The Somatosensory System

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

Receptor potential genesis

St: stimulus RP: response

Me: Merkel cell; Ne: nerve ending 6 Mechanosensitive channels

7 Modeling the skin I: Continuum Mechanical Model

physical quantities closely related to local membrane stretch were most predictive of the observed afferent responses.

8 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 Exploring the skin: Magnetic resonance elastography (MRE)

10 II. Major somatosensory pathways: Lemniscus medialis & Tractus spinothalamicus

Touch, proproception Pain, temperature

11 II.a. Topographic organization Labeled lines

12 The vibrissal somatosensory pathway of rodents

13 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 The dermatomes

15 II.b. Functional representations Thalamocortical loop

16 The cortical somatosensory map

17 The hand representation area

18 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 Functional vibrotactile maps: submodalities

20 Sub-barrel column direction map

21 Further cortical processing

22 III: Receptive field (RF) organization

23 Subcortical origin of surround RF

24 RF characteristics

25 Orientation and direction sensitivity

26 Spatio-temporal dynamics of RF

Lagged inhibition: 30 ms delay

27 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 Comparison of Peripheral and cortical RFs

29 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 IV. Encoding stimulus attributes in SI: Texture discrimination

31 Velocity invariance

32 Vibrotactile discrimination

33 Population response

34 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