Somatosensory System Touch - mechanoreception

1. Somatosensory System Touch - mechanoreception Limb posture movements and forces – proprioception (Haptics – object recognition through touch + proprioception, usually with the hand) Readings: Squires et al Ch 25 Supplementary reading: Kandel et al Chs 21-23 http://www.youtube.com/watch?v=FKxyJfE831Q

3. + Hair follicle afferents Most Sensitive to deformation 20-50 corpuscles/afferent ?? RA SA11 PC SA1 Slipping object/large rf’s High density Form/braille Texture-hardness roughness Transmitted vibration textures Stretch?

4. Reaction to slippage: 65 msec - 45 msec is peripheral nerve and motor response time. Applied forces in response to shape of grasped object - 100 msec.

5. Mechanoreceptors Differ in Morphology and Skin Location RA SA1 SAII

6. merkel meissner ruffini pacinian

7. Dorsal root ganglion cells serve a dual role of transduction and information transmission. Multiple fibers from the axon branch to form a large receptive field. Receptive fields in the skin overlap.

8. Large variation in two-point threshold across the body surface The Spatial Resolution Varies Because the Density of Mechanoreceptors Varies

9. RA mechanoreceptor responds to sinusoidal mechanical stimuli with a single action potential for each cycle. The lowest stimulus intensity that evokes one action potential per cycle of the sinusoidal stimulus is called the receptor's “tuning threshold.” Merkel: 5-15 Hz Meissner: 20-50 Hz Pacinian: 60-400 Hz 25 Hz vibratory stimulus Tuning thresholds for vibration RA’s – lower thresholds

10. Spatial characteristics of stimuli depend on activity across population of receptors.

11. Stimulus intensity in encoded by spike rate Neural response Perceived magnitude (pressure)

12. Proprioceptors in muscle spindles and tendons

13. dorsal column-medial lemniscalsystem for tactile sensations and proprioception and anterolateral system for pain and temperature

17. Serial + parallel processing

20. Tactile information from the fingertips is used faster than can be readily explained by rate codes. (Johansson & Birznieks NN 2004) Reaction to slippage: 65 msec - 45 msec is peripheral nerve and motor response time. Applied forces in response to shape of grasped object - 100 msec.

21. Receptive fields in area 3b of S1

22. Velocity invariance of cell’s responses.

23. Receptive fields of neurons in S1 are larger than those of the sensory afferents. Each of the hand figurines shows the receptive field of an individual neuron in areas 3b, 1, 2, and 5, based on recordings in alert monkeys.Colored regions indicate the region where light touch elicits action potentials. Neurons at later stages of cortical processing (areas 1 and 2) have larger receptive fields and more specialized inputs than neurons in area 3b. The neuron from area 2 is directionally sensitive to motion toward the fingertips. Neurons in area 5 often have symmetric bilateral receptive fields at mirror image locations on the contralateral and ipsilateral hand. Ablation of area 3b: general losses Ablation of area 1: loss of texture discrim Ablation of area 2: loss of 3D form discrim Direction sensitivity emerges in Area 1 and 2, not present in 3b

24. Orientation tuning in SII receptive fields. Finger pads have similar tuning. Results from lateral inhib, not composition of inputs as in vision Ablation of SII: Loss of discrim of shape and texture Doesn’t affect SI responses Ablation of SI silences SII Responses modulated by attention

25. Effect of lesion in area 2 - loss of ability to coordinate fingers, plus sensory deficits.

26. Somatotopic organization of SI

27. http://hebb.mit.edu/courses/8.515/lecture3/sld005.htm