Chapter 7 Somatosensory System

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Chapter 7 Somatosensory System

• Chris Rorden
• University of South Carolina
• Norman J. Arnold School of Public Health
• Department of Communication Sciences and
  Disorders
• University of South Carolina

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Overview

• gt20 types of receptors in skin touch,
  temperature, stretch, etc
• 2 pathways to brain
• Dorsal columns
• Precise touch, joint angle, etc.
• Crosses side at medulla
• Antero-lateral Tract
• Coarse information regarding pain and temperature
• Convergence of information
• Crosses side at entry in spinal column

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Early Somatosensation

• PNS detection of
• Pain
• Temperature
• Touch
• Conscious proprioception
• Transfer of information to CNS

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Cross section of spinal cord
Muscle
Motor Cell
Efferent Fibers
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Hierarchy of Sensory Fibers

• Specialized Receptors
• (Stimuli to Neural Signal)
• Single Nerve Fiber
• Sensory Fiber Bundle
• Spinal Nerve
• Dorsal Root Ganglia
• Dorsal Column Nuclei
• Spinal Motorneurons or Reticular Formation
• Thalamus
• Primary and Association Cortex (Parietal Lobe)

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Organization

• Each tract mediates specific modalities of
  sensation, somatotopic organization in tracts and
  cortex
• Mechanoreceptive
• Mechanical displacement of nerve endings
• Touch (fine and diffuse), pressure, vibration,
  kinesthesia
• Thermoreceptive
• Cold and Heat
• Nociceptive
• Pain

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Specialized Receptors

• Receptors specialize by type of stimulus
• Adaptiveness Reduction of response to sustained
  stimuli
• Basic Types of Sensory Receptors
• Encapsulated Endings
• Adapting (tactile)
• Pacinian corpuscle deep pressure touch and high
  frequency vibration.
• Meissners corpuscle light touch, such as the
  fingertips, palms, soles, lips, tongue, face
• Free Nerve Endings (pain, temp, some tactile)
• Nonadapting
• Expanded Tip Endings (tactile, temp)
• Moderately adapting

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Three neuron Organization

• 1st Order
• Dorsal Root Ganglion
• 2nd Order
• Enter CNS at spinal cord or brainstem
• Project to opposite side crossing midline to
  thalamus
• 3rd Order
• Thalamus neurons which project to cortex

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Discriminative Touch
Bipolar or multipolar
Cerebral Cortex
3
Thalmus
2
1
Brainstem/spinal cord
Unipolar nerve
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Anatomical Divisions

• Dorsal Column-Medial Lemniscal (or Epicritic
  System)
• Fine discriminative touch, vibration, limb
  position, kinesthesia deep pressure
• Position sense
• Proprioception - Awareness of limb position
• Kinesthesia - Awareness of limb movement
• Anterolateral (or Protopathic System)
• Pain, temperature and diffuse touch
• Lateral spinothalamic tract
• Anterior spinothalamic tract
• Dorsal Column-Medial Lemniscal System

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Dorsal Column-Medial Lemniscal System

• Important for skilled movements
• Stereognosis - Fine touch discrimination
• Graphesthesia - Recognizing numbers written on
  body
• Two and multiple point touch
• Deep touch
• Receptors
• Meissners and Pacinian Corpuscles
• Encapsulated end receptors
• Highly sensitive and adaptable
• Muscle Spindle Organs
• Kinesthesia
• Proprioception

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Neural Pathways

• Neural Pathways
• Fasciculus Gracilis
• Fasciculus Cuneatus
• Path
• Spinal Ganglion (1)
• Gracilis or Cuneatus Nucleus (2)
• Through Medial Lemniscus to Thalamus (3)
• Thalamus to Cortex

Mediate discriminative Touch from different Body
areas follow three-neuron organization
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Levels of Reception

• Fasciculus Gracilis
• Sacral to Midthoracic Level
• Lower Body
• Fasciculus Cuneatus
• Above Midthoracic Level
• Upper Body

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Pathway

• Spinal Cord
• Brainstem Nuclei
• Thalamus (N. Ventral Posterolateralis)
• Thalamus through Internal Capsule to Primary
  Sensory Parietal Cortex
• Primary to Association Cortex
• Mapped spatially (homunculus)

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Dorsal Column-Medial Lemniscal System

• In the PNS/Spine

Pacinian corpuscle
Cervical Thoracic Lumbar Sacral
Fasciculus cuneatus
Fasciculus gracilis
Meissners corpuscle
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Dorsal Column-Medial Lemniscal System

• Pons and Medulla

Nucleus gracilis (lower body)
Nucleus cuneatus (upper body)
Medulla
Decussation
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Dorsal Column-Medial Lemniscal System

• Midbrain-Cortex

Homonculus
Thalamus
Midbrain
Medial lemniscus
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The homunculus (little man)

• The motor strip (red, frontal cortex) spatially
  map corresponding portions of the contralateral
  hemisphere.

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Clinical Considerations

• If injury is inferior to decussation, deficit can
  be ipsilateral (same side)
• If injury is suprior to decussation, deficit will
  be contralateral (opposite side)
• Tests
• Two Point Discrimination
• Stereognosis ID object with eyes closed
• Graphesthesia number or letter on skin
• Vibratory Tuning fork on bony surface
• Romberg standing with eyes closed
• Kinesthesia movement identified
• Association Identification of object

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Anterolateral system

• Pain, Temperature, Diffuse Touch
• Three-tier neuron organization system
• Enter at spinal ganglion (1st)
• Cross in spinal tract (2nd order)
• Ventral posterolateral n. of thalamus (3rd)
• Two Tracts
• Lateral Spinothalamic
• Anterospinothalamic

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Lateral Spinothalamic Tract

• Receptors - Free Nerve Endings
• Neural Pathway
• Nocioceptors (pain)
• Dorsolateral spinal cord (up or down several
  segments)
• spinal cord entrance
• Substantial Gelatinosa and Proprius
• Where 1st order neurons connect with 2nd order
  neurons
• Lateral Spinothalamic Tract
• Cross Midline (2nd order)
• Spinal Lemniscus (brainstem)
• Thalamus (VPL) to Cortex
• Collaterals to Subcortical structures

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Pain and Temperature

• Anterolateral System

Cerebral Cortex
3
Thalmus
2
1
Brainstem/spinal cord
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Clinical Considerations (lesion locations)

• PNS or spinal before midline cross results in
  problems ipsilaterally.
• Spinal or Brainstem lesion results in problems
  contralaterally.
• Chordotomy (surgical lesion) to reduce pain
• Dermatomes Failure to perceive pain

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Dermatome

• Dermatome Refers to the body area innervated by
  the neurons in a single dorsal root ganglion
  (dorsal part of the spinal nerve)

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Dermatome

• Can help distinguish between psychiatric and
  neurological injury.
• Psychiatric conversion disorder often
  glove/stocking anesthesia
• Neurological disorder follows dermatomes

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Other Considerations

• Referred pain one site has pain but felt in
  another site
• Drugs can suppress pain sensitivity or block
  pathway
• Analgesia No sensation
• Hypalgesia Decreased pain (higher threshold)
• Hyperalgesia Increased pain (lower threshold)

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Anterospinothalamic Tract

• Discrimination of Diffuse touch
• Receptors All three types
• Encapsulated endings
• Free nerve endings
• Expanded tip endings
• Neural Pathway
• Skin to ganglia (1st)
• Dorsolateral spinal cord (up and down seg)
• Proprius and Substantia Gelatinosa (2nd)
• Go to spinothalamic tract to VPL (thalamus) to
  postcentral gyrus
• Collaterals to subcortical structures
• Clinically, interruption causes no obvious
  deficit

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Collaterals in the axon
Cortex
VPL in thalamus
Subcortical structures
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Sensation from the head

• Face and Head area
• face
• forehead
• anterior half of scalp
• dura mater
• orbital cavities
• nasal and oral cavities
• Epicritic and Protopathic Systems

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Facial sensation

• Three Neuron Levels
• 1st order Semilunar ganglion of Trigeminal Nerve
• 2nd order Principal sensory nucleus and
  trigeminal spinal tract nucleus
• 3rd order VPL in thalamus to lower third of
  postcentral gyrus

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Fine Discriminative Touch

• Neural Pathway
• Encapsulated receptors in facial and head skin
• Semilunar ganglion and trigeminal nucleus
• Medial Lemniscus Thalamus to cortex

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Cranial Proprioceptive and Kinesthetic Sensation

• Teeth, periodontium palate, TMJ, muscles of
  mastication
• Involves mesencephalic N. and follows similar
  pattern
• Mechanism for jaw reflex and bit control

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Cranial Sensation Clinical Considerations

• Lesions can affect only one branch
• Ophthalmic
• Maxillary
• Mandibular
• Or one half of the face
• Tests the same for discrimination

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Pain and Temperature from Face

• Neural Pathway
• Nocioceptors
• Semilunar ganglion to
• nucleus of spinal trigeminal tract (moves
  caudally)
• chief sensory nucleus
• Cross midline to thalamus and some stay
  ipsilateral
• Postcentral Gyrus

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Trigeminal Cranial Nerve
Cerebral Cortex
3
Thalmus
2
1
Brainstem Spinal Cord
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Clinical Considerations

• Inflammation of semilunar ganglion causes severe
  pain
• Tic douloureux - severe pain
• Assessment of normal function
• pinching to cause pain
• Quality assessment by patient

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Diffuse Touch from Face

• Neural Pathway
• Dorsal and ventral secondary trigeminal tract
• Some to spinal trigeminal tract nucleus
• Some to chief sensory nucleus
• To ventral posteromedial nucleus of thalamus
• To sensory cortex

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Unconscious Proprioception

• Conscious proprioception by dorsal column-medial
  lemniscal system
• Unconscious involved in walking, articulating,
  writing, swallowing, and eye movement.
• Two order neural system
• Tracts
• Dorsal Spinocerebellar
• Cuneocerebellar
• Ventral Spinocerebellar
• Receptors
• Muscle spindles and Golgi tendon organs located
  in muscles and limb joints

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Ventral Spinocerebellar Tract

• Mediates unconscious proprioception
• Lower limbs to bilateral cerebellum
• Sacral and Lumbar levels through ventrolateral
  Spinocerebellar tract to opposite cerebellar
  hemisphere

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Dorsal Spinocerebellar Tract

• Mediates unconscious proprioception
• Lower limbs and middle regions of body to to
  bilateral cerebellum
• Spinal ganglion to nucleus dorsalis of Clark at
  third lumbar segment
• Do not cross and enter ipsilateral cerebellar
  hemisphere

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Cuneocerebellar Tract

• Mediates upper limbs and neck
• Uncrossed fibers to ipsilateral external cuneate
  nucleus to cerebellum
• Clinical Considerations
• Romberg used to determine some function
• Difficult to test clinically