The Somatosensory System. Structure and function

Transcription

1 The Somatosensory System Structure and function L. Négyessy PPKE, 2011

2 Somatosensation Touch Proprioception Pain Temperature Visceral functions

3 I. The skin as a receptor organ Sinus hair Merkel endings are also found in the sinus hair (2,5). Note the clustering and annular distribution of receptors (2,5,6) in the sinus follicle. 3

4 Touch receptors Meissner Merkel Paccini Ruffini Free nerve ending ridges: 0.5 mm, grooves: 0.25 mm 4

5 The Merkel cell - structure Cross section of the skin. Arrows indicate the localization of Merkel cells deep in the epidermis at the border with the dermis. Scanning electron micrograph. Cell surface processes: diam.: 0,1-0,25 mm length: ~1 mm (<2,5 mm) 5

6 The Merkel cell - function Receptor potential genesis St: stimulus RP: response Me: Merkel cell; Ne: nerve ending 6

7 Mechanosensitive channels 7

8 Modeling the skin I: Continuum Mechanical Model physical quantities closely related to local membrane stretch were most predictive of the observed afferent responses. 8

9 Modeling the skin II: 3D Finite Element Model the elastic behavior of skin is nonlinear and can be divided into three regions: an initial region of low elastic modulus, a transition region, and a final region with high elasticity 9

10 Exploring the skin: Magnetic resonance elastography (MRE) 10

11 II. Major somatosensory pathways: Lemniscus medialis & Tractus spinothalamicus Touch, proproception Pain, temperature 11

12 II.a. Topographic organization Labeled lines 12

13 The vibrissal somatosensory pathway of rodents 13

14 Spinal cord The spinal rootlets, contributing to one nerve, arise from one spinal segment. Each segment is a functional unit, related to a region of the body. Limited independence - controlled by the CNS (brain stem and cortex) - descending tracts Intersegmental coordination - ascending fibres to higher centres - propriospinal fibres within the cord 14

15 The dermatomes 15

16 II.b. Functional representations Thalamocortical loop 16

17 The cortical somatosensory map 17

18 The hand representation area 18

19 Somatotopy and the funneling ilusion A merging index (MI) was designed to measure the spatial shift in the activation spot location. The merging index (MI) ranges from 1 (cortical location of one digit) to 0 (centre between two digits) to 1 (cortical location of other digit). Under two digit stimulation conditions, the center of digit activation can shift either towards the center (MI < 1 ) or away from the center (MI > 1 ). 19

20 Functional vibrotactile maps: submodalities 20

21 Sub-barrel column direction map 21

22 Further cortical processing 22

23 III: Receptive field (RF) organization 23

24 Subcortical origin of surround RF 24

25 RF characteristics 25

26 Orientation and direction sensitivity 26

27 Spatio-temporal dynamics of RF Lagged inhibition: 30 ms delay 27

28 The 3 component RF model Fix components: -orientation selectivity -spatial filter (selectivity for spatial features, patterns) Lagged inhibition: -stimulus gradient selectivity -direction selectivity Note the fixed relative position of the excitatoiry and fixed inhibitory components. Only the lagged inhibitory component changes its position. 28

29 Comparison of Peripheral and cortical RFs 29

30 Summary of RF organization The three-component RFs predicted orientation sensitivity and preferred orientation to a scanned bar accurately. The orientation sensitivity was determined most strongly by the intensity of the coincident RF inhibition in relation to the excitation. The fixed excitatory and inhibitory components of each neuron function as a spatial filter, conferring selectivity for particular spatial features or patterns regardless of scanning direction and velocity. The lagged inhibitory component confers selectivity for stimulus gradients in the scanning direction, regardless of that direction. To the extent that its lag center is displaced from the center of excitation, it also functions, at least theoretically, as a basis for directional sensitivity. 30

31 IV. Encoding stimulus attributes in SI: Texture discrimination 31

32 Velocity invariance 32

33 Vibrotactile discrimination 33

34 Population response 34

35 SUMMARY Vibrotactile receptors Somatosensory pathways, labeled lines Vibrotactile cortical maps RF organization (3 component model) RF characteristics: selectivity for stimulus features Neural correlates of texture and vibrotactile discrimination Basics of population coding 35