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
Afferent Fibers
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

Dorsal root ganglion (spinal ganglion)
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Discriminative Touch
Cerebral Cortex
3
Bipolar or multipolar
Thalmus
2
Dorsal root ganglion
Medulla
Receptors (skin, muscle, joints)
1
Spinal cord
Pseudo-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

• Fasciculus Gracilis
• (slender, graceful)
• Fasciculus Cuneatus
• (wedge-shaped short)
• (think cuneiform writing)
• Path
• Spinal Ganglion (1)
• Fasciculus Gracilis/Cuneatus tracts (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 superior 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 (antero-lateral)
Bipolar or multipolar
Cerebral Cortex
3
Dorsal root ganglion
Thalmus
2
Receptors (skin, muscle, joints)
1
Spinal cord
Pseudo-Unipolar nerve
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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 (Dorsal) and Protopathic
  (Anterolateral) 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

Dorsal spinocerebellar tract information about
movement (sensory feedback) Ventral
spinocerebellar - internally generated
information about the movement.
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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

Romberg Test Ask individual to stand straight
with feet together and hands by the sides.
Compare balance with eyes open versus eyes
closed. If less steady with eyes closed
(positive), ataxia is sensory spinal injury. If
there is no difference (negative) it suggests
cerebellar problem.
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MCQ

• Which is the nucleus?
• A
• B
• C
• D

A
B
C
D
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MCQ

• Which is the node of ranvier?
• A
• B
• C
• D

A
B
C
D
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MCQ

• Which is the nucleus?
• A
• B
• C
• D

A
B
C
D