Somatosensation

1. Somatosensation Lesson 17

2. Somatosensation • Sensory info from body • Cutaneous senses • exteroceptors • touch / pain • Kinesthesia • interoceptors • body position & movement ~

3. 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 ~

4. The Cutaneous Senses • Primarily mechanical stimulation • Several types of receptors • Several submodalities • Touch & pressure • Kinesthesis • Haptic System • Temperature • Pain~

5. Somatosensory Receptors • Several types • mechanoreceptors • nociceptors • thermoreceptors • chemoreceptors • proprioceptors • Transduction of environmental energy ~

6. Mechanoreceptors • Unmyelinated axon branches • Free nerve ending • glabrous & hair regions • pain sensation • Basket cells • Hairy regions ~

7. Mechanoreceptors • Encapsulated end organs • Glabrous (hairless) skin • Non-neural tissue • Merkel’s Disks • Meissner’s corpuscle • Pacinian corpuscles • Ruffini endings ~ ~

8. 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 ~

9. Somatosensory Pathways • Touch & Proprioception • Dorsal column-medial lemniscal pathway • Pain and Temperature • Spinothalamic system • Trigeminal pathway • face & neck • cranial nerve V, also others ~

10. Dorsal dorsal columns lateral columns Ventral Spinal Cord

11. 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 ~

12. M1 PPC S1

13. Somatosensory Cortex • Topographic representation • Body Image – S1 • Space around us – PPC • Coordinates for movement • Phantom limbs & pain • after amputation • Contralateral neglect • PPC damage ~

14. Kinesthesia • Body Position & Movement • proprioception • Joint information • Pacinian corpuscles & Ruffini endings • Muscle & tendon information • changes in tension • Golgi tendon organ • muscle spindle fibers ~

15. 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 ~

16. 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 ~

17. 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 ~

18. 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” ~

19. Gamma Motoneuron Dorsal M S + + Ventral

20. Golgi Tendon Organ • Gauges muscle tension • high threshold • Stretch receptor • safety mechanism • controlled muscle contraction ~

21. GTO + - Dorsal Inhibits alpha motor neuron + Ventral

22. 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 ~