Somatosensation Part 1

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SomatosensationPart 1

• Lecture 14

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Somatosensation

• Sensory info from body
• Kinesthesia
• interoceptors
• body position movement
• Cutaneous senses
• exteroceptors
• touch / pain

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The Cutaneous Senses

• Physical events next to organism
• info about stimuli in direct contact
• surfaces, objects, energy
• body position
• Feedback critical for skilled movements
• walking after foot falls asleep
• talking after a shot of novocaine
• no feedback from touch

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The Cutaneous Senses

• Primarily mechanical stimulation
• Several types of receptors
• Several submodalities
• Touch pressure
• Pain
• Temperature
• Kinesthesis
• Haptic System

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Function of Skin

• Largest organ
• 9 lbs. 1.8 m2 , 0.5-4 mm thick
• Barrier
• Protection from foreign agents
• Containment of vital fluids
• Regulation of body temperature

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Function of Skin

• Sensory organ
• Cutaneous Sensitivity
• Sensory effect of skin stimulation
• pressure, temperature, pain
• not uniformly sensitive

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Somatosensory Receptors

• Several types
• thermoreceptors
• chemoreceptors
• mechanoreceptors
• nociceptors
• proprioceptors
• Transduction of environmental energy

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Mechanoreceptors

• Unmyelinated axon branches

• Free nerve endings
• glabrous hair regions
• pain sensation

• Basket cells
• Hairy regions

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Mechanoreceptors

• Encapsulated end organs
• Glabrous (hairless) skin
• Non-neural tissue
• Merkels Disks

• Meissners corpuscle
• Pacinian corpuscles
• Ruffini endings

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Receptive Fields

• Skin region influencing single neuron
• small, sharply defined
• fine spatial detail, tactual acuity
• large, less distinct regions
• fine acuity not as important

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Receptor Adaptation

• Different rates of adaptation
• response to continued stimulation
• Slowly adapting (SA)
• steady pattern of firing
• Rapidly adapting (RA)
• fire only at onset of stimulus

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Receptive Field Size
Small Large
Meissners Corpuscle
Pacinian Corpuscle
Rapid Slow
Adaptation
Ruffini Ending
Merkels Disks
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Thresholds for Touch

• Absolute threshold
• skin displacement 0.00004 inch
• sensitivity varies
• face gt trunk gt fingers arms
• Point localization
• also varies by region
• hands mouth 2 mm
• upper arm, thigh, back 1 cm

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Thresholds for Touch

• Two-point Threshold
• minimum separation between 2 points perceived as
  separate
• mobile regions 4 mm
• immobile regions 48 mm

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Primary Afferent Nerves

• Receive information from receptors
• Project to CNS
• Parallel pathways
• touch proprioception
• pain temperature

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Primary Afferent Nerves

• Size large ---gt small
• from sensory receptors Aa, Ab, Ad, C
• from muscles I, II, III, IV
• C small, unmyelinated
• pain temperature
• Aa
• not associated with skin receptors
• muscles sensory info

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Primary Afferent Nerves

• Speed of conduction
• larger is faster
• myelinated faster
• slowest C fibers

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Somatosensory Pathways

• Touch -
• Dorsal column-medial lemniscal pathway
• Pain and Temperature -
• Spinothalamic system
• Trigeminal pathway
• face neck
• cranial nerve V, also others

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Somatosensory Pathways

• Spinal Cord Columns
• Ascending tracts of axons
• Dorsal columns
• touch proprioception
• Lateral columns
• pain temperature

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Spinal Cord
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Somatosensory Pathways

• Thalamus
• ventral posterior nuclei (VP)
• pain touch still segregated
• pain also to intralaminar nuclei
• also input from cortex
• integration of info
• Input to S1

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Dorsal column-medial lemniscal pathway

• Submodalities
• Touch
• vibration
• 2 point discrimination
• proprioception

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Touch
S1
Dorsal column-medial lemniscal pathway
Thalamus - VP
Medulla
DRG
R
Spinal Cord
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Somatosensory cortex

• S1 - Postcentral Gyrus
• Somatotopic Organization
• topographic representation of body
• Distorted Homunculus
• disproportionate amount of cortex for body parts
• high sensitivity large cortical area

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Somatosensory Cortex

• Formation of a Body Image
• Does not simply respond to sensory input
• Phantom Limbs
• after amputation
• also pain
• stimulation of nerves in stump?
• surgery only temporary relief

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Posterior Parietal Cortex - PPC

• Constructs spatial coordinates for behavior
• Apraxia
• inability to purposefully organize movements
• Left parietal apraxia
• bilateral inability to perform requested
  movements
• Constructional apraxia - damage to PPC

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Spatial Neglect

• Contralateral neglect
• neglect of left side of body and world
• damage to right PPC
• neglect left side of sensory world