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Somatosensation
Lesson 17
Somatosensation
Sensory info from body
 Cutaneous senses
 exteroceptors
 touch / pain
 Kinesthesia
 interoceptors
 body position & movement ~

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 ~

The Cutaneous Senses
Primarily mechanical stimulation
 Several types of receptors
 Several submodalities
 Touch & pressure
 Kinesthesis
 Haptic System
 Temperature


Pain ~
Somatosensory Receptors
Several types
 mechanoreceptors
 nociceptors
 thermoreceptors
 chemoreceptors
 proprioceptors
 Transduction of environmental
energy ~

Mechanoreceptors
Unmyelinated axon
branches
 Free nerve ending
 glabrous & hair regions
 pain sensation
 Basket cells

Hairy regions ~
Mechanoreceptors

Encapsulated end organs
Glabrous (hairless) skin

Non-neural tissue
Merkel’s Disks
Meissner’s corpuscle
Pacinian corpuscles

Ruffini endings ~


~

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 ~

Somatosensory Pathways
Touch & Proprioception
 Dorsal column-medial lemniscal
pathway
 Pain and Temperature
 Spinothalamic system
 Trigeminal pathway
 face & neck
 cranial nerve V, also others ~

Spinal Cord
dorsal columns
Dorsal
lateral columns
Ventral
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 ~

PPC
M1
S1
Somatosensory Cortex
Topographic representation
 Body Image – S1
 Space around us – PPC
 Coordinates for movement
 Phantom limbs & pain
 after amputation
 Contralateral neglect
 PPC damage ~

Kinesthesia
Body Position & Movement
 proprioception
 Joint information
 Pacinian corpuscles & Ruffini endings
 Muscle & tendon information
 changes in tension

Golgi tendon organ
muscle spindle fibers ~
Cutaneous Receptors
Stretching of the skin
 Limited role in proprioception
 Knee: anesthesia no affect
 Mouth, hands, & feet proprioception
significantly reduced by anesthesia
 Ruffini Endings
 slow adapting
 population of neurons responding
simultaneously ~

Muscle Receptors
Major role in proprioception
 Stretch receptors
 detect changes in tension
 2 types of receptors
 Muscle spindles & Golgi tendon organs
 differences in threshold & location ~

Muscle-Spindle Receptors
Muscle length detectors
 Parallel with extrafusal fibers
 Low threshold
 Monosynaptic stretch reflex
 Postural adjustments
 Muscle tonus
 Sensory neuron  alpha motor neurons

monosynaptic excitation

disynaptic inhibition ~

Intrafusal Muscle Fibers
Innervated by gamma motor
neurons
 active concurrently with alpha
motor neurons
 Control tension on muscle spindles
 Not force for movement
 “Load the Spindles” ~

Dorsal
Gamma
Motoneuron
M
S
+
+
Ventral
Golgi Tendon Organ
Gauges muscle tension
 high threshold
 Stretch receptor
 safety mechanism
 controlled muscle contraction ~

Dorsal
GTO
Inhibits alpha
motor neuron
-
Ventral
+
GTO: Function
Inhibits muscle contraction
 Control of motor acts
 slow contraction as force increases
 e.g., holding an egg
 Autogenic inhibition
 safety mechanism
 too much tension  damage ~
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