. Double simultaneous
  stimulation of right and left hands was perceived
  only on the right (extinction). Motor examination
  revealed left hemiparesis.

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Case Study 1 (contd)

• In bed, the patient failed to cover his left side
  wih either a robe or a sheet. When his left arm
  was brought across the midline into his right
  visual field, the patient denied that it was his
  own hand and arm (neglect).

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Case Study 1 (Question)

• The patient probably has a
• A. Peripheral neuropathy
• B. Lesion of the dorsal roots
• C. Lesion of the dorsal columns
• D. Lesion restricted to the primary motor and
  primary sensory (SI) cortex
• E. Lesion involving the posterior parietal
  association cortex

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Dermatome Skin served by sensory spinal or
cranial nerve Myotome Muscle served by motor
spinal or cranial nerve
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Sensory transduction at mechanoreceptors
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DCML and Trigeminal Analogs Thalamus VPL Body,
VPM Head
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DCML first, second, third and fourth order neurons
SI for conscious perception, posterior parietal
association cortex for meaning of stimulus
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Trigeminal Analogs of DCML System
Third order VPL, VPM
Cross in CNV Lemniscus
First order DRG V
Second order N. cuneatus, CNV
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Columnar Organization of SIFunctional Elements
of Cortex

• Small vertical columns (lt 1 mm diameter)
• RFs overlap
• Respond to same modality of stimuli
• Same response latencies to simultaneous
  stimulation

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Topographic organization at all levels of DCML
Spinal cord
Medulla
Fibers projecting to higher levels separate
more permitting more precise localization of
lesions
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SI Face/hand lateral, Leg/foot medial same as
in spinal cord and medulla
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Second, third and fourth order neurons have RFs
with an inhibitory surround component to focus
activity
Inhibits spread due to divergence in ascending
pathways
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RFs of labial receptors are smaller than gingival
receptors
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Clinical Features of DCML LesionsSensory Deficits

• Increased two-point discrimination threshold
• Increased vibratory sensation threshold
• Decrease in position sense and kinesthesia
• Loss of ability to recognize objects by touch
  (astereognosis)
• Failure to perceive the stimulus on one side
  during double simultaneous stimulation
  (unilateral extinction)
• Loss of ability to recognize letters or numbers
  traced on skin (agraphesthesia)

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Dermatome Pattern of Sensory Deficits
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Case Study 1 (Answer)

• The presence of neglect, denial of illness,
  astereognosis and agraphesthesia (in the absence
  of profound somatosensory loss) are strongly
  characteristic of lesions that involve the
  posterior parietal association cortex. A lesion
  here influences higher order somatosensory
  processing, interfering more with the sense of
  body scheme and the ability to perceive the
  meaning of the stimulus than with mere perception
  of a stimulus. These findings appear more
  frequently when the lesion is in the right
  cerebral hemisphere than when it is in the left.

Shortcut to ovation
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Lewis Carroll, Alices Adventures in Wonderland

• Alice, speaking to the Cheshire Cat
• Would you tell me, please, which way I ought to
  go from here?
• That depends a good deal on where you want to
  get to, said the Cat.
• I dont much care where, said Alice.
• Then it doesnt matter which way you go, said
  the Cat.
• so long as I get somewhere, Alice added as an
  explanation.
• Oh, youre sure to do that, said the Cat, if
  you only walk long enough.

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